PRIMARY DRINKING WATER FROM WASTEWATER EFFLUENT: PILOT TEST EXPERIENCE AT THE MIAMI DADE SOUTH DISTRICT WWTP

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1 PRIMARY DRINKING WATER FROM WASTEWATER EFFLUENT: PILOT TEST EXPERIENCE AT THE MIAMI DADE SOUTH DISTRICT WWTP Paul J. Vinci, P.E. 1, Richard Cisterna, P.E. 1, Robert Harris 1, John Chorlog, P.E. 2, Joseph Mazzarese 1, Steve Kronheim 2 1 Hazen and Sawyer 4000 Hollywood Boulevard, 750N Hollywood, Florida Miami-Dade Water and Sewer Department ABSTRACT The Miami-Dade Water and Sewer Department (MDWASD) owns and operates the South District Wastewater Treatment Plant in Goulds, Florida. This is a 225 mgd peak flowrate secondary oxygen activated sludge treatment facility that discharges effluent via deep injection wells. Based upon the detection of nitrogen species in the monitoring wells for the deep injection well system, MDWASD entered into a Consent Order with the Florida Department of Environmental Protection (FDEP) to address these concerns. The Consent Order requires a significant increase in the level of treatment for the facility. The resulting effluent standards and requirements for this facility include high level disinfection (HLD) as defined by the FDEP, Primary Drinking Water Standards, addressing peak flowrates, and future participation in the Comprehensive Everglades Restoration Program (CERP). Primary Drinking Water Standards were addressed by a series of analytical efforts plus the implementation of an anoxic zone in the fifth of six stages of the oxygenation tanks at this facility. Peak flowrates were analyzed and are being addressed in the near term via secondary treatment plus HLD treatment for 285 mgd of peak hourly flowrate, and in the future, via the use of high rate clarifiers plus HLD treatment for peak flowrates above 285 mgd and up to a potential future peak hourly flowrate of 450 mgd. HLD standards per the regulations are based around filtration to less than 5 mg/l of TSS on a continuous basis plus disinfection. With the need for continuous compliance to a standard that has typically only been applied to reuse systems, which typically have alternate means of effluent disposal, it was apparent that these conditions were unique. To address the unique circumstances, a literature search and review of similar facilities was performed. These investigations resulted in the need to pilot test deep bed sand and disk filtration processes in order to confirm potential for continuous compliance and define appropriate design criteria. Initial pilot test screening of these two alternative filtration approaches resulted in elimination of the disk filters from further consideration and testing since the data did not support compliance at acceptable loading rates (3 gpm/sf and higher). Deep bed mono-media sand filter tests resulted in supporting their application for this HLD upgrade and the use of a 6 gpm/sf hydraulic loading rate as a full-scale demonstration design value. The Consent Order allows this one year 3387

2 demonstration with a recommended loading rate with possible, subsequent process supplementation. Criteria from the pilot testing also supported the need to ensure about 15 % redundancy of filters, plus capability to backwash and air scour up to 15% of the filters to address solids and minimize the potential for filter plugging. On-line TSS metering devices were also tested. Results of this testing indicated benefit for control of pilot filter target influent TSS levels, but inconclusive data for filter effluent TSS monitoring. The data indicated that the on-line TSS meters should be further tested in the full-scale system with less variability of loading conditions and generally better and more focused control of the meters. KEYWORDS Deep bed filters, disk filters, high level disinfection, deep injection wells, pilot testing, primary drinking water standards, on-line TSS monitoring INTRODUCTION Miami-Dade Water and Sewer Department (MDWASD) owns and operates three major wastewater treatment facilities that manage about 370 mgd on an annual average basis. Their South District Wastewater Treatment Plant (SDWWTP) located in Goulds, Florida provides secondary treatment to about 90 mgd of domestic wastewater and discharges the resulting effluent via a deep injection well system. This facility is currently rated and permitted capacity is mgd annual average flowrate, 225 mgd on a peak hourly basis. Future wastewater discharges and lift station improvements result in an anticipated increase in treatment needs to mgd annual average flowrate, with 450 mgd on a peak hourly basis. Based upon the detection of nitrite in the injection well monitoring system, MDWASD entered into a consent order with the Florida Department of Environmental Protection (FDEP) in April of Concerns for potential leakage of treated effluent into a possible underground source of public drinking water led to the Consent Order requirements for implementing a High Level Disinfection (HLD) system and for producing effluent that meets primary drinking water (PDW) standards prior to disposal. The C.O. indicates that HLD must be placed in service by April 29, Further, recent EPA regulations require that all Deep Injection Well implement HLD treatment by the end of The final rule is entitled Revision of Federal Underground Injection Control Requirements for Class I Municipal Disposal Wells in Florida. These regulations make HLD requirements widespread in their impacts, not just an impact to the MDWASD SDWWTP. Compliance with the majority of the PDW Standards will not be an issue for the existing SDWWTP. Also, MDWASD was able to expeditiously implement an anoxic zone in their high purity oxygenation system to address the nitrogen concerns. However, compliance with the Consent Order requires expeditious design and construction of secondary treatment process improvements for reliability and of HLD upgrades. HLD Upgrades are defined by the Regulations which indicate the need for filtration followed by disinfection. Of key concern for ensuring compliance with the HLD standards imposed by the Consent Order is the requirement for effluent Total Suspended Solids (TSS) less than 5 mg/l on a continuous basis under various filter influent 3388

3 loading conditions. Typically, WWTP s with reuse capability that are required to meet these HLD standards also have alternate methods of effluent disposal which provides relief from the continuous compliance concern. In addition to filter influent loadings based upon typical secondary clarifier effluent TSS levels of 20 mg/l or less (about mg/l), the filters must also be capable of handling Influent TSS of 45 mg/l, which is the weekly average TSS permit limit for the secondary WWTP, as well as higher TSS levels which are expected, although infrequently. In addition to primary drinking water standards and HLD requirements, the C.O. required MDWASD to address peak flows at the SDWWTP and develop a peak flow proposal. The peak flow proposal that resulted defined the use of high rate clarifiers (HRC s) for addressing peak flows (flows above the capacity of the existing biological treatment system). Pilot testing conducted at the SDWWTP for implementation of HLD upgrades also addressed testing of HRC s for raw wastewater treatment. Testing conditions included HRC effluent blended with the effluent from the secondary process plus filtration as well as effluent from the HRC s plus secondary clarifiers blended and then filtered. As a result, pilot system drawings and aerial photos show the HRC s. However, these HRC tests are associated with potential future conditions and were considered as preliminary and not as critical as filtration tests of secondary effluent in order to address HLD requirements. Based upon the need to expedite the HLD upgrades, inconsistent preliminary results of testing with HRC s and other considerations, HRC pilot tests were not fully completed as part of this program and therefore are not addressed further in this paper (other than inconsequential references / illustrations on some figures). A literature search and check on WWTP s with treatment standards similar to HLD standards determined that the C.O. required continuous TSS compliance limit is unique, and that specific design data for compliance with these standards are not available. This preliminary investigation, however, did support that deep bed filters (a more traditional technology) and disk filters (new, potentially economical technology) were most suitable for further investigation. As a result, a pilot test program was planned and conducted with these two selected technologies, utilizing trailer-mounted pilot filter units, sized for roughly at 0.3 mgd. GOALS AND OBJECTIVES Pilot testing focused on the following criteria and conditions for the implementation of deep bed mono-media filters at the SDWWTP: Continuous compliance with effluent Total Suspended Solids (TSS) of 5 mg/l under various filter influent loading criteria including typical domestic loading, weekly permit limits and upset conditions. Test of a new technology membrane disk technology (side-by-side with deep beds). Selection of the filter system design hydraulic loading rate. Confirmation of deep filter media type and effective size selection. Identification of anticipated filter run times. Confirmation of filter backwash rates and durations. 3389

4 Confirmation of compressor scour rates and durations for backwashing. Confirmation of filter bumping rates and durations for extending filter run times. Impacts of filter bumps on filter performance and on run times. Use of flocculation upstream of the filter system with and without polymers. Testing of polymers for improvement and confirmation of types and dosages for design selection. Obtaining sufficient data in order to support filter suppliers so that they will provide a filter system performance guarantee. Testing of new on-line TSS metering equipment in low TSS level service; comparison of with laboratory TSS measurements as well as on-line turbidimeter measurements. Development of approaches to control and apply TSS concentrations to the test filters at (or at least close to) pre-determined target values. TESTING PROGRAM Study Plan A Testing Program was developed in an effort to focus on meeting the pilot testing goals in an expeditious and phased manner due to the C.O. time limitations and the large amount of data collection required. Figure 1 provides a summary of the Testing Program as originally planned. This program was adapted as the pilot program progressed to address numerous considerations including: Results of testing (revisions were made based upon results to maximize the efficiency of meeting the programs goals, eliminate un-necessary testing, repeat testing when insufficient or contradictory data were obtained, etc.) Availability of pilot testing units Pilot test unit operating and maintenance issues Availability of chemicals MDWASD data needs to best address their compliance efforts As a result, the as-tested testing program is considerably different, especially in terms of sequence. This as-tested program is summarized in Table

5 Table 1 - HLD Upgrade Pilot Testing, Originally Proposed Testing Program Summary Testing Period Test Description Initial Shakedown and Stabilization - Build Up to Standard Loadings Standard Secondary Wastewater Loadings to Filters - 3 loadings, with and without coagulant Re-Stabilize Secondary Weekly Limit Loadings to Filters - with and without coagulant Re-Stabilize Secondary Biological Upset Conditions to Filters - with and without coagulant Re-Stabilize Possible Additional Testing - To be determined Duration (days) Total Key Assumptions: 1 Two or three alternative filter technologies may be tested; target loading rates will be adjusted based upon selected technologies for testing. 2 Target Loading Rates selected and testing period sequence may be modified during test based upon preliminary results. 3391

6 Figure 1 As-Tested Program Summary 3392

7 Pilot Plant Construction The pilot system was planned, designed, permitted, constructed, started-up and placed into service as a cooperative effort between Hazen and Sawyer and MDWASD staff. Figure 2 illustrates the process flow of the pilot system and Figure 3 is the construction drawing overall site layout utilized for this pilot system. Also, an overhead photograph of the pilot system site is provided in Figure 4. This overhead photograph shows some construction activity and the nearly complete pilot plant. (As noted previously, HRC pilot units are reflected in some of these figures, although these are not addressed further in this document). Figure 2 - Peak Flow/HLD Pilot Testing, Preliminary General Process Flow Schematic 3393

8 Figure 4 Pilot System Construction Drawing 3394

9 Figure 5 Pilot Site Aerial Photograph Test Conditions and Equipment A pilot system was constructed in proximity to the western plant s grit removal and oxygenation systems. An influent tank with a mixer was provided at the upstream end of the pilot system to allow control of influent feed for the various testing conditions described subsequently. An online TSS meter was installed just downstream of this tank for monitoring and adjustment of the influent TSS levels to the pilot system to most closely match the target TSS levels addressed subsequently. The on-line TSS meters for this pilot unit were purchased from Cerlic Environmental, Inc. On-line turbidimeters were purchased from the Hach Company and installed at the filter influent as well as the effluent lines for each filter. Valving and flow metering (flowmeters purchased from ABB, Inc.) from the influent tank were utilized on each pilot filter unit influent line to ensure that influent hydraulic loading rates most closely matched their respective target values and were consistent to each pilot unit as well. A flocculation tank was provided just upstream of the filters with a slow stir mixing device to optimize solids floc formation either with or without polymer prior to filtration. In-line static mixers were provided for mixing polymer upstream of the floc tank. Based upon hydraulics, pilot unit configurations, site conditions, etc., pumps were needed to lift the flow to the pilot filter units. Peristaltic pumps were selected for 3395

10 this application to ensure that minimal to no floc damage was incurred in lifting the flow to the pilot filters. Peristaltic pumps for this pilot were purchased from Eccentric Pumps, Inc. On-line Turbidimeters were installed on the effluent of each pilot filter unit. On-line TSS meters were also installed at the filter effluent of the filter system. Tanks were furnished by the PolyProcessing Company. The F.B. Leopold Company, Inc. and Severn Trent Services, Inc. (Tetra) provided pilot deep bed filters and I. Kruger, Inc. (Hydrotech) provided a pilot disk filter for the testing. Schematics / dimensional drawings of a deep bed filter and a disk filter are provided in Figures 5 and 6. Photographs of each individual pilot unit used in the study are provided in Figures 7, 8 and

11 Figure 5 Deep Bed Tetra Filter 3397

12 Figure 6 Kruger Disk Filter (Typical) 3398

13 Figure 7 Leopold Pilot Unit Figure 8 Tetra Pilot Unit 3399

14 Figure 9 Kruger Hydrotech Disk Filter Pilot Unit Filter media / membrane sizes used in the pilot testing are summarized in Table 2: Table 2 - Filter Media Summary Filter Technology Leopold Deep Bed Tetra Deep Bed Kruger Disk Media Summary Five 3-inch layers of mixed gravel 3/4 x 1/2 inch, 1/2 x 1/4, 1/4 x1/8, 1/2 x 1/4, 3/4 x 1/2 ; 6 feet of No. 8 x No. 12 sand (effective size 1.70 mm 2.36 mm) 6 feet of 6 x 9 mesh (effective size 2 3 mm) 10 micron membrane It is also important to note that the deep bed pilot filters were backwashed in a manner consistent with full-scale system and using a backwash rate of about 6 gpm/sf, for a period of about

15 minutes. For relatively high influent TSS and hydraulic loading conditions, longer backwash durations were provided when needed. Further, filter bumping was automatically programmed, at a rate of about 8 gpm/sf for a duration of about 3 minutes. Bumps were usually set to run every 4 hours, but under higher solid or hydraulic loading rates were adjusted to occur every 2 hours. Filter pilot units and other suppliers participated by tracking results, providing input to H&S and addressing issues and questions on an as-needed basis. Suppliers for all pilot equipment were identified to acknowledge their efforts in this pilot test program, but not for the purpose of product endorsement. The testing was conducted for the purpose of technology assessment and design criteria confirmation, not for specific product / supplier selection. Hazen and Sawyer directed and conducted the pilot testing including operation and maintenance (O&M), on-line monitoring of key parameters (flowrate, turbidity and TSS), sampling and some analyses. MDWASD staff assisted with O&M and sampling and performed the majority of the lab analyses. Certain specific analyses were performed by Severn-Trent Laboratories (Miramar, FL). Figure 10 provides the Sampling and Analytical Schedule utilized during the pilot testing program. 3401

16 Figure 10 - Sampling and Analytical Schedule 3402

17 Filter pilot tests were performed on a side-by-side basis (identical influent and loading conditions via monitoring and controls as described previously). Pilot filters were tested at several influent TSS levels that duplicate actual anticipated conditions, and at several hydraulic loading conditions in an effort to assist with the selection of the most appropriate design hydraulic loading rate. Influent TSS conditions applied included typical secondary treatment performance with target Influent TSS of about 10 to 20 mg/l, existing WWTP weekly permitted effluent TSS limits of 45 mg/l as target filter influent TSS, and Upset TSS conditions with target influent TSS levels of 75 mg/l and 120 mg/l. While typical secondary effluent is anticipated for filter influent TSS levels for the majority of the operating conditions, a review of historical performance data at the SDWWTP supports that TSS levels equal to or higher than 45 mg/l are incurred refer to Figure 11. Figure 11 - Highest Effluent TSS Values (mg/l) for SDWWTP Improvements to the secondary process are planned as part of this upgrade; however, since the required effluent standards do not allow any relief from the 5 mg/l TSS limit, there is no other effluent discharge option (except for emergencies) and storage and re-treatment of unacceptable effluent is not feasible, it was determined that high influent TSS levels needed to be addressed in the pilot testing and design of the filter system for this facility. For test conditions other than typical secondary clarifier performance levels (10-20 mg/l), return activated sludge (RAS) from the nearby oxygenation trains was blended with secondary effluent 3403

18 to simulate the required target higher influent TSS levels. This RAS blend allowed for better control over the influent solids than using RAS alone. In general, to ensure representative data and define filter run time capabilities, testing was conducted on a 24 hour per day and 7 day per week basis. RESULTS Results presented in this document are limited to those considered of key importance to the goals of the pilot study and focus around the ability of the filter units to meet the TSS limit of less than or equal to 5 mg/l, as well as filter run times. Initial Screening Tests In an effort to expedite pilot testing, initial pilot testing focused towards screening out of filter types, and loading rates to allow more focused testing where it is considered most applicable and critical to expediting the design process. For this initial testing phase, deep bed filters and disk filters were tested in parallel. Test conditions (target Influent TSS levels and hydraulic loading rates) and results in terms of compliance with the required filter effluent TSS limit of 5 mg/l for this initial testing phase are provided in Table

19 Table 3 - Deep Bed and Disk Filter Performance Comparison Run No. Run Duration (days) Target Influent TSS (mg/l) 1 Actual Influent TSS (mg/l) Deep Bed /Disk Hydraulic Loading Rate (gpm/sf) Compliance with TSS 5 mg/l Limit Leopold Deep Bed Filter Tetra Deep Bed Filter Kruger Disk Filter / 3 Yes Yes Yes / 4 Yes Yes No / 4.2 and 3 Yes Yes No / 2.5 and 2 Yes Yes No 5 0 / / 2 No Yes No / / 2 Yes Yes No This summary shows that the disk filters did not meet the required filter effluent TSS limits, except at relatively low influent TSS levels and relatively low hydraulic loading rates On this basis, disk filters were eliminated from further testing. Traditional considerations related to filter run times for deep bed filters are not really applicable to disk filters since disk filters are backwashed every few minutes. Disk filters are limited by overflow liquid level (plugging condition). Disk filter overflows occurred at relatively high influent TSS levels and hydraulic loading rates. Under these conditions for a full-scale WWTP application, with continuous overflow mode / filter bypass for cleaning, disk filters would be unacceptable in terms of compliance with the C.O. continuous TSS limit of 5 mg/l. Deep bed filter run times during this side-by-side comparison with the disk filter were acceptable, with a minimum run time of 29 hours, but often lasting far longer. Deep Bed Filter Testing For convenience and evaluative purposes, deep bed filter testing has been grouped by test conditions. Results of deep bed filter testing are provided by test condition group in this section. It is noted that some effluent TSS data unintentionally collected in conjunction with filter bumps were not included in this analyses presented below since they are considered not representative. Based upon previous experience and assuming careful filter operation / control, it is expected 3405

20 that filter bumping on a full-scale system will not result in deterioration of effluent quality as it did for a few instances. It is also noted that these tests were conducted with flocculation but without the use of coagulants. Test Condition 1 Typical Secondary Effluent - Target Influent TSS = 10 to 20 mg/l: Using typical secondary effluent as filter influent, deep bed filters were tested at hydraulic loading rates ranging from 4 gallons per minute per square foot (gpm/sf) to 12 gpm/sf. Results are summarized in Figure 12. Figure 12 - Effluent Quality for Deep Bed Filters Treating Secondary Effluent As shown in this Figure, with typical secondary effluent, with actual Filter Influent TSS = 10 to 17 mg/l (as compared to the target of 10 to 20 mg/l), the deep bed system was generally able to meet or come very close to meeting the effluent TSS limit of 5 mg/l throughout this large range of hydraulic loading rates, and even at the very high end. As shown in Figure 13, the minimum filter run times for this test condition ranged from 6 to 76 hours for the 12 gpm/sf to 4 gpm/sf test conditions, respectively. Typical design standards favor use of roughly 20 to 24 hours for an acceptable run time. Using this run time criteria only, with typical secondary effluent, hydraulic loading rates of up to 9 gpm/sf would be acceptable. 3406

21 Figure 13 - Run Time for Deep Bed Filters Treating Secondary Effluent Test Condition 2 Secondary Effluent Spiked for Weekly TSS Limit Target Influent TSS = 45 mg/l: Using spiked secondary effluent as filter influent, with a target influent TSS of 45 mg/l, deep bed filters were tested at 5 and 6 gpm/sf hydraulic loading rates. Results are summarized in Figure

22 Figure 14 - Effluent Quality for Deep Bed Filters Treating Target TSS 45 ppm As shown in this Figure, with actual Filter Influent TSS = 38 to 46 mg/l (as compared to the target of 45 mg/l), the deep bed system was able to meet the effluent TSS limit of 5 mg/l at the loading rates tested. As shown in Figure 15, the minimum filter run times for this test condition ranged from 9 to 17 hours for the 6 gpm/sf to 5 gpm/sf test conditions, respectively. Typical design standards favor use of roughly 20 to 24 hours for an acceptable run time. Using this run time criteria only and, hydraulic loading rate of 5 gpm/sf, the minimum run time approaches the acceptable level; however, in general, lower hydraulic loading rates would be needed to ensure operation within the standard run time criteria. 3408

23 Figure 15 Run Time for Deep Bed Filters Treating Target TSS 45 ppm Test Condition 3 - Secondary Effluent Spiked for Slight Upset Conditions Target Influent TSS = 75 mg/l: Using spiked secondary effluent as filter influent, with a target influent TSS of 75 mg/l, deep bed filters were tested at a hydraulic loading rate 6 gpm/sf. Results are summarized in Figure

24 Figure 16 - Effluent Quality for Deep Bed Filters Treating Target TSS 75 ppm As shown in this Figure, with actual Filter Influent TSS = 63 mg/l (as compared to the target of 75 mg/l), the deep bed system was unable to meet the effluent TSS limit of 5 mg/l. As shown in Figure 17, the minimum filter run time for this test condition was 6 hours, an unacceptable run time considering a standard run time is 20 to 24 hours. 3410

25 Figure 17 - Run Time for Deep Bed Filters Treating Target TSS 75 ppm Test Condition 4 Secondary Effluent Spiked for Upset Conditions - Target Influent TSS = 120 mg/l: Using spiked secondary effluent as filter influent, with a target influent TSS of 120 mg/l, deep bed filters were tested at 3 and 4 gpm/sf hydraulic loading rates. Results are summarized in Figure

26 Figure 18 - Effluent Quality for Deep Bed Filters Treating Target TSS 120 ppm As shown in this Figure, with actual Filter Influent TSS = 69 to 75 mg/l (as compared to the target of 120 mg/l), the deep bed system was generally unable to meet or even come close to meeting the effluent TSS limit of 5 mg/l - filter effluent values averaged around mg/l. Also, under these test conditions, filter run times were not readily defined since plugging and related overflows were experienced. Overall Deep Bed Filter Performance Results Based upon pilot test results and the opportunity for full-scale one year performance demonstration, a design hydraulic loading rates of 6 gpm/sf as selected for this application. This hydraulic loading rate is considered somewhat aggressive, but supported for full-scale testing in terms of effluent TSS performance. Regarding filter run times, this hydraulic loading rate is considered more aggressive and needs to be addressed in design criteria with respect to the number of filters and backwashing capacity. Other Considerations Coagulant Aids: Inorganic and polymer coagulant aids were tested for potential filter performance improvement. Figure 19 shows results for the use of ferric chloride. In general, inorganic and polymer coagulants did not provide consistent improvement in performance of 3412

27 either technology tested, in terms of effluent quality and filter run times. Figure 20 shows results for the use of polymer. Figure 19 - Effects of 2.5 mg/l Ferric Chloride Dosage to Secondary Effluent on Filter Effluent Quality Figure 20 - Effects of Polymer Dosage to Secondary Effluent on Filter Effluent Quality 3413

28 Operation and Maintenance Considerations: The disk filters incurred a seal leak shortly after startup. The Leopold deep bed filters experienced an air compressor failure during the first phase of the pilot testing. The Tetra deep bed filter experienced a control problem related to the liquid level sensor. On-Line TSS Metering Results: On-line TSS meter results did not track well with laboratory wet TSS measurements, and therefore, were not used for performance assessment. These online TSS data were used for influent TSS feed adjustments. Pilot testing included a lot of variability in on-line liquid being analyzed. Further testing of on-line TSS meters on a fullscale system with less variability is needed. Influent TSS Control: A system of pilot system RAS feed tanks, pumps and on-line TSS metering allowed for capability to control pilot influent TSS levels reasonably to meet test objectives. Figure 21 illustrates in schematic form the Influent TSS feed control system utilized. Figure 22 shows a photograph of the RAS feed tanks. This system generally allowed TSS levels to be controlled to within about 25 percent of the target value for testing. TSS values were easier to control at lower TSS doses and due to the variability in RAS concentration, solids control proved difficult on a few occasions. Depending on the testing scenario, TSS could be added to either the influent mix tank or the floc tank, but not to both at the same time. Figure 21 Influent Solids Feed System Schematic 3414

29 Figure 22 Influent Solids Feed System CONCLUSIONS AND RECOMMENDATIONS The following conclusions and recommendations were identified. Exclude the disk filter for this application. Use an initial deep bed filter hydraulic loading rate of 6 gpm/sf, per C.O. flexibility for an initial aggressive loading rate that can be modified after one year of full-scale operation. Use filter run time of about 6 to 12 hours (allow for 15 % of filters out of service at a time for backwashing); size the backwash water supply, backwash wasting and air scour systems for 6 to 12 hours (15% of filters out of service at a time for backwashing). These criteria are conservative but justified when considered in conjunction with the aggressive filter hydraulic loading rates and potential high influent TSS levels. Although not directly supported by pilot data, provide capability for polymer addition based upon regulatory requirements and other related experience. 3415

30 Provide for backwash equalization prior to return to the WWTP oxygenation train or secondary clarifiers for further treatment. Install the two on-line TSS meters from the pilot testing in the full-scale system for further testing (full-scale demonstration testing). ACKNOWLEDGMENTS In addition to the Suppliers defined previously in this document, MDWASD staff are acknowledged. The MDWASD staff worked as a team with Hazen and Sawyer to ensure the success and timely completion of this pilot program, enduring basically continuous 24 hour per day and 7 day per week efforts and meeting very tight schedule constraints. REFERENCES American Public Health Association; American Water Works Association; Water Environment Federation (2005) Standard Methods for the Examination of Water and Wastewater Metcalf & Eddy (2003) Wastewater Engineering, Treatment and Reuse 3416