Wet Weather Conveyance and Treatment Evaluation

Transcription

1 Wet Weather Conveyance and Treatment Evaluation SausalitoMarin City Sanitary District February 2008 Innovative Solutions for Water and the Environment

2 SausalitoMarin City Sanitary District Wet Weather Conveyance and Treatment Evaluation Report Prepared by: Water and Environment February 2008

3 Table of Contents ES Executive Summary... i ES1.1 Purpose of Report and Project Background... i ES1.2 Existing Conveyance and Treatment Facilities... i ES1.3 Findings Regarding Current Wastewater System...iii ES1.3.1 Wet Weather Flows to be Managed...iii ES1.3.2 Capacity of Existing Conveyance System... iv ES1.3.3 Statistics on Current Blending Practice at Treatment Plant... v ES1.3.4 Impact of Peak Flows on Plant Performance... v ES1.3.5 Hydraulic Capacity of Plant... vi ES1.3.6 Process Capacity of Plant...vii ES1.4 SystemWide Improvement Alternatives to Handle Wet Weather Flows...vii ES1.4.1 SystemWide Alternative 1: Conveyance with I/I Reduction...vii ES1.4.2 SystemWide Alternative 2: Conveyance with Storage at TCSD...vii ES1.4.3 SystemWide Alternative 3: Wet Weather Storage...viii ES1.4.4 SystemWide Alternative 4: Wet Weather Storage with I/I Reduction...viii ES1.4.5 SystemWide Alternative 5: Wet Weather Storage with Pumped Diversion from LSPS... ix ES1.4.6 Estimated Cost of Alternatives... ix ES1.5 Comparison of Alternatives... x ES1.6 Alternatives without TCSD...xii ES1.7 Recommendations...xii Chapter 1 Introduction Background and Purpose of Study Scope of Study and Report Organization...11 Chapter 2 Existing Facilities Conveyance System Treatment Plant...27 Chapter 3 Design Flows Continuous Simulation Methodology Design Events and Flows...32 Chapter 4 Conveyance System Analysis Model Development Model Data Model Calibration Predicted Performance During Design Events...47 Chapter 5 Wastewater Treatment Plant Evaluation Current Wet Weather Blending Analysis Current Wet Weather Treatment Plant Performance Hydraulic Evaluation Process Capacity Evaluation...55 Chapter 6 Conveyance System Improvement Alternatives Potential Conveyance Improvements Flow Reduction Conveyance System Improvements Upstream Storage Facilities Model Scenarios and Results Real Time Control Strategies...69 Chapter 7 Treatment Alternatives...71 February 2008

4 7.1 Hydraulic Improvements Replace Effluent Flow Meter Weir Remove Primary Clarifier Flow Meter Equalization and Storage Process Improvement Alternatives No Treatment Improvements Alternative Physical/Chemical Treatment Alternatives Biological Treatment Initial Screening of Process Alternatives Project Alternatives to Be Evaluated...78 Chapter 8 SystemWide Alternatives (Conveyance + Treatment) Description of SystemWide Alternatives SystemWide Alternative 1: Conveyance with I/I Reduction SystemWide Alternative 2: Conveyance with Storage at TCSD Systemwide Alternative 3: Wet Weather Storage Systemwide Alternative 4: Wet Weather Storage with I/I Reduction Systemwide Alternative 5: Wet Weather Storage with Pumped Diversion from LSPS Alternatives without TCSD Chapter 9 Comparison of SystemWide Alternatives Discussion of Alternative 1: Conveyance with I/I Reduction Alternative 1a Alternative 1b Alternative 1c Discussion of Alternative 2: Conveyance with Storage of TCSD Flows Discussion of Alternative 3: Wet Weather Storage Discussion of Alternative 4: Wet Weather Storage with I/I Reduction Discussion of Alternative 5: Wet Weather Storage with Diversion from LSPS Recommended Alternative(s) Alternative 1b vs. Alternative Alternative 2 vs. Alternative Recommendations...97 References... R February 2008

5 List of Figures Figure 21: Service Area 22 Figure 22: Conveyance System 23 Figure 23: Conveyance System Schematic 24 Figure 24: Photos of Locust Street Pump Station 25 Figure 25: Photos of Scotty s Wet Weather Pump Station 26 Figure 26: Photos of Main Street Pump Station 27 Figure 27: SMCSD Process Flow Schematic 27 Figure 28: Existing SMCSD Facility Layout 29 Figure 31: Design Rainfall Events 33 Figure 32: Flow Hydrographs for 5Year Design Event (Unrestricted System) 35 Figure 41: Profile of Modeled Conveyance System 42 Figure 42: Tributary Basins 43 Figure 43: TriTriangular Unit RDI/I Hydrograph 44 Figure 44: Hydraulic Profile Upstream of LSPS Under 5Year Design Event 47 Figure 51: NDPES Permit Exceedances Excluding, Illicit Discharge, Pilot Testing, Mech. Failure and Disinfection Exceedances (April 2003 to March 2007) 53 Figure 52: Historical San Francisco Bay Tide Data 54 Figure 53: SMCSD WWTP Hydraulic Profile 56 Figure 61: Potential Storage Site Martin Luther King Park 64 Figure 62: Potential Storage Site Marinship Park 65 Figure 63: Potential Storage Site Dunphy Park 65 Figure 64: Potential Storage Site Vacant Lot at Bridgeway & Locust 66 Figure 65: Potential Storage Site Community Center near Bell Lane PS 66 Figure 71: Membrane Filtration Module 73 Figure 72: Existing Sand Media Filters at SMCSD 74 Figure 73: HighRate Clarification 75 Figure 74: Enhanced Secondary Sedimentation 76 Figure 75: Membrane Bioreactor Schematic 77 Figure 76: Existing Fixed Film Reactors and Secondary Clarifiers at SMCSD 77 Figure 77: Enhanced Secondary Sedimentation Footprint Requirement 79 Figure 78: HighRate / Ballasted Flocculation Clarifier Layout 710 Figure 79: MBR Process Layout 712 Figure 81: Conveyance with I/I Reduction Alternative Piping Schematic 83 Figure 82: Location of Proposed MLK Park Wet Weather Storage Facility 86 Figure 83: Piping Schematic for Wet Weather Storage Alternative 87 Figure 84: Proposed MLK Park Storage Basin and Pump Station Conceptual Layout 88 Figure 85: Proposed MLK Park Storage Basin and Pump Station Elevation View 88 Figure 86: Pumped Diversion from LSPS Alternative Piping Schematic at LSPS 812 February 2008

6 List of Tables Table 31: SMCSD Design Flows (Unrestricted System) 34 Table 41: Modeled Pump Stations 41 Table 42: Basin Flow Parameters 46 Table 43: RDI/I Hydrograph Parameters 46 Table 51: Blending Events Analysis Annual Totals 51 Table 52: Process Loading Comparison 57 Table 61: Results of Conveyance System Model Scenarios 68 Table 71: Preliminary Comparison of Alternatives 78 Table 81: SystemWide Alternatives 81 Table 82: Treatment Alternatives for SystemWide Approach 82 Table 83: Estimated Cost of SystemWide Alternative 1 84 Table 84: Estimated Cost of SystemWide Alternative 2 85 Table 85: Estimated Cost of SystemWide Alternative Table 86: Estimated Cost of SystemWide Alternative Table 87: Estimated Cost of SystemWide Alternative Table 88: Estimated Cost of Alternatives with TCSD Removed from SMCSD System 813 Table 91: Cost Comparison of SystemWide Alternatives 91 Table 92: Summary Comparison of Alternatives 92 Table 93: Possible Cost Allocations for Alternatives 2 and 5 97 Appendices Appendix A Detailed Cost Estimates February 2008

7 List of Abbreviations ADWF Average Dry Weather Flow BOD Biological Oxygen Demand CDO Cease and Desist Order District SausalitoMarin City Sanitary District EPA U.S. Environmental Protection Agency FFR Fixed Film Reactor GGNRA Golden Gate National Recreation Area GIS Geographic Information System HP Horsepower I/I Infiltration/Inflow KW Kilowatt LSPS Locust Street Pump Station MBR Membrane Bioreactor MG Million Gallons MGD Million Gallons per Day MLK Park Martin Luther King Park MLKPS Martin Luther King Pump Station MOST Model for Optimization of Storage and Treatment MSPS Main Street Pump Station NFAA No Feasible Alternatives Analysis NOAA National Oceanic and Atmospheric Administration NPDES National Pollutant Discharge Elimination System OD Outside Diameter PDWF Peak Dry Weather Flow PICS Program for I/I Continuous Simulation PS Pump Station PWWF Peak Wet Weather Flow RDI/I RainfallDependent Infiltration/Inflow Regional Board Regional Water Quality Control Board RMC RMC Water and Environment RTC Real Time Control SASM Sanitation Agency of Southern Marin SMCSD SausalitoMarin City Sanitary District SSO Sanitary Sewer Overflow TCSD Tamalpais Community Services District TM Technical Memorandum TSS Total Suspended Solids VFD Variable Frequency Drive WERF Water Environmental Research Foundation WSPS Whiskey Springs Pump Station WWTP Wastewater Treatment Plant February 2008

8 Executive Summary

9 Executive Summary ES Executive Summary This report summarizes the evaluation of alternatives to address capacity issues related to peak wet weather flows in the SausalitoMarin City Sanitary District (SMCSD, District) wastewater conveyance and treatment systems. This report was prepared by RMC Water and Environment (RMC) under a contract with the District dated December 21, ES1.1 Purpose of Report and Project Background SMCSD experiences significant increases in flows in its wastewater system during wet weather events. In very large events, such as the major storm that occurred on December 31, 2005, the wastewater treatment plant (WWTP) has reached or very nearly reached its maximum hydraulic capacity. High peak flows have also resulted in sanitary sewer overflows (SSOs) in the upstream conveyance system during some storm events. The District has also had effluent discharge violations at the treatment plant, some of which have occurred during wet weather, and has been subject to enforcement actions by the San Francisco Bay Regional Water Quality Control Board (Regional Board). The District is currently under an administrative order from the U.S. EPA to correct effluent discharge violations. In addition, in order to continue its current practice of blending (bypassing a portion of the primary effluent flow around the secondary treatment process during peak wet weather events), the District will be required to conduct a detailed analysis of alternatives to blending (referred to as a No Feasible Alternatives Analysis or NFAA), including potential collection system and treatment plant improvements and/or construction of storage facilities. Prior to this study, the District completed a wet weather capacity evaluation for the upper portions of the conveyance system tributary to the District s Locust Street Pump Station, which identified potential improvements to that portion of the system for handling wet weather flows. This study builds upon the previous efforts by developing systemwide alternatives, including both conveyance and treatment plant improvements. The analyses conducted as part of this study will provide information to support the District s response to the requirements of the EPA Order, provide the data needed for the NFAA, and provide overall strategies for dealing with peak wet weather flows in the SMCSD system. ES1.2 Existing Conveyance and Treatment Facilities The SMCSD service area includes the City of Sausalito and the unincorporated community of Marin City in Marin County, California. Under contract, the District also provides wastewater conveyance and treatment for flows from the Tamalpais Community Services District (TCSD) and for the U.S. governmentowned property at Fort Baker, Fort Cronkite, and Fort Barry in the Golden Gate National Recreation Area (GGNRA). The SMCSD wastewater system includes the wastewater collection system serving Marin City; the wastewater conveyance system that conveys flows from Sausalito, Marin City, and TCSD to the SMCSD treatment plant; and a 1.8MGD treatment facility and outfall located off of Fort Baker Road on property owned by the U.S. National Park Service on which SMCSD holds an easement. The City of Sausalito owns and operates its own wastewater collection system, which discharges to the SMCSD system at several locations. Flows from TCSD are pumped into the SMCSD system from TCSD s Bell Lane Pump Station. Flows from the GGNRA area are pumped directly to the SMCSD treatment plant via a pump station and force main owned and operated by the National Park Service. Figure ES1 shows the SMCSD conveyance system. The major facilities include 4.5 miles of gravity and force main interceptor pipelines extending south to the treatment plant, roughly paralleling the western shore of Richardson Bay; and two major pump stations located at Locust Street and Main Street. February 2008 i

10 Executive Summary Figure ES1: Conveyance System SMCSD Wet Weather Conveyance and Treatment Plant Evaluation Flow from TCSD & Marin City Scotty's PS From Gate 5 PS Figure ES1 Conveyance System From Whiskey Springs PS Bridgeway Blvd Locust St. PS 101 From Anchor St PS From Princess St PS Main St. PS Legend Modeled PS Force Main City of Sausalito Sewer Gravity Interceptor (diameter) 17.3 inch lined 20.3 inch lined all other gravity pipes Miles Flow from Ft. Baker (Not in Service) WWTP February 2008 ii

11 Executive Summary There is also a small wet weather pump station (Scotty s) located at Bridgeway and Coloma Street to which peak flows that exceed the capacity of the gravity interceptor can be diverted during wet weather events. Portions of the 21inch gravity interceptor upstream of the Locust Street Pump Station (LSPS) have previously been rehabilitated with 17.3 and 20.3 inch liners, reducing the original capacity of the pipeline. Figure ES2 shows a flow schematic of the wastewater treatment plant. The treatment units include a primary clarifier, fixed film reactors, secondary clarifiers, an effluent screen, sidestream granular media filters (up to 1 MGD), chlorine contact basins, and an outfall that discharges to San Francisco Bay. The plant was designed to provide treatment for 1.8 MGD of average dry weather flow, 6.0 MGD of peak day flow, and a peak instantaneous flow of 10 MGD. Although the fixed film reactor feed pumps can pump up to 7.8 MGD, primary effluent flow above 6 MGD is routed around the fixed film reactors and directed to the secondary clarifiers, a practice known as blending that is allowed under the District s current NPDES permit. Figure ES2: SMCSD Wastewater Treatment Plant Process Flow Schematic ES1.3 Findings Regarding Current Wastewater System The following findings regarding the capacity and performance of the existing wastewater system were used as the basis for developing alternatives to handle peak wet weather flows. ES1.3.1 Wet Weather Flows to be Managed The design flows for the SMCSD system were determined from flow monitoring data and an analysis methodology that utilized a technique called longterm continuous simulation to develop estimates of peak flows for various recurrence frequencies. Flows in the SMCSD system are metered at the major pump stations (Bell Lane, Marin City, Locust Street, and Main Street), as well as at the treatment plant. In addition, temporary flow meters and rain gauges were installed at several locations in the system during the 2004/05 wet weather season as part of the Locust Street Pump Station study. These monitoring locations provided data with which to estimate current dry and wet weather flows in the SMCSD system and calibrate a longterm continuous simulation model that was used to assess the frequency, duration, and volume of wet weather flows over a modeled period of 58 years. For this study, a 5year recurrence frequency event was selected as the primary event for evaluation of system capacity and design of system improvements. By definition, a 5year event is one that has a probability of 1 in 5, or 20 percent, of being exceeded in any given year. Based on precedent sent by February 2008 iii

12 Executive Summary other agencies in the San Francisco Bay region, a 5year event has been generally accepted by regulators as an appropriate criterion for evaluation and design of wastewater facilities for wet weather conditions. Analysis of the longterm flow simulations indicated that the large storm event of December 3031, 2005 was equivalent to a 5year event in terms of peak flow rate and flow volume within the SMCSD system. This storm event was used to assess the performance of the various alternative improvements presented herein. Table ES1 presents the estimated current dry weather flow rates and the wet weather flow rates that would be expected from this storm event at various points in the wastewater system, if there were no flow restrictions or overflows. As shown in this table, the 5year peak instantaneous (15minute) flow rate at the treatment plant would be approximately 13.8 MGD. This study assumes that the service area is predominantly built out; therefore flows would not increase in the future due to growth. Similarly, it is assumed that wet weather infiltration/inflow (I/I) will not increase substantially in the future, and that any such increases would be controlled through normal sewer system maintenance, inspection, and rehabilitation. Table ES1: SMCSD Design Flows (Unrestricted System) Facility ADWF (MGD) PDWF (MGD) 5year PWWF (MGD) Bell Lane PS (TCSD) Locust St. Pump Station Locust St. PS + Scotty's PS Main St. PS WWTP ADWF = Average Dry Weather Flow PDWF = Peak Dry Weather Flow PWWF = Peak Wet Weather Flow ES1.3.2 Capacity of Existing Conveyance System The capacity of the existing conveyance system was evaluated by developing a computer hydraulic model of the system and running the model for the 5year design event selected based on the continuous simulation analysis (December 3031, 2005 storm). The model identified the extent of surcharge or potential overflows due to capacity limitations in the pipelines and/or pump stations. The key findings of this modeling indicate the following: The lined 21inch gravity sewer upstream of the LSPS would be significantly surcharged under these conditions, potentially resulting in an overflow upstream of Coloma Street. This result is consistent with the findings of the Locust Street Pump Station study. There would be no significant surcharge in the gravity interceptor in between LSPS and Main Street Pump Station (MSPS), largely because the capacity restrictions upstream limit the magnitude of the peak flow that can reach the downstream facilities. Under this scenario, the peak flow reaching the LSPS would be approximately 4.6 MGD, with an approximate 11.9 MGD peak at the MSPS. (If upstream constrictions were relieved, however, flow rates at these points would be greater, as shown in Table ES1.) February 2008 iv

13 Executive Summary ES1.3.3 Statistics on Current Blending Practice at Treatment Plant As part of the wet weather blending analysis required by the EPA, an assessment of the frequency, duration, and volume of current blending events is required. Blending occurs when primary effluent flows are bypassed around the fixed film reactors (FFRs). Although the FFRs were designed to treat up to 6.0 MGD, the FFR feed pumps are believed to be able to feed up to 7.8 MGD to the FFRs. However the maximum allowable flow to the FFRs is regulated by the control system for the pumps. It is believed that historically, the FFR pumps were not allowed to operate at their full capacity of 7.8 MGD during wet weather, however the pumps may have been allowed to send more than 6.0 MGD to the FFRs. Therefore, for the purposes of this evaluation historical blending event where estimated using both 6.0 MGD and 6.8 MGD FFR feed capacities. Using both 6.0 MGD and 6.8 MGD secondary treatment capacities for the analysis should provided a reasonable range of estimated historical blending events. Due to concern over the treatment effectiveness and potential impacts to the FFRs, it is recommended that feed to the reactors be limited to the original design value of 6.0 MGD. The blending analysis listed under 6.0 MGD secondary treatment capacity could be used as indication of the frequency, duration and volume of potential future blending events. The results from the blending frequency analysis are presented in Table ES2. Wet Weather Year Table ES2: Blending Events Analysis Annual Totals 6.0 MGD Secondary Treatment Capacity (Future Operation) Frequency (# of Events/ Year) a Cumulative Duration (Hours/Year) Cumulative Volume (MG/Year) 6.8 MGD Secondary Treatment Capacity Frequency (# of Events/ Year) a Cumulative Duration (Hours/Year) Cumulative Volume (MG/Year) b Average (not including ) Notes: a Unique wet weather blending events were defined based on one hour intervals (i.e. not blending for 1 hour followed by blending was counted as a new blending event) b wet weather season includes influent flow data through January 24, 2007 The analysis of blending events indicates that when blending events do occur (average 7.0 per year under future operations), they last for a short duration (average 4.3 hours/event) and result in a relatively small volume (average 0.27 MG/event) of primary effluent that does not receive biological treatment. Based on the average blending frequency presented in Table ES2, for 6.0 MGD secondary treatment capacity, 99.7 percent of the annual flow into the SMCSD treatment plant receives full biological treatment. ES1.3.4 Impact of Peak Flows on Plant Performance A key factor to be evaluated as part of this report is whether peak wet weather flows impact treatment plant performance. During the past few years, the SMCSD plant has had difficulty meeting its NPDES effluent limits. An analysis of recent permit exceedances was performed to determine whether exceedances were more common in wet weather, which would indicate reduced plant performance. February 2008 v

14 Executive Summary From April 1, 2003 through March 30, 2007 there were 76 NPDES permit exceedances. Overall the treatment plant has had difficulty consistently meeting NPDES permit limits. However some of these exceedances can be attributed to specific one time events such as equipment problems, illegal discharges to the conveyance system, and chemically enhanced sedimentation pilot testing problems. These events were documented in the District s annual reports to the Regional Water Quality Control Board and USEPA, which characterize treatment plant performance and the District s compliance with its waste discharge requirements. In order to obtain a more accurate representation of wet weather impacts on NPDES permit compliance (i.e. plant performance), the 45 permit limit exceedances due to these one time events where not used in this analysis. The remaining 31 NPDES permit exceedances were related to BOD and suspended solids exceedances. Figure ES3 presents the 31 BOD and TSS NPDES permit limit exceedances from April 1, 2003 through March 30, 2007, along with the average daily flow at that time. As shown in Figure ES3, eight of the 31 permit exceedances occurred during periods when influent flow to the plant exceed 1.5 MGD, while 23 occurred at influent flows less than 1.5 MGD. On this basis, it could be said that permit exceedances occur throughout the year even when there is normal flow to the plant and that exceedances do not occur more frequently during the high flow (wet weather) events. Figure ES3: NDPES Permit Exceedances Excluding, Illicit Discharge, Pilot Testing, Mech. Failure and Disinfection Exceedances (April 2003 to March 2007) 7.0 Average Day Flow (MGD) Monthly Weekly Daily/Inst Flow (MGD) /1/2003 7/2/2003 1/1/2004 7/1/ /31/2004 7/1/ /31/2005 7/1/ /31/2006 ES1.3.5 Hydraulic Capacity of Plant A hydraulic model of the SMCSD plant was developed to evaluate its hydraulic capacity. Hydraulic capacity is dependant on tide level because the SMCSD treatment plant discharges through an outfall to the San Francisco Bay. Based on observed conditions and the hydraulic modeling, the hydraulic capacity of the plant is believed to be 13.0 MGD during a peak high tide event and approximately 13.5 MGD during a normal high tide. February 2008 vi

15 Executive Summary ES1.3.6 Process Capacity of Plant The capacity of the individual process units at the plant were analyzed by comparing actual loadings with typical treatment design criteria for wet weather. Based on the comparison, the primary clarifier exceeds the typical range of surface loading rates at flows above 7.1 MGD. The fixed film reactors approach the upper limit of the typical hydraulic loading range at 6.0 MGD, which is the current design capacity for the fixed film reactors. The surface loading rate to the secondary clarifiers exceeds the typical design range during peak flows above 5.6 MGD. Although some processes exceed the typical design value, it should be noted that original 1986 wastewater treatment plant expansion was designed to handle an instantaneous peak flow of 10 MGD. On this basis the peak day process capacity of the plant was estimated to be 6 MGD consistent with the design capacity of the plant, with the hydraulic loadings to the primary sedimentation, fixed film reactor, and secondary clarifiers being the limiting factors. ES1.4 SystemWide Improvement Alternatives to Handle Wet Weather Flows Five systemwide alternatives were developed to reduce system overflows and bypasses based on the system capacities and performances described above. The alternatives consisted of different combinations of the following components: I/I flow reductions obtained by rehabilitation of collection system sewers and service laterals Storage of peak wet weather flows to reduce peak flow rates Treatment plant improvements to increase plant process capacity beyond the current capacity of 6 MGD ES1.4.1 SystemWide Alternative 1: Conveyance with I/I Reduction Alternative 1 includes I/I flow reductions, minor conveyance system improvements, and improvements to the treatment plant to increase its peak treatment capacity. It results in peak wet weather flows of up to 13 MGD to the treatment plant. This flow is considered the instantaneous hydraulic capacity of the plant and outfall and is achieved through I/I reductions in the collection system. I/I reduction is assumed to be achieved through rehabilitation of public sewer mains and the connected service laterals located within the public rightofway (lower laterals). In this case, the sewer rehabilitation is assumed to be focused in the portion of the City of Sausalito downstream of the LSPS, which appears to contribute the highest I/I to the system. Conveyance system improvements would include upgrading the City of Sausalito s Whiskey Springs Pump Station to function as both a dry and wet weather facility (replacing the existing Scotty s PS), which would allow diverting additional wet weather flows away from the capacitydeficient lined gravity interceptor. This alternative has three variations with the differences being how to treat peak flows at the treatment plant. Alternative 1a (full biological secondary treatment for entire flow) and Alternative 1b (ballasted flocculation for peak flows greater than 6.0 MGD) are intended to comply with EPA s blending policy by providing two different types of secondary treatment up to the 13 MGD peak flow. The third variation, Alternative 1c, continues the current practice of bypassing flows above 6 MGD around the FFRs. This complies with the District s current permit. ES1.4.2 SystemWide Alternative 2: Conveyance with Storage at TCSD Alternative 2 was developed to assess whether eliminating TCSD flows from the SMCSD system during peak wet weather periods would reduce flows sufficiently to avoid the need for I/I reduction, conveyance system improvements, or major modifications to the treatment plant. Although it might be possible for TCSD to permanently redirect all of its flows now going to SMCSD to another treatment facility, for February 2008 vii

16 Executive Summary purposes of comparing the cost of alternatives, it was assumed that elimination of TCSD flows during peak wet weather periods could effectively be accomplished by construction of a 2 MG storage facility within the TCSD. This facility would store all of TCSD s flows during peak wet weather events, allowing peak flow rates to the treatment plant to be limited to 10.5 MGD. This flow rate is within the hydraulic capacity of the plant, but exceeds the secondary capacity of 6 MGD by 4.5 MGD. Therefore, the alternative calls for a 4.5 MGD ballasted flocculation facility to treat primary effluent flows above current secondary capacity, which if approved by the EPA, would not be considered as blending. ES1.4.3 SystemWide Alternative 3: Wet Weather Storage Alternative 3 was developed to assess whether flow storage, of a size that could be sited within the collection system, could avoid the need for I/I reductions and minimize the improvements needed at the treatment plant. Hydraulic analysis indicates that without I/I reductions, peak flows to the treatment plant would be reduced to 8.1 MGD, if approximately 3.5 MG of storage were sited within the collection system. The site selected as the best potential location for the storage facility was the Martin Luther King (MLK) Park on Coloma Street west of Bridgeway, located just west of the existing Scotty s Pump Station. A conceptual location for this facility is shown on the aerial photograph in Figure ES4. The facility would also include a new pump station to replace both the Scotty s PS and Whiskey Springs PS, which both have operational deficiencies. Under this alternative, treatment plant modifications would be relatively minor and would include a ballasted flocculation facility with a capacity of 2.1 MGD. This modification would have a very small footprint, would be relatively low in cost, and potential could fit within the plant site and function as a replacement for the existing sand filters.. Figure ES4: Conceptual Location of MLK Park Wet Weather Storage Facility ES1.4.4 SystemWide Alternative 4: Wet Weather Storage with I/I Reduction Alternative 4 reduces peak flows to the treatment plant s existing secondary capacity of 6 MGD, thereby avoiding the need for treatment capacity upgrades. It limits the flow rate with a combination of I/I flow reductions and 3.5 MG of storage located within the collection system. The I/I flow reductions required February 2008 viii

17 Executive Summary in this alternative (2.2 MGD, or 50 percent I/I reduction in the target area downstream of the LSPS) are greater than those achieved in Alternative 1 (1.3 MGD, or 30 percent I/I reduction), and are assumed to be achieved through rehabilitation of the upper (private) laterals in addition to the mains and lower laterals. ES1.4.5 SystemWide Alternative 5: Wet Weather Storage with Pumped Diversion from LSPS Alternative 5 increases flow storage to 4.5 MG, which reduces peak flows to the treatment to 6 MGD. Additional diversions to storage are accomplished by pumping back flows that reach the LSPS to the proposed storage facility at MLK Park. This alternative therefore eliminates the need to rely on I/I reduction in the collection system to reduce peak flows, and avoids the need to increase the secondary treatment capacity beyond its current capacity of 6 MGD. ES1.4.6 Estimated Cost of Alternatives The estimated costs of the alternatives are summarized in Table ES3. Table ES3: Cost Comparison of SystemWide Alternatives Alt. 1a 1b 1c 2 Description Conveyance with I/I Reduction; Full Secondary Treatment Conveyance with I/I Reduction; Ballasted Flocculation Conveyance with I/I Reduction; Continue Blending Conveyance w/storage at TCSD Peak Flow at WWTP (MGD) I/I Reduction a Estimated Capital Cost ($ millions) Conveyance Improvements and Flow Storage b Treatment Plant Improvements Total Wet Weather Storage Wet Weather Storage with I/I Reduction Wet Weather Storage plus Pumped Diversion to Storage a I/I reduction costs include rehabilitation or replacement of local sewer mains, lower laterals and, in some cases, upper laterals, and would be the responsibility of the collection system owner (the City of Sausalito in the case of the alternatives above) and/or private property owners. b Does not include improvements that may be required at LSPS and MSPS to address other deficiencies not specifically related to wet weather capacity. In conjunction with such upgrades, Alternatives 1 and 2 would also require an increase in capacity at MSPS. February 2008 ix

18 Executive Summary ES1.5 Comparison of Alternatives Table ES4 presents a summary comparison of the systemwide alternatives. The implications of this comparison are discussed below. Alternative 1a (I/I reduction with full secondary treatment for 13 MGD peak flow) cannot be recommended within the context of managing wet weather flows because of its very high cost. However, unlike all of the other alternatives, this alternative results in a new treatment plant with a 20 to 30 year design life, with substantial benefits to dry weather treatment performance. Alternative 1b (I/I reduction with 7 MGD ballasted flocculation for peak flows) and Alternative 5 (wet weather storage with pumped diversion) have very similar costs. However, Alternative 5 provides future benefits that Alternative 1b does not. Alternative 5 reduces by 50 percent the capacity that would be needed with a future overhaul/replacement of the plant and thereby offers considerable future cost savings. These savings have not been included in the analysis because of the speculative nature of estimating when a major treatment plant overhaul/replacement would need to occur. Alternative 5 minimizes the physical size of a future plant overhaul/replacement, which is an advantage because of the constrained plant site and difficulty of obtaining additional area from the National Park Service. On this basis, Alternative 1b cannot be recommended when compared to Alternative 5. Alternative 1c (I/I reduction with continuation of current blending practice) has the lowest cost, but cannot be recommended because it continues the existing blending practice which results in severely overloaded process units throughout the plant. Furthermore, the EPA trend is to minimize this practice in general, and the poor performance of the SMCSD plant in this alternative makes this alternative non viable in the longterm. Alternative 4 (wet weather storage with I/I reduction) cannot be recommended because of cost. It is similar to Alternative 5 in that it relies heavily on flow storage and avoids the need to upgrade the capacity of the treatment plant. However, it requires a large amount of I/I flow reduction through collection system rehabilitation, including potentially private laterals, which causes the cost of this alternative to be twice the cost of Alternative 5 without offering mitigating advantages. Furthermore, the large amount of required sewer system rehabilitation would involve substantial disruption to residents and businesses in the community, and carries with it the associated uncertainties in cost and effectiveness. Based on the above discussion, Alternative 2 (Conveyance with storage at TCSD) and Alternative 5 (Wet Weather Storage plus Pumped Diversion) are considered the most advantageous alternatives. Alternative 2 offers cost savings in the near term when compared to Alternative 5. However, it requires upgrading the capacity of the existing plant from 6 MGD to 10.5 MGD with the addition of a ballasted flocculation facility. This facility could fit within the existing plant site, but it would add to an already congested site. Alternative 2 would also require that an eventual overhaul or replacement of the existing plant to provide 10.5 MGD of capacity, which is not as advantageous as Alternative 5 which requires a longterm plant capacity of 6 MGD. However, Alternative 2 is highly dependent upon TCSD s willingness to construct a 2 MG storage facility within its service area or otherwise redirect its flows elsewhere. A storage facility located at TCSD has an estimated capital cost of $7.9 million. It should be noted that while none of the apparent best alternatives involves a targeted I/I reduction program, rehabilitation of sewers within the tributary collection systems of Sausalito, Marin City, and TCSD will still be needed in the long term to address maintenance and structural problems in aging infrastructure, reduce saltwater intrusion in lowlying areas, and prevent further increases in I/I due to continued system deterioration. These rehabilitation requirements must also be considered in funding for longterm sewer system improvements. February 2008 x

19 Executive Summary Table ES4: Summary Comparison of Alternatives Alt. Description Purpose of Alternative Storage Volume Needed (MG) WWTP Capacity Needed (MGD) Upgrade Needed at Treatment Plant Method of Meeting Blending Rule at Treatment Plant Treatment Reliability Est. Capital Cost ($ mil) 1a Conveyance with I/I Reduction; Full Secondary Treatment Assess the ramifications of conveying up to 13 MGD to treatment plant, and using full secondary treatment to treat all flows without blending. none 13 major Full biological secondary treatment for all flows very high $ 48 1b Conveyance with I/I Reduction; Ballasted Flocculation for Peak Wet Weather Flows Same as Alternative 1a, except that all flows above 6 MGD would receive treatment in a ballasted flocculation process rather than in a biological secondary treatment process. none 13 moderate Ballasted flocculation of peak flows high $ 17 1c Conveyance with I/I Reduction; Continue Blending Assess whether I/I flow reductions alone would provide a viable means of managing wet weather flows. none 13 minor Continue current blending practice low $ 11 2 Conveyance w/storage at TCSD Assess whether additional flow reduction, storage, or conveyance improvements would be needed if TCSD flows were eliminated, and assess the implications for the treatment plant. 2 (at TCSD) 10.5 moderate Ballasted flocculation of peak flows high $ 13 3 Wet Weather Storage Assess whether flow storage, without I/I rehabilitation, would avoid the need for major capacity upgrades at the treatment plant moderate Ballasted flocculation of peak flows high $ 20 4 Wet Weather Storage with I/I Reduction Assess whether flow storage, in conjunction with I/I rehabilitation could avoid major improvements at the treatment plant minor Full biological secondary treatment of all flows high $ 33 5 Wet Weather Storage plus Pumped Diversion to Storage Assess whether additional diversions into flow storage could eliminate the need for I/I rehabilitation and treatment capacity upgrades minor Full biological secondary treatment of all flows high $ 19 February 2008 xi

20 Executive Summary ES1.6 Alternatives without TCSD For each of the storage alternatives described above (Alternatives 3, 4, 5), the storage volume and cost of facilities could be reduced if flows from TCSD were permanently diverted away from the SMCSD system. Without TCSD, the required storage volume at the MLK Park facility would be reduced by about 2 MG and the influent piping to the storage facility could also be reduced in size. Under Alternative 2, the storage facility at TCSD would not be a component of SMCSD system improvements. However, under all alternatives, the peak wet weather flows to the WWTP and associated treatment plant improvements (if required) would remain the same. Table ES5 presents the estimated costs of these alternatives (denoted as Alternatives 2x, 3x, 4x, and 5x) without TCSD. These alternatives are presented for informational purposes only and are not analyzed in detail in this report. Table ES5: Estimated Cost of Alternatives with TCSD Removed from SMCSD System Alt. 2x 3x 4x Description Peak Flow at WWTP (MGD) I/I Reduction Estimated Capital Cost ($ millions) Conveyance Improvements and Flow Storage Treatment Plant Improvements Conveyance without a 10.5 b TCSD Wet Weather Storage without TCSD Wet Weather Storage with I/I Reduction without TCSD Total c x Wet Weather Storage plus Pumped Diversion to Storage w/out TCSD a No storage at TCSD. b Minor weir modifications to allow diversion of more flow to Scotty s PS; cost would be minor. c I/I reduction costs include rehabilitation or replacement of local sewer mains, lower laterals and, in some cases, upper laterals, and would be the responsibility of the collection system owner (the City of Sausalito in the case of this alternative) and/or private property owners. ES1.7 Recommendations Because of the significantly lower total costs associated with Alternative 2, it is recommended that TCSD s willingness to construct the storage facilities be assessed. If TCSD is willing to construct these facilities, Alternative 2 could be recommended. Projectspecific approval from the EPA to use ballasted flocculation to meet its blending policy would also be recommended if Alternative 2 is to be implemented. Although the ballasted flocculation facility could also provide benefits under dry weather flows, there is also a possible risk that future changes in regulations might preclude use of this treatment technology to comply with secondary treatment requirements for wet weather flows. If TCSD is unwilling to construct storage facilities or redirect its flows elsewhere, Alternative 5 would be recommended. This alternative allows the District to meet the EPA s blending policy by reducing peak wet weather flows to the treatment plant s current secondary treatment capacity; it promises the lowest longterm cost when an eventual plant overhaul/replacement is considered; and by minimizing the capacity needed by the longterm plant, it minimizes the amount of treatment plant site area needed in the longterm. February 2008 xii

21 Executive Summary As indicated in Table ES5, the costs of potential alternatives could be significantly reduced if TCSD flows were permanently removed from the SMCSD system. Termination of the agreement between SMCSD and TCSD would have other cost implications relating to revenue generation and cost sharing. Should either the District or TCSD wish to consider termination of their agreement in the future, the alternatives presented in this report should be reevaluated under this new scenario. It should also be noted that the success of the recommended wet weather strategies depends on wet weather flows not increasing in the future. The District, in conjunction with its satellite agencies, should continue to monitor flows in the system and implement appropriate best management practices for sewer system maintenance, inspection, and rehabilitation to control I/I and gradually replace old and defective sewers and service laterals. February 2008 xiii

22 Section 1 Introduction Wet Weather Conveyance and Treatment Evaluation

23 Introduction Chapter 1 Introduction This report summarizes the evaluation of alternatives to address capacity issues related to peak wet weather flows in the SausalitoMarin City Sanitary District (SMCSD, District) wastewater conveyance and treatment systems. This evaluation was conducted by RMC Water and Environment (RMC) under a contract with the District dated December 21, Background and Purpose of Study Like many other communities in Marin County, SMCSD experiences significant increases in flows in its wastewater system during wet weather events. In very large events, such as the major storm that occurred on December 31, 2005, the wastewater treatment plant (WWTP) has reached or very nearly reached its maximum hydraulic capacity. High peak flows have also resulted in sanitary sewer overflows (SSOs) in the upstream conveyance system during some storm events. The District has had effluent discharge violations at the treatment plant, some of which have occurred during wet weather, and have been subject to enforcement actions by the San Francisco Bay Regional Water Quality Control Board (Regional Board). The District is currently under an administrative order from the U.S. EPA to correct violations of the Clean Water Act. The EPA Order requires the District to document historical SSOs, treatment unit bypasses, and effluent violations; and submit reports detailing plans to eliminate SSOs and prohibited bypasses, achieve effluent limits, and ensure accurate flow measurement and appropriate operations and maintenance procedures. In addition, in order to continue its current practice of blending (bypassing a portion of the primary effluent flow around the secondary treatment process during peak wet weather events), the District will be required to conduct a detailed analysis of alternatives to blending (referred to as a No Feasible Alternatives Analysis or NFAA), including potential collection system and treatment plant improvements and/or construction of storage facilities. The analyses conducted as part of this study will provide information to support the District s response to the requirements of its NPDES permit and EPA Order, provide the data needed for the NFAA, and provide overall strategies for dealing with peak wet weather flows in the SMCSD system. Prior to this study, the District completed a wet weather capacity evaluation for the upper portions of the conveyance system tributary to the District s Locust Street Pump Station (Technical Memorandum, Locust Street Pump Station Wet Weather Flow and Capacity Study, MWH, February 23, 2006), which identified potential improvements to that portion of the system for handling wet weather flows. The District has also conducted various evaluations of facilities and processes at the treatment plant, but none specifically addressing wet weather flow issues. This study builds upon the previous efforts by developing systemwide alternatives, including both conveyance and treatment plant improvements, and evaluating feasible strategies for handling peak wet weather flows in the SMCSD system. 1.2 Scope of Study and Report Organization The scope of work for the study included in the following major tasks: Task 1 Update Design Flows and Peak Flow Recurrence Frequency Task 2 Assess Capacity of Conveyance System and Wastewater Treatment Plant Task 3 Identify Potential Wet Weather Improvements Task 4 Evaluate Alternatives and Develop Wet Weather Conveyance System Strategy Task 5 Conduct No Feasible Alternatives Analysis for Wet Weather Blending Task 6 Prepare Technical Memorandum Task 7 Project Management February

24 Introduction A preliminary Technical Memorandum (TM) on Existing Wet Weather Hydraulic, Treatment Capacity and Wet Weather Diversion Analysis was prepared in February 2007 to support the District s initial submittal of the NFAA as required for its NPDES permit renewal application. This report provides information that can be used to update those portions of the NFAA and summarizes the overall results and recommendations of this study. The report is organized into the following sections: ES. Executive Summary presents a summary highlighting the key findings and recommendations of the study. 1. Introduction discusses the background and purpose of the study, study scope, and report organization. 2. Existing Facilities describes the existing conveyance system and wastewater treatment plant facilities. 3. Design Flows presents the basis for determining appropriate peak design flows for analysis and design of system facilities. 4. Conveyance System Analysis describes the hydraulic model used to analyze the capacity of the conveyance system and the results of the analysis for current peak flow conditions. 5. Wastewater Treatment Plant Evaluation describes the current hydraulic and process performance of the treatment plant. 6. Conveyance System Improvement Alternatives identifies and analyzes potential improvements to the conveyance system, including sewer rehabilitation to reduce I/I, upgraded pipelines and pump stations, and potential flow storage. 7. Treatment Alternatives identifies and evaluates potential hydraulic and process improvements at the wastewater treatment plant to handle peak wet weather flows. 8. SystemWide Alternatives (Conveyance + Treatment) presents detailed descriptions and estimated costs of five overall systemwide alternatives for handling peak wet weather flows in the SMCSD system. 9. Comparison of SystemWide Alternatives presents a comparison of the alternatives and recommended alternatives for further consideration by SMCSD. February

25 Section 2 Existing Facilities Wet Weather Conveyance and Treatment Evaluation

26 Existing Facilities Chapter 2 Existing Facilities The SMCSD service area, shown in Figure 21, includes the City of Sausalito and the unincorporated community of Marin City in Marin County, California. Under contract, the District also provides wastewater conveyance and treatment for flows from the Tamalpais Community Services District (TCSD), which serves the Tam Valley area north of SMCSD, and for the U.S. governmentowned property at Fort Baker, Fort Cronkite, and Fort Barry in the Golden Gate National Recreation Area (GGNRA). The SMCSD wastewater system includes the wastewater collection system serving Marin City; the wastewater conveyance system that conveys flows from Sausalito, Marin City, and TCSD to the SMCSD treatment plant; and a 1.8MGD treatment facility and outfall located off of Fort Baker Road east of the northern end of the Golden Gate Bridge on property leased from the U.S. National Park Service. The City of Sausalito owns and operates its own wastewater collection system, which discharges to the SMCSD system at several locations. Flows from the GGNRA area are pumped directly to the SMCSD treatment plant via a pump station and force main owned and operated by the National Park Service. The following subsections describe the facilities in the SMCSD conveyance and treatment systems, respectively. 2.1 Conveyance System The SMCSD conveyance system, shown in Figure 22, includes 4.5 miles of gravity and pressure interceptor pipelines and seven wastewater pump stations. The interceptor generally parallels the western shore of Richardson Bay. Figure 23 is a schematic diagram of the conveyance system showing key facilities and major flow input points. At the northern end of the system, flow from TCSD is pumped from TCSD s Bell Lane Pump Station via an 18inch outside diameter (OD) force main, which also receives pumped flow from Marin City and several other smaller tributary areas. South of Marin City, the force main follows Bridgeway and transitions to a 21inch gravity interceptor at Gate 5 Road. The gravity interceptor extends 6,800 feet further downstream to the SMCSD Locust Street Pump Station (LSPS), following Bridgeway, Marinship Way, and through easements parallel to Bridgeway. The upper 3,370 feet of the 21inch sewer has been lined with a 9mm wall thickness curedinplace lining, resulting in an inside diameter of 20.3 inches; and the lower 2,430 feet was lined with an 18.7inch OD slipliner, resulting in an inside diameter of 17.3 inches. The last 1,000 feet of the gravity sewer upstream of the LSPS is 24inch diameter pipe that has not been lined. Flows from the City of Sausalito system discharge to the gravity interceptor upstream of the LSPS at several locations, including pumped discharges from the City s Gate 5 and Whiskey Springs Pump Stations (the latter is located on Bridgeway on the south side of Coloma Street), and gravity sewer connections at Nevada, Spring, Litho, and Locust Streets. February

27 Existing Facilities Figure 21: Service Area February

28 Existing Facilities Flow from TCSD & Marin City Scotty's PS From Gate 5 PS Figure 22: Conveyance System <Replace page with full size figure> SMCSD Wet Weather Conveyance and Treatment Plant Evaluation Figure 22 Conveyance System From Whiskey Springs PS Bridgeway Blvd Locust St. PS 101 From Anchor St PS From Princess St PS Main St. PS Legend Modeled PS Force Main City of Sausalito Sewer (Not in Service) Gravity Interceptor (diameter) 17.3 inch lined 20.3 inch lined all other gravity pipes Miles Flow from Ft. Baker WWTP February

29 Existing Facilities Figure 23: Conveyance System Schematic February

30 Existing Facilities The LSPS, located on the sidewalk on the east side of Bridgeway at the corner of Locust Street, was built in 1953 and is known to be in need of upgrades. The existing pump station is a wet pit/dry pit facility containing two centrifugal pumps (one 20 HP and one 50 HP). In addition, there is a single 35HP emergency submersible pump installed in a manhole immediately upstream of the pump station wet well. In 1999, the District contracted with Nute Engineering to prepare a predesign study and 90 percent design for upgrade of the pump station. Nute estimated the existing pumping capacity at 6.6 to 7.1 MGD with the two centrifugal pumps operating together, and 7.8 to 8.3 MGD with all three pumps operating. Nute proposed to relocate two 35HP submersible pumps (currently temporary pumps ahead of Main St. PS) and install two new 40HP submersible pumps. Nute estimated that these four new pumps operating together would increase the peak pumping capacity to 8.8 to 9.3 MGD, with more reliability. These improvements have not been constructed pending completion of the overall system wet weather studies. Figure 24: Photos of Locust Street Pump Station At Coloma Street there is a side overflow weir in the 21inch pipe that allows high flows during wet weather to be diverted through a 15inch pipe to the Scotty s Wet Weather Pump Station, located on the northwest corner of the Bridgeway/Coloma intersection. From the Scotty s PS, flows are pumped through a 14inch OD (12.9inch inside diameter) force main and discharged immediately downstream of the LSPS to the LSPS force main (the force main can be valved to discharge to the LSPS wet well; however, this mode of operation has generally not been used). The Scotty s PS, constructed in 1987 as an ejector type station, contains a single 35HP submersible pump with a design capacity of 2.9 MGD in a circular 8foot diameter concrete vault. Actual pumping capacity is lower (2.3 MGD or less, depending on flow through LSPS) when valved to pump to the LSPS force main. The control panel is located adjacent to the vault. The station has no permanently installed standby power. February

31 Existing Facilities Figure 25: Photos of Scotty s Wet Weather Pump Station Downstream from Locust Street, the flow is pumped through approximately 4,000 feet of 20diameter force main, which transitions to a 24inch (23.3inch inside diameter) gravity sewer approximately 700 feet south of Princess Street. Two offline pump stations owned by the District discharge flows from areas of Sausalito to the LSPS force main at Anchor Street and Princess Street, respectively. Downstream of Princess Street, there are several additional gravity connections from the City s system to the interceptor. The Main Street Pump Station (MSPS), originally constructed in 1952, is located at eastern end of Main Street east of Second Street. New pumps were installed when this facility was improved in the late 1990s. This pump station is a wet pit/dry pit facility currently containing one 100HP, 2.5 MGD pump and two 250HP, 7.5 MGD pumps. All three pumps have variable frequency drives. The smaller pump was again replaced in about 2005 after the new pump experienced significant cavitation. An evaluation of the MSPS completed in 2001 indicated significant deficiencies in the facility, including lack of pump redundancy, poor access to equipment, poor ventilation, and close proximity to nearby residences with associated issues of noise and odors. Therefore, future replacement of this pump station may be required. Two additional temporary 35HP submersible pumps are located in a junction structure (called the Rock Catcher ) upstream of the pump station. The pumps are currently not in use. The Rock Catcher pumps discharge to an older force main (called the Beach Force Main), which was the original force main for the MSPS. There are still a few connections into this older force main which flow back to the Rock Catcher junction structure; hence it has not been abandoned. The City of Sausalito has plans to remove these connections from the force main, but the necessary project remains to be implemented. The MSPS now discharges to a 20inch force main which extends south along Second Street and Alexander Avenue. The last 265foot section of the interceptor is a very steep pipe which drops over 100 feet from Fort Baker Road to the primary sedimentation tank influent line at the treatment plant. February

32 Existing Facilities Figure 26: Photos of Main Street Pump Station 2.2 Treatment Plant The SMCSD wastewater treatment plant was built in 1953 on the shoreline of central San Francisco Bay. The treatment plant was expanded in 1986 to provide secondary treatment. The existing liquid treatment process flow diagram for the SMCSD plant is shown in Figure 27. Figure 27: SMCSD Process Flow Schematic As illustrated in Figure 27, there are two source of wastewater flow into the treatment plant. The majority flow is sent to the treatment plant from the MSPS. Flow from Ft. Baker and the Marin Headlands also arrives at the plant via a pump station and force main. The treatment plant includes the following treatment units: primary clarifiers, fixed film reactors (FFR), secondary clarifiers, effluent screens, sidestream granular media filters (up to 1 MGD), chlorine contact basins, and an outfall which discharges to San Francisco Bay. Solids handling facilities at the SMCSD plant included a gravity thickener, primary and secondary digesters, a belt press and sludge hauling facilities February

33 Existing Facilities The treatment plant was designed to provide treatment for 1.8 MGD of average dry weather flow, 6.0 MGD of peak day flow 1, and a peak instantaneous hydraulic flow of 10 MGD. Although the FFR feed pumps are capable of sending up to 7.8 MGD to the FFRs, at flows above 6.0 MGD primary effluent is routed around the FFRs and directed to the secondary clarifiers. When flow into the plant is less than 6.0 MGD, some FFR effluent is recycled back with primary effluent and fed back through the FFRs. The layout of the existing SMCSD treatment plant is shown in Figure 28. Several treatment processes such as the secondary clarifiers, chlorine contact basins and primary digester were constructed underneath other process structures. The location and configuration of the plant have resulted in extremely constrained site conditions. The SMCSD treatment plant has had difficulty in consistently meeting its targeted effluent limits in both dry and wet weather. SMCSD has previously performed studies to analyze treatment plant performance and capacity during dry weather conditions. SMCSD is currently implementing several projects to improve treatment capacity, including adding chemically enhanced primary treatment, and switching to motorized distributors on the fixed film reactors. 1 The original secondary treatment plant design drawings show a peak day rating of 5.5 MGD for the overall plant, and 6 MGD for the fixed film reactors. Since the biological process is the limiting process at the plant, 6 MGD is listed as the peak day treatment capacity. February

34 Existing Facilities Figure 28: Existing SMCSD Facility Layout N February

35 Section 3 Design Flows Wet Weather Conveyance and Treatment Evaluation

36 Design Flows Chapter 3 Design Flows The design flows for the SMCSD system were determined from flow monitoring data and an analysis methodology that utilized a technique called longterm continuous simulation to develop estimates of peak flows for various recurrence frequencies. Flows in the SMCSD system are metered at the major pump stations (Bell Lane, Marin City, Locust Street, and Main Street), as well as at the treatment plant. In addition, a flow meter was placed in the 21inch gravity sewer and pump runtime recorders were installed at the Scotty s and Gate 5 Pump Stations and on the LSPS submersible pump during the 2004/05 wet weather season as part of the Locust Street Pump Station study. Temporary rain gauges were also placed at the LSPS and Bell Lane PS during that wet weather season and at the Bell Lane PS during the 2005/06 season (as part of work being conducted for TCSD). These monitoring locations provided data with which to estimate current dry and wet weather flows in the SMCSD system. 3.1 Continuous Simulation Methodology Longterm continuous simulation is a technique that is used to develop a relationship between rainfall and infiltration/inflow (I/I) flows in a sewer system, which can then be applied to a historical rainfall record (on the order of 50+ years) to synthesize wet weather flows over a longterm period. The continuous simulation analysis is conducted using a hydrologic model that is calibrated to actual flow and rainfall monitoring data (typically several recent years of wet weather flows). Statistical analyses are applied to the longterm modeled flows to develop estimates of peak flows and volumes for various recurrence frequencies. A continuous simulation model was developed for SMCSD as part of the Locust Street Pump Station Study. The model is called PICS/MOST, which is a combination of two applications: Program for I/I Continuous Simulation (PICS), which generates the longterm flow hydrograph based on historical rainfall; and Model for the Optimization of Storage and Treatment (MOST), which performs the statistical calculations. PICS/MOST was used in the Locust Street Pump Station Study to identify 20, 10, 5, and 2year frequency peak flow events for the SMCSD service area. The model was calibrated based on flows measured over three wet weather seasons at the LSPS and treatment plant, and rainfall measured in nearby Mill Valley. The historical record for the rain gauge located on Mount Tamalpais (57 years of data from 1948) was used for the longterm simulation, with appropriate adjustments made to approximate the lower rainfall over the SMCSD service area. The continuous simulation methodology was described in detail in an appendix to the Locust Street Pump Station Study TM. For this study, the PICS/MOST model was updated using additional rainfall data for the 2005/06 wet weather seasons, which was one of the wettest seasons in recent years. The addition of the latest rainfall data and some adjustments to the PICS model developed in the previous study resulted in some changes to the predicted events for the various return periods. Specifically, for this study, the longterm simulation was run without dry weather base flow so that the flows represented I/I flows only, and therefore the ranking of the historical events relates to the return frequency of peak I/I flows. This methodology allows the selection of appropriate historical storms to be used to represent specified recurrence frequency wet weather events. Based on the PICS/MOST analysis, the simulated peak flows for the 58year historical rainfall record were ranked to identify various recurrence frequency peak flow events. For example, the approximate 20year event is the event that results in the 3rd highest peak I/I flow in the 58year simulation; while the approximate 5year event results in the 11 th highest flow. Based on this analysis, the storm of December 31, 2005 was determined to be an approximate 5year I/I event for SMCSD. Similarly, PICS/MOST can also be used to predict the recurrence frequency of peak volume events. These events are critical in sizing potential storage facilities. Similar to the rankings for peak flow, MOST produces rankings of the modeled events based on total volume (specifically, the ranking is based February

37 Design Flows on volume exceeding a specified treatment capacity, which in the case of the SMCSD treatment plant, was assumed to be 6 MGD, the capacity of the secondary treatment process). Based on the MOST rankings of storage events, the event of December 31, 2005 was also determined to be an approximate 5 year event based on volume. 3.2 Design Events and Flows For this study, a 5year recurrence frequency event was selected as the primary event for evaluation of system capacity and design of system improvements. Based on precedent sent by other agencies in the San Francisco Bay region, including the Central Marin Sanitation Agency, East Bay Municipal Utility District, and Vallejo Sanitation and Flood Control District, a 5year event has been generally accepted by regulators as an appropriate criterion for evaluation and design of wastewater facilities for wet weather conditions. For SMCSD, a 10year event was also selected as a more conservative secondary event, to be used to verify that overflows would not likely occur during such a larger storm. Specifically, for this study, the storm of December 30 to 31, 2005 is used as the 5year design event, and the storm of February 2 to 3, 1998 as the 10year event. Figure 31 shows the hourly rainfall distribution for the design 5 and 10year events. February

38 Design Flows Figure 31: Design Rainfall Events Year Event December 3031, Rainfall (in.) :00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 Time Year Event February 23, Rainfall (in.) :00 6:00 9:00 12:00 15:00 18:00 21:00 0:00 3:00 6:00 9:00 12:00 Time In determining total design flows for the 5 and 10year events, the actual timing of those events and resultant I/I flows are used in conjunction with the normal diurnal base wastewater flow pattern based on actual flows measured in the system. Table 31 presents the estimated dry and wet weather flows in the SMCSD system based on recent flow data and modeled wet weather flows for the design events (the hydraulic model used for the study is described in the next section of this report). Note that the flows presented in the table represent the flows that would be predicted to reach each of the facilities listed if there were no upstream capacity restrictions. Actual peak wet weather flows under current system conditions would be lower than those shown in the table because pumping and conveyance February

39 Design Flows limitations currently restrict flows that can reach facilities further downstream. Figure 32 shows the flow hydrographs at the MSPS, LSPS, and Bell Lane PS for the 5year event with flow restrictions relieved, as well as the hydrograph of the combined flow from LSPS and Scotty s PS. These hydrographs correspond to the peak flows listed in Table 31 for the 5year event. Table 31: SMCSD Design Flows (Unrestricted System) Facility ADWF (MGD) PDWF (MGD) 5year PWWF (MGD) 10year PWWF (MGD) Bell Lane PS (TCSD) Inteceptor U/S of Coloma St. Diversion Structure Locust St. Pump Station Locust St. PS + Scotty's PS Main St. PS WWTP * 14.8* * Includes estimate of PWWF from Ft. Baker based on recorded flows during Dec. 3031, 2005 storm. ADWF = Average Dry Weather Flow PDWF = Peak Dry Weather Flow PWWF = Peak Wet Weather Flow February

40 Design Flows Figure 32: Flow Hydrographs for 5Year Design Event (Unrestricted System) Main St. PS Locust St. + Scotty's PS Locust St. PS Bell Lane PS (TCSD) Fllow (MGD) :00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 Time It should also be noted that this study assumes that the service area is predominantly built out; therefore flows would not increase in the future due to growth. Similarly, it is assumed that wet weather I/I will not increase substantially in the future, and that any such increases would be controlled through normal sewer system maintenance, inspection, and rehabilitation. February

41 Section 4 Conveyance System Analysis Wet Weather Conveyance and Treatment Evaluation

42 Conveyance System Analysis Chapter 4 Conveyance System Analysis The analysis of the conveyance system was performed using a hydraulic model. This section of the TM describes the development of the model and the modelpredicted performance of the existing system under the 5year design event. 4.1 Model Development An initial model of the system upstream of the LSPS was developed as part of the Locust Street Pump Station study. Specifically, this model included the gravity sewer starting at Gate 5 Road and extending downstream to the LSPS, plus the diversion to the Scotty s Wet Weather PS. The LSPS and Scotty s PS were represented in the model in a simplified fashion. The model utilized EPA SWMM5 software and was calibrated to the flow monitoring data collected during the winter 2004/05 wet weather season. Flow inputs to the model were based on metered flows for Bell Lane PS (TCSD), Marin City PS, and estimates of flows for the various flow input points to the interceptor from the City of Sausalito. These estimates were made by roughly allocating the total flow at the LSPS (minus TCSD and Marin City flow) to the locations where City flows enter the system along the gravity sewer upstream of LSPS. For this study, the SWMM5 model developed in the Locust Street Pump Station study was converted to InfoWorks CS (Wallingford Software) and expanded to include the conveyance system downstream of LSPS, specifically the LSPS and Scotty s PS force mains, the gravity sewer upstream of the MSPS, the MSPS, and the MSPS and Beach force mains to the treatment plant Model Data Data for the modeled facilities were obtained from record drawings and other documentation provided by the District. Pump curves and actual pump on/off settings, as well as wet well dimensions based on pump station plans, were used to more accurately represent the operation of the LSPS, MSPS, and Scotty s pump stations. Table 41 lists key data for each of the modeled pump stations and Figure 41 shows a profile of the modeled system from InfoWorks. Note that other District pump stations (e.g., Marin City, Princess Street, and Anchor Street) and nondistrict facilities (e.g., Bell Lane, Gate 5, Whiskey Springs, and Fort Baker pump stations) are not explicitly modeled but are represented as flow inputs to the major conveyance system. Facility Main St. PS 3 Locust St. PS 3 Table 41: Modeled Pump Stations No. of Pumps Pump No. Pump Type 1 2& d VFD VFD VFD VFD Fixed Speed Pump Capacity a (MGD) PS Firm Capacity b,c (MGD) PS Total Capacity b (MGD) e e e f f Scotty's PS 1 1 Fixed Speed g a Rated pump capacity except as indicated. b Capacity calculated based on manufacturer's pump curves and system curve under peak wet weather flow and high wet well level; assumes 10 percent reduction in theoretical pump performance. c Firm capacity based on largest pump out of service. d Emergency pump located in manhole upstream of pump station wet well. e Estimated by Nute Engineering (Locust St. Pump Station Improvement Project Design Memorandum, September 24, 1999). f High value assumes no flow from Scotty's PS; low value assumes ~2.5 MGD flow from Scotty's PS discharging downstream of LSPS. g High value assumes no flow to LSPS; low value assumes ~5 MGD flow through LSPS. February

43 Conveyance System Analysis Figure 41: Profile of Modeled Conveyance System Refinements were also made to the estimated flow inputs from the City of Sausalito collection system by digitizing sewer basins based on GIS files of the sewer system and parcel base mapping provided by the City. These files identify each manhole based on its basin designation, enabling the tributary area to each connection point on the District s interceptor to be delineated. The estimated area of each tributary basin could then be determined and used to allocate the total system flows to the various basins. Measured flows at the LSPS and MSPS were used for this allocation. Figure 42 shows the basins tributary to the SMCSD interceptor. February

44 Conveyance System Analysis Figure 42: Tributary Basins <Replace page with full size figure> February

45 Conveyance System Analysis Model Calibration Model calibration for a wet weather study primarily involves a process that determines parameters that can be used to represent the shape and magnitude of the rainfalldependent I/I (RDI/I) hydrograph entering the modeled system from each tributary basin. For each basin, the total RDI/I volume is represented as a percentage of the rainfall falling on the basin. This percentage is referred to as the R value. The model employs a tritriangular hourly unit hydrograph to convert the RDI/I volume for each basin into a hydrograph. Under this unit hydrograph model, the total RDI/I volume is distributed over time into three unit hydrographs, corresponding to fast, medium, and slow response. The parts of the total R value placed into each of these unit hydrographs are called R1, R2, and R3, respectively. Figure 43 illustrates the tritriangular unit hydrograph model. Figure 43: TriTriangular Unit RDI/I Hydrograph Each unit hydrograph is defined by a time to peak (T) and a recession constant (K). The model calibration process is used to determine the three sets of T and K values and the fraction of the total RDI/I volume that is assigned to each of the unit hydrographs for each basin. This is done by determining the February

46 Conveyance System Analysis combination of R, T, and K values for each of the three unit hydrographs that results in the best match of model flow (base wastewater diurnal flow plus RDI/I) to observed flows at locations in the system where flow measurements are available. As noted previously, the original SWMM5 model of the LSPS area was calibrated based on the flow monitoring data collected in the 2004/05 wet weather season, specifically to the storm event that occurred in late December The original scope intent of this study was to use this same event to calibrate the downstream portion of the system added to the model. Since the original focus of the Locust Street Pump Station study was to determine conveyance capacity deficiencies and appropriate peak design flows for the LSPS, the original model calibration emphasized accurate simulation of peak wet weather flows. However, in reviewing the model calibration for this study, it was observed that while the model was reasonably calibrated for peak flows, it did not accurately represent the shape and total volume of the wet weather hydrographs. This is particularly important for evaluating wet weather storage options, a focus of this study. Therefore, to refine the model calibration, the model was recalibrated to refine the hydrograph parameters for the original modeled area, as well as to determine appropriate RDI/I hydrograph parameters for the tributary basins downstream of the LSPS. The model calibration was also verified based on the actual flows for the December 31, 2005 storm as measured at the various permanent monitoring points in the SMCSD system. Table 42 lists the flow parameters for each of the basins shown in Figure 42, including average dry weather base wastewater flow and RDI/I hydrograph reference. Table 43 lists the R, T, and K values for each RDI/I hydrograph, which correspond to the major drainage areas shown in Figure 42. As discussed previously, flows were allocated uniformly for the basins upstream of the LSPS (based on recorded LSPS flows and flows measured at the temporary meter installed on the interceptor in the 2004/05 wet weather season) and the basins tributary to the system in between the LSPS and MSPS (based on recorded MSPS flows) in proportion to basin area, as there are no available flow monitoring data to distinguish the flows from individual basins. As shown in Table 43, the results appear to indicate that the Sausalito basins downstream of the LSPS (RDI/I hydrograph Main ) appear to have significantly higher RDI/I contributions than upstream areas. February

47 Conveyance System Analysis Basin (Load MH ID) Table 42: Basin Flow Parameters Approx. Area (acres) ADWF (MGD) RDI/I Hydrograph (see Table 53) Main Main Main Main Main Locust Locust Locust Locust Locust Locust Locust Locust Locust Locust Meter * Gate Meter Meter TCSD MarinCity * Gate 5 Road area; flows are strongly influenced by tides. Unit Hydrograph ID R total (%) R1 (%) Table 43: RDI/I Hydrograph Parameters T1 (hrs.) K1 R (%) T2 (hrs.) K2 R3 (%) T3 (hrs.) Main Locust Meter Gate TCSD MarinCity K3 Once calibrated, the model was ready to be used to simulate flows in the system under different rainfall events. Specifically, the model was applied to the 5 and 10year design events, as presented in the following subsection, and for various improvement scenarios, as described in subsequent sections of this report. February

48 Conveyance System Analysis 4.2 Predicted Performance During Design Events The capacity of the existing conveyance system was evaluated by running the hydraulic model for the 5 year design event selected based on the continuous simulation analysis (December 31, 2005 storm). The model identifies the extent of surcharge or potential overflows due to capacity limitations in the pipelines and/or pump stations. Figure 44 presents the hydraulic profile of the gravity interceptor upstream of the LSPS under 5year design peak wet weather flow (PWWF) conditions. As shown in the figure, the lined 21inch gravity sewer upstream of the LSPS would be significantly surcharged under these conditions, potentially resulting in an overflow upstream of Coloma Street. This result is consistent with the findings of the Locust Street Pump Station study. The model indicates no significant surcharge in the gravity interceptor between LSPS and MSPS, largely because the capacity restrictions upstream limit the magnitude of the peak flow that can reach the downstream facilities. Under this scenario, the peak flow reaching the LSPS is approximately 4.6 MGD, with an approximate 11.9 MGD peak at the MSPS. Figure 44: Hydraulic Profile Upstream of LSPS Under 5Year Design Event To determine the true design flows in the system (as listed previously in Table 31 and shown in Figure 32) if capacity limitations did not restrict the peak flows reaching downstream facilities, improvements were added to the model to relieve capacity constrictions. Specifically, the 17.3inch inside diameter sliplined section of the 21inch gravity interceptor upstream of LSPS was assumed to be replaced with a 24 inch pipe, and the weir at the Coloma Street diversion structure was adjusted to divert 3.2 MGD to the Scotty s Wet Weather PS. Under this scenario, much of the surcharge upstream of the LSPS is relieved, and even under the 10year design event, the model predicts the system would surcharge by only about ½ February

49 Conveyance System Analysis foot (to within 2.8 feet of the ground) upstream of Coloma Street but would not overflow. However, by relieving the capacity restrictions in the gravity interceptor, the increased flow conveyed downstream (4.8 MGD at LSPS plus 3.2 MGD from Scotty s PS) would produce about 1 foot of surcharge (to within about 3 feet of the ground) in the gravity sewer upstream of the MSPS. The peak flow reaching the MSPS under this scenario is approximately 13.1 MGD. February

50 Section 5 Wastewater Treatment Plant Evaluation Wet Weather Conveyance and Treatment Evaluation

51 Wastewater Treatment Plant Evaluation Chapter 5 Wastewater Treatment Plant Evaluation The treatment plant was evaluated on the basis of its hydraulic capacity (the peak flow rate that can be handled) and on the basis of its process treatment capacity (the peak flow rate that can receive adequate treatment). The performance of the plant when it is in blending mode of operation (flows greater than the treatment capacity of the fixed film reactors are by passed around these units) was also analyzed to assess the frequency and duration of this mode of operation. 5.1 Current Wet Weather Blending Analysis As part of the wet weather blending analysis required by the EPA, an assessment of the frequency, duration, and volume of current blending events is required. Blending occurs when primary effluent flows are bypassed around the fixed film reactors (FFRs). Although the FFRs were designed to treat up to 6.0 MGD, the FFR feed pumps are believed to be able to feed up to 7.8 MGD to the FFRs. However the maximum allowable flow to the FFRs is regulated by the control system for the pumps. It is believed that historically, the FFR pumps were not allowed to operate at their full capacity of 7.8 MGD during wet weather, however the pumps may have been allowed to send more than 6.0 MGD to the FFRs. Therefore, for the purposes of this evaluation historical blending event where estimated using both 6.0 MGD and 6.8 MGD FFR feed capacities. Using both 6.0 MGD and 6.8 MGD secondary treatment capacities for the analysis should provided a reasonable range of estimated historical blending events. The District has previously investigated the possibility of increasing the design loading rate to the fixed film reactors. However, due to the age of the media and potential treatment process failure or upset it was determined that increasing the design loading rate may not be achievable. Due to concern over the treatment effectiveness and potential impacts to the FFRs, it is recommended that feed to the reactors be limited the original design value of 6.0 MGD. The blending analysis listed under 6.0 MGD secondary treatment capacity could be used as indication of the frequency, duration and volume of potential future blending events. The results from the blending frequency analysis are presented in Table 51. Wet Weather Year Table 51: Blending Events Analysis Annual Totals 6.0 MGD Secondary Treatment Capacity (Future Operation) Frequency (# of Events/ Year) a Cumulative Duration (Hours/Year) Cumulative Volume (MG/Year) 6.8 MGD Secondary Treatment Capacity Frequency (# of Events/ Year) a Cumulative Duration (Hours/Year) Cumulative Volume (MG/Year) b Average (not including ) Notes: a Unique wet weather blending events were defined based on one hour intervals (i.e. not blending for 1 hour followed by blending was counted as a new blending event) b wet weather season includes influent flow data through January 24, 2007 February

52 Wastewater Treatment Plant Evaluation As summarized in Table 51, influent flow data for the wet weather seasons from the years 2002/03 through 2006/07 show that with a secondary treatment capacity of 6.8 MGD, SMCSD blended primary effluent on an average of 5.3 times per year, lasting a cumulative total of 19.2 hours per year with a total volume of primary effluent that bypassed the fixed film reactors of 1.1 MG per year. For future operation (assuming 6.0 MGD secondary capacity) it is estimated that there would be approximately 7 blending events per year with a duration of 29.9 hours (<0.3% of the time) and a total volume of bypassed primary effluent of 1.9 MG/year (<0.3% of total annual flow). The analysis of blending events indicates that when blending events do occur (average 7.0 per year), they last for a short duration (average 4.3 hours/event) and result in a relatively small volume (average 0.27 MG/event) of primary effluent that does not receive biological treatment. Based on the average blending frequency presented in Table 51, for 6.0 MGD secondary treatment capacity, 99.7 percent of the annual flow into the SMCSD treatment plant receives full biological treatment. 5.2 Current Wet Weather Treatment Plant Performance A key factor to be evaluated as part of this report is whether peak wet weather flows impact treatment plant performance. During the past few years, the SMCSD plant has had difficulty meeting its NPDES effluent limits. An analysis of recent permit exceedances was performed to determine whether exceedances were more common in wet weather, which would indicate reduced plant performance. From April 1, 2003 through March 30, 2007 there were 76 NPDES permit exceedances. Overall the treatment plant has had difficulty consistently meeting NPDES permit limits. However some of these exceedances can be attributed to specific one time events such as equipment problems, illegal discharges to the conveyance system, and chemically enhanced sedimentation pilot testing problems. These events were documented in the District s annual reports to the Regional Water Quality Control Board and USEPA, which characterize treatment plant performance and the District s compliance with its waste discharge requirements. In order to obtain a more accurate representation of wet weather impacts on NPDES permit compliance (i.e. plant performance), the 45 permit limit exceedances due to these one time events where not used in this analysis. The remaining 31 NPDES permit exceedances were related to BOD and suspended solids exceedances. Figure 51 presents the 31 BOD and TSS NPDES permit limit exceedances from April 1, 2003 through March 30, 2007, along with the average daily flow at that time. As shown in Figure ES3, eight of the 31 permit exceedances occurred during periods when influent flow to the plant exceed 1.5 MGD, while 23 occurred at influent flows less than 1.5 MGD. On this basis, it could be said that permit exceedances occur throughout the year even when there is normal flow to the plant and that exceedances do not occur more frequently during the high flow (wet weather) events. February

53 Wastewater Treatment Plant Evaluation Figure 51: NDPES Permit Exceedances Excluding, Illicit Discharge, Pilot Testing, Mech. Failure and Disinfection Exceedances (April 2003 to March 2007) 7.0 Average Day Flow (MGD) Monthly Weekly Daily/Inst Flow (MGD) /1/2003 7/2/2003 1/1/2004 7/1/ /31/2004 7/1/ /31/2005 7/1/ /31/ Hydraulic Evaluation A hydraulic model of the SMCSD wastewater treatment plant was developed to evaluate the hydraulic capacity of the existing plant. The hydraulic model was set up specifically to evaluate wet weather operation and identify the maximum hydraulic capacity of the plant facilities. Flow passes through the SMSCD treatment plant by gravity with the exception of the fixed film reactors, which have feed pumps to distribute flow to the top of the reactors. Because the plant is located directly on and discharges to the San Francisco Bay, tide level was also included as a boundary condition in the hydraulic model. San Francisco Bay tide data from 2001 through 2006 were reviewed to determine an appropriate maximum peak high tide elevation for use in the hydraulic model. Tide data were obtained from the National Oceanic and Atmospheric Administration s (NOAA) Tides Online database for the San Francisco Bay Station (Station ID ), which is located near Crissy Field. High and low tide data for the San Francisco Bay Station from 2000 to 2006 are shown in Figure 52. February

54 Wastewater Treatment Plant Evaluation Figure 52: Historical San Francisco Bay Tide Data 6.00 High Tide Low Tide Tide Level /1/00 12/31/00 12/31/01 12/31/02 12/31/03 12/30/04 12/30/05 12/30/06 Date Note: Tide level shown relative to NGVD 1929 datum. Only high and low tide levels are shown. During this time period, the high tide level averaged approximately 2.6 feet. The highest tidal elevation from 2000 to 2006 was 5.5 feet, which occurred on January 8, The low tide averaged approximately 1.5 feet, with the lowest level being 4.8 feet on June 4, Figure 52 also illustrates that the annual peak high tide level typically occurs at the end of December or beginning of January. The timing of this peak high tide event is due to the earth s orbit around the sun. Both the sun and moon influence tide levels. The sun s influence is greatest when the earth s orbit is closest to the sun, which occurs around January 2 nd of each year. The net impact of the sun and moon on tide is greatest when the moon is between the earth and sun, also known as a new moon (NOAA). Maximum Hydraulic Capacity Using a high tide elevation of 5.5 feet for the boundary condition for the hydraulic model, the calculated hydraulic capacity of the existing treatment plant is approximately 13.0 MGD. The results from the hydraulic model were compared to recent plant operating data to verify the results. During the wet weather season there was one storm event on December 31, 2005 (also determined to be the 5year frequency design event for SMCSD), which had both a very high tide level (10.8 feet relative to the NGVD 1929 datum) and a peak wet weather flow into the plant. The peak high tide level was likely due to the coincidence of the earth being near its closest point to the sun and a new moon, which occurred on the evening of December 30, The fact that the peak high tide level occurs during the middle of winter is important because of the increased chance that it may coincide with a peak wet weather storm event. February

55 Wastewater Treatment Plant Evaluation Although the flow meter reading at the treatment plant exceeded its maximum reading of 10 MGD during the event, it is believed that the actual flow into the treatment plant was between 12.5 and 13.0 MGD based on flow measurements for the Main Street and Ft. Baker Pump Stations, which feed directly into the treatment plant. The 13.0 MGD flow rate is within the estimated hydraulic capacity estimated by the hydraulic model. Also, during the estimated 13.0 MGD event in the wet weather season, visual observations of water surface elevations at various locations in the plant indicate that the treatment plant was approaching its maximum hydraulic capacity. The hydraulic model was also run at a more typical high tide level of 3.0 feet, which resulted in an estimated hydraulic capacity of 14.0 MGD. An updated hydraulic profile for the SMCSD plant, which shows estimated water surface elevations throughout the plant at 13.0 MGD peak wet weather flow, is shown as Figure 53. Based on observed conditions and the hydraulic modeling, the hydraulic capacity of the plant is believed to be 13.0 MGD during a peak high tide event, and approximately 13.5 MGD during a normal high tide Process Capacity Evaluation An evaluation of the SMCSD treatment plant process capacity during peak wet weather events was completed using an approach similar to the average dry weather flow analysis completed by CH2M Hill (Operational Audit Preliminary Design Report, March 2006). The initial evaluation included a comparison of current process operating parameters during wet weather to the original plant design criteria and typical design criteria. The process criteria comparison is presented in Table 52. The specific design criteria presented in Table 52 are those that are particularly applicable to processes that would be impacted by peak wet weather conditions and are not inclusive of all process in the plant. For example, peak wet weather solids handling is not expected to be significantly different from dry weather solids handling and is therefore not discussed in this TM. It is important to note that this wet weather evaluation is focused on wet weather issues (i.e. treatment plant performance issues that occur due to increased flows to the plant). Dry weather treatment issues that were previously identified in the CH2MHill report will also be a concern during wet weather. These issues are considered overall plant performance issues and are not addressed further in this evaluation. Process Loadings The influent solids loadings (BOD and TSS) used in this analysis were developed using treatment plant operating data from 2000 to Daily mass loadings (in pounds per day) were calculated from the data set using the daily influent flow and the influent BOD and TSS concentrations. The daily mass loadings were then sorted for BOD and TSS, independently, to determine the top ten values for each parameter. The top ten BOD daily mass loadings varied from 6,847 to 13,130 lbs/day. A representative value of 9,000 lbs/day of BOD was selected for this analysis. The top ten TSS daily mass loadings varied from 14,201 to 20,529 lbs/day. A representative value of 18,000 lbs/day of TSS was selected for this analysis. The typical design criteria shown in Table 52 were taken from Metcalf and Eddy, Inc. Wastewater Engineering, Treatment, Disposal and Reuse (3 rd and 4 th Edition) and the WEF Manual of Practice 8.. Each process is discussed in more detail in the following sections. Although some processes exceed the typical design value, it should be noted that original 1986 wastewater treatment plant expansion was designed to handle an instantaneous peak flow of 10 MGD. Primary Treatment Surface overflow rates were calculated using measured flows and surface area of each clarifier. At current peak day flows, the primary treatment surface overflow rate is 2,609 gal/dayft 2, which is slightly above the typical design value of 2,500 gal/dayft 2. February

56 Wastewater Treatment Plant Evaluation Figure 53: Treatment Plant Hydraulic Figure 53: SMCSD WWTP Hydraulic Profile Figure 53 February 2008 February

57 Wastewater Treatment Plant Evaluation At current instantaneous peak wet weather flows (instantaneous PWWF), the primary treatment surface overflow rate is more than double typical design value for wet weather flow. As noted in the CH2M Hill evaluation, the side water depth of the primary clarifier is shallow at 9.5 feet, which could lead to reduced performance during peak wet weather flows. The District is currently in the process of upgrading the existing primary clarifier to enhance primary sedimentation through the addition of ferric chloride into the main influent line at Fort Baker Road before entering the plant and polymer into the influent line along the shoreline at the plant, prior to the entering the primary clarifier. The District anticipates that the enhanced primary treatment modification will improve suspended solids and BOD removal efficiencies through the primary clarifier. Flow Table 52: Process Loading Comparison Units Original Design Criteria Average Dry Weather Peak Day Wet Weather Current Peak Day Current Instantaneous Peak Typical Peak Flow Design Value a Peak Wet Weather Flow MGD Primary Treatment Surface Area ft 2 2,376 Surface Overflow Rate gal/dayft ,315 2,609 5,471 2,0003,000 Fixed Film Reactors (FFR) b Total Surface Area of Units ft 2 2,514 Organic Loading Rate lbs BOD/ day1000 ft Hydraulic Loading Rate gal/minft Secondary Clarifiers Total Surface Area ft 2 3,520 Surface Overflow Rate gal/dayft ,563 1,761 3,693 1,0001,600 Solids Loading Rate c Granular Media Filters d lbs TSS/day ft Total Surface Area ft Surface Loading Rate gal/minft Disinfection Contact Time min Notes: a Typical design criteria taken from Metcalf and Eddy, Inc, Wastewater Engineering Treatment, Disposal and Reuse (3 rd and 4 th edition) and WEF Manual of Practice 8. b Maximum flow through the Fixed Film Reactors (FFRs) is 6.0 MGD. Primary effluent flow greater than 6.0 MGD is routed around the FFRs and recombined with the FFR effluent prior to the secondary clarifier. c Estimated based on assumed solids removal through the primary clarifier and solids produced from the FFRs. d The granular media filters were added as a side stream treatment process after the secondary treatment plant was constructed. Maximum flow through the granular media filters is 1.0 MGD February

58 Wastewater Treatment Plant Evaluation Fixed Film Reactors (FFR) The fixed film reactors were designed to treat a maximum flow of 6 MGD. Flows higher than 6 MGD are passively routed around the FFR and recombined with the FFR effluent prior to the secondary clarifiers. Note that the current peak day flow is just over 6 MGD while the instantaneous PWWF is more than twice the maximum flow. Organic loading rates were calculated using media depth in the reactors and the average of the maximum day BOD from 2000 to At both the current peak day flow and instantaneous PWWF, the FFR organic loading rate is within the typical design values of 37 to 200 lbs BOD/day1000 ft 3. Hydraulic loading rates were calculated using measured flows up to the maximum flow of 6 MGD and the total surface area of all the duty filters. At both the current peak day flow and instantaneous PWWF, the FFR hydraulic loading rate is within the typical design values of 1.0 to 2.0 gal/minft 2. As discussed in Section 5.1, District review of past operating data and pump curves for the film reactor feed pumps indicates that historically up to 7.8 MGD of flow could be sent to the fixed film reactors, which is above the original design value for the secondary treatment plant. Because of the plant s difficulty in consistently meeting its NPDES limits and the potential for impacts to performance and risk of failure of the FFRs, it is recommended that the District limit the maximum flow the FFRs to 6.0 MGD. However, the District may consider stress testing the FFR s to make a more accurate determination of the FFRs actual treatment capacity. Secondary Clarifiers At current peak day flow, the surface overflow rate is slightly above the typical design values of 1,000 to 1,600 gal/dayft 2. During blending events, when flow into the plant exceeds the FFR capacity, blended flow is still routed through the secondary clarifiers. Therefore the peak wet weather flow through the secondary clarifiers is the same as the peak flow into the treatment plant. At the current hourly PWWF, the surface overflow rate is more than double the typical design values. The increased overflow rate could result in treatment plant performance issues, especially when coupled with other issues at the plant such as bay water intrusion and poor settleability of the FFR suspended solids. The District may want to consider routing primary effluent around the secondary clarifiers during blending events, which would help to reduce the peak wet weather overflow rate through the secondary clarifiers. Granular Media Filters The granular media filters were added as a side stream process that can treat a maximum flow of 1.0 MGD. The surface loading rate on the filters at the maximum flow rate of 1.0 MGD is 2.7 gal/minft 2, which is within the typical design values of 28 gal/minft 2. However, during peak wet weather flows the sand filters are only capable of treating a small portion of the flow and therefore do not have a significant effect on plant performance. Disinfection At current peak day flow, the contact time in the chlorine contact tanks falls within typical design values of 15 to 90 minutes. At the current hourly PWWF, the contact time is below the typical design range of values. To adjust for the reduced contact time, SMCSD staff must adjust the chlorine dose accordingly to maintain adequate disinfection. February

59 Section 6 Conveyance System Improvement Alternatives Wet Weather Conveyance and Treatment Evaluation

60 Conveyance System Improvement Alternatives Chapter 6 Conveyance System Improvement Alternatives Potential conveyance system improvements were developed to eliminate system capacity deficiencies and/or reduce flows to the treatment plant, and the hydraulic model was used to evaluate the impact and potential benefits of the improvements. The following subsections describe these potential improvements in more detail and present the model results. 6.1 Potential Conveyance Improvements The potential improvements considered for this study included combinations of various approaches, as listed below: Flow Reduction Reduce I/I in the collection system Eliminate TCSD from the SMCSD system Conveyance System Improvements Upsize pipelines Modify diversion to Scotty s PS and/or increase pumping capacity Route Whiskey Springs Pump Station (WSPS) flow to Scotty s PS and increase pumping capacity Upstream Storage Facilities Provide inline storage by upsizing pipes Provide offline storage basins Flow Reduction Reducing flows in the system would provide dual benefits of potentially alleviating conveyance capacity deficiencies as well as reducing the peak flows that must be treated at the treatment plant. Two approaches were considered, as described below. I/I Reduction I/I in the system can potentially be reduced by rehabilitating sewers and/or sewer laterals to prevent the entry of extraneous groundwater and storm water into defective pipes and manholes. Additional I/I reduction could potentially also be achieved by eliminating all sources of direct storm water inflow (e.g., roof or area drains that are directly connected to sewer laterals, crossconnections between storm drains and sanitary sewers, manholes that are subject to surface inflow or ponding of water during rainfall events). Assuming that the collection systems tributary to SMCSD are similar to other systems in the Bay Area, it is likely that there are not a significant number of direct inflow sources, but rather, the major portion of I/I is a result of defective sewer mains and laterals that allow extraneous water to enter through cracks and open or offset pipe joints. Therefore, reducing I/I would require repair, rehabilitation, or replacement of those facilities. The current consensus of the industry is that significant reductions in I/I can only be achieved if the system is rehabilitated comprehensively, meaning that all sewers and manholes in a given area are rehabilitated, including the private service laterals. Under such programs, flow reductions of up to 70 percent or higher may be achievable. If service laterals are not included, then potential I/I reduction (i.e., 30 percent) is much lower. Documentation of I/I reductions achieved through sewer system rehabilitation has been attempted by many agencies, with results varying widely due to differences in rehabilitation methods, system components addressed, as well as variations in system conditions (soils, topography, groundwater, rainfall patterns) and analysis methods. A recent Water Environment Research Foundation (WERF) study attempted to analyze a number of rehabilitation projects throughout the country using a rigorous and February

61 Conveyance System Improvement Alternatives standardized analysis methodology (WERF, Reducing Peak RainfallDerived Infiltration/Inflow Rates Case Studies and Protocol, 2003). The WERF case studies documented reductions ranging from zero to 78 percent in peak RDI/I flows for various levels of sewer rehabilitation. The results indicated that rehabilitation of public sewers alone resulted in relatively small reductions in peak RDI/I flows, whereas higher reductions were achieved where private laterals were also addressed. The WERF study concluded that rehabilitation programs run the risk of achieving insufficient RDI/I reductions if they do not address laterals. However, it appeared that rehabilitation of public sewers may have benefit in reduction of overall I/I volumes rather than peak flows. Because of the uncertainties in quantifying potential I/I reduction and the institutional barriers to implementing private sewer lateral rehabilitation on a large scale, I/I reduction alternatives in this study were focused primarily on potential rehabilitation of public sewer facilities only, and the I/I reduction benefit was assumed to be 30 percent. (Note: the City of Sausalito has a lateral compliance ordinance which requires testing and repair of private laterals at the sale of property or major remodel; however, this program is not coupled with public sewer rehabilitation and is therefore not likely to result in significant quantifiable shortterm flow reductions in defined areas of the system.) To simulate the I/I reduction scenario, model runs were made with each of the three RDI/I hydrograph components (R1, R2, and R3 values) for all areas reduced by 30 percent. While some studies indicate that rehabilitation of public sewers may result in greater reductions in total I/I volume (e.g., higher reductions in the slower RDI/I components than in the fast component), there is insufficient information to quantitatively predict what those reductions might be; therefore, varying RDI/I reduction by component was not considered for this study. Elimination of TCSD Flows SMCSD currently provides conveyance and treatment for a portion of flows from TCSD; the remaining portion are conveyed to the Sanitation Agency of Southern Marin (SASM) treatment plant. The agreement between SMCSD and TCSD states that either agency may cease their contractual arrangement. TCSD must provide SMCSD at least one year s notice of its intent to terminate the agreement, whereas SMCSD must give sufficient notice for TCSD to secure alternate comparable service. Several years ago, TCSD conducted an evaluation of the possibility of routing all of its flows to SASM; however, at the time, it was not found to be cost effective to do so. For this study, the alternative of eliminating TCSD flows from the SMCSD system was evaluated by assuming that TCSD s wet weather flows would be stored until wet weather flows from the remainder of the SMCSD service area had passed through the wastewater system. As noted later in this report. this alternative revealed the costs of managing wet weather flows in the SMCSD service area if TCSD s wet weather flow contribution were eliminated. (It also revealed the potential cost of storing TCSD s flow.) Conveyance System Improvements Conveyance system improvements are focused on eliminating constrictions in the system that may cause surcharging and overflows. Conveyance system improvements alone would not, of course, reduce peak flows to the treatment plant (in fact, would be likely to increase the peak flows conveyed downstream to the plant). However, conveyance system improvements could be combined with flow reduction and/or storage improvements to provide both conveyance capacity relief and treatment plant flow reduction benefits. The conveyance improvements considered for this study are described below. Upsize Pipelines As discussed in Section 4.2, the sliplined section of the 21inch gravity interceptor upstream of the LSPS is currently a significant capacity restriction in the system. Upsizing this pipe to 24inch diameter would eliminate the surcharging and potential upstream overflows caused by this constriction. (Note that adding February

62 Conveyance System Improvement Alternatives a parallel sewer could also provide the required capacity. However, due to the constricted nature of the existing pipeline corridor, construction of a parallel pipe was not considered a viable option.) Scotty s Diversion and Pump Station Improvements The current height of the side overflow weir in the 21inch gravity sewer at Coloma Street determines the amount of the flow that is diverted to the Scotty s PS during wet weather events. Lowering the height of the weir could increase the amount of flow diverted to Scotty s. Increasing the diverted flow would also require an increase in pumping capacity for the Scotty s PS. Increasing the flow diverted to Scotty s PS in peak wet weather events would have the benefit of reducing the flow in the capacitydeficient downstream gravity sewer between Coloma Street and the LSPS, thereby possibly eliminating the need to upsize that pipe, and also reducing the flow that ultimately reaches the LSPS, reducing the capacity requirements for the LSPS improvements. Note that up to about 6.8 MGD of flow could be pumped through Scotty s PS based on the capacity of the existing 14inch OD force main (assuming a maximum 10 feet per second velocity). Diversion of Whiskey Springs PS Flow to Scotty s The existing City of Sausalito s Whiskey Springs PS (WSPS) is located on the southwest corner of Bridgeway and Coloma Street, just across the street from the Scotty s PS. This wet pit/dry pit facility, which is maintained by SMCSD, has two 10HP centrifugal pumps, each rated at 0.6 MGD. The pump station is located adjacent to some apartment buildings; therefore noise and odors are significant concerns. The station has an existing generator set (genset) to provide standby emergency power, but the genset needs replacement, and the day tank has insufficient fuel storage capacity. The estimated 5year design storm PWWF to the WSPS is approximately 0.63 MGD based on the model flow allocation and calibration. As shown in Figure 23, the pump station discharges to the adjacent 21 inch gravity interceptor. Thus, diverting these flows and combining them with the flows to the Scotty s PS could further reduce the flow in the 21inch interceptor. Therefore, the alternative of combining the WSPS and Scotty s PS flows into a single, improved pumping facility was evaluated for this study Upstream Storage Facilities Similar to flow reduction, providing peak flow storage in the collection system would have the dual benefits of potentially alleviating conveyance capacity deficiencies as well as reducing the peak flows that must be treated at the treatment plant. Storage can be provided in the conveyance system itself, by oversizing pipelines that would otherwise only be sized for conveyance of peak flows, called inline storage; or by providing offline storage basins to which flow is diverted during peak flow periods and then released back into the system after peak flows recede. Each of these storage options is described below. InLine Storage As noted previously, the existing lined 21inch gravity sewer upstream of the LSPS is a significant capacity constriction in the existing conveyance system. Upsizing this pipe, and oversizing it to provide additional storage capacity, could potentially reduce the peak flows conveyed downstream to the treatment plant. Such an inline storage option was also evaluated for the Locust Street Pump Station study. In that case, it was assumed that the entire gravity pipeline upstream of LSPS to Coloma Street would be replaced with a new pipe ranging in size from 24 inches at the upstream end to 72 inches at the downstream end. The same inline storage alternative was evaluated for this study. OffLine Storage Offline storage involves construction of a tank at a suitable location upstream in the system to provide storage of wastewater during peak flow events. In urban areas, such a facility would be expected to be an underground tank with an associated pumping facility to either pump the flow into or out of the tank. A nonpumped diversion into the tank would be preferred, with a pumped release of flow from storage that February

63 Conveyance System Improvement Alternatives can be timed to occur after the peak flow period has passed. Such a configuration is appropriate for the SMCSD system as the sewer pipelines are relatively shallow. Storage in the SMCSD system would provide the greatest benefit where the maximum amount of flow could be diverted and stored. Therefore, a number of potential locations for an offline storage basin facility were initially identified. These included city parks (Martin Luther King, Marinship, and Dunphy Parks), as well as a vacant lot east of Bridgeway at the intersection of Locust Street. All of these sites have available power and relatively large open areas under which a storage basin could potentially be constructed. Potential storage basin locations further downstream, e.g., near the MSPS, were investigated, but no feasible sites were found. Martin Luther King (MLK) Park. This park is located on the north side of Coloma Street west of Bridgeway. Although further upstream than other sites considered, this site appears to be the most promising due to its large size, flat topography and proximity to the Bridgeway gravity sewer, Scotty s force main, and Scotty s and Whiskey Springs Pump Stations. This location also facilitates providing a new dry weather pump station adjacent to the storage basin that could replace both the WSPS and Scotty s PS. However, the site is fairly close to the Bay, hence bay mud and high groundwater are likely to be design and construction considerations. Therefore, the storage basin may need to be pile supported. Figure 61: Potential Storage Site Martin Luther King Park Marinship Park. This park is located on the east side of Marinship Way north of the Bay Model. The site is smaller than the MLK Park site but is flat. It is also closer to the Bay than the MLK site; therefore, bay mud and high groundwater are more likely to be design and construction considerations, and the storage basin may need to be pile supported. Access is not as convenient as to the MLK site since it is necessary to go from Bridgeway along Marinship to reach it. February

64 Conveyance System Improvement Alternatives Figure 62: Potential Storage Site Marinship Park Dunphy Park. This park is located on the shoreline east of Bridgeway between Napa Street and Locust Street. This site is also smaller than the MLK Park site and has rolling topography as well as numerous trees that may limit where a storage basin could be located. The eastern boundary of the site is located at the bay shore; therefore, bay mud and high groundwater are likely to be design and construction considerations. (The south boundary of the site appears to be a wetlands.) The storage basin therefore may need to be pile supported. Access from Bridgeway is good by entering a court east of Napa Street and parking lot. Figure 63: Potential Storage Site Dunphy Park Vacant lot east of Bridgeway at Locust Street. This site is located across from the Police Station at the intersection of Locust Street. The site is smaller than the MLK Park site and has uneven topography, as well as fill areas that may contain rip rap which may limit where a storage basin could be located. The eastern boundary of this site is located at the bay shore; therefore bay mud and high groundwater are likely to be design and construction considerations, and the storage basin may need to be pile supported. Access from Bridgeway is restricted due to needing to drive along a gravel lane south of the Police Station and north of a wharf/warehouse area. The vacant lot is currently used for parking and appears to be for sale. February

65 Conveyance System Improvement Alternatives Figure 64: Potential Storage Site Vacant Lot at Bridgeway & Locust Based on field visits to each of the sites, the MLK Park site was considered the best location for upstream storage, with Marinship Park as a secondary location. While the two sites further downstream could potentially capture more flow, the site conditions would be much less desirable for underground construction. Furthermore, locating a storage facility further downstream would require construction of additional pipeline capacity to convey the flow to the storage sites. The MLK Park site also has the added advantage of proximity to the Scotty s PS and WSPS, which provides opportunities for consolidation of these pump stations and the storage basin pump station into a single, improved facility. Potential Storage Locations at TCSD. Another potential location for storage would be at or near the Bell Lane PS in TCSD. While the pump station site itself is congested, there is an adjacent community center that could potentially provide adequate space for an underground storage basin. This site is located immediately to the east of the Bell Lane PS and a garbage truck parking facility. The Community Center has a large flat asphalt paved parking area that would be a suitable storage basin location. Access would be from Marin Avenue just off of Tennessee Valley Road. Diversion piping could readily be routed from the pump station through the garbage truck parking lot. Though the site is in close proximity to Coyote Creek and within about ½ mile from the Bay, it may be far enough inland that pile support for the basin would not be required. Figure 65: Potential Storage Site Community Center near Bell Lane PS As noted previously, in terms of strategies to handle peak wet weather flows in the SMCSD system, storage at Bell Lane would provide benefits equivalent to eliminating TCSD flows from the system. February

66 Conveyance System Improvement Alternatives 6.2 Model Scenarios and Results Table 61 presents the results of model scenarios incorporating various combinations of the improvements described above for the 5year design event. For each scenario, the table describes the improvements included; the resulting peak flow at the Scotty s PS, LSPS and MSPS (the flows to the MSPS are essentially the same as the flows to the treatment plant except for Fort Baker flows); and the remaining surcharge (height above the crown of the existing pipe and depth of the water level below ground) at critical locations in the existing system. Based on these model runs, several alternatives were eliminated from further consideration. The Marinship Park storage site does not offer any advantages in terms of significant additional flow volume available for storage than the MLK Park site, yet would require pipeline upgrades in order to convey the flow to that site. Inline storage (oversizing the pipe upstream of the LSPS) would be very expensive and disruptive to construct and would provide only limited storage volume. Diverting more flow to Scotty s PS and combining the WSPS and Scotty s PS flows does appear to be a costeffective approach for dealing with conveyance system capacities, but has no benefits in terms of reducing downstream peak flows. This approach, however, could be combined with upstream storage at MLK Park to address both conveyance and treatment plant peak flow issues. February

67 Conveyance System Improvement Alternatives Scenario Description Table 61: Results of Conveyance System Model Scenarios Improvements Included Pipe Length / Dia PS Capacity Storage Volume PWWF (mgd) Diversion to SPS LSPS MSPS Surcharge (above crown/below ground) Upstream of Coloma (MH 48000) Upstream of LSPS (MH 37700) Upstream of MSPS (MH 17001) Existing Depth of Pipe Crown below Ground>> Existing System None OF 4.8/ /2.7 Existing System w/out None / /3.7 TCSD Existing System w/30% Rehabilitation of all public sewers / / /3.4 I/I Reduction Existing System w/30% Rehabilitation of all public sewers /4.4 I/I Reduction, w/out TCSD Upsize gravity pipe Replace 17.3" lined portion of 21" 2,420' / / /1.2 gravity sewer 24" Upsize gravity pipe, divert more flow to Scotty's PS Replace 17.3" lined portion of 21" gravity sewer, lower diversion weir, upgrade SPS 2,420' 24" Scotty's 3.2 mgd / / /0.9 Divert Whiskey Springs PS flow to Scotty's PS Divert more flow to Scotty's PS, w/out TCSD Inline storage upstream of LSPS Lower diversion weir, diversion pipe from WSPS to SPS, reconstruct SPS Scotty's 4.5 mgd / /1.0 Lower diversion weir / /3.6 Replace 21" & 24" gravity sewers, LSPS limited to 6 mgd Storage near LSPS Replace 17.3" lined portion of 21" gravity sewer, lower diversion weir, upgrade SPS, construct storage tank near LSPS Storage at Marinship Park Storage at Marinship Park, w/out TCSD Storage at MLK Park Storage at MLK Park w/30% I/I Reduction Downstream of Locust St. PS Replace gravity sewer from Coloma to Marinship Park; no flow to SPS; construct storage tank at Marinship Park Construct storage tank and pump station at MLK Park. Abandon SPS and WSPS. Construct storage tank at MLK Park; rehabilitation of all public sewers downstream of LSPS. 5,920 24" to 72" 2,420' 24" 6,020 27" Scotty's 3.2 mgd ~0.6 MG / / / / MG MG MG / / MG / /3.4 February

68 Conveyance System Improvement Alternatives I/I reduction alone, even if accomplished throughout the entire system, including TCSD, would not provide sufficient flow reduction to eliminate surcharge conditions in the gravity sewer upstream of LSPS, and peak flows to the treatment plant would still be fairly high. Furthermore, the amount of sewer rehabilitation required would be very costly and could not likely be accomplished within a reasonable timeframe. However, I/I reduction in targeted portions of the system, specifically the area downstream of LSPS where I/I contributions appear to be highest, could provide sufficient flow reductions such that, when used in combination with upstream storage facilities, significant peak flow reductions to the treatment plant could be achieved. Elimination of TCSD flows clearly provides significant benefits to SMCSD in terms of eliminating conveyance system capacity deficiencies; however, this would still be insufficient to reduce flows to the treatment plant to the existing secondary treatment hydraulic capacity. The results of the conveyance system improvement analyses were used to develop systemwide alternatives, which are presented in a subsequent section of this report. 6.3 Real Time Control Strategies Improvements that involve diversions to offline storage basins require methods to control the timing and amount of the flow diverted, as well as the timing and rate of release of stored flow back into the system after the peak flow period has passed. For modeling of offline storage alternatives, controls were assumed to be implemented through use of weirs or sluice gates, with the opening and/or closing of those control facilities based on conditions in the system. The concept of controlling the operation of facilities (e.g., opening or closing of gates or valves, turning of pumps on or off) based conditions in the system (e.g., flow depth or flow rate), often at another location, is called real time control (RTC). The parameters that govern how the facilities operate under these conditions are called real time control rules. Since the primary need for storage is to reduce high flows at the treatment plant, flow at the treatment plant was the main consideration for this study in developing real time control strategies for controlling flow diversions to storage and release of flow from storage. The real time control scenarios developed for this study were based on the 5year design event described in Section 3. Note that prior to design of any storagebased wet weather facilities, additional storm scenarios should also be analyzed to further develop the control strategies, as other considerations or control rules may be appropriate for different types of storms. Based on the continuous simulation modeling described in Section 3, the design storm used (December 31, 2005) created both a 5year peak flow event, as well as a 5year storage (peak volume) event. This storm is therefore fairly peaky, i.e., there is relatively rapid rise in flow in response to the rainfall, followed by a relatively rapid decrease in flow after the peak rainfall ends. Based on the model simulations, the flow coming to the treatment plant from the MSPS would increase from 4 MGD to 6 MGD within approximately 45 minutes, and increase further from 6 MGD to 7 MGD within approximately 15 minutes. Because of the high peak flows resulting from the storm, all of the flow upstream of the Coloma Street diversion, plus flow currently going to WSPS, would need to be diverted to storage during the peak of the storm in order to significantly reduce peak flows to the wastewater treatment plant. Based on the above considerations, upstream diversions would need to be initiated well in advance of flows reaching 6 MGD at the treatment plant. Therefore, 4 MGD was considered the initial trigger flow at the treatment plant. Based on the hydraulic modeling, if diversions were not initiated at 4 MGD, the flows reaching the treatment plant would exceed 6 MGD before the effects of the diversion would be experienced at the plant. Therefore, once flow increased to 4 MGD at the plant, all flow from upstream of the Coloma Street diversion, as well as flow from the area served by Whiskey Springs PS, was assumed to be diverted to storage. Flow diversions were assumed to stop after the storm event had passed and flow February

69 Conveyance System Improvement Alternatives at the treatment plant had receded to 4 MGD. After diversions to storage stop, additional time must be allowed for flows to recede further prior to pumping the wastewater volume out of storage and back into the conveyance system. Therefore, for the storage solutions considered, RTC would be needed for the following operations: Timing for starting and stopping flow diversions from the interceptor Timing for pumping stored volume out of storage In reality, this level of flow control would need to be automated given the rapid response time of the collection system to rainfall. Active flow control devices would be needed rather than simple passive type devices. Maintenance of these devices, which are up in the collection system, would be more involved than with passive devices, and would be critical to the operation of the treatment plant. That is, more than under current practice, wet weather operation of the conveyance system would be integral to the operation and performance of the treatment plant. February

70 Section 7 Treatment Alternatives Wet Weather Conveyance and Treatment Evaluation

71 Treatment Alternatives Chapter 7 Treatment Alternatives Potential treatment plant improvements are presented in the following subsections. The improvements include a list of potential hydraulic improvements that could be implemented to reduce the hydraulic headloss through the plant and thereby increasing the hydraulic capacity of the plant. However since treatment performance is the most important aspect of the treatment plant, the majority of discussion is centered around alternatives to improve treatment process performance during wet weather. 7.1 Hydraulic Improvements In general, the hydraulic capacity of the existing treatment plant is in line with the treatment capacity, therefore improvements designed to solely increase the hydraulic capacity of the treatment plant are not warranted without accompanying treatment capacity improvements. However, based on the review of existing hydraulics, some minor flow restrictions were identified. Removal of these restrictions could provide a small amount of hydraulic relief, but probably would not result in any meaningful level of increase in hydraulic capacity. The hydraulic improvements presented below are specifically meant to address flow restrictions through the plant during peak wet weather conditions in order to improve flow through the plant Replace Effluent Flow Meter Weir Currently an effluent weir is installed at the end of chlorine contact basin. Coupled with level measurement, the weir is used to determine the flow over the weir. The weir was designed to provide a large span of level ranges in order to maximize the accuracy of the flow measurement readings. During peak weather flow events, the weir creates additional headloss that results in higher water surface elevations in the chlorine contact basins. If the weir were no longer used for flow measurement, the weir could be modified to provide additional weir length and therefore lower headloss during peak flow conditions. However, the District is required to measure effluent flow as part of its selfmonitoring program. As an alternative to the effluent weir the District could investigate the feasibility of installing flow meter on the 30inch diameter pipeline from the effluent screens to the chlorine contact basins Remove Primary Clarifier Flow Meter Prior to the secondary treatment expansion, a primary effluent flow meter was used to measure flow through the plant. Similar to the effluent flow meter, the primary effluent flow meter utilizes a weir to measure flow. The primary effluent flow meter is no longer used however the weir is still used to divert flow to the primary effluent screens. Removing this unused flow meter would relieve the flow restriction, which impacts flow through the primary clarifier. If the weir is required, the District may consider looking at replacing the weir with a downward opening weir gate, which could divert flow in dry weather, and could also be lowered during wet weather to relieve the flow restriction Equalization and Storage Another option that would improve the plant s ability to handle peak wet weather flow would be to provide equalization and/or storage at the existing treatment plant site. Storage alternatives for the conveyance system were presented in Section of this report. Storage at the treatment plant would reduce the instantaneous peak through the plant, however this option is likely infeasible due to the constrained site conditions at the plant currently. 7.2 Process Improvement Alternatives The EPA s No Feasible Alternatives Analysis (NFAA) guidance document requires that the District evaluate alternatives for treating bypass flow during wet weather. Providing treatment for primary effluent bypass flows (i.e. when plant influent flow exceeds 6 MGD) would be expensive and would February

72 Treatment Alternatives require expansion of the existing plant site. For the purpose of this evaluation, it was assumed that the existing plant can be expanded. The discussions below provide a description of each treatment option followed by a preliminary screening evaluation. Based on the preliminary screening, only those treatment options that could be reasonably implemented to treat bypass flows are carried forward. The potential treatment plant options for treating primary effluent are discussed in the following paragraphs No Treatment Improvements Alternative The District could continue to operate under its existing operational mode, which requires blending flow around the secondary treatment process during peak wet weather flows. Continuing with this mode of operation requires that the treatment plant consistently meet its NPDES discharge limits, not only in wet weather, but also in dry weather. In addition, the No Treatment Improvement Alternative requires the District to continue to rely on blending, which the District may be required to demonstrate is needed every five years during NPDES permit renewal. If blending were no longer allowed, the District would then need to implement a project to eliminate blending. In addition, the no treatment improvements alternative could also be implemented if the peak wet weather flow to the treatment plant could be reduced to less than 6.0 MGD through use of upstream storage facilities. If the peak flow to the plant can be reduced, then wet weather improvements at the plant would not be required and blending would no longer be necessary Physical/Chemical Treatment Alternatives Physical/chemical treatment alternatives include membrane filtration, media filtration, high rate clarification/ballasted flocculation, and enhanced secondary sedimentation. These alternatives are described below. Membrane Filtration This alternative would involve installing a new microfiltration membrane system, which would be used to filter primary effluent bypass flows during wet weather. An example of a membrane filtration module is shown as Figure 71. Although the membrane system would have a relatively small footprint, the membranes would require extensive pretreatment including fine screening and possibly grit removal. Membrane filtration of primary effluent is a relatively new approach without a long installation history. Even with screening, there would still be a potential for the membranes to foul due to grease and build up of solids and other material in the primary effluent. For this reason, membrane filtration may not be a reliable treatment alternative. February

73 Treatment Alternatives Figure 71: Membrane Filtration Module Permeate to Disinfection Wastewater with Suspended Material Membrane Fibers Source: Koch Membrane Systems (modified) Media Filtration The District currently has continuously backwashing sand filters (shown in Figure 72), which are used to remove additional suspended solids from the secondary effluent. Additional sand filters could be added to treat the primary effluent bypass flow. Sand filters would have a relatively low operational cost and could be started up quickly when needed during wet weather. However, the District has had a history of performance issues with their existing sand filters. Additional sand filters to treat the primary effluent bypass flow of 6 MGD would require approximately 6 times the amount of space of the existing filters and would likely have even more performance issues due to the increased solids loading associated with filtering primary effluent. The increased solids loading would lead to more backwash flow and potentially blinding of the filter. February

74 Treatment Alternatives Figure 72: Existing Sand Media Filters at SMCSD Highrate Clarification / Ballasted Flocculation Another treatment process that could be used to treat bypassed primary effluent is highrate clarification. Treated primary effluent from the highrate clarifier would be blended back with the FFR effluent and sent to disinfection. The highrate or flocculating clarifiers are manufactured units consisting of a flash mixing vessel, a chemical reaction vessel and a clarification/thickening vessel. Coagulant is added before the flash mix vessel. In some versions of this process, ballast in the form of sand is added to provide ballasted flocculation. Coagulated flow then moves to the reactor vessel where polymer is added and flocculation occurs. The coagulated/flocculated water then moves to the clarification/thickening vessel. Clarified water is collected through a series of launders and discharged into the effluent trough. Solids could either be thickened in the highrate clarifier and sent directly to digestion or they could be sent to the gravity thickener. The highrate clarifier can either be installed in concrete basins or in steel tanks. Highrate clarification would be effective at removing suspended solids and BOD associated with those solids from the primary effluent, however it would not be effective at removing soluble BOD. An illustration of a highrate clarifier is shown as Figure 73. The use of ballasted flocculation for treatment of flows that bypass biological secondary treatment has been cited by EPA as a tentative means of meeting their blending policy. Obtaining their formal projectspecific approval of its use to meet their blending policy is recommended. February

75 Treatment Alternatives Figure 73: HighRate Clarification Source: Infilco Degremont, Inc. Enhanced Secondary Sedimentation Under current operation, primary effluent that bypasses the fixed film reactor is sent to the inlet of the secondary clarifiers. If the District decides to continue with this mode of operation, the addition of chemicals could improve solids removal in the secondary clarifier. Enhanced sedimentation would consist of ferric or ferrous chloride addition to the influent flow to the secondary clarifiers. Polymer may also be used; however the District has previously reported process issues associated with the use of polymers (in particular polymer foaming and floating sludge). It is uncertain whether the EPA would consider this as an additional treatment process for blended flows. Enhanced secondary sedimentation could also be used during dry weather to improve sedimentation in the secondary clarifier when needed. An illustration of the enhanced secondary sedimentation configuration is presented in Figure 74. February

76 Treatment Alternatives Figure 74: Enhanced Secondary Sedimentation Biological Treatment A biological treatment alternative represents the maximum level of treatment for bypassed wet weather flows and would provide treatment similar to the existing secondary treatment process at the SMCSD plant. The addition of biological treatment to treat peak wet weather flows would increase the overall capacity rating of the plant. For example the existing FFR and secondary clarifiers were originally designed to treat up 6.0 MGD peak wet weather flow and an average dry weather flow of 1.8 MGD. Increasing the FFR and secondary clarifier peak wet weather flow capacity to 13.0 MGD would increase the average dry weather capacity of the SMCSD plant to approximately 4.0 MGD. Alternatives for providing additional biological capacity are presented in the following sections. Membrane Bioreactor (MBR) A membrane bioreactor (MBR) is a relatively new technology, which combines the activated sludge treatment process with membrane filtration process. Combining these two process allows for the elimination of the secondary clarifiers and a higher mean cell residence time, which results in a more compact treatment footprint. An MBR would be similar to the membrane filtration process discussed in Section including fine screening, however it would also include additional tank volume and processes equipment prior to the membrane filtration step. In essence, the MBR option would be equivalent to adding a new activated sludge treatment plant at SMCSD (not including solids handling). The MBR process would be able to produce high quality effluent within a compact footprint. Because the MBR is a biological process, it would need to be started about a month prior to the wet weather season to get the biological process going. A schematic illustrating how a MBR would be used is presented as Figure 75. February

77 Treatment Alternatives Figure 75: Membrane Bioreactor Schematic Additional Fixed Film Reactors and Secondary Clarifiers This treatment alternative would expand the secondary capacity of the existing treatment plant with new fixed film reactors and clarifiers. Although this alternative would provide treatment for the bypass flows, it would be difficult and expensive to implement, especially given the constraints of the existing site and the operational challenges of the existing fixed film reactor process at the plant. Because the fixed film reactors are a biological process, the additional fixed film reactors would have to be started up well before they would be needed to treat the peak wet weather flows. In essence, the District would need to operate the wet weather treatment facility during the wet weather season, whether it is needed or not. The existing fixed film reactors are shown in Figure 76. Figure 76: Existing Fixed Film Reactors and Secondary Clarifiers at SMCSD February

78 Treatment Alternatives Initial Screening of Process Alternatives A comparison of the potential process alternatives is presented in Table 71, which shows a qualitative description of each alternative in terms of capital cost, footprint, operational issues and treatment performance. Table 71: Preliminary Comparison of Alternatives Treatment Alternative Capital Cost Footprint Requirement Potential Operational Issues Treatment Performance Improvement No Project None None None None Membrane Filtration High Low High (Fouling) Moderate Media Filtration Moderate Moderate High (Fouling) Low HighRate Clarification Moderate Moderate Low Moderate Enhanced Secondary Sedimentation Low None/Low Low Low MBR High High High (Startup Time) High Fixed Film Reactor Expansion High High High (Startup Time) Moderate As stated in Section 5.1, assuming a secondary capacity of 6.0 MGD, primary effluent flow would be bypassed around the secondary treatment process approximately 7 times per year on average. Treatment of bypassed flow would only be required for a small amount of time each year, which represents a cumulative volume of approximately 1.9 MG per year. The infrequent use, short duration and relatively small volume of flow were taken into account as part of the initial screening process. Based on an initial screening of treatment options, it is recommended that the following treatment options be developed in further detail: No Project Alternative Enhanced Secondary Sedimentation Highrate clarification /ballasted flocculation MBR (only as part of a longterm solution for both dry weather and wet weather performance) The alternatives are listed in order of increasing treatment performance, which range from continuing the current method of operation (No Treatment Improvements Alternative) to provide full secondary treatment (MBR). The increasing level of performance also coincides with increasing capital cost Project Alternatives to Be Evaluated More detailed information for each of the four alternatives, including estimated footprint requirements is presented in the following sections. No Treatment Improvements Alternative As discussed previously, this alternative would require no upgrades to the existing treatment plant and therefore would have no capital cost or footprint requirement. Operation during wet weather would either continue to rely on blending, which would be dependent on continued regulatory approval to do so or a reduction in peak wet weather flows to the treatment plant using upstream flow storage to limit peak flow reaching the plant during large storm events. The fact that the District is currently in the process of implementing enhanced primary treatment, which would provide additional treatment during wet weather, may be worth noting during discussions with the RWQCB. February

79 Treatment Alternatives Enhanced Secondary Sedimentation Enhanced secondary sedimentation would be relatively easy to implement. The improvement would mainly consist of additional chemical metering equipment and chemical storage for injecting ferric chloride and possibly polymer into the process flow prior to the secondary clarifiers. It may be possible to integrate the required equipment and/or chemical storage into the District s existing chemical storage areas or as part of the enhanced primary sedimentation system, which is currently being installed. Implementation would also require pilot testing to identify the optimum chemical dose, polymer type and dose, as well as the most suitable location for injecting the chemical. Depending on the dose required it is likely that a 275 gallon tote for each chemical would be sufficient onsite storage for the enhanced secondary sedimentation system. For the purposes of this evaluation it is assumed that new chemical metering pumps, polymer mix/feed units and tote storage is required. The estimated footprint for the chemical facility is approximately 10 feet by 14 feet and is shown in Figure 77. Figure 77: Enhanced Secondary Sedimentation Footprint Requirement Polymer Tote Ferric Chloride Tote 10 Polymer Feed Unit Ferric Metering Pump 14 Scale ¼ = 1 HighRate Clarification / Ballasted Flocculation During peak wet weather flows greater than 6 MGD, highrate clarification would provide additional removal of suspended solids from primary effluent that is routed around the fixed film reactors. Highrate clarification equipment is typically supplied by equipment manufacturers as packed systems. The equipment can either be installed in tanks provided by the manufacturer or can be installed in concrete tanks constructed on site. An example layout for a highrate clarifier is shown in Figure 78. February

80 Treatment Alternatives Figure 78: HighRate / Ballasted Flocculation Clarifier Layout A 7.0 MGD capacity highrate clarifier and screens would have a footprint of approximately 26 feet by 56.3 feet and a height of approximately 20 feet. The highrate clarifier would be treating primary effluent and would require fine screening, which could be provided just ahead of the clarifier or as part of a larger treatment plant headworks improvement. Chemical storage and metering equipment would also be needed and would be installed next to highrate clarifier. In addition, to providing wet weather treatment of bypassed flows, the highrate clarifier could also be used during dry weather as a polishing step for removal of suspended solids from the secondary effluent. Highrate clarifiers are commonly used for suspend solids removal as a tertiary treatment process for recycled water production. If used in this manner, the highrate clarifier could replace the function of the existing sand filters. Also depending on the capacity needed, the highrater clarifier may be able to be installed where the existing sand filters are located. February

81 Treatment Alternatives Membrane Bioreactor As mentioned previously, the membrane bioreactor would represent a major facility upgrade. Installation of a membrane bioreactor would in essence result in the replacement of the existing treatment facility. The MBR alternative represents the most extensive upgrade and would be capable of providing both physical and biological treatment. An appropriately sized MBR plant would allow SMCSD to provide full primary and secondary treatment for the entire flow 100% of the time. Fine screening and grit removal would be required prior to the MBR process, in order to prevent damage t to the membranes. As was illustrated previously in Figure 75, the MBR alternative would consist of a series of process tanks. In addition, process equipment areas and a blower/electrical building would also be required. A potential layout and footprint requirement for a 13.0 MGD peak hour wet weather flow (5.0 MGD ADWF) MBR facility is shown in Figure 79. A new 5.0 MGD MBR facility would be approximately 105 feet wide by 215 long and would roughly double the footprint of the existing treatment plant site. Figure 79 shows the MBR alternative on a new platform south of the existing plant site. Some existing treatment facilities would be reused, including solids handling and disinfection. Construction of such a large expansion would also be difficult due to site conditions and potential regulator approvals that would be required. Including time for approvals, design and construction a new MBR facility could take more than five years to complete. February

82 Treatment Alternatives Figure 79: MBR Process Layout February

83 Section 8 SystemWide Alternatives (Conveyance + Treatment) Wet Weather Conveyance and Treatment Evaluation

84 SystemWide Alternatives (Conveyance + Treatment) Chapter 8 SystemWide Alternatives (Conveyance + Treatment) Based on the analysis of potential conveyance and treatment improvements, systemwide alternatives were developed to address both conveyance system and treatment plant capacity limitations. The conveyance system alternatives include improvements that would result in 5year design peak flows to the treatment plant ranging from 6 MGD (current design secondary capacity of the plant) to about 13 MGD (estimated current peak hydraulic capacity of the plant). The treatment plant alternatives are designed to eliminate bypasses of the secondary facilities by increasing secondary treatment capacity or providing alternative treatment processes to achieve enhanced treatment of primary effluent that is bypassed around the secondary treatment process. 8.1 Description of SystemWide Alternatives Table 81 lists the five systemwide alternatives, and Table 82 lists the treatment alternatives for providing the additional treatment capacity required under each of the systemwide alternatives. The alternatives and their estimated costs are described in detail in the subsections below. Backup for the cost estimates is provided in Appendix A. Table 81: SystemWide Alternatives Alt. Description 1 Conveyance with I/I Reduction WWTP PWWF (mgd) Storage (MG) Pump Station Improvements c Collection System Rehabilitation 13 Upgrade WSPS and pump to Scottys FM, Rehabilitate all mains and lower laterals in abandon exist. Scotty's lower Sausalito (or PS equivalent) Flow Controls Weir elev. for diversion to new WSPS from 21" gravity during peak flows 2 Conveyance w/storage at TCSD a Weir elev. for diversion to Scotty's PS 3 Wet Weather Storage b New PS at MLK Park, abandon WSPS and Scotty's PS RTC for diversion to storage and pump back after storm 4 Wet Weather Storage with I/I Reduction 5 Wet Weather Storage plus Pumped Diversion b New PS at MLK Park, abandon WSPS and Scotty's PS b New PS at MLK Park, abandon WSPS and Scotty's PS; configure LSPS to pump upstream to storage Rehabilitate all mains, lower laterals, and upper laterals in lower Sausalito RTC for diversion to storage and pump back after storm RTC for diversion and pumping to storage and pump back after storm RTC = Real Time Control a Storage at TCSD b Storage at MLK Park c Additional firm wet weather capacity at MSPS would be required under Alternatives 1 and 2. However, because previous studies have indicated that capacity expansion of the existing facility is not feasible and replacement of the station would likely be required in order to address numerous other deficiencies, a specific improvement project for MSPS is not included in this analysis. February

85 SystemWide Alternatives (Conveyance + Treatment) Table 82: Treatment Alternatives for SystemWide Approach Treatment Alternative Influent Feed Flow Range No Treatment Improvement Project Not Applicable Up to 13.0 MGD Enhanced Secondary Sedimentation High Rate Clarification / Ballasted Flocculation Blended FFR and Primary Effluent Primary Effluent Up to 13.0 MGD Up to 7.0 MGD Membrane Bioreactor Raw Influent 13.0 MGD The design capacity for the treatment alternative selected will be dependent on the peak wet weather flow into the plant (i.e. how much the peak flow can be reduced through conveyance system improvements). For the development of systemwide alternatives, any of the treatment alternatives could be used along with conveyance system improvements, with one exception. Because of the large capital investment required, the MBR alternative was developed as a standalone alternative that would be capable of treating the full 13.0 MGD peak wet weather flow at the plant. The No Treatment Improvement Project alternative could be implemented with any influent flow condition. For systemwide alternatives with an influent flow above 6.0 MGD the No Treatment Improvement Project alternative would require that the District continue to rely on blending. However, blending would not be required for systemwide Alternatives 4 and 5, which reduce the flow to the treatment plant to 6.0 MGD. Enhanced secondary treatment could provide some additional solids removal, but only for flow that passes through the secondary clarifiers. The enhanced secondary sedimentation alternative could be implemented with any of the systemwide alternatives to help improve treatment plant performance. Highrate clarification / ballasted flocculation would be used to treat primary effluent that is bypassed around the fixed film reactors and therefore could be combined with systemwide Alternatives 1, 2 and 3. For the wet weather evaluation, only a 13.0 MGD peak wet weather capacity MBR facility was considered since it represents the most expensive treatment alternative. Costs for small MBR alternatives would be similar in order of magnitude cost. If the District were to pursue a MBR treatment facility for other reasons (e.g. to address dry weather performance or aging infrastructure), then the District could look at optimizing the MBR sizing at that time. For this evaluation, the MBR alternative was only included as part of systemwide Alternative SystemWide Alternative 1: Conveyance with I/I Reduction This alternative would provide for conveyance of all system flows to the treatment plant. To eliminate capacity constrictions in the conveyance system that could potentially result in sewer overflows, improvements would be constructed in the upstream portion of the system to divert additional flow away from the existing capacitydeficient gravity sewer upstream of the LSPS, and sewer rehabilitation would be conducted in the portion of the City of Sausalito s collection system downstream of LSPS to reduce I/I and thereby limit the peak flow to the treatment plant to 13 MGD. The upstream conveyance improvements would involve upgrading the City s WSPS to serve as both a dry weather and a wet weather diversion PS, replacing the existing Scotty s PS. The existing weir diversion would be modified to divert more flow to the upgraded WSPS under peak wet weather conditions. The new WSPS would be upgraded to handle the additional wet weather flows as well as to correct current deficiencies that also impact dry weather operation (e.g., undersized wet well and fuel storage tank, concerns about level control and generator reliability, tight access, and close proximity to adjacent apartment building with February

86 SystemWide Alternatives (Conveyance + Treatment) associated noise and odor concerns), and the existing WSPS force main would be upsized. During wet weather, the upgraded WSPS would discharge into the existing Scotty s force main. Piping Revisions The proposed piping scheme is shown in Figure 81. Sausalito flow that now goes to the existing WSPS would continue to be conveyed in the existing 8inch sanitary sewer in Coloma Street to the manhole located in front of the pump station wet well. A new 15inch sanitary sewer would be provided from the manhole located just upstream of the Scotty s PS at the northwest corner of the Bridgeway/Coloma intersection and extended to the WSPS wet well (about 80 feet). The existing WSPS 6inch force main that connects the pump station to the 21inch gravity interceptor in Bridgeway would be upsized to 14 inches. A tee and two 14inch valves would be installed where this pipe crosses the existing Scotty s 14 inch OD FM. This piping arrangement would allow dry weather flow to continue to be discharged to the 21inch gravity sewer. During wet weather, the valve feeding the 21inch sanitary sewer would be closed and the other valve opened to allow flow to enter the Scotty s force main. Figure 81: Conveyance with I/I Reduction Alternative Piping Schematic Pump Station Improvements The existing WSPS is a wet pit/dry pit type; submersible pumps would be preferred for an upgraded facility. Under this alternative, the existing two 10HP dry pit pumps would be replaced with two 40HP submersible pumps. The 8foot diameter wet well is too small for installation of a third pump; however, February

87 SystemWide Alternatives (Conveyance + Treatment) having two pumps still provides one duty pump and one standby. (The existing Scotty s PS currently has only one pump.) Use of submersible pumps would also minimize noise. In order to install submersible pumps, the existing hatch in the pump room floor would be enlarged. An opening would also be saw cut in the pump station roof above this hatch to enable removing the pumps. Existing pump discharge piping would be replaced. New controls and variable frequency drives (VFDs) would be provided. A canister type odor control unit would be provided. A new engine generator would need to be about 175 KW, which would be too large to fit within the pump room. Therefore, it is proposed to remove the existing engine generator and to provide the 175 KW unit in a weatherproof enclosure located at the south end of the site, likely at the end of the driveway. The generator would be a Whisperquiet type insulated to minimize noise. A belly tank would be provided beneath the generator to avoid the need for a separate fuel storage tank. Sewer Rehabilitation Sewer rehabilitation is assumed to consist of rehabilitation or replacement of all sewer mains and the lower portion of the service laterals within the five lower basins of the Sausalito system that discharge to the gravity interceptor upstream of the MSPS (basins associated with drainage area Main in Figure 42). Based on City of Sausalito sewer and parcel GIS mapping, it is estimated that there are approximately 33,000 feet of sewer mains and 1,070 laterals in this area. For purposes of estimating the cost of this alternative, sewer rehabilitation is assumed to involve pipe bursting or opencut removal and replacement of existing sewers and lower laterals. Treatment Plant Improvements As noted above, the peak flow to the treatment plant under Alternative 1 would be approximately 13 MGD. Treatment plant improvements could take three different directions, leading to three variations of this alternative. Alternative 1a would install a new MBR facility sized to treat a peak flow of 13 MGD. This would provide full biological secondary treatment to all flows. Alternative 1b would provide a 7 MGD ballasted flocculation facility to treat all flows above the 6 MGD capacity of the existing secondary treatment facilities. (Total treatment capacity would be 13 MGD.) Alternative 1c would continue the existing blending practice wherein flows above 6 MGD are bypassed around the fixed film reactors. Cost of Alternative Table 83 summarizes the estimated capital cost of the system improvements that comprise Alternative 1. As can be seen in this table, the costs vary from about $48 million for Alternative 1a to $11 million for Alternative 1c. The implications of these alternative variations are discussed in the Comparison of SystemWide Alternatives in Chapter 9. Table 83: Estimated Cost of SystemWide Alternative 1 Estimated Capital Cost ($ million) Alternative 1b (7MGD Ballasted Flocculation) Alternative 1c (Current Blending Practice) Alternative 1a (13MGD Component MBR) I/I Reduction (Sewer Rehabilitation) Conveyance System Improvements Treatment Plant Improvements Total February

88 SystemWide Alternatives (Conveyance + Treatment) SystemWide Alternative 2: Conveyance with Storage at TCSD This alternative would reduce peak wet weather flows by eliminating TCSD flows from the SMCSD system during peak wet weather periods. For purposes of estimating the cost of this alternative, it is assumed that TCSD would construct facilities to store its wastewater flow during peak flow events. Conveyance improvements would be limited to weir modifications in the Coloma Street diversion structure to allow diversion of more flow to the Scotty s PS in order to minimize surcharging in the gravity sewer upstream of the LSPS. Treatment Plant Improvements The peak flow to the treatment plant under Alternative 2 would be approximately 10.5 MGD. Treatment plant improvements would include a 4.5 MGD highrate clarifier (ballasted flocculation) for side stream treatment of primary effluent that is bypassed around the fixed film reactors. Cost of Alternative The costs of the components that comprise Alternative 2 are summarized in Table 84. Table 84: Estimated Cost of SystemWide Alternative 2 Component Estimated Capital Cost ($ million) I/I Reduction (Sewer Rehabilitation) 0 Conveyance System Improvements/ Flow Storage 7.9 Treatment Plant Improvements 5.2 Total Systemwide Alternative 3: Wet Weather Storage This alternative would provide offline storage in the conveyance system at MLK Park, with an adjacent new pump station to replace the WSPS and Scotty s PS. The proposed location of the facility is at the southeast corner of MLK Park, as shown in Figure 82. The storage facility would be designed to handle the entire flow reaching the Coloma Street diversion structure and the WSPS under peak storm conditions. Under these conditions, flows reaching the LSPS would be limited to only those discharges from the Sausalito system that enter the interceptor downstream of Coloma Street. The new MLK Pump Station (MLKPS) would be located adjacent to the storage facility and would function as both a dry weather pump station (replacing the existing WSPS) and a wet weather pump station (replacing the existing Scotty s PS) that could be used in smaller storm events when diversions to storage are not required. Figure 83 shows a flow and piping schematic of the proposed facilities, and Figure 84 shows a more detailed layout of the MLKPS and storage basin. A conceptual elevation view of the facilities is shown in Figure 85. February

89 SystemWide Alternatives (Conveyance + Treatment) Figure 82: Location of Proposed MLK Park Wet Weather Storage Facility February

90 SystemWide Alternatives (Conveyance + Treatment) Figure 83: Piping Schematic for Wet Weather Storage Alternative February

91 SystemWide Alternatives (Conveyance + Treatment) Figure 84: Proposed MLK Park Storage Basin and Pump Station Conceptual Layout Figure 85: Proposed MLK Park Storage Basin and Pump Station Elevation View February