Tracy WWTP Solids Master Plan

Tracy WWTP Solids Master Plan Prepared for City of Tracy June 2006 2485 Natomas Park Drive, Suite 600 Sacramento, CA 95833 179201_TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC

179201_TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC I

Table of Contents Table of Contents... ii Table of Figures... iii Table of Figures... iii Introduction...1 Background and Organization...1 Existing Conditions and Biosolids Management Facilities...1 Existing Thickening Processes...2 Existing Stabilization Processes...2 Anaerobic Digesters...2 Dewatering Facilities...6 Preliminary Planning Efforts...8 Thickening Alternatives...10 Alternative 1 WAS Thickening with DAFT...11 Alternative 2 WAS Thickening with GBT...13 Alternative 3 WAS Thickening with Centrifuges...16 Alternative 4 WAS Thickening with Rotary Drum Concentrator...20 Alternative 5 - Co-Thickening of PS and WAS with GBT...22 Biosolids Stabilization Alternatives...28 Alternatives 1 12, Digestion...28 Alternative 13 Co-Thickening with Incineration...29 Dewatering Alternatives...30 Biosolids Conditioning...30 Biosolids Dewatering...31 Class A Biosolids...43 Class A Biosolids...43 Vector Attraction Reduction Requirements...46 Class A Biosolids Alternatives...48 Alternatives Selected for Further Consideration...60 Thickening Alternatives:...66 Alternatives Costs...67 Recommended Plan...67 Thickening...67 Stabilization...67 Dewatering...67 Class A Biosolids...68 CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC II

Table of Figures Figure 1 Corroding Valve at Digester Building...4 Figure 2 Gas and Moisture Compressors at Digester Building...5 Figure 3 Corrosion at Gas and Moisture Compressors at Digester Building...5 Figure 4 Gas Conditioners at Digester Building...6 Figure 5 Sludge Drying Beds...7 Figure 6 Typical Belt Filter Press...35 Figure 7 Cut-Away View of a Centrifuge...39 Figure 8 Flow Schematic of a Direct Heat Drying System...50 Figure 9 Alkaline Stabilization Schematic...55 Figure 10 Typical Schematic Showing MHF and Auxiliary Equipment...61 Figure 11 Typical Schematic Showing Hot Windbox FBI and Auxiliary Equipment...61 Figure 12 Typical MHF Cross Section...62 Figure 13 Typical FBI Cross Section...63 CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC III

List of Exhibits Exhibit 1 Alternative 1 WAS Thickening DAFT Thickener...14 Exhibit 2 Alternative 2 WAS Thickening Gravity Belt Thickener...17 Exhibit 3 Alternative 3 WAS Thickening Centrifuge Thickener...19 Exhibit 4 Alternative 4 WAS Thickening Rotary Drum Concentrator...21 Exhibit 5 Alternative 5 Co-Thickening of PS and WAS Gravity Belt Thickener...24 Exhibit 6 Alternative 6 Co-Thickening of PS and Was Gravity Thickener...27 Exhibit 7 Alternative 7 Biosolids Dewatering Sludge Drying Beds...33 Exhibit 8 Alternative 8 Biosolids Dewatering Belt Filter Press...38 Exhibit 9 Alternative 9 Biosolids Dewatering...42 Exhibit 10 Alternative 10 Class A Biosolids Sludge Drying Beds...49 Exhibit 11 Alternative 11 Class A Biosolids Centrifuge Dewatering and Head Drying...53 Exhibit 12 Alternative 12 Class A Biosolids Centrifuge Dewatering Lime Stabilization...59 Exhibit 13 Alternative 13 Co-Thickening and Incineration...65 Exhibit 14 Thickening Alternatives...69 Exhibit 15 Dewatering Alternatives...72 Exhibit 16 Class A Alternatives...73 Exhibit 17 Gravity Belt Thickener Cost...74 Exhibit 18 WAS with Gravity Belt Thickener...75 Exhibit 19 Modified Anaerobic Digester...76 Exhibit 20 New Anaerobic Digester...77 Exhibit 21 Drying Bed Dewatering...78 Exhibit 22 Centrifuge Dewatering...79 Exhibit 23 Drying Beds...80 CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC IV

Introduction This report summarizes the Evaluation of Biosolids Management Alternatives for the City of Tracy Wastewater Treatment Plant (WWTP). The purpose of this report is to provide an assessment of biosolids management alternatives available to the City, and to evaluate a number of biosolids management alternatives considered to be feasible after an initial screening exercise. Background and Organization The current Tracy WWTP expansion program consists of two elements: upgrade existing treatment facilities to provide tertiary treatment and expand the plant from 9 mgd to 16 mgd. The expansion will be conducted in four phases to achieve the primary goals of increasing the treatment capacity of the plant as the City of Tracy grows, and to meet new and anticipated treatment standards. As an additional objective, the City is taking proactive steps to identify facilities necessary to meet Title 22 of the California Code of Regulations for unrestricted water reuse. The Phase 1B Project will provide the required facilities and process modifications necessary to achieve these objectives. Prior reports utilized as background for this report include: City of Tracy WWTP Expansion Phase 1B Pre-Design Report This report is organized as follows: Background and Organization Existing Conditions and Biosolids Management Facilities Development of Biosolids Management Alternatives Evaluation of Biosolids Management Alternatives Existing Conditions and Biosolids Management Facilities The City currently utilizes the following processes for biosolids management at the WWTP: Sludge produced at the WWTP is primary sludge (PSD), waste activated sludge (WAS), and filter backwash solids (FBS). PSD is removed from the primary clarifiers at approximately 2-3% solids concentration. Those solids are pumped directly to the anaerobic digesters. WAS is removed from the secondary clarifiers at approximately 1% solids concentration and thickened via a dissolved air flotation thickener (DAFT) into a thickened WAS (TWAS) of about 3% solids concentration. TWAS is pumped directly to the anaerobic digesters. The TWAS and PSD undergo anaerobic digestion to produce a Class B biosolids liquid at approximately 2% solids. The Class B digested biosolids are dewatered and further stabilized via onsite sludge drying beds. The drying beds are capable of producing a Class A biosolids product. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 1

In 2005, the City land applied its biosolids generated at the WWTP using a private contractor for the hauling and land application. Although the drying beds are capable of producing Class A biosolids, the biosolids are disposed of as Class B. There is a desire by the City to produce a Class A biosolids product. Class B biosolids can be land applied only on permitted and restricted sites, as compared to Class A products that are generally less regulated in their distribution and have less restrictions on their uses. Existing Thickening Processes Primary Sludge Primary sludge is thickened in the primary clarifiers and pumped directly to the primary digesters. The City currently operates two rectangular primary clarifiers. Primary sludge is collected from the cross rake mechanisms and suction lines are routed to the primary sludge pump station. The current primary sludge pump station contains three air operated diaphragm (ODS) pumps that pump primary sludge directly to the anaerobic digesters. Primary solids are limited to approximately 3% before the suction lines of the primary sludge pumps clog and halt pumping. Secondary Sludge Secondary sludge is removed from three secondary clarifiers. Two of the secondary clarifiers were constructed in 1984 and one secondary clarifier was constructed in 2001 as part of the phase 1A Expansion project. WAS pumps are currently located in the two RAS pump stations. Suction to the WAS pumps is provided off of the RAS suction header and the WAS is discharged directly to the existing DAFT units for thickening. WAS pumps draw suction from the discharge of the RAS pumps and are pumped directly to the DAFT for thickening. Polymer is added to the DAFT inlet piping to condition the WAS. Dissolved air flows through the conditioned WAS, bringing the TWAS to the surface. A rotating flight mechanism removes the TWAS from the surface at a maximum of 3- percent solids. The TWAS is discharged into a wet well, which is discharged into the existing digesters via the progressing cavity TWAS pumps. The City installed two daft units in 1984. They currently operate one DAFT unit, with the second unit on standby. The DAFT units were constructed to thicken the WAS prior to digestion and reduce the required digestion volume. The DAFT units are at the end of their useful life. The units have been patched by the City but require more improvements to prolong their operation. These units are also maintenance intensive for plant operators and require significant time commitments to keep in operation. High dosages of polymer are also required to produce modest solids concentrations compared to other technologies. Existing Stabilization Processes Anaerobic Digesters In 1984, two 80-foot anaerobic digesters with floating covers were constructed to stabilize primary and secondary solids from the new primary clarifiers and the trickling filter activated sludge (TFAS) process. The digesters have historically been operated in a parallel CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 2

mode as primary digesters, with each having both primary and secondary sludge fed to them. Both of the digesters have floating aluminum covers. The floating cover on Digester No. 2 has completely failed. The edges of the cover are buried under several feet of accumulated soil, making the cover ineffective for regulating level and gas pressure. Continued operation may create a dangerous situation where the gasses become too highly pressurized. The aluminum cover also has corroded valves and piping that are beyond repair. There is also concrete damage on the overflow boxes. Digester mixing is accomplished with liquid ring gas compressors discharging compressed gas to each digester through nozzles. Three gas compressors were installed in 1984 for digester mixing. Two of the three original gas compressors were replaced in 2004 and 2005 with different compressor models because the original models were no longer available. The third compressor was installed as a redundant compressor, but it also needs replacement and the City currently operates with little redundancy. From a functional standpoint, the gas mixing system is adequate for the current sludge quantities and low sludge feed concentrations. However, from an operational standpoint, the system is extremely maintenance intensive. Discussions with Plant staff indicate that the City has been replacing corroding parts, pumps, valves, and piping as necessary to keep the system operating. Many valves and pumps have become obsolete in the market, creating situations where older equipment has been replaced with new equipment models that are not necessarily the best fit. Recent compressor failures, gas leaks, and other operational issues reported by plant staff show that the existing gas mixing system is inadequate and unreliable to the point that its operation should not be continued in the future. The digesters were installed along with a reciprocating engine-based cogeneration system and a watertube boiler system to burn digester and/or natural gas as a part of a sludge handling project. The cogeneration system was designed to operate on digester gas, supplemented as needed by natural gas to provide heat for the digesters and electrical power for the plant. The boiler system was provided as backup for the cogeneration system and could run on either digester or natural gas to provide heat to the digesters. The cogeneration system was operated until the mid-1990s, providing a substantial cost savings along with heat for the digesters and electrical power for the plant. Repeated failures of the gas conditioners and difficulty in maintaining the gas conditioners in service to provide suitable quality digester gas to the engine generator resulted in continual operational and maintenance problems of both systems. Costs to maintain the engine generator accumulated, eventually requiring a $100,000 overhaul. These high operation and maintenance costs resulted in the City taking the cogeneration system out of service. With the cogeneration system offline, the boiler system, originally the backup to the cogeneration system became the primary method of burning digester and natural gas to heat the digesters. After years of operating the boiler on digester gas severe plugging around the tubes was created by the high moisture along with the corrosive nature of the digester gas. The plugging required an extensive and difficult cleaning of the tubes, and caused rapid deterioration of the boiler. Because of this, the boiler was no longer used to burn digester gas. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 3

Currently, the digesters flare the digester gas. The digesters are being heated by the boiler system, operating on natural gas. As an emergency change order to the Phase 1B Expansion project, the boiler was finally replaced with a new boiler. No backup to that boiler system currently exists. The City would like return to operating the boiler system on the more cost effective digester gas. Future boiler additions should be designed to operate on a dual gas mixture of natural gas and digester gas. The digester gas is an energy resource that can provide a significant O&M cost savings to the plant, which is currently being flared into the atmosphere. The City currently spends approximately $83,000 annually for natural gas. In general, cogeneration systems have a high initial cost and require substantial maintenance due to their complexity of operation. Many WWTP s have neither the expertise nor time allotted in their staff plans to maintain a cogeneration system. Two possible alternatives that may be worthy of further consideration are as follows: First, an evaluation of the existing cogeneration engine could be completed to study the cost and feasibility of rehabilitation to bring it back on-line. Second, it may be worth investigating the potential for a third party interest to do the design, installation, and operation of the existing, or a new cogeneration system under contract with the City. The City of Tracy may be able to competitively bid the operation of the cogeneration unit and the related maintenance to a third party specializing in this area. Figure 1 Corroding Valve at Digester Building CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 4

Figure 2 Gas and Moisture Compressors at Digester Building Figure 3 Corrosion at Gas and Moisture Compressors at Digester Building CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 5

Dewatering Facilities Figure 4 Gas Conditioners at Digester Building Sand drying beds are used for dewatering and drying biosolids. Digested sludge is pumped in a batch operation from the digesters using sludge transfer pumps located in the basement of the digester complex. Dewatering of the digested sludge is accomplished by infiltration of the filtrate through the sand bed and into the underdrain system as well as through decant rings located at the water surface. After infiltration and decanting of water, natural CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 6

evaporation thickens the solids. Solids are wind rowed with a front loader to expose wet areas and enhance the drying process. Over time the sludge is turned over, piled, and dried. Testing of the dewatered biosolids shows that the drying beds are capable of producing Class A biosolids. However, the City of Tracy currently disposes of these solids as a Class B product. Figure 5 Sludge Drying Beds Each sand bed has an underdrain system that is piped into the Plant. Discussions with Plant staff indicate that many of the underdrain system pipes are plugged and do not drain effectively, thus limiting the amount of water that can quickly be removed from the sludge which extends drying time and limits capacity. The underdrain decanting system is located approximately six feet from the sides of the drying beds, with two drains located on each drying bed opposite of the feed inlet pipe. The current decanting system is difficult to use and has experienced a lot of wear and tear throughout the past twenty years. Decant rings and piping are susceptible to damage when they are forgotten in-place and hit with the front loader. Manipulation of the decant rings requires operators to put on waders and wade out into the drying bed in several feet of digested sludge. Often times operators are required to fish for the decant rings via a metal pole. Discussions with Plant staff indicate that a different method of decanting is necessary. Historically the drying beds were operated on a rotational basis and only half of the beds were required to process the sludge on an annual basis. However, with 25% of the beds taken out of commission as part of the Phase IB expansion, and with the growing development within the City of Tracy, the City will need to utilize all of the drying beds to accomplish dewatering. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 7

Preliminary Planning Efforts The Pre-design effort for the Phase 1B WWTP expansion included some preliminary evaluation of biosolids management alternatives. During the predesign four phases of plant expansion were identified. Each phase would incrementally increase the plant capacity from 9 mgd to 16 mgd. The Pre-design assumed that the existing anaerobic digestion facilities and the existing drying bed facilities would continue to be used for stabilization and dewatering respectively. Under these assumptions, the following biosolids management projects were identified for each phase. Phase 1B Expansion Project Anticipated handling of the primary sludge in a similar manner to the current operations. The Phase 1B project will add one new 100-foot diameter circular primary clarifier. The clarifier will contain a spiral rake mechanism for enhanced removal of primary sludge. It is anticipated that only the new primary clarifier will be necessary for normal operations and the existing rectangular clarifiers will be utilized only for extreme wet weather events. Added one new ODS pump in the existing primary sludge pump building dedicated to the new primary clarifier for primary sludge pumping. A new primary sludge pipeline routed from the primary sludge pump station to the digester was also added. Anticipated that primary sludge would always be pumped directly to the digester for stabilization and that no further primary sludge thickening would be required through the phased expansion process to reach 16 mgd. Modified the WAS pumping scenario by placing one new WAS pump in each of the two new galleries. WAS pumps will draw suction from the discharge of the RAS pumps and be pumped directly to the DAFT for thickening. The new WAS pumps were designed to accommodate discharge pressures required for that of an in line polymer mixer and a potential gravity belt thickener. DAFT units will have the ability to thicken combined solids streams including WAS, Secondary Scum (SSM), and Tertiary Filter backwash solids (FBS). WAS enters the DAFT units at approximately 1-percent solids. Secondary scum is metered and fed into the WAS solids stream at a rate less than 25% of the total WAS flow. FBS may or may not be thickened in the DAFT as they can also be recycled to the head of the plant and removed as primary sludge, or they may be removed from the process by discharging the solids directly to the drying beds. A new natural fired gas boiler was added as a change order to the Phase 1B project to replace the existing boiler which had failed. Removed four sludge drying beds to make room for the construction of expanded secondary treatment facilities. Paved two sludge drying beds. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 8

Constructed new dewatering filtrate pump station that pumps filtrate back into the aeration basins. Phase 2 Initially the Phase 1B project included a design to replace the two existing DAFT units with two gravity belt thickeners (GBTs). This work was removed from the scope of the Phase 1B project due to overall cost considerations and moved to Phase 2. The project would consist of two GBTs, GBT in operation and the second on standby. The GBT design would simply replace the DAFTs to provide a more effective thickening process with less operation and maintenance costs. Addition of a new polymer feed system utilizing liquid polymer in totes. Phase 2 No biosolids management projects identified Phase 4 Construct a new anaerobic digester to meet the Phase 4 flow and load requirements. Development of Biosolids Management Alternatives Knowledge has been gained primarily from CH2M HILL s prior work on the Tracy WWTP Expansion Project, supplemented by meetings with Plant staff from February 6-10, 2006. During those meetings, background information on biosolids management was shared and preferences of the City for its biosolids management program were discussed. CH2M HILL has used the background knowledge and City preferences to develop alternatives for thickening, stabilization, and biosolids management. Drivers for the Tracy WWTP to study alternative biosolids management alternatives include: Rapid growth in the City of Tracy and the expansion of the treatment facility from its current capacity of 9 mgd to 16 mgdthe condition assessment of the current facilities has identified many shortcomings of the existing system. Those issues will need to be resolved in order for the City to continue processing biosolids with the existing facilities. Anticipated regulatory standards for land applying and land filling biosolids are becoming expected to become more stringent. Less land application sites are anticipated to be available for land application. The City would like to upgrade its biosolids management program to provide biosolids that meet Class A beneficial-use criteria, but the City may consider other alternatives of Class B land application or landfilling, especially as back-up options to a Class A biosolids process, if one is implemented. The City has concerns with the long-term sustainability of producing only Class B biosolids for land application. The City has a strong interest in evaluating the potential for utilizing their existing drying bed process to produce a Class A biosolids product. This would likely require adding additional drying beds. A Class A biosolids product is CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 9

better suited for distribution to broader and less restricted markets, such as nurseries, landscapers, or golf courses, in addition to larger agricultural uses. The City has expressed an interest in the continued use of existing infrastructure to provide the most cost effective biosolids management alternatives. They prefer to continue to use the existing anaerobic digestion facilities for solids stabilization. However, they also want to consider the technical and environmental merits of cogeneration or boiler system with energy recovery via use of the digester gas. Operating costs are saved by producing digester gas as a fuel for heating the digesters. Thickening Alternatives Raw wastewater entering a treatment plant is quite dilute. Suspended solids concentrations in untreated wastewater generally range from 200 to 300 milligrams per liter (mg/l) or parts per million (ppm) (approximately 0.02-percent to 0.03-percent solids by weight). Settling that occurs in a primary treatment process can produce a thickened sludge that is 1 to 4-percent solids by weight. The actual solids concentration depends on the wastewater characteristics and how quickly the primary sludge is removed from the tank. For the City of Tracy a limitation on primary sludge solids results from the long primary sludge suction lines. At primary sludge concentrations above about 3.5 percent solids, the primary sludge suction lines may clog requiring significant plant operator time to clear the blockage. For this reason this study assumes that the primary sludge solids concentration will be maintained at a maximum of 3.5 percent. During treatment, an equivalent amount of additional solids are generated in the secondary treatment process due to cell synthesis of BOD. The Phase 1B Expansion project will considerably improve the secondary sludge settling characteristics. For this reason it was assumed that the minimum waste activated sludge concentration will be 0.8 percent solids. This value was used for sizing thickening facilities where applicable to secondary solids. Thickening processes can further concentrate solids removed through primary and secondary treatment. Not all thickening alternatives include thickening of the primary sludge beyond 3.5 percent, as most thickening alternatives only assume thickening of the waste activated sludge. Thickening is often essential because many downstream solids processing systems are based upon detention time, so a thicker material can significantly reduce the size of downstream processes by removing excess water. Each thickening technology will result in a different thickened sludge concentration, depending upon the technology and what type of sludge is being thickened. These assumptions are listed in the individual alternative descriptions. Based on discussions with plant staff and available appropriate technologies, the following thickening alternatives were developed for consideration. All thickening processes, except incineration, assume anaerobic digestion for stabilization. A schematic for each alternative is shown in Exhibits 1 through 6. A solids balance for each alternative is shown in Exhibit 14. Each alternative is described in detail below. Alternative 1 WAS Thickening with DAFT Alternative 2 WAS Thickening with GBT CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 10

Alternative 3 WAS Thickening with Centrifuges Alternative 4 WAS Thickening with Rotary Drum Concentrator Alternative 5 Co-Thickening of PS and WAS with GBT Alternative 6 Co-Thickening of PS and WAS with Gravity Thickener Alternative 1 WAS Thickening with DAFT Dissolved air flotation thickening is used to concentrate biosolids that have greater tendencies to float than to settle. Dissolved air flotation thickening is used primarily for WAS, but also has been applied to aerobically digested solids, blended primary solids and WAS, and other similar solids. In the dissolved air flotation thickening process, air is added to incoming flow at a pressure in excess of atmospheric pressure. High pressure causes oxygen to dissolve into the flow stream. When the pressure is reduced as the flow enters the process tank, excess air is released from the solution as very small bubbles. The bubbles adhere to the suspended particles or become enmeshed in the solids matrix. The density of the solids-air aggregate is less than that of water, thereby causing it to float to the surface. Water drains from the float, increasing the solid concentration. Float is continuously removed from the surface of the thickener by skimmers. Bottom collectors are also used to remove any settled solids or grit that may accumulate. There are several ways of adding pressurized air, including adding it to the entire solids flow stream, adding it to only a part of the solids flow stream, or adding it to a recycled portion of the clarified effluent (or alternate source containing little suspended matter). Because pressurization of a relatively clear recycle stream eliminates clogging problems in pressurization pumps and minimizes high shear conditions in the floc, it is the most commonly used method in the United States. Dissolved air flotation thickeners can be either rectangular or circular. Design criteria for dissolved air flotation thickeners depend on the nature of the solids being thickened and the specific features of the equipment being used. Some typical design criteria are listed in Table 1. TABLE 1 Typical Design Criteria for Dissolved Air Flotation Thickening Evaluation of Biosolids Management Alternatives, City of Tracy, CA Criteria Values Air Pressure Air to Solids Ratio Solids Loading Rate Recycle Flow Rate Hydraulic Loading Rate 40 to 80 psig (2.7 to 5.4 bars) 0.02 0.1 weight ratio [depends on solids volume index (SVI)] 2 3 lbs/hour/sf (9 to 13.5 kg/hour/sq m)(with coagulant addition) 0.4 1.2 lbs/hour/sf (1.8 to 5.4 kg/hour/sq m)(without coagulant addition) Depends on manufacturer 0.8 2.5 gpm/sf (30 to 90 liters/minute/sq m) (depends on whether or not recycle is included) CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 11

TABLE 1 Typical Design Criteria for Dissolved Air Flotation Thickening Evaluation of Biosolids Management Alternatives, City of Tracy, CA Criteria Values Air Pressure Air to Solids Ratio Solids Loading Rate 40 to 80 psig (2.7 to 5.4 bars) 0.02 0.1 weight ratio [depends on solids volume index (SVI)] 2 3 lbs/hour/sf (9 to 13.5 kg/hour/sq m)(with coagulant addition) 0.4 1.2 lbs/hour/sf (1.8 to 5.4 kg/hour/sq m)(without coagulant addition) The concentration of solids produced by dissolved air flotation thickening of WAS will vary, but generally can be expected to be in the range of 3 to 5-percent solids by weight. Removal efficiency will also vary but can be up to 90-percent or greater when flocculating chemicals are used and the system is optimized. To improve solids-capture efficiency and reduce the size of the units, most dissolved air flotation facilities use a flocculent aid. The most common chemicals used are cationic polyelectrolytes (polymers). Polymers neutralize particle surface charges, causing the particles to coagulate so that air bubbles can attach to them. With the use of polymers, the size of the dissolved air flotation unit may be reduced and solids capture improved. With respect to operation and maintenance, operator attention is required to maintain the chemical feed, recycle, and pressurization pumps, skimmers, and bottom-solids removal equipment. Because of air entrainment in the float, there can also be difficulties in pumping the thickened biosolids if the correct pumps are not selected. Because of the oxygen content in the thickened solids, the potential for odors is less than with gravity-thickening processes. With respect to power and labor, a general indication of the requirements for two different surface areas is shown in Table 2. TABLE 2 Typical Operations and Maintenance Requirements for Dissolved Air Flotation Thickeners Evaluation of Biosolids Management Alternatives, City of Tracy, CA Criteria 100 sf (10 s m) of DAF Surface Area 1,000 sf (100 sq m) of DAF Surface Area Annual Labor (hours) 400 2,500 Annual Power (kwh) 100,000 700,000 The major advantages of dissolved air flotation thickening are: Provides better solids-liquid separation than gravity thickening For WAS, yields higher solids concentration than gravity thickening Requires relatively little land area Offers excellent solids equalization control Solids are maintained in aerobic condition, reducing potential odors CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 12

Can remove grit from solids processing system Removes grease Relatively high reliability Proven track record Relatively high solids loading rates are possible The major disadvantages are: Operating costs for dissolved air flotation are higher than for gravity thickening, especially for coagulants and power Has little solids storage capacity Thickened solids concentration is significantly less than from a centrifuge or gravity belt thickeners Requires more land than a centrifuge or gravity belt Maintenance cost are typically higher than for centrifuge and gravity belt thickeners Optimal performance requires expensive polymer addition and dosages are typically higher than for gravity belt thickeners Alternative 1 is shown schematically in Exhibit 1 and depicts the City of Tracy s current operations for sludge thickening. For this alternative primary sludge would be pumped directly to the digesters at 3.5 percent solids. WAS at 0.8 percent solids would be fed to a DAFT and thickened to approximately 3.5 percent solids before being pumped to the digester. An 80 percent solids capture rate was assumed. The existing DAFT units have reached the end of their useful life and would need to be replaced with new units. New polymer feed facilities would also be required. It was also assumed that the existing TWAS hopper could be used, however, the existing TWAS pumps would need to be replaced with new TWAS progressing cavity pumps. With this thickening alternative the digesters would have adequate treatment capacity through Phase 1B flow and loads. After that the existing digester capacity would become limiting on sludge retention time and a new digester would be necessary for Phase 2 flow and loads. This is because the DAFT units only thicken WAS solids to approximately 3.5 percent, rather than the 6 percent solids assumed in the Phase 1B Predesign using a gravity belt thickener. These units also require more operation and maintenance attention, significantly higher polymer dosages than a gravity belt thickener, and are more expensive to operate. For these reasons it is recommended that thickening utilizing the existing or new DAFT units not be considered further. Alternative 2 WAS Thickening with GBT Gravity belt thickening is a solids-liquid separation process that relies on coagulation and flocculation of solids in a dilute slurry, and drainage of free water from the slurry through a moving fabric-mesh belt. It is essentially a modification of the upper gravity drainage zone of the belt filter press, which can be used for dewatering as described below. Gravity belt thickening has been used on a variety of solids having initial solids concentrations as low as 0.4-percent and as high as 8.0-percent. The process is polymer dependent and can achieve 95-percent or greater solids capture. Because of the relatively open-mesh filter belts that are used, a relatively high dose of polymer is required to create flocs large enough to be trapped CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 13

PRIMARY CLARIFIER NO.1 PRIMARY SLUDGE 3.5% SOLIDS PRIMARY CLARIFIER NO.3 PUMP STATION EXISTING DIGESTER PRIMARY NO.1 CLARIFIER NO.4 SECONDARY SCUM SECONDARY CLARIFIER NO.1 WAS PUMP STATION THICKENED PRIMARY SCUM EXISTING SECONDARY CLARIFIER NO.2 0.8% SOLIDS DAFT THICKENER TWAS PUMPS DIGESTER NO.2 WAS PUMP STATION ALTERNATE SECONDARY CLARIFIER NO.3 DISCHARGE TO THICKENING 1 2 3 4 5 6 7 8 FILTER BACKWASH EQUALIZATION BASIN FILTER BACKWASH SOLIDS PUMP STATION TO DRYING BEDS NEW DIGESTER NO.3 TERTIARY FILTERS NOTES: 1. REQUIRES MIXING IMPROVEMENTS IN DIGESTER. EXHIBIT 1 ALTERNATIVE 1 WAS THICKENING DAFT THICKENER 00nf001d.dgn 10-JUL-2006 16:04:24 City of Tracy - WWTP Expansion Phase B

by the mesh. The type and amount of polymer used is dependent upon the type of solids and the particular machine to be used. Cationic polymers are generally successful in these applications. Gravity belt thickening systems are designed based on site-specific applications. However, some typical design criteria for thickening are listed in Table 3. Although gravity belt thickeners are relatively simple to operate, and produce good results with relatively little operator attention, there are a few operational issues that need to be addressed to meet thickening objectives. Probably the most important is proper type and mixing of polymer. Other operational features such as the solids feed rate, belt speed, and thorough washing of the belt are also important. Belt washing is particularly important to prevent binding of the belt. Odor control within the facility is necessary and usually requires that gravity belt thickeners be installed in enclosed buildings with high ventilation rates. Odor control facilities are typically not required for on the exhaust from the building if only WAS is thickened provided the treatment plant is not located in an extreme odor sensitive area. TABLE 3 Typical Design Criteria for Gravity Belt Thickeners Evaluation of Biosolids Management Alternatives, City of Tracy, CA Criteria Values Hydraulic Loading Rate Solids Loading Rate 100 to 220 gpm (380 to 1,140 liters/minute) per meter of belt width Up to 1,100 lbs (500 kg) /hour per meter for WAS thickening Up to 1,700 lbs (770 kg) /hour/meter for thickening digested solids Thickened Solids Concentration Solids Capture Efficiency Flocculation Time 5 to 7-percent solids by weight 90 to 98-percent 30 seconds, minimum The major advantages of gravity belt thickeners are: Relatively low space requirements Low power usage Moderate capital costs compared to other thickening processes Simple operation, requiring little operator attention The major disadvantages are: Generally requires moderate to high dosages of polymer May produce odors and may require enclosure and odor control CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 15

May have fairly large variations in thickened solids concentration with fluctuations in characteristics of feed solids Alternative 2 is shown schematically in Exhibit 2. For this alternative primary sludge would be pumped directly to the digesters at 3.5 percent solids. WAS at 0.8 percent solids would be fed to a GBT and thickened to approximately 6.0 percent solids before being pumped to the digester. Two new 1.5 meter gravity belt thickeners would be required for a 16 hour per day operation, with one unit being completely redundant. Both units could be operated to reduce the hours per day of operation down to 8.0 hours. A maximum hydraulic loading of 220 gpm per meter of gravity belt thickener was used for sizing. The City may be able to get by with two 1 meter gravity belt thickeners which is borderline for capacity and would require 24 hour per day of operation with one unit out of service, however, the differential cost of the 1.5 meter machines is small and is recommended. A 95 percent solids capture rate was assumed. This alternative would require new polymer feed facilities consisting of a polymer tote, neat polymer mixing, polymer feed, and polymer dilution facilities. The gravity belt thickeners would also require a conveyor to collect the thickened solids and convey them to the existing TWAS hopper. The existing TWAS pumps would need to be replaced with new TWAS progressing cavity pumps. With this thickening alternative the digesters would have adequate treatment capacity through the Phase 3 flow and loads. After that the existing digester capacity would become limiting on sludge retention time and a new digester would be necessary for Phase 4 flow and loads. This alternative provides very cost effective treatment, has low operational and maintenance costs, has modest polymer addition costs, and is extremely reliable. For these reasons it is recommended that thickening utilizing new gravity belt thickeners be considered further. Alternative 3 WAS Thickening with Centrifuges Centrifuges have been used to thicken a wide range of solids. Their operation is based on the application of centrifugal force to a liquid-solids stream, which accelerates the separation of the liquid and solid fractions based upon specific gravity differences. The process involves both clarification of the centrate stream and compaction of the solids. Solid bowl conveyor-type centrifuges are typically used to thicken and dewater municipal biosolids. This centrifuge unit operates with a continuous feed and discharge. The solids, which may be conditioned with polymer, are fed into the rotating bowl which has a conical shape at one end and an end plate at the other. The end plate has holes in it for the discharge of the centrate. These holes are equipped with adjustable weir plates to control the operating level of the liquid in the bowl. A motor drives the bowl at speeds ranging from 2,000 to 3,000 rpm. This spinning action creates the centrifugal forces required to concentrate the solids against the bowl wall. To remove these solids, a spiral conveyor in the bowl rotates at a slightly differing speed than the bowl and conveys the solids towards the conical solids discharge. The centrate water is discharged over the weir plates at the opposite end of the centrifuge and conveyed back to the treatment process. Typical solids concentrations resulting from thickening using conventional centrifuges are roughly in the range of 6 to 10-percent dry solids, depending on the type of solids being thickened and the amount of polymer added. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 16

PRIMARY CLARIFIER NO.1 PRIMARY SLUDGE 3.5% SOLIDS PRIMARY CLARIFIER NO.3 PUMP STATION EXISTING DIGESTER PRIMARY NO.1 CLARIFIER NO.4 SECONDARY SCUM SECONDARY CLARIFIER NO.1 WAS PUMP STATION THICKENED PRIMARY SCUM EXISTING SECONDARY CLARIFIER NO.2 0.8% SOLIDS GRAVITY BELT THICKENER TWAS PUMPS DIGESTER NO.2 WAS PUMP STATION ALTERNATE SECONDARY CLARIFIER NO.3 DISCHARGE TO THICKENING 1 2 3 4 5 6 7 8 FILTER BACKWASH EQUALIZATION BASIN FILTER BACKWASH SOLIDS PUMP STATION TO DRYING BEDS NEW DIGESTER NO.3 TERTIARY FILTERS NOTES: 1. REQUIRES MIXING IMPROVEMENTS IN DIGESTER. EXHIBIT 2 ALTERNATIVE 2 - WAS THICKENING GRAVITY BELT THICKENER 00nf002d.dgn 10-JUL-2006 11:55:14 City of Tracy - WWTP Expansion Phase B

Centrifuges have historically required a substantial level of maintenance, and frequent repairs and considerable downtime have been common. However, with recent advances, modern centrifuges are much more reliable than in the past. An important part of centrifuge maintenance is frequent internal cleaning. If a unit is to be shut down for more than a couple of hours, it is important that the solids inside be removed before they have a chance to dry. Newer centrifuges incorporate an automatic water flushing step as a part of the shutdown procedure. Dry solids can cause load imbalance. Centrifuges also may require a substantial amount of flocculent aid. Because centrifuges are totally enclosed, odors are usually minimal. Power and labor requirements are highly variable depending on the type of centrifuge used. The major advantages are: Contained process minimizes housekeeping and odor considerations Continuous operation provides flexible control capability for process performance Moderate or highly thickened solids concentration Relatively small area requirements Moderate to high throughput capabilities versus space requirements Low operator attention requirements High solids capture The major disadvantages are: High capital costs Requires skilled maintenance personnel and a fairly high degree of maintenance Centrate may precipitate struvite (primarily when thickening anaerobically digested biosolids), which increases operation and maintenance costs High power requirements Moderate to high polymer requirements (thickening can be done without polymer, but the capture efficiency is reduced to 85 to 90-percent) High operating speeds High noise potential Alternative 3 is shown schematically in Exhibit 3. For this alternative primary sludge would be pumped directly to the digesters at 3.5 percent solids. WAS at 0.8 percent solids would be fed to centrifuges and thickened to approximately 6.0 percent solids before being pumped to the digester. Three new 125 gpm centrifuges would be required, with one unit being completely redundant and two units operating 25 hours per day, which is recommended. A maximum hydraulic loading of 125 gpm per unit was used for sizing. Three smaller units are preferred over two larger units so as to provide more redundancy and reduce costs. A 95 percent solids capture rate was assumed. This alternative would require new polymer feed facilities consisting of a polymer tote, neat polymer mixing, polymer feed, and polymer dilution facilities. The centrifuges would also require a conveyor to collect the thickened solids and convey them to the existing TWAS hopper. The existing TWAS pumps would need to be replaced with new TWAS progressing cavity pumps. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 18

PRIMARY CLARIFIER NO.1 PRIMARY SLUDGE 3.5% SOLIDS PRIMARY CLARIFIER NO.3 PUMP STATION EXISTING DIGESTER PRIMARY NO.1 CLARIFIER NO.4 SECONDARY SCUM SECONDARY CLARIFIER NO.1 WAS PUMP STATION THICKENED PRIMARY SCUM EXISTING SECONDARY CLARIFIER NO.2 0.8% SOLIDS CENTRIFUGE THICKENING TWAS PUMPS DIGESTER NO.2 WAS PUMP STATION ALTERNATE SECONDARY CLARIFIER NO.3 DISCHARGE TO THICKENING 1 2 3 4 5 6 7 8 FILTER BACKWASH EQUALIZATION BASIN FILTER BACKWASH SOLIDS PUMP STATION TO DRYING BEDS NEW DIGESTER NO.3 TERTIARY FILTERS NOTES: 1. REQUIRES MIXING IMPROVEMENTS IN DIGESTER. EXHIBIT 3 ALTERNATIVE 3 - WAS THICKENING CENTRIFUGE THICKENER 00nf003d.dgn 10-JUL-2006 11:55:46 City of Tracy - WWTP Expansion Phase B

With this thickening alternative the digesters would have adequate treatment capacity through the Phase 3 flow and loads. After that the existing digester capacity would become limiting on sludge retention time and a new digester would be necessary for Phase 4 flow and loads. For thickening applications centrifuges may be somewhat over kill as they require significant power consumption resulting in high operation costs. However, their relatively small footprint and low maintenance requirements still make them a good thickening alternative. This alternative would meet all of the process performance criteria, and for this reason, this alternative is recommended for further consideration. Alternative 4 WAS Thickening with Rotary Drum Concentrator A rotary drum thickener operates similarly to a gravity belt thickener, with free water draining through a moving porous medium while flocculated solids are retained on the medium. A rotary drum thickener consists of an internally fed rotary drum with an integral internal screw for transporting thickened solids out of the drum. The drum rotates and is driven by a variable or constant speed-drive. Generally, rotary drum thickeners are used in small treatment plants for WAS thickening. They are particularly well suited for high-fiber solids, such as those found in the pulp and paper industry. As with gravity belt thickeners, they are highly dependent upon polymer addition to achieve thickening objectives. The addition of large amounts of polymer, however, can be a concern because of cost, floc sensitivity, and the shear potential in the rotating drum. There are many factors that influence the design of rotary drum thickeners, and generally pilot testing is performed to determine design criteria. The drums generally rotate at 5 to 20 revolutions per minute (rpm). With the proper polymer application and feed rate, rotary drum thickeners can produce a thickened solids concentration of 4 to 8-percent and a solids capture rate of 90 to 95-percent. The major advantages of rotary drum thickeners are: Relatively low space requirements Low power usage Moderate capital costs Ease of enclosure, which improves housekeeping and odor control Good performance on a variety of solids The major disadvantages are: Floc sensitivity and shear potential in the rotating drum Limited size units available, restricting use to small facilities Requires higher dosages of polymer than gravity belt thickeners Alternative 4 is shown schematically in Exhibit 4. For this alternative primary sludge would be pumped directly to the digesters at 3.5 percent solids. WAS at 0.8 percent solids would be fed to rotary drum thickener and thickened to approximately 4.5 percent solids before being pumped to the digester. Three new 120 gpm drum thickeners would be required, with one unit being completely redundant, and two units operating 24 hours per day which is recommended. A maximum hydraulic loading of 120 gpm per unit was used for sizing. A 90 percent solids capture rate was assumed. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 20

PRIMARY CLARIFIER NO.1 PRIMARY SLUDGE 3.5% SOLIDS PRIMARY CLARIFIER NO.3 PUMP STATION EXISTING DIGESTER PRIMARY NO.1 CLARIFIER NO.4 SECONDARY SCUM SECONDARY CLARIFIER NO.1 WAS PUMP STATION THICKENED PRIMARY SCUM SECONDARY CLARIFIER NO.2 0.8% SOLIDS ROTARY DRUM CONCENTRATOR TWAS PUMPS EXISTING DIGESTER NO.2 WAS PUMP STATION ALTERNATE SECONDARY CLARIFIER NO.3 DISCHARGE TO THICKENING 1 2 3 4 5 6 7 8 FILTER BACKWASH EQUALIZATION BASIN FILTER BACKWASH SOLIDS PUMP STATION TO DRYING BEDS NEW DIGESTER NO.3 TERTIARY FILTERS NOTES: 1. REQUIRES MIXING IMPROVEMENTS IN DIGESTER. EXHIBIT 4 ALTERNATIVE 4 - WAS THICKENING ROTARY DRUM CONCENTRATOR 00nf004d.dgn 10-JUL-2006 11:56:06 City of Tracy - WWTP Expansion Phase B

This alternative would require new polymer feed facilities consisting of a polymer tote, neat polymer mixing, polymer feed, and polymer dilution facilities. The drum thickeners would also require a conveyor to collect the thickened solids and convey them to the existing TWAS hopper. The existing TWAS pumps would need to be replaced with new TWAS progressing cavity pumps. With this thickening alternative the digesters would have adequate treatment capacity through the Phase 2 flow and loads. After that the existing digester capacity would become limiting on sludge retention time and a new digester would be necessary for Phase 3 flow and loads, accelerating this project relative to Alternatives 2 and 3. Drum thickeners have large drums that are susceptible to damage and have limited capacity, requiring three units to be installed. These units would require a significantly larger building footprint. They also utilize more polymer than gravity belt thickeners and centrifuges. For these reasons this alternative should not be considered further. Alternative 5 - Co-Thickening of PS and WAS with GBT This alternative is similar to Alternative 2 except rather than pumping primary sludge directly to the digester; the primary sludge is pumped to a blend tank where it is mixed with the secondary sludge. Alternative 5 is shown schematically in Exhibit 5. Primary sludge is stored in the tank over night until processing operations begin. Secondary sludge is pumped to the blend tank and mixed with the primary sludge when thickening operations begin. Both primary and secondary sludge are then pumped to the gravity belt thickener and thickened to approximately 6.0 percent solids. Two new 2.0 meter gravity belt thickeners would be required for a 12 hour per day operation, with one unit being completely redundant. Both units could be operated to reduce the hours per day of operation down to 6.0 hours. A maximum hydraulic loading of 220 gpm was used for sizing the gravity belt thickeners. This alternative would require new polymer feed facilities consisting of a polymer tote, neat polymer mixing, polymer feed, and polymer dilution facilities. The gravity belt thickeners would also require a conveyor to collect the thickened solids and convey them to the existing TWAS hopper. The existing TWAS pumps would need to be replaced with new TWAS progressing cavity pumps. With this thickening alternative the digesters would have adequate treatment capacity based on sludge retention time through the Phase 4 flow and loads. However, the two existing digesters would become overloaded from a volatile solids loading perspective and a third digester would be required to meet the Phase 4 flow and loads regardless. From a digester standpoint this alternative provides no additional benefit over thickening only the WAS sludge. Several other disadvantages include: 1. The larger gravity belt thickeners would require a larger building, thus driving up cost 2. The polymer dosage for the additional primary sludge would almost double polymer consumption with no added benefit of digester capacity. 3. Placing primary sludge over the gravity belt thickeners would trigger a Class 1 Division 1 space which either requires additional ventilation to declassify and/or explosion proof motors and switches. CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 22