TO: DESALINATION TASK FORCE FROM: PROGRAM MANAGERS SUBJECT: SWRO DESALINATION FACILITY DESALINATION PROCESS CONFIGURATION DATE: MAY 18, 2011

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1 DESALINATION TASK FORCE MEMORANDUM TO: DESALINATION TASK FORCE FROM: PROGRAM MANAGERS SUBJECT: SWRO DESALINATION FACILITY DESALINATION PROCESS CONFIGURATION DATE: MAY 18, 2011 RECOMMENDATION: That the scwd 2 Desalination Task Force review and provide comment on draft Technical Memorandum No. 2 from Camp Dresser & McKee (CDM) titled Desalination Process Preliminary Design, accept as final with comments incorporated as appropriate, and support staff recommendation on the desalination process. BACKGROUND: At its September 2010 meeting the Task Force approved the scope of work with Camp Dresser & McKee for design of a seawater reverse osmosis desalination facility. CDM s first task (Task 1.1 of their scope of work) was to recommend a pretreatment process(es) ahead of the seawater desalination system. CDM s first deliverable, Technical Memorandum No. 1, summarized their evaluation of six pretreatment alternatives and recommended DAF + pressurized MF/UF as the preferred pretreatment process for the project. At its December 2010 meeting the Task Force supported this staff recommendation on the pretreatment process. CDM s next task, Task 1.2, was to advance the design of the desalination plant downstream of the pretreatment process to the point of connection to the potable distribution system. DISCUSSION: The SWRO Pilot Test Program recommended a single stage reverse osmosis (RO) system as the main desalination components to achieve water quality goals with a future potential second pass to provide flexibility. The CDM design team and technical advisory committee revisited the SWRO Pilot Test data, assumptions and goals and confirmed that this proposed configuration remains appropriate. Because the recommendation on the desalination process was consistent with the recommendations from the pilot work, there was no workshop necessary to vet different alternatives as was done with the pretreatment process. Rather, staff met with CDM on several occasions to review the evaluation process, review and modify the TM, and concur with its findings. While the focus of Task 1.2 was design of the RO process, other components upstream and downstream of the RO process were also advanced in their level of design to adequately inform the project and the Environmental Impact Report. There are many design considerations that will be further evaluated as the project proceeds; several are shown below. 8

2 1. As mentioned previously to the task force, no standby power will be provided for the main process equipment. Because the facility provides supplemental water supply, periodic, short-term interruptions in plant operation caused by power outages will be mitigated by existing raw water supplies and treated water storage. 2. The current area required for the proposed treatment facility is approximately 4 to 5 acres. This will be refined as the actual site alternatives is narrowed; decisions are made about functionality of the Control Building (currently oversized at 7,000 sf to accommodate meeting and office space and other amenities that may be reduced and/or eliminated); and the overall design progresses. 3. As the design proceeds, additional consideration will be given to flexibility, reliability and redundancy with an eye towards reducing costs. CDM is currently on hold until the site selection process is able to eliminate those sites with significant space limitations. Once sites are eliminated, CDM will resume work on completing preliminary design so as to fully inform the EIR. (The site selection process is discussed in a separate agenda item.) FISCAL IMPACT: Costs are inclusive of all major SWRO treatment facility elements and ancillary systems known at this time. (I.e., costs shown below do not include intake facilities, or other infrastructure improvements leading to and from the SWRO treatment facility.) Construction costs include a 30% contingency to allow for additional costs that cannot be estimated without detailed engineering design drawings and were escalated at 3% per year from January 2011 to the midpoint of construction assumed to be June The engineer s opinion of probable construction cost at this level of design effort is $47 to $71 million. The purpose for presenting a range of costs is to address the uncertainties and variability associated with: design considerations; site location; permitting; environmental constraints; economic conditions; availability of equipment, material and labor; and, other factors that may impact construction costs. Operation and maintenance costs include a 10% contingency and were escalated at 3% per year to The engineer s estimate is $2.75 million. ATTACHMENTS: Final Draft Technical Memorandum No. 2 (without appendices) 9

3 Draft Technical Memorandum No. 2 (TM-2) Desalination Process Preliminary Design Purpose The primary purpose of Task 1.2 Desalination Process Preliminary Design is to advance the design of the desalination plant from the equalization basin ahead of the reverse osmosis (RO) process to the connection to the potable distribution system. Other components before and after the RO process are discussed and advanced in their design as appropriate at this level of evaluation. Preliminary design criteria and process flow diagrams are presented in this memorandum for: Desalination and Energy Recovery Disinfection and Treated Water Pumping Chemical Systems Residuals Handling Also included are more detailed process and instrumentation diagrams and electrical one-line diagrams for the following components that comprise the desalination system: Microfiltration/Ultrafiltration (MF/UF) Filtrate and RO Feed Water Equalization Basin Contents: Purpose... 1 Summary... 2 Production and Water Quality Goals... 8 Summary of Functional Analysis Overview of Preliminary Design Reverse Osmosis Membrane System and Energy Recovery Devices Post-Treatment, Disinfection and Product Water Pumping Chemical Storage and Feed Systems RO Concentrate Storage and Pump Station Residuals Production, Handling and Disposal Projected Energy Use Desalination System Construction and Operating Cost Estimates Appendices A. TM-2A Summary of Functional Analysis B. Preliminary Design Criteria Tables C. Preliminary Design Drawings D. Source Water Quality Summary E. Membrane Projections F. Acronym Table Single-Stage High Pressure RO Feed Pumps Single-Stage RO Membrane Skids RO Cleaning System Calcite Contactor, Chlorine Contact Tank and Distribution Pumps. In Task 1.1 Pretreatment Process Evaluation and Recommendation, the project team evaluated six pretreatment alternatives: 1) slow sand filtration; 2) MF/UF with no chemical addition or clarification; 3) dissolved air floatation (DAF) and MF/UF; 4) flocculation, sedimentation and MF/UF; 5) DAF and granular media filtration (GMF); and 6) flocculation, sedimentation and GMF. The evaluations were conducted with assumptions made regarding the number, size and operating criteria for each alternative and for the ancillary equipment (such as residuals handling and chemical systems) required for each alternative. The CDM Team made these assumptions based on pilot testing results, experience at other similar facilities and treatment practices throughout the drinking water industry. scwd 2 selected DAF and MF/UF based 10 TM2-1

4 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District several factors including performance, cost, reliability, energy efficiency, area requirements and other considerations. In Task 1.2, the design team developed more detailed preliminary design criteria for the pretreatment facilities, residuals handling system, chemical systems and other ancillary equipment. This information was used to develop recommendations for the primary electrical power system, prepare electrical single-line diagrams for the facility and develop area requirements and conceptual equipment layouts for the plant. Summary Desalination System The scwd 2 Pilot Testing Program recommended a single-stage RO system with a future partial second pass as the main treatment facilities to meet the water quality and supply objectives for the Project. In Tasks 1.1 and 1.2, the CDM Team reviewed this recommendation, confirmed that the treatment techniques are appropriate and developed more complete preliminary design information. The recommended desalination process (illustrated in Figure 1) features four equally sized, single-stage RO skids, each with a design treatment capacity of 0.83 million gallons per day (mgd). The firm treatment capacity is 2.5 mgd with three units in service and one unit in standby mode. Provisions for a future second pass of low-pressure RO membranes have been incorporated into the preliminary design should future regulations or water quality objectives require additional treatment. Under normal raw water quality conditions (characterized by low turbidity and low algae concentrations), pretreatment will consist of chemical coagulation, MF/UF and cartridge filtration. When storm events (high turbidity), moderate algae blooms or red tide events occur, DAF pretreatment units will clarify the seawater to reduce particulate and organic loading to the MF/UF membranes and to mitigate potential fouling of the RO membranes. As shown in Figure 1, the CDM team recommends pump water injection rather than a static mixer for rapid mixing and placing a strainer before the MF/UF system rather than upstream of the DAF units. The rationale for these recommendations is found later in this TM. The RO system will meet the scwd 2 water quality goals in an energy efficient manner through a combination of high rejection and low energy RO elements. The RO system will operate at flux rates ranging from 8 to 10 gallons per day per square foot of membrane area (gfd) and permeate recovery rates of 40 to 50 percent. The optimal RO system recovery for energy efficiency will be approximately 42.5 percent. This range of flux and recovery values allows flexibility to achieve production during anticipated source water quality variations and to temporarily produce more water from an individual unit as necessary. Energy efficiency will be realized through the use of high-efficiency pumps and motors and energy recovery devices mounted on each of the four RO skids. The anticipated average energy use for the entire treatment process from raw seawater pumping through treatment and distribution of the finished water is 14.1 kwh/kgal. This estimate is similar to the average energy use estimates included in the Pilot TM2-2 11

5 Raw Seawater Intake & Screen Seawater Supply System Intake Pumps Desalination Facility Ferric Chloride Chlorine (Optional) Dissolved Air Flotation Transfer Pumps Strainer MF/UF Dechlorination Transfer Pumps Reclaimed Washwater Coagulation & Rapid Mixing Solids to Thickener for Disposal to Sewer MF/UF Feed Water Equalization Basin Backwash to Washwater Equalization RO Feed Water Equalization Basin Bypass for Direct Filtration Antiscalant (Optional) Caustic Soda (Optional) Mixing Cartridge Filters High Pressure Booster Pumps Single-Stage RO System Chlorine Carbon Dioxide Calcite Contactor Corrosion Inhibitor Carbon Dioxide Chlorine Desalination Facility High Service Pump Station Pipeline Improvements To Distribution System Limestone ERD Pressure Booster Pump RO Feed Water High Pressure RO Concentrate Bypass for Future Partial 2nd Pass RO System RO Concentrate Transfer Pumps Bypass for Split Stream Remineralization Chlorine Contact Tank/Clearwell To WWTF Outfall RO Feed Water Energy Recovery System RO Concentrate Equalization Basin W:\REPORTS\Santa Cruz City of\desalination Project_10\TM-2_Jan11\Figures\Figure 1_SWRO Desalination Plant Treatment Process.ai 03/31/11 JJT 12 Figure 1 SWRO Desalination Plant Treatment Process

6 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Program Report and the Pretreatment Technical Memorandum (TM-1). Energy use will continue to be evaluated and updated as the preliminary design progresses. Post Treatment Because reverse osmosis removes many of the minerals from the water, desalinated water tends to taste flat, and the lack of hardness and alkalinity makes the water more corrosive to concrete and other materials. To improve the taste and reduce the corrosive properties of the desalinated water, the permeate will be treated with carbon dioxide, then pass through calcite contactors to increase the calcium content and alkalinity. The treated permeate will pass through a chlorine contact basin to meet regulatory requirements for primary disinfection. After disinfection, a corrosion inhibitor, identical to the chemical currently used for corrosion control at the Graham Hill Water Treatment Plant, will be added before pumping the finished water into the distribution system. Residuals Handling and Disposal Residual streams produced at the plant include: 1) a mixture of the floating solids and clarified water from the DAF basins; 2) used washwater from the MF/UF system; 3) cleaning solutions from the MF/UF and RO systems; and 4) RO concentrate. The DAF residuals stream and MF/UF used washwater stream contain significant concentrations of suspended solids and will require treatment before recovery of the water. As shown in Figure 2, the DAF residuals and used washwater will be clarified and thickened in two parallel treatment units. Thickened solids from the two clarifiers/thickeners will be sent to the City s sanitary sewer system for disposal. Clarified water from the two units will be recycled to the plant influent for treatment. Chemical cleaning solutions from the MF/UF and RO systems will be dechlorinated and/or adjusted to neutral ph (6 to 9 units), then disposed of to the sanitary sewer. RO concentrate disposal is discussed in the following section. 13 TM2-3

7 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Figure 2 Residuals Handling System Flow Schematic RO Concentrate Storage and Disposal RO concentrate from the desalination process will be produced at rates ranging from approximately 0.83 to 3.8 mgd and salinity will range from approximately 60,000 to 75,000 mg/l depending on raw water quality and recovery rates for the RO system. The concentrate will be pumped from the desalination plant site and mixed with treated effluent from the City s Wastewater Treatment Facility. Preliminary design for the 2.5 mgd desalination plant includes a 600,000 gallon concentrate storage basin as recommended in the technical memorandum, Dilution Analysis for Brine Disposal via Ocean Outfall, prepared by Brown and Caldwell for scwd 2 in The concentrate storage basin will store concentrate during the night, when WWTF effluent flowrates are typically low. The stored concentrate would then be discharged during the day when effluent flowrates increase. This approach permits maintaining the appropriate concentrate to effluent ratio to meet the outfall discharge requirements. Electrical Power System Two essential power supply components lay the groundwork for electrical distribution system design: 1) the horsepower (HP) of the high pressure RO feed pumps; and 2) primary metering of the electrical service. To maintain continuity with the electrical metering approach recently installed at the Graham Hill WTP, CDM assumed that the City would prefer to take advantage of the reduced energy costs associated with primary metering of the PG&E electrical supply. The plant electrical service equipment (vacuum circuit breaker and service transformer) will be City owned and maintained. For purposes of this preliminary document, the primary voltage is assumed to be 21 kv. This assumption will be revisited when the exact plant site is identified and details are discussed with PG&E. Transformers will convert the 21 kv primary service to medium voltage (4,160 volts alternating current or VAC) to power the RO feed pumps, and to low voltage (480 VAC) for all the other loads at the plant. Preliminary electrical distribution singleline diagrams based on these concepts are shown on attached Drawings E-1 and E-2 in Appendix C. Other relevant electrical design concepts include the following: Standby power for the main process equipment is not proposed. Because the desalination plant provides a supplemental water supply, scwd 2 anticipates that periodic, short-term interruptions in plant operations caused by power outages will be mitigated by existing treated water storage in the distribution system and the existing primary water supplies such as the City s Graham Hill Surface Water Treatment Plant, Beltz Groundwater Treatment Plant, or the District s groundwater wells and treatment facilities. TM2-4 14

8 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design A small emergency diesel engine generator will be provided for operation of critical life safety systems and RO flush valves and pumps. UPS (Uninterruptible Power Supplies) will be provided for all programmable logic controllers (PLCs) and operator-machine interface (OMI) hardware. All facility lighting will be designed to comply with California Title 24 Energy Efficiency requirements. A photovoltaic system (PV) may be included in the project. It is assumed that PV panels will be mounted on the Control Building and other roofs as appropriate. Sizing and other PV system concepts will be developed in Task 1.10 Solar Power Evaluation. Concepts for auxiliary systems such as security, access control, in-plant communications, the City s wide area network (WAN), fire alarm, and others will be developed in Task 1.8 Overall Desalination Facility Preliminary Design. Process Instrumentation System The Plant Control System (PCS) will be based on PLC (Programmable Logic Controllers) and OMI (Operator Machine Interface) software to closely match the recently upgraded system at the City s Graham Hill Water Treatment Plant. Multiple PLCs and associated input and output (I/O) signals will be located throughout the plant to interface with field equipment. The OMI software will run on personal computers located in the control room. Additional OMI computers will be located near the process areas as appropriate to assist the plant staff in operating and monitoring the process equipment. Other relevant instrumentation and control concepts include the following: PLC system will be Modicon hardware to match the Graham Hill WTP. OMI system will be WonderWare System Platform software to match the Graham Hill WTP. In general, all field equipment connections to the PCS will be via hardwired connections to I/O modules. Data highway connections to field equipment are not proposed. Typically a Local / Off / Remote hardware switch will be provided at the motor controller for each major component. Local allows manual operation and remote allows automated operation from the PCS ma will be used for all analog field signals. 24 VDC will be used for all discrete field signals. The PCS major components (PLC and OMI) will typically be interconnected with fiber optic cabling utilizing Ethernet protocol. 15 TM2-5

9 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District A system supplier approach will be used for the RO and MF/UF to maximize unit responsibility to provide effective system performance and warranty requirements. Instrumentation and control components directly affected by the system mechanical equipment will be furnished by the system supplier. Plant Area Requirements The required area for the proposed treatment facility will be approximately 4 to 5 acres. This is an increase over the 3.5 acres estimated in the Pilot Test Program Report and TM-1. Reasons for the increases include: 1. The Control Building area increased from an assumed size of approximately 2,000 sf to approximately 7,000 sf; scwd 2 is currently evaluating the proposed uses and room sizes. 2. In previous work, CDM assumed concentrate storage facilities to store approximately 200,000 gallons onsite; however, a recent study by Brown and Caldwell recommends 600,000 gallons of onsite storage for the proposed 2.5 mgd plant, and 2,000,000 gallons at the plant s potential future expanded capacity (4.5 mgd). 3. For more effective and reliable operation, current design of the residuals handling system provides approximately one week of onsite storage of sludge in the clarifier/thickener units under worst-case conditions; and the redundancy of one duty and one standby clarifier/thickener. Previous design assumptions assumed smaller, high-rate clarifiers with limited thickening ability and minimal onsite sludge storage (i.e., less than one day). 4. In anticipation of the California Coastal Commission requiring more stringent stormwater handling practices, the CDM team used 25-year storm event data to size retention and pervious areas instead of 10-year storm event data as assumed in earlier planning. 5. Visual buffering areas and vehicle access routes for staff, visitors and chemical deliveries are larger for the three sites now being evaluated. Three potential sites are currently being evaluated by scwd 2 and the CDM Team under Task 1.3 Site Investigations and Task 1.6 Site Layout Plans. Figure 3 presents a layout for the 2.5 mgd plant and includes major process units, buildings and ancillary equipment. Potential future equipment additions such as second pass RO units and process units to expand the plant to an ultimate capacity of 4.5 mgd are also shown. Estimated Construction and Operating Costs The engineer s opinion of probable construction cost range for the desalination plant is $47 to $71 million (shown in Table 1) and assumes the project will be bid in 2014 and the midpoint of construction will be June TM2-6 16

10 420 Stormwater Retention Basin Stormwater Detention Basin Solids Handling Building Solids Clarifiers and Thickeners Solids Clarifiers and Thickeners RO Concentrate EQ Basin Future RO Concentrate EQ Basin Future RO Concentrate EQ Basin Reclaimed Water PS Sludge Transfer Station DAF Solids PS Air Saturator Administration Building Strainers MF/UF Skids ERD ERD SWRO SWRO SWRO SWRO SWRO SWRO Air Rapid Mix Room BW/CEB/ CIP Area 2nd Pass RO 2nd Pass RO RO CIP Used Washwater MF/UF RO Feed Water Pumps Feed Pumps Transfer Pumps DAF Recycle Pumps EQ Basin EQ Basin EQ Basin Calcite Contactors Future Water Channel DAF Clarification Basins Future 420 HP Pump HP Pump Cartridge Filters CO2 System Outdoor Chemical Loading and Containment Area Permeate Flush/ 2nd Pass Feed EQ Basin High Service PS, Surge Tank and Wet Well Emergency Generator Electrical Switch Gear Electrical Room Mech. Room FeCl 3 NaClO Bilsulfite Antiscalant NaOH Corrosion Inhibitor Stormwater Retention Basin Clearwell Stormwater Detention Basin W:\REPORTS\Santa Cruz City of\desal Pilot_Final Report_09\Graphics\Figure 3_Preliminary Site Layout.ai 03/30/11 JJT 17 Feet Figure 3 Preliminary Site Layout (4 acre site)

11 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Operating costs for the desalination plant are estimated at $2.75 million per year, as shown in Table TM2-7

12 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 1: Engineer s Opinion of Probable Construction Costs in Millions ($2015) System, Process or Building Description Construction Cost Estimate Control Building $3.2 DAF Facility $2.7 MF/UF Facility $4.1 RO Facility $5.8 Calcite Contactors $0.5 Chlorine Contact Basin, Clearwell and Flush Tank $0.9 Treated Water Pumping $0.8 Residuals Handling Facilities $1.3 RO Concentrate Storage and Pumping System $0.9 Chemical Systems $1.8 Electrical Equipment, Instrumentation and Controls $5.5 Yard Piping, Site Development and Miscellaneous Facilities $4.4 Land (1) $5.0 Subtotal $ % contingency $11.1 Subtotal $48.0 Escalation at 3% per year to June 2015 $6.8 Total $54.8 Engineer s Opinion of Probable Construction Costs Range $47-71 (1)Land costs based on required land for treatment facilities to produce initial capacity of 2.5 mgd treated water and additional land for potential future expansion to 4.5 mgd treated water; land cost assumed to be $1 million per acre. Table 2: Estimated Annual O&M Costs in Millions ($2015) Estimated Annual Cost (1) Annual Cost Components/Descriptions ($million per year) Energy $1.50 Labor $0.42 Pretreatment chemicals $0.27 Consumable replacement (cartridge filters, filter media, UF & RO membranes) $0.21 Solids disposal $0.10 Sub-total $ % contingency $0.25 Estimated Annual O&M Costs Total $2.75 (1)Estimates assume an annual average production rate of 1.6 mgd. This concludes the summary portion of this technical memorandum. TM2-8 19

13 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Production and Water Quality Goals Desalination Facility Operation and Production Goals The City (SCWD) and District (SqCWD) propose to operate the 2.5 mgd seawater desalination facility to provide water to each agency at various times throughout each year to meet the different agencies needs and objectives. For Task 1.2, CDM assumed that the water produced by the facility will be allocated according to a monthly priority system similar to Table 3 which was prepared by scwd 2 and included in the Request for Proposal for the project. The priority system identifies the maximum plant output, but production may be scaled back at the discretion of each party within its priority. When neither party requests water according to its priority, then the plant will be put in standby mode. Additional information presented by scwd 2 at the project s Quality Management Program (QMP) Workshop in October 2010 indicated that the plant must reliably produce 2.5 mgd of drinking water during droughts; and during nondrought conditions, the average production from the Desalination Plant could range from less than 1 mgd to 2 mgd. CDM considered the entire range of the potential plant production rates in the development of TM-2. Agency with 1 st Priority Table 3: scwd 2 Desalination Facility Production Priority System January February March April May June SqCWD SqCWD SqCWD SCWD/ SqCWD SCWD SCWD Quantity (mgd) each Agency with 2 nd Priority SCWD SCWD SCWD SCWD/ SqCWD SqCWD SqCWD Quantity (mgd) each July August September October November December Agency with 1 st Priority SqCWD SCWD SCWD SCWD SCWD/ SqCWD Quantity (mgd) each 2.5 Agency with 2 nd Priority SCWD SqCWD SqCWD SqCWD SCWD/ SqCWD Quantity (mgd) each 2.5 SqCWD SCWD Water Quality Goals and Design Criteria Table 4 summarizes the product water quality objectives for the facility. These goals were established during the pilot testing phase of the project and the boron and chloride concentration goals are being reevaluated and confirmed as a part of Task 1.2. Designing the SWRO system to meet the boron and bromide concentration goals under expected source water quality conditions will produce product water that meets the TDS, sodium and chloride goals. The plant will meet product water hardness, alkalinity, ph, chlorine residual, and phosphate residual requirements during the post-treatment process prior to distribution. 20 TM2-9

14 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 4: scwd 2 Desalination Facility Product Water Quality Goals Parameter Units Criteria TDS mg/l <300 Sodium mg/l <80 Chloride mg/l <150 Boron mg/l <1.0 Bromide mg/l <0.5 Hardness mg/l as CaCO3 30 to 60 Alkalinity mg/l as CaCO3 30 to 60 ph ph Units Match ph in distribution system (approx. 7.3) Chlorine residual mg/l Match residual in distribution system (approx. 1.0) Phosphate residual mg/l Match GHWTP dose (approx. 1.0) Table 5 summarizes the range of anticipated raw seawater quality used to develop the preliminary design for the Regional Desalination Plant. The values were developed during the pilot testing phase and are based on both historical water quality data and water quality sampling data collected at the pilot plant intake during the scwd 2 Seawater Reverse Osmosis Desalination Pilot Test Program (March 2008 to April 2009); and the proposed plant intake during monitoring for the Watershed Sanitary Survey (January 2008 to December 2009). This sampling data is summarized in Appendix D. Table 5: Raw Seawater Data Description units Average (Range) Source Water TDS mg/l 36,000 (35,000-37,000) Temperature Degrees C 14 (10 18) ph Source Water RO feedwater ph Units 8.0 ( ) 7.6 ( ) Source water turbidity NTU 5 (1 100) Source water TOC mg/l 3.0 (1 20) Source water chloride mg/l 19,000 (18,000 20,000) Source water bromide mg/l 70 (60 80) Source water boron mg/l 4.5 ( ) TM

15 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Summary of Functional Analysis For this project, functional analysis refers to the quantitative method used to estimate the available production capacity for the treatment facility based on the planned and unplanned outages for individual treatment processes, specifically DAF, MF/UF and RO. The CDM team developed four production scenarios for the plant based on information from scwd 2 communicated in the project description in the Request for Proposal and in discussions at the Pretreatment Workshop: Drought 2.5 mgd- Maximum plant production rate. High 2.0 mgd- Upper end of daily average production requirements in non-drought months. Average 1.6 mgd- Approximate average of daily production requirements in non-drought months. Low 0.83 mgd- Lowest average daily production requirement of approximately 1 mgd as described by scwd 2 and based on the smallest RO skid evaluated in the functional analysis. Next, the team developed nine process alternatives (3 DAF, 2 MF/UF and 4 RO), each consisting of various numbers and sizes of major equipment items. Design and operating assumptions were documented and estimates for the frequencies and durations of planned and unplanned outages were prepared for each major equipment item. Probability analysis techniques were applied to determine the estimated outages for each unit process and the impacts on overall plant production. The alternatives were compared with respect to relative reliability, construction and operating costs, area and building space requirements and other factors. The following are CDM s recommendations for the number and size of treatment units for the plant: DAF- Two basins (3.1 mgd each; 6.2 mgd nominal design capacity) MF/UF- Four racks (2.1 mgd each; 6.2 mgd nominal design capacity) RO- Four skids (0.83 mgd each; 2.5 mgd nominal design capacity) Table 6 presents a summary of the results of the functional analysis in terms of the estimated days per year that each unit process and the plant can operate at treated water production capacities up to 2.5 mgd. A more thorough discussion of the assumptions, alternatives, methodology and results is presented in Draft TM-2A Summary of Functional Analysis (provided as Appendix A). 22 TM2-11

16 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Production Scenario Table 6: Summary of Functional Analysis Results for Recommended Alternatives Estimated Days per Year that Each Treatment System and the Plant Can Meet the Specified Production Goals Treated Water Production Goal (mgd) DAF 1, 2 (2 Basins, 3.1 mgd each; 2 Duty, 0 Standby) MF/UF 3 (4 Racks, 2.1 mgd each; 3 Duty and 1 Standby) RO 4 (4 Skids, 0.83 mgd each; 3 Duty and 1 Standby) Drought High Average Low Plant (Production Rate) 356 (2.5 mgd) 361 (2.0 mgd) 361 (1.6 mgd) 365 (0.83 mgd) 1 DAF design surface loading rate (SLR) with two DAF basins in service at 6.2 mgd clarified water (2.5 mgd treated water) is 10 gpm/sf; manufacturers report that DAF units can operate effectively at SLRs up to 20 gpm/sf. 2 Because DAF is only required to operate during adverse storm events, algae blooms or red tides and each event typically lasts from approximately 1 week to 3 months, DAF performance is acceptable when followed by MF/UF. 3 MF/UF membranes design flux with three MF/UF racks in service at 6.2 mgd filtered water (2.5 mgd treated water) is 40 gfd; MF/UF membranes can operate at flux up to 44 gfd based on results of scwd 2 Desalination Pilot Test Program. 4 RO design flux with three RO skids in service at 2.5 mgd permeate/treated water is 8 gfd; RO membranes can operate at flux up to 10 gfd. The results presented in Table 6 indicate that during a given year, the plant is expected to produce: 2.5 mgd or more 356 days per year, 1.6 to 2.0 mgd for 6 days per year, and 0.83 mgd for the remaining 3 days per year. The City and District indicated during the Pretreatment Workshop that 2.5 mgd is expected from the facility during summer months during a drought, but that reduced production from the facility during other times of the year is acceptable for short periods of time. The reason that the RO system is not expected to be as reliable as MF/UF or DAF is that RO has different operational requirements and is more apt to be offline than more traditional water treatment processes. The CDM team considered adding a second redundant RO unit to improve anticipated reliability; however, the additional cost could not be justified for the minor increase in reliability provided by an additional unit. Overview of Preliminary Design Process Description A process flow drawing for the scwd 2 seawater RO treatment facility is presented in Figure 1. Seawater intake alternatives for this facility are currently being investigated by scwd 2 under a separate contract. While this study is investigating both an open ocean intake alternative and subsurface intake alternatives, the process flow drawing reflects that seawater will be withdrawn from an open ocean intake. This TM

17 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design scenario will require pretreatment facilities and treatment process at least as comprehensive as a subsurface intake scenario. If a subsurface intake is determined to be viable for this project, naturally occurring sand/soil layers or pre-engineered fill material may provide filtration for the removal of suspended solids and isolation of the intake from algal blooms. In this case, it may be possible to eliminate some of the pretreatment unit processes, such as the DAF. However, the remaining desalination and post-treatment facilities would be similar to those shown in the drawing. From this perspective, this drawing represents a more comprehensive treatment process which may be simplified if a subsurface intake can be developed. The SWRO desalination plant will likely include the following components. These recommendations are based on the assumption that the plant will be supplied from an open ocean intake. Pretreatment: Rapid mixing, coagulation, DAF clarification. Pressurized MF/UF: Feed pumps, auto-backwasing strainers, MF/UF membranes, backwash, cleaning system and ancillary support systems. Provisions will be included to allow bypassing the flocculation and/or DAF clarification process and to operate the MF/UF system in a direct filtration mode. SWRO Desalination: Equalization basin, RO feed transfer pumps, cartridge filters, high pressure RO feed pumps, RO units, energy recovery system, RO cleaning and flushing systems. Provisions will be included to accommodate the installation of a partial second pass RO treatment system in the event that it is determined that higher levels of boron removal will be desired in the future. Post-treatment and Distribution: Carbon dioxide (CO 2 ) system, calcite contactors, chlorine contact basin, clearwell, and high service pump station. Provisions will be made to facilitate the installation of onsite product water storage facilities if they are determined to be desired in the future. Residuals Handling: Washwater equalization basin, solids clarifiers/thickeners, reclaimed washwater pump station, solids transfer pump station, and a solids disposal pump station or solids mechanical dewatering system (if discharging solids to the SCWD s sewer system is not feasible). Concentrate Disposal: Concentrate equalization basin andpotentially a concentrate disposal pump station (depending on the final plant elevation). Chemical Systems: Sodium hypochlorite, sodium bisulfite, ferric chloride, antiscalant, caustic soda, CO 2, sodium hypochlorite, corrosion inhibitor, provisions for additional MF/UF and RO cleaning chemicals including, citric acid, caustic soda, strong acid, and other cleaners. Provisions will also be included for future chemicals. Miscellaneous Facilities: Operations and maintenance rooms, electrical equipment room, yard piping, stormwater detention/handling, and other miscellaneous items. 24 TM2-13

18 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District In TM-1, 100 to 120-micron strainers were recommended before the DAF units. These strainers would protect both the DAF and MF/UF systems from debris that could potentially damage the process equipment. As the preliminary design has progressed, the design team assumed that the DAF basins would be outdoors and uncovered. With uncovered basins, debris (such as sand or pine needles) could enter the basins, be pumped to the MF/UF system and damage the membranes. Based on discussions with the potential MF/UF system suppliers and experience at other MF/UF facilities, CDM recommends locating strainers after the DAF units and MF/UF feed pumps, and immediately before the MF/UF membranes. This revised strainer location will prevent debris larger than approximately 100 microns from reaching the MF/UF membranes when the DAF units are in operation or when raw or coagulated water is bypassed around the DAF basins and sent directly to the MF/UF membranes. CDM will continue discussions with potential DAF equipment suppliers to determine if screens, strainers or other particle removal equipment should also be installed before the DAF units. Similarly, static mixers were shown for rapid mixing in the Pilot Test Program Report and TM 1. In TM-2, CDM recommends pumped water injection because it will provide the following benefits over static mixers or impellor-type mixers: 1) more effective mixing over the entire range of anticipated flows; 2) less susceptible to fouling and fewer maintenance requirements; 3) closed pipe system can be located indoors or outdoors and will not produce potentially corrosive salt water aerosols; 4) low energy use; and 5) preference expressed by City s operating staff. Reverse Osmosis Membrane System and Energy Recovery Devices Reverse Osmosis Introduction In the seawater reverse osmosis (SWRO) process, seawater is pumped through high rejection seawater membranes at high pressure to produce a high purity stream (permeate) that meets established drinking water quality standards. High pressure RO pumps are used to increase the feed pressure to the seawater reverse osmosis membranes to the range of 800 to 1,000 pounds per square inch (psi). Operation at these high feed pressures overcomes the natural osmotic pressure of seawater and allows high purity water to flow through the membranes while removing over 95 percent of the dissolved salts. In a typical seawater RO system, 40 to 50 percent of the seawater that is fed to the reverse osmosis system passes through the membranes and becomes high purity drinking water. Because approximately half of the water is removed from the remaining seawater stream while the majority of the dissolved salts are retained, this stream becomes more concentrated and is called the concentrate (a.k.a., brine) stream. The concentrate stream leaves the RO at a pressure only 20 to 40 psi lower than the feed pressure to the membranes. For this reason there is a significant amount of energy remaining in the concentrate stream and energy recovery devices are used to recover over 95 percent of the energy in this process stream, which greatly improves the overall energy efficiency of the process. This section will describe the components of the RO system and the energy recovery system. TM

19 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Reverse Osmosis Process Description Figure 4 illustrates the proposed desalination process, and Table 7 shows the mass balance for desalination system at maximum daily plant flow. Filtered seawater from the MF/UF process will be pumped by transfer pumps from the RO feed equalization basin to the seawater RO process. This filtered feedwater will first pass through cartridge filter vessels to remove any suspended particles that may have accumulated downstream of the MF/UF process or in the MF/UF clearwell. A chemical supply line and static mixer with injection ports will be supplied for dosing antiscalant, caustic soda, or sodium bisulfite ahead of the RO system as necessary 1. Downstream of the cartridge filters, a portion of the feedwater flow, equal to the RO concentrate flow, will be directed to the energy recovery devices (ERD s). The remaining feedwater flow will be directed to the high pressure RO pumps. These pumps will pressurize the RO feedwater up to approximately 900 psi at average salinity and temperature conditions and up to approximately 1,000 psi for the condition of maximum feedwater salinity at minimum temperature and a five year membrane life. The feedwater passing through the ERD devices will be pressurized by recovering energy from the RO concentrate using isobaric pressure exchangers with an energy recovery efficiency greater than 95 percent. The feedwater stream leaving the ERD s will require minor pressure boosting by an ERD booster pump to compensate for the pressure losses experienced from the feed/concentrate stream passing through the feedwater channels of the RO membranes, any pressure loses in the piping, and ERD energy transfer inefficiencies. The necessary pressure boost will typically be in the range of 30 to 35 psi. The feedwater from the ERD boost pumps will be combined with feed water from the high pressure RO pump discharge for treatment in the RO membranes. The RO membranes will be a hybrid arrangement of high boron rejection seawater RO membranes in the three (3) lead element positions and low energy consumption seawater RO membranes in the four (4) tail element position of the RO pressure vessels. This hybrid arrangement provides the optimum balance of boron removal to meet treatment objectives while minimizing RO system energy requirements. The RO system will have the ability to operate from 40 to 50 percent recovery, however, it is anticipated that the RO system will typically operate in a recovery range of 40 to 45 percent. 1 The Technical Advisory Committee recommends that provisions for antiscalant, caustic soda, and sodium bisulfate be included upstream of the RO system. Antiscalant addition may be desired to reduce iron fouling of the RO membranes when an iron based coagulant is added during pretreatment. Caustic soda addition may be desired to temporarily reduce boron concentrations in the desalinated water. Sodium bisulfate addition will be necessary for dechlorination when chlorine is added upstream for biofouling control and/or MF/UF cleaning. 26 TM2-15

20 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Figure 4 Desalination System ID No. Table 7: Mass Balance for Desalination System at Maximum Daily Plant Flow Description Flow TDS TSS (gpm) (mg/l) (mg/l) ,000 <0.1 1 Pretreated/RO feed water from MF/UF pretreatment via the RO feedwater equalization basin 2 Split RO feedwater stream through high pressure pumps ,000 <0.1 3 Split RO feedwater stream through energy recovery system ,000 <0.1 4 Recombined RO feedwater ,750 <0.1 5 RO permeate <0.1 6 RO concentrate ,500 <0.1 TM

21 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design The low pressure concentrate stream leaving the ERD s will pass through a flow control valve which regulates the flow through the ERD s and will flow to a concentrate equalization basin. Transfer pumps will pump the concentrate from the equalization basin to a pipeline where it will blend with the treated effluent from the waste water treatment facility prior to disposal in the ocean through the existing outfall line. Membrane Performance Projections Membrane performance projections have been developed to determine the optimum membrane selection to meet the finished water quality goals based on the anticipated operating ranges of seawater salinity, temperature, and water quality as outlined in the Water Quality, Production and Operational Goals section of this TM. For a specified plant capacity, the membrane performance projections will calculate the required operating pressure and predicted permeate quality for the selected membrane element model(s) and the specified RO system configuration based on input information concerning feedwater quality and temperature. All the major membrane manufacturers (e.g., Hydranautics, Toray, Dow Filmtec) provide software for generating membrane performance projections for their products. Because of the importance of energy recovery on the overall performance of a seawater RO system, several have made provisions in their projection programs to include various energy recovery devices. At the present time, the membrane projection software from Hydranautics appears to be the most advanced in integrating a number of features of importance in the design of seawater RO systems including isobaric energy recovery devices, split partial permeate stream splitting which can significantly increase product quality or reduce second pass size in a two pass system, hybrid membrane configurations with isobaric energy recovery, etc. Since pilot testing and membrane performance projections developed during the pilot phase of this project indicated that a hybrid membrane element configuration using isobaric energy recovery devices would be the optimum design for this project, the Hydranautics software was considered the best membrane projection software for further developing the design criteria for this project. Now that the initial projections have been developed using the Hydranautics program, projected ERD performance from this work will help to facilitate performing similar analyses using projection software from other major membrane manufacturers. As discussed in the functional analysis section of this memo, membrane unit capacities in the range of 0.83 mgd to 1.25 mgd would be a logical choice to provide a balance between capital and operating costs and plant reliability. The selection of the type of high pressure RO pump may also impact the selected capacity for the membrane treatment units as described in the High Pressure RO feed pump section of this TM. Membrane projections defining the anticipated operating envelope for this project are presented in Appendix E. As shown in the projections, the maximum TDS and minimum temperature condition at the maximum assumed membrane age requires the highest membrane operating pressure. This pressure is used in the calculation to determine the required total dynamic head (TDH) for the design of the HP RO 28 TM2-17

22 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District pump. The maximum TDS and maximum temperature case at maximum membrane age represents the worst case membrane permeate quality. Performance at average conditions reflect typical plant operating values for parameters such as power and chemical consumption that can be used as a basis for evaluating typical system operating costs. The minimum TDS and maximum temperature case with new membranes reflects the lowest head condition for the HP RO pump and defines the turndown ratio of this pump from the maximum design TDH to the minimum required TDH. It is intended that the design of this facility provide sufficient flexibility to allow operation over a range of 40 to 50 percent recovery to provide the optimum balance between plant operating costs and capital cost. For operation over a wide range of the anticipated salinity and temperature conditions a recovery rate of 42.5 to 45 percent should be optimum. However, at extreme conditions of maximum salinity and minimum temperature, it may be desirable to operate at approximately 40 percent recovery. Similarly under certain conditions, there may be benefits in terms of reduced feed flow requirements or concentrate discharge requirements associated with operation at higher recovery. Experience with other seawater RO projects indicates that operation at an average design flux of approximately 8 gfd results in optimum balance of power cost versus capital cost. The system could be operated at an average flux of up to 10 gfd at conditions of lower salinity and higher temperature; however, as shown in the membrane projections, flux and recovery will be limited as the membranes age and foul. For this design, it was determined that a combination of high boron rejection and low energy seawater RO membranes would provide the optimum solution to meet the established boron treatment goal at the lowest energy consumption. For the membrane performance projections, a hybrid design was selected that utilizes three high boron rejection Hydranautics SWC4B membranes in the lead element positions and four lower energy consumption Hydranautics SWC5 membranes in the tail end positions of each pressure vessel, which is where TDS will increase and feed pressure will decrease as seawater flows through the vessel. Although the performance projections were based on Hydranautics RO elements, seawater RO elements from other qualified manufacturers can also meet the treatment goals and the RO elements will be competitively bid during the procurement phase of the project. The Hydranautics RO performance projection program also provides the flexibility for the user to adjust how membrane performance might vary over time with respect to membrane flux decline and salt passage increase. Hydranautics published RO membrane design guidelines give conservative, typical, and aggressive values of membrane flux decline factor and salt passage increase factors based on the application (brackish, seawater, groundwater, surface water, etc.) and the level of pretreatment required. These membrane projections are based on Hydranautics typical design guidelines for an open seawater intake with MF/UF pretreatment ahead of the RO system. The typical design guidelines for MF/UF pretreatment ahead of the RO system are an RO membrane flux decline factor of 7 percent per year and a salt passage increase factor of 10 percent per year. For end of life membrane performance, an average membrane life of 5 years has been used in these projections; therefore the projections are conservative. The RO membranes are often replaced systematically at frequencies that maintain the 5-year life, but TM

23 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design reduce the average membrane age to less than 5 years; in these cases, water quality may be better than indicated by the projections for 5-year membrane life. Table 8 provides a summary of membrane performance at maximum, average, and minimum operating conditions. RO Feed Equalization and Transfer While the RO units tend to operate continuously with infrequent offline cleanings, the MF/UF units cycle back and forth between filtration mode and backwash mode at frequencies of once or twice per hour. To compensate for the time out of service during backwash mode, the MF/UF units operate at a capacity higher than the required feed flow to the RO units. This higher production rate compensates for production lost while the units are out of service for backwashing, chemically enhanced backwashes, integrity testing, etc. For this reason, it is common practice to install a RO feed equalization tank between the MF/UF units and the RO units. This equalization tank provides buffer capacity to compensate for the differences in the production rates of the MF/UF units and the feed requirements for the RO units. This basin can also serve as a reservoir for backwash water for the MF/UF system. While it is possible to coordinate operation of the MF/UF and RO units so that filtrate from MF/UF units can go directly to the suction of the HP RO pumps without the use of an RO feed equalization basin, feed transfer pumps or cartridge filters, this design approach (a.k.a., direct coupling) will require more complex programming for the MF/UF system and more close coordination with the RO system. This approach may require (1) having additional MF/UF units online, (2) the MF/UF units may need to operate at higher flow rates to maintain full RO feed flow when a MF/UF unit goes into backwash, and (3) a recirculation loop from the MF/UF filtrate line back to the clearwell after the DAF system. It was considered that the more conventional and easier to operate system that includes the equalization tank between the MF/UF and RO systems should be used as the basis of the preliminary design. Direct coupling of the MF/UF and RO systems will be investigated during the detailed design phase if scwd 2 elects to pursue this option. Sizing information and design criteria for the RO equalization basin is provided in Table 9. A total of 3 RO feed transfer pumps will be provided with two pumps required for operation at full plant capacity with the third unit serving as an installed spare. The RO feed transfer pumps will be designed to provide a suction pressure of 20 to 30 psi at the suction side of the HP RO pump allowing for approximately 15 psi pressure drop for dirty cartridge filters, static mixer and piping losses. It is anticipated that these pumps will be vertical turbine wet pit type pumps. These pumps will be of superduplex stainless steel construction for seawater service. Table 8 indicates that these pumps will be equipped with variable frequency drives; however, these pumps will be further evaluated during the detailed design to determine whether constant speed pumps will provide adequate coverage over the required operating range. 30 TM2-19

24 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 8: Membrane Performance Summary Parameter Units RO Membrane Type Membrane Elements Selected To Run Projections RO Membrane Model Nos. Hydranautics SWC4B or equal (3 Lead Elements) Hydranautics SWC5 or equal (4 Tail Elements) Pressure and Flow Design Values Operating Condition Maximum Design Average Design Pressure Conditions Pressure Conditions Assumed Element Age years Feedwater Temperature Degree C Assumed Fouling Allowance Per Year 1 units 7% Flux Decline/10% 7% Flux Decline/ Salt passage increase 10% Salt Rejection Minimum Design Pressure Conditions 7% Flux Decline/ 10% Salt Rejection Pressure Vessels in Single Stage, Single Pass Array number SWRO Elements per Pressure Vessel number Total Elements per Skid number Permeate Recovery Rate 2 percent 40% 42.5% 42.5% Permeate Capacity per Skid mgd Permeate Capacity per Skid gpm Average Flux gfd Maximum Single Element Flux gfd Feedwater Pressure (w/fouling Allowance) psi Total Permeate Back Pressure psi Feedwater ph units Permeate Water Quality Values Operating Condition 5-Year Water Quality Average Water Quality Projections 4 Projections Initial Water Quality Projections Permeate Water Quality Assumed Element Age years Permeate Recovery Rate 2 percent 40% 42.5% 45% Average Flux gfd Modified RO Feedwater Temperature Conditions at Criteria Above Degree C Calcium mg/l Magnesium mg/l Sodium mg/l Bicarbonate mg/l Sulfate mg/l Chloride mg/l Boron mg/l Bromide mg/l Total Alkalinity mg/l as CaCO Total Hardness mg/l as CaCO Total Dissolved Solids mg/l ph units Concentrate Water Quality Temperature Condition Degree C Calcium mg/l Magnesium mg/l ,256 Sodium mg/l 20,105 19,135 18,997 Bicarbonate mg/l Sulfate mg/l 5,047 6,744 4,774 Chloride mg/l 36,170 33,670 34,280 Boron mg/l Total Alkalinity mg/l as CaCO Total Hardness mg/l as CaCO3 11,889 12,186 11,097 Total Dissolved Solids mg/l 65,549 63,815 62,059 ph units Notes: 1. Allowances are for flux decline and salt passage increase. 2. Operation at recovery rates less than 42% will typically be associated with low feedwater temperature and advanced membrane age. 3. Maximum feedwater pressure assumes maximum TDS, minimum temperature, and membrane age of 5 years. 4. Maximum TDS and Maximum Temperature Conditions Assumed to Indicate Maximum Salt Concentrations in RO permeate. TM

25 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Table 9: RO Feedwater Equalization Basin and Low Pressure RO Feed Pumps Design Criteria Parameter Units Initial 2.5 mgd Facility Potential Expansion to 4.5 mgd MF/UF Filtrate/ RO Feedwater EQ Basin Type Concrete Concrete Rated Capacity (Minimum-Maximum) mgd Number of Chambers no. 2 2 Nominal Rated Capacity gpm 4,340 7,813 Volume Required for 10 Minutes of Equalization at Future Rated Capacity gallons 78,000 78,000 MF/UF Backwash Volume Required gallons 17,000 28,000 Dimensions Each Chamber Length feet Width feet Height feet Water Depth (Maximum) feet Total Volume gallons 109, ,000 Detention Time at Rated Capacity minutes RO Feedwater Low Pressure Transfer Pumps Type Vertical Wet Pit with VFD Vertical Wet Pit with VFD Number of Pumps in Operation No. 2 3 Number of Pumps in Installed No. 3 4 Design Flow per Pump gpm 2,170 2,604 Firm Capacity gpm 4,340 7,813 Design Differential Pressure (TDH) psi Motor Size HP Cartridge Filters & Static Mixer Cartridge filtration will be provided upstream of the RO units as a safety precaution to remove any sand, silt, pipe shavings, and other suspended solids that may enter the system downstream of the MF/UF units and foul or physically damage the membrane elements. The filter cartridges will measure 40 inches in length, 2.5 inches in diameter, and have a nominal pore size of 5 microns. The cartridge filter vessels accommodate either string-wound cartridge filters or rigid-structure cartridge filters. With MF/UF filtration ahead of the cartridge filter, it is anticipated that the feedwater will be of high quality in terms of low sand and suspended solids levels. In this case, the string-wound cartridges should be adequate for normal operating conditions; however, with the low fouling potential and relatively long times between cartridge change out, either string-wound or rigid-structure cartridges could be used cost effectively. For seawater RO systems there are several alternate materials of construction that have been used for cartridge filter housings. High alloy stainless steel construction such as AL6XN, 254 SMO, or super 32 TM2-21

26 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District duplex stainless steel have proven to give reliable service in seawater RO plants; however, these alloys can cost two to four times as much as 316L SS housings. On large international design/build/operate projects some contractors have supplied carbon steel rubber lined vessels. For the size of cartridge filter housings for the scwd 2 project, fiberglass cartridge filters can be a cost effective solution that provides a high degree of corrosion resistance. As shown in Table 10, three cartridge filter vessels will be provided. Each housing will hold the equivalent of 472, 10-inch long filters. This will result in loading rates of 3.1 gpm per 10-inch cartridge with 3 cartridge vessels in service and 4.6 gpm per 10-inch cartridge with 2 vessels in service when producing 2.5 mgd at 40 percent recovery. Cartridge Filters Table 10: Cartridge Filter Design Criteria Parameter Units Initial 2.5 mgd Facility Number of Filter Housings (in service) Type Horizontal Fiberglass Housings Potential Expansion to 4.5 mgd Horizontal Fiberglass Housings No. 2 4 Number of Filter Housings (total) No. 3 5 Design Flow per Vessel Loading Rate All-in-service Loading Rate One-out-of-service mgd gpm 2,170 2,604 gpm per 10 in. of filter gpm per 10 in. of filter Length of Each Cartridge Filter inches Nominal Pore Size micron 5 5 Pressure Drop, Clean psi 1 to 2 1 to 2 Pressure Drop, Dirty psi 10 to to 15 High Pressure RO Pumps For seawater treatment plants in the capacity range envisioned for the scwd 2 project there are two broad classifications of pumps, positive displacement and centrifugal, that can be used for the high pressure (HP) RO pumps. Selection of the type of high pressure RO pump may also impact the selected capacity for the membrane treatment units. For example positive displacement pumps while typically providing high operating efficiencies may be limited to membrane unit capacities of 1.0 mgd or less. On the other hand, centrifugal RO pumps can accommodate much larger capacity membrane units and would tend to have increasing efficiency as membrane unit size increases. Budgetary quotes received from vendors indicated that the positive displacement pump would have an efficiency of 88 percent while the centrifugal pump would have an efficiency of 78 percent. These quotes assumed seawater RO membrane unit capacities of 0.83 mgd and 1.25 mgd for the positive displacement (PD) and centrifugal pumps respectively. In comparison, the centrifugal HP RO pumps for the Blue Hills, Bahamas plant, which has a design permeate production of 1.2 mgd per unit, have an efficiency of 82 percent. In this case it is TM

27 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design anticipated that the efficiency difference between centrifugal pumps is due to differences in pump models rather than differences in the pump capacity. Consolidated Water supplied both the 2.6 mgd Windsor and the 7.2 mgd Blue Hills desalination plants as well as over a dozen other seawater RO plants in the Caribbean in the size range of 0.2 mgd to 7.2 mgd. Some of these facilities use positive displacement HP RO pumps while others use centrifugal pumps. Consolidated advised that based on their experience with both positive displacement and centrifugal HP RO pumps, that even with approximately $0.35/kWh (USD) electrical costs (in several Caribbean islands) we are moving away from reciprocating PDs and going exclusively to centrifugal pumps, due to overall cost effectiveness and reliability. Until a final decision is made concerning the type of HP RO pump to be used for this project, the design criteria has been based on the use of centrifugal pumps which result in a more conservative design approach in terms of power consumption and configuration of the electrical power system. These options will be investigated further in the detail design phase to determine whether improvements can be made on energy efficiency and the cost of the electrical power system. One dedicated horizontal multistage split case HP RO pump with variable frequency drive will be provided for each RO membrane unit. A total of four HP RO pumps are provided with three pumps normally in service when the plant is operating at full plant capacity with one installed standby unit, which is dedicated to the standby RO unit. The design criteria data for the HP RO pumps are based on a budgetary pump quote for horizontal multistage split case (HMSC) centrifugal pumps. This is the pump type currently used in Blue Hills, Tampa, Trinidad and numerous other seawater desalination plants. Another option for centrifugal HP RO pumps is the radially split multistage centrifugal pump. These pumps tend to be somewhat lower in capital cost but not quite as efficient (76 percent) as the HMSC pumps. The HMSC pumps are also considered to have higher reliability and are easier to maintain. As noted in the discussion of membrane performance projections, HP RO pumps would be designed to operate over a range of conditions of RO unit feed pressure from initial (0-year) conditions at minimum TDS and maximum temperature to the long term (5-year membrane life) maximum TDS and minimum temperature conditions. Due to the anticipated range in TDS and temperature conditions and initial and long-term membrane performance conditions, each HP RO pump will require a wide operating range. As discussed previously, the RO feed transfer pumps will be designed to provide a suction pressure of 20 to 30 psi at the suction side of the HP RO pump allowing for approximately 15 psi pressure drop for dirty cartridge filters, static mixer and piping losses. The HP RO pumps will require a TDH to pump the RO feed water from the suction pressure provided by the RO feed transfer pumps to the required operating pressure of the RO membrane unit based on the feed salinity and temperature conditions and the degree of fouling of the membrane elements. Operating conditions are expected to range from 587 gpm at a total dynamic head of approximately 756 psi for initial (0 yr) conditions at minimum TDS and maximum temperature to 589 gpm at a total dynamic head of 967 psi for the long term (5 year membrane life) maximum TDS and minimum temperature condition. 34 TM2-23

28 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District To optimize operating efficiency over this range of operating flow and pressure, a variable frequency drive will be provided on each HP RO pump. The variable frequency drive will allow the RO membrane unit to maintain water production under varying water quality conditions, temperatures, and fouling conditions. These variable frequency drives conserve energy by allowing the pump and motor to operate at the minimum energy required for that particular operating condition. The HP RO pumps will be installed in a separate pump room to minimize pumping noise in the rest of the membrane plant. The materials of construction will be superduplex stainless steel or equal in terms of corrosion resistance. Seawater Reverse Osmosis Units As discussed in the functional analysis section of this memo, membrane unit capacities in the range of 0.83 mgd to 1.25 mgd would be the most logical choice to provide the best balance between capital and operating costs and plant reliability for the initial design phase and potential expansion for this project. As discussed in the previous section, the HP RO pump selection may impact the selected treatment capacity of the membrane units. For the purposes of this analysis a membrane unit size of 0.83 mgd was selected as providing the flexibility to use either positive displacement or centrifugal pump options. The RO units will be configured as independent membrane units with a dedicated HP RO pump and energy recovery system for each membrane unit. Each membrane unit will be arranged in a single pass configuration with the membrane vessels in that pass installed in parallel in a single stage configuration. Each membrane unit will have 37 pressure vessels installed and will be configured to accommodate the installation of a total of 42 vessels. Each pressure vessel will contain 7 membrane elements, for a total of 259 elements per skid. Membrane elements will be arranged in a hybrid configuration with high rejection boron elements in the lead positions and low energy elements in the tail positions of each vessel to optimize the balance between boron removal and power consumption. The configuration reflected in the membrane projections is based on three high boron rejection SWC4B membranes followed by four SWC5 low energy elements in each vessel. The design basis reflects the use of standard 8-inch diameter by 40- inch long seawater RO elements with 400 sf membrane area per element. Due to the potential for biofouling associated with algal bloom events, this design envisions the use of standard 400 sf rather than higher membrane area elements. In addition, it is recommended that membrane elements with 31 mil or larger feed/brine spacers be selected to reduce pressure drops across the membrane elements during biofouling events as compared to standard 28 mil spacers. It is recommended that biostatic spacers be provided where available. The larger feed/brine spacers reduce pressure drop per element which translates to lower energy loss. The larger feed/brine channels tend to be less susceptible to biofouling buildup and facilitate membrane element cleaning. Each membrane unit will include all pressure vessels, membrane elements, supporting frame, sample panels, on-board instrumentation and associated panels, piping, valves and actuators, and all necessary appurtenances. TM

29 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design A total of four 0.83 mgd permeate production capacity membrane units will be provided for the initial design phase. Three membrane units would normally be in operation with the fourth unit representing an installed spare. Energy Recovery System A high-efficiency, isobaric pressure exchanging energy recovery system will be provided for each RO membrane unit to significantly reduce the overall power consumption of the SWRO system. The energy recovery system described in Table 11 is based on pressure exchanger type ERDs as manufactured by Energy Recovery Inc. (ERI); additional ERDs will be evaluated in future tasks. A total of 4 ERI model PX- 300 ERD s will be provided per RO membrane unit. In this configuration, three units would normally be in service at RO recoveries of 43 to 50%; the fourth unit would only be necessary at RO recoveries of 40% to 42%. The design flow rate per ERD will be 261 gpm at an operating recovery of 42.5 percent resulting in design flow rate of 783 gpm per energy recovery system. The system will have the operating flexibility to operate at a recovery down to 40 percent. At 40 percent recovery, the total concentrate flow will be 868 gpm and the flow per unit would be gpm. Table 11: SWRO System Design Criteria Parameter Units Initial 2.5 mgd Facility Potential Expansion to 4.5 mgd RO High Pressure Feed Pumps Type Horizontal, Split-case, Horizontal, Split-case, Centrifugal with VFD Centrifugal with VFD Number of Duty Pumps No. 3 5 Number of Pumps Installed No. 4 6 Feed Flow per RO Unit gpm 1,447 1,563 Feed Flow per HPRO Pump gpm Maximum Feed Pressure psi 993 1,000 ft 2,310 2,310 Maximum Motor Size HP RO Treatment Skids Type Skid-mounted; singlestage; single pass array Skid-mounted; singlestage; single pass array Total Permeate Capacity mgd gpm 1,736 3,125 No. of Units per Skid No. 2 2 Number of Duty Units in Operation No. 3 5 Number of Units Installed No. 4 6 Permeate Capacity per Unit mgd gpm Expected Design Recovery % Elements per Pressure Vessel No. 7 7 Active Pressure Vessels in (per Unit) No Non-active vessels per unit No. 5 5 Total Vessels per Unit No TM2-25

30 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 11: SWRO System Design Criteria Parameter Units Initial 2.5 mgd Facility Potential Expansion to 4.5 mgd Number of Elements per Unit No Area per Element sf Average Permeate Flux gfd Energy Recovery Devices Type Isobaric Isobaric Manufacturer and Model Number Name ERI PX-300 ERI PX-300 Number of Duty ERD's per Unit No. 4 5 Number of ERD's per Skid No. 4 5 Concentrate Flow Rate per ERD gpm Feed Flow Rate per ERD gpm PX Lubrication Flow % PX Efficiency % ERD Booster Pumps Type Vertical Inline; Centrifugal with VFD Vertical Inline; Centrifugal with VFD Number of Duty Pumps No. 3 5 Number of Pumps in Installed No. 4 6 Design Flow per Pump gpm Design Differential Pressure (TDH) psi Motor Size HP The feedwater stream leaving the ERD s will require minor pressure boosting by an ERD booster pump to compensate for the pressure losses experienced from the feed/concentrate stream passing through the feedwater channels of the RO membranes, any pressure loses in the piping, and ERD pressure transfer energy inefficiencies. Each energy recovery system will have a dedicated vertical in-line centrifugal ERD booster pump. Each pump will be designed to deliver 925 gpm at a total dynamic head of 55 psi. Each pump will be supplied with a 50 HP motor and a variable frequency drive to maintain water production under different fouling conditions. RO Cleaning System Because RO membranes are very effective in removing dissolved and suspended solids, they tend to foul or become dirty over time and periodic cleaning is needed. A clean-in-place (CIP) system will be provided for periodic cleaning of the RO membranes. As shown in Table 12, the primary components of the CIP system include cleaning tanks with a tank heater, cleaning pumps, and cartridge filters. Two cleaning tanks are recommended to provide operating flexibility and reduce cleaning time. With two tanks, the operator can be cleaning with a low ph solution in one tank and can be preparing a high ph cleaning solution in the other tank. Having this flexibility can save a couple of hours on every cleaning operation. A small batching tank will also be provided. This tank will allow chemicals to be batched in smaller quantities closer to ground level and then transferred to the cleaning tanks. TM

31 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design The primary RO cleaning chemicals anticipated for use in the facility will be acid for low ph cleaning and sodium hydroxide for high ph cleaning. This selection of cleaning chemicals is based on foulants that might migrate past the MF/UF units in the pretreatment. Other acids or bases may need to be used for less common cleaning applications. Various common detergents may also be used in combination with the acidic and basic solutions. After use, these high ph and low ph cleaning solutions will be neutralized to a ph range of 6 to 9 prior to being discharged to the sanitary sewer for treatment at the City wastewater treatment plant. 38 TM2-27

32 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 12: RO Cleaning and Flush System Design Criteria Potential Expansion to Parameter Units Initial 2.5 mgd Facility 4.5 mgd RO Cleaning (CIP) System Type RO Clean-in-Place RO Clean-in-Place Number of Storage Tanks (cleaning solution mix tank and waste neutralization tank ) No. 2 2 Storage Tank Capacity Each gallons 8,500 8,500 Number of Mixers per Tank No. 1 1 Number of Cleaning Pumps No. 2 (1 duty; 1 standby) 2 (1 duty; 1 standby) Design Flow per Pump gpm 1,680 1,680 Design Discharge Pressure psi Motor Size HP RO Flushing/2 nd Pass RO Feed Tank Type Concrete; connected to clearwell Concrete; connected to clearwell Number of Chambers No. 1 1 Length ft Width ft Water Depth ft Height ft Volume gallons 122, ,000 Number of Flush Pumps No. 1 1 Design Flow per Pump gpm 1,680 1,680 Design Discharge Pressure psi Motor Size HP RO Flushing System Because of high concentrations of dissolved salts in seawater and the concentrate, it is recommended to flush the RO concentrate out of the system when an RO membrane unit is shut down for any reason to reduce the potential for membrane scaling and to reduce corrosion potential. The flushing will be done with RO permeate from a tank that is kept full and renewed periodically. This simple permeate flushing system will consist of a tank and a flush pump that is similar to the cleaning pumps, along with the appropriate controls and valves to automatically flush a membrane unit on shutdown including those initiated by plant operators, system alarms and power outages. TM

33 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Post-treatment, Disinfection, and Product Water Pumping RO Permeate Re-mineralization and Stabilization The desalination process will produce high quality water with very low concentrations of minerals such as calcium, magnesium, and bicarbonate. Because of the low concentrations of these minerals, the desalinated water will taste flat. Additionally, introducing desalinated water into a distribution system can impact the formation and release of existing corrosion scale within the distribution system. To address taste and corrosion concerns, calcium and alkalinity are added to the desalinated water. Posttreatment typically includes reintroducing calcium carbonate into the water in the form of lime or limestone and carbon dioxide addition. Blending with another potable water source, ph adjustment with caustic soda, and the addition of phosphate-based corrosion inhibitor are additional post-treatment methods used at desalination plants. Calcite contactors, carbon dioxide, and corrosion inhibitor are the post-treatment processes selected during the scwd 2 Seawater Reverse Osmosis Desalination Pilot Program to meet the post-treated water goals summarized in Table 13. Carbon dioxide addition will add alkalinity and lower the ph to increase calcium uptake from the calcite, which adds hardness and increases ph. Figure 5 illustrates the posttreatment process. Table 13: Post-treatment Water Quality Goals Parameter Units Design Goal Design Range ph ph Units 7.3 to match GHWTP water Alkalinity mg/l as CaCO Hardness mg/l as CaCO Phosphate (corrosion inhibitor) mg/l 1.0 to match GHWTP water Figure 5 40 TM2-29

34 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Calcite Contactors Post-Treatment and Distribution System Calcium hardness will be added to the RO permeate as it flows through the calcite contactor bed. The calcite slowly dissolves and must be periodically replenished. The rate of calcium uptake will be determined by the ph and hardness of the RO permeate. Continuous carbon dioxide will be necessary to reduce ph to meet the target alkalinity and hardness goals. The carbon dioxide dose will vary to a slight degree as the RO membranes age. Calcite can be delivered as needed or stored onsite as dry chemical. Options for loading the calcite into the contactors will be investigated as the design advances; options include pneumatic loading, lifting equipment with bag splitters, and conveyor systems. Calcite loading is expected to occur between once and twice a year per contactor. Table 14 summarizes design criteria for the calcite contactors. The criteria assumes the calcite contactors will be sized to treat 2.5 mgd; split-stream re-mineralization will be evaluated in more detail during subsequent tasks. Table 14: Calcite Contactor System Design Criteria Parameter Units Initial 2.5 mgd Potential Expansion to Facility 4.5 mgd Maximum Flow Basis mgd No. of Vessels Online No. 5 9 No. of Vessels Installed No Design Flow per Vessel gpm Filtration Area/Vessel sf Normal Loading Rate gpm/sf Vessel Material Type FRP FRP Vessel Diameter ft Vessel Height ft Vessel Pressure Rating psi Calcite Particle Diameter mm Calcite Depth ft Calcite Volume/Filter cf 1,131 1,131 Calcite Volume/Filter gallons 8,465 8,465 Empty Bed Contact Time (EBCT) minutes Average Calcite Consumption mg/l Maximum Calcite Consumption mg/l Purity as Delivered % 95% 95% Maximum Day Usage lbs/day 1,250 2,252 Duration between Loadings at Max Usage Per Contactor 1 days Average Daily Usage lbs/day 1,043 1,877 Duration between Loadings at Avg. Usage Per Contactor 1 days TM

35 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design 1 Assumes that calcite will be replenished when approximately half of the volume is remaining. Corrosion Inhibitor and ph Adjustment Following the calcite contactors, chlorine and corrosion inhibitor will be added before the re-mineralized RO permeate is ready to be pumped into the distribution system. Sodium hypochlorite (chlorine) will be added before and/or after the calcite contactors for disinfection. Corrosion inhibitor will be added after the calcite contactors to provide additional stabilization to reduce corrosion in the distribution system and to match addition at the Graham Hill Water Treatment Plant (GHWTP). Final ph adjustment will be achieved through addition of sodium hydroxide (to increase ph as needed) or carbon dioxide (to decrease ph as needed) downstream of the calcite contactors. It is expected that carbon dioxide to lower ph will be required more often than caustic soda based on water quality model calculations. Design criteria for carbon dioxide, corrosion inhibitor, and caustic soda storage and feed systems are presented in the Chemical Systems section of this Technical Memorandum. Disinfection Pathogen Inactivation Requirements Disinfection to meet 0.5 log Giardia and 2.0 log virus inactivation will be required by the California Department of Public Health (CDPH) to provide a multiple barrier approach against pathogens entering the distribution system. CDPH measures inactivation credits for various disinfectants as CT, the product of the disinfectant residual C (in mg/l) and contact time T. The range of CT values required to meet these levels of inactivation using free chlorine is summarized in Table 15. The table indicates that the CT required for 0.5 log of Giardia inactivation at minimum temperature and maximum ph conditions will dictate the design of the disinfection process. Table 15: Range of CT Values to Meet Pathogen Inactivation Based on Projected Treated Water Temperature and ph Pathogen Inactivation (log) CT at 1.0 mg/l of free chlorine, ph = 8.0 1, and temperature = 10 o C (mg/l x minutes) CT at 1.0 mg/l of free chlorine, ph = 6.0 2, and temperature = 20 o C (mg/l x minutes) Giardia Virus Although the product water ph goal is approximately 7.2, a conservative ph value was selected because ph will vary during the post-treatment process. 2. A ph of 6.0 assumes that chlorine is added prior to the calcite contactors. 42 TM2-31

36 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Clearwell/Chlorine Contact Tank The clearwell is sized to provide full CT credit assuming that at times CT credit will not be achieved by adding chlorine upstream of the calcite contactors. This will provide operations staff the flexibility to optimize the post-treatment and disinfection processes separately as needed. A maximum depth of 20 feet was selected for the clearwell so that it can be buried or partially buried to allow gravity flow from the calcite contactors. Table 16 summarizes the design criteria to meet the required CT credit entirely in the chlorine contact tank. Table 16: Clearwell Design Criteria Parameter Units Initial 2.5 mgd Potential Expansion Facility to 4.5 mgd Nominal Rated Capacity mgd gpm 1,736 3,125 Number of Chambers No. 1 2 Water Depth ft Chamber Length (each) ft Chamber Width (each) ft Chamber Height ft Disinfection Volume per Chamber gallons 101, ,000 Detention Time at Design Flow minutes Design Baffling Factor % T10 minutes Min Chlorine Dose at Design Flow mg/l 1 1 Design CT credit mg/l x minutes Distribution High Service Pump Station and Wetwell An additional chamber will be included at the end of the clearwell to provide a wetwell for a high service pump station to pump water into the distribution system. It is assumed that the pumps will be installed on top of the wetwell to reduce footprint. The design pressure assumes that the pumps will be able to lift water to the overflow of the new Bay Street Storage tanks and is based on initial estimates provided by SCWD. Table 17 summarizes the design criteria for the high service pumps and wetwell. TM

37 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Table 17: High Service Pump Station and Wetwell Design Criteria Parameter Units Initial 2.5 mgd Facility Potential Expansion to 4.5 mgd Number of Pumps in Operation No. 2 3 Number of Pumps in Installed No. 3 4 Rated Capacity gpm 1,736 3,125 Design Flow per Pump gpm 868 1,042 Total Installed Capacity gpm 2,604 4,167 Design Differential Pressure (TDH) psi ft Motor Size (HP) HP Assumed Efficiency % 75% 75% Pump and Drive Type Vertical Turbine with Variable Frequency Drive Wetwell Length ft Wetwell Width ft Wetwell Water Depth ft Chamber Sidewall Height ft Wetwell Volume gallons 30,000 30,000 Wetwell Detention Time minutes Chemical Storage and Feed Systems Multiple chemicals will be required for treatment, disinfection, and membrane cleaning at the desalination facility. The following section provides a summary of these requirements. Chemical Description and Application Points Table 18 provides a summary of the chemicals, application points, and chemical doses selected for the preliminary design phase of the desalination facility. Antiscalant Table 18: Chemical and Application Point Summary Chemical Description of Use Application Points Carbon Dioxide Corrosion Inhibitor Continuous dispersant addition to minimize SWRO membrane scaling and/or iron fouling. Continuous alkalinity addition and ph reduction to improve calcium uptake during post-treatment. Continuous phosphate addition to inhibit corrosion in the distribution system. Dose (mg/l) Average Range RO feedwater RO permeate before calcite contactors Product water TM2-33

38 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Table 18: Chemical and Application Point Summary Chemical Description of Use Application Points Ferric Chloride Sodium Bisulfite Sodium Hydroxide Sodium Hypochlorite Citric Acid and Potential MF/UF and RO Cleaning Chemicals Spare Continuous coagulant addition to improve removal of suspended particulates and dissolved constituents during pretreatment. Intermittent use as a reducing agent to dechlorinate RO feedwater and prior to disposal of chlorinated discharges and cleaning solutions. It is also used to preserve RO membranes during extended periods of downtime. Intermittent use to improve boron rejection, control product water ph, clean MF/UF and RO membranes, and neutralize acidic cleaning solutions. Continuous use as a disinfectant. Intermittent use as a pre-oxidant to improve pretreatment and to clean MF/UF membranes. Citric acid will be used intermittently as a chelating agent to clean MF/UF and RO membranes. Potentially, other membrane cleaning chemicals (EDTA and detergent chemicals) may be recommended in addition to citric acid. Space included to allow up to two chemicals for future treatment or cleaning requirements. Raw water Used washwater RO feedwater Membrane cleaning tanks Neutralization tank RO Concentrate disposal EQ basin RO Feedwater Product water Membrane cleaning tanks Neutralization tank Raw water MF/UF Membrane cleaning tanks Product water Limestone Contactors MF/UF Cleaning Tanks RO Cleaning Tanks Raw water, RO feedwater, and/or Product water Dose (mg/l) Average Range Batch process; Varies TBD Batch process; Varies TBD Chemical Use and Storage Requirements Table 19 provides a summary of the chemical use and storage requirements assuming average and maximum doses in Table 18 above, an average monthly production of 1.6 mgd, and maximum monthly production of 2.5 mgd. The values for carbon dioxide are presented in terms of pounds instead of gallons because this is the industry standard for gaseous chemicals. TM

39 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Table 19: Chemical Use and Storage Use (gpd) Storage Chemical Design Design Operating Days at Avg. Tanks Average at Maximum at 2.5 Volume Use at 1.6 (No.) 1.6 mgd mgd (gallons) mgd Antiscalant Carbon Dioxide 400 lbs/day 1,040 lbs/day 1 25,200 lbs Corrosion Inhibitor , Ferric Chloride , Sodium Bisulfite , Sodium Hydroxide , Sodium Hypochlorite , Citric Acid and Potential MF/UF and RO Cleaning Chemicals Days at Max. Use at 2.5 mgd These chemicals are typically ordered as needed and stored temporarily in totes or pallets before use. Space will be included in the MF/UF and RO CIP equipment areas. Spare TBD TBD 2 TBD TBD TBD 1 The upcoming preliminary design phase will include an evaluation of an onsite generation system for sodium hypochlorite similar to the systems at the City s Beltz and Graham Hill treatment plants. RO Concentrate Storage and Pump Station Approximately 40 to 50 percent of the seawater entering the desalination facility will be treated to drinking water standards. The remaining 50 to 60 percent is commonly referred to as brine or RO Concentrate. The concentrate will be returned back to Monterey Bay via the City s existing outfall after combining with the effluent from the City s Wastewater Treatment Facility (WWTF). The allowable blending percentage of concentrate and effluent is determined by the City s National Pollution Discharge Elimination System (NPDES) discharge permit (Permit No. CA ) with a minimum initial dilution ratio (MIDR) of 139:1. Therefore, concentrate will be stored onsite to reduce discharge during periods of low effluent discharge (e.g., night hours during low rainfall periods) from the WWTF. Work performed by Brown and Caldwell from a separate study indicates that up to 600,000 gallons of concentrate storage should be provided for a facility producing 2.5 mgd and space should be provided to allow up to 2,000,000 gallons for a potential future expansion to 4.5 mgd. Table 20 summarizes the results of the Brown and Caldwell Study. 46 TM2-35

40 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Desalination Plant Production Flow (mgd) Table 20: Estimated RO Concentrate Storage Requirements RO Concentrate Flow Based on 45 Percent Recovery (mgd) Minimum WWTF Effluent Flow Required During Summer/Fall Months (mgd) RO Concentrate Storage Volume During Summer/Fall Months (mg) Minimum Effluent Flow Required During Winter Months (mgd) RO Concentrate Storage Volume During Winter Months (mg) Table 21 summarizes the preliminary design criteria for the concentrate equalization basin and discharge pumps. Brine EQ Basin Design Flow Basis Table 21: RO Concentrate Disposal System Summary Initial 2.5 mgd Facility Potential Future Expansion to 4.5 mgd gpm 2,600 4,688 mgd Length ft Width ft Water Depth ft Height ft Volume gallons 674,000 2,020,000 Detention time hours RO Concentrate Discharge Pumps Number of Pumps (Duty/Standby) No. 2/1 3/1 Design Flow per Pump gpm 1,950 1,950 Total Installed Capacity gpm 3,906 6,250 Design Differential Pressure psi Pump Type Vertical Turbine with Variable Frequency Drive Motor Size HP TM

41 City of Santa Cruz & Soquel Creek Water District Draft TM2: Desalination Process Preliminary Design Figure 6 illustrates the concentrate storage and disposal system. The concentrate equalization basin will also be designed to capture and discharge saline overflows from plant processes such as raw seawater before coagulant addition and RO feedwater following pretreatment, and overflows from the solids clarifier thickeners. Overflows from the DAF and MF/UF feedwater equalization basin will be captured and transferred to the solids clarifier thickeners for treatment before overflow to the concentrate equalization basin. Residuals Production, Handling and Disposal Figure 6 RO Concentrate Storage and Disposal System Description System The major residual streams that will be produced at the desalination plant will include DAF solids, used washwater from the MF/UF system, and chemical cleaning solutions from clean-in-place (CIP) procedures conducted periodically at the MF/UF and RO membrane units. The recommended Residuals Handling System is shown schematically in Figure 2 and consists of one (1) Solids Transfer Pump Station, one (1) Washwater Equalization Basin, two (2) 40-foot diameter Clarifier Thickeners, and one (1) Reclaimed Water Pump Station. The system will: separate liquids and solids in the DAF Solids and MF/UF washwater; recycle the clarified (reclaimed) water to the plant influent for treatment; and dispose of the thickened solids to the sanitary sewer. CIP wastes will be produced intermittently and in much smaller volumes than the DAF Solids and MF/UF Washwater. CIP wastes can also contain high levels of metals, organic constituents, chlorine and other compounds that can disrupt the coagulation and treatment processes, hinder (MF/UF and RO) membrane performance and/or contribute to membrane fouling. For these reasons, CIP wastes will be dechlorinated and neutralized to ph of 6 to 9 units and disposed of through the sanitary sewer. The mass balance for the Residuals Handling System, operating at the maximum daily flow and highest anticipated solids loading, is presented in Figure 7 and Table 22. Additional information used to develop the preliminary design of the Residuals Handling System are presented in the paragraphs that follow. 48 TM2-37

42 Draft TM2: Desalination Process Preliminary Design City of Santa Cruz & Soquel Creek Water District Figure 7 Mass Balance Schematic for Residuals Handling System ID No. Table 22: Mass Balance for Residuals Handling System at Maximum Daily Plant Flow (1) Description Flow TDS TSS (gpm) (mg/l) (mg/l) (lbs/day) , Used Washwater from MF/UF to Solids Clarifier/ Thickener No. 1 2 Clarified MF/UF Used Washwater from Solids , Clarifier/Thickener No. 1 3 Thickened Solids from Clarifier/Thickener No. 1 1 (3) 36,000 20, DAF Waste to Clarifier/Thickener No (2) 36,000 2,259 2,967 5 Clarified DAF Waste from Solids Clarifier/Thickener No , Thickened Solids from Clarifier/Thickener No (3) 36,000 20,000 2,964 7 Combined Reclaimed Water to Plant Influent , Combined Thickened Solids to Sanitary Sewer 13 (3) 36,000 20,000 3,281 (1) Assumptions for Mass Balance include: Raw Seawater Flow = 7 mgd Raw Seawater TSS = 45 mg/l Ferric Chloride Dose = 30 mg/l MF/UF Efficiency = 90%; used washwater production = 10% RO Recovery = 40% DAF Efficiency = 98%; DAF waste production = 2% Thickened Sludge = 2% by weight (2) DAF Waste flow of 109 gpm is the daily average; actual flows will be intermittent and are anticipated to range from 150 to 600 gpm. (3) Thickened sludge flows from clarifier thickeners are daily averages; actual flows will be intermittent and are anticipated to range from 150 to 600 gpm for a 4-inch to 6-inch diameter discharge line to the sanitary sewer. TM