tracy wwtp solids master plan · draft tracy wwtp solids master plan cvo/tracy wwtp solids master...

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

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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


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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


IV

DRAFT TRACY WWTP SOLIDS MASTER PLAN

CVO/TRACY WWTP SOLIDS MASTER PLAN_FINAL.DOC 1

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.



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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



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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.



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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



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Figure 2 – Gas and Moisture Compressors at Digester Building

Figure 3 – Corrosion at Gas and Moisture Compressors at Digester Building



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Figure 4 – Gas Conditioners at Digester Building

Dewatering Facilities 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



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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.



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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.



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• 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



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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



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• 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 40 to 80 psig (2.7 to 5.4 bars)

Air to Solids Ratio 0.02 – 0.1 weight ratio [depends on solids volume index (SVI)]

Solids Loading Rate 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)

Recycle Flow Rate Depends on manufacturer

Hydraulic Loading Rate 0.8 – 2.5 gpm/sf (30 to 90 liters/minute/sq m) (depends on whether or not recycle is included)



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TABLE 1 Typical Design Criteria for Dissolved Air Flotation Thickening Evaluation of Biosolids Management Alternatives, City of Tracy, CA

Criteria Values

Air Pressure 40 to 80 psig (2.7 to 5.4 bars)

Air to Solids Ratio 0.02 – 0.1 weight ratio [depends on solids volume index (SVI)]

Solids Loading Rate 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



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• 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

PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

3.5% SOLIDS

ALTERNATE

DISCHARGE TO

THICKENING

FILTER

BACKWASH

EQUALIZATION

BASIN

FILTER

BACKWASH

SOLIDS PUMP

STATION

TO DRYING BEDS

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2DAFT

THICKENER

TWAS

PUMPS

0.8% SOLIDS

REQUIRES MIXING IMPROVEMENTS

IN DIGESTER.

1.

NOTES:

00nf001d.dgn 10-JUL-2006 16:04:24

EXHIBIT 1

ALTERNATIVE 1

WAS THICKENING DAFT THICKENER

City of Tracy - WWTP Expansion Phase B

NEW

DIGESTER

NO.3



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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 100 to 220 gpm (380 to 1,140 liters/minute) per meter of belt width

Solids Loading Rate 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 5 to 7-percent solids by weight

Solids Capture Efficiency 90 to 98-percent

Flocculation Time 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



16

• 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.

PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

3.5% SOLIDS

ALTERNATE

DISCHARGE TO

THICKENING

FILTER

BACKWASH

EQUALIZATION

BASIN

FILTER

BACKWASH

SOLIDS PUMP

STATION

TO DRYING BEDS

WAS

PUMP

STATION

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2TWAS

PUMPS

0.8% SOLIDS


IN DIGESTER.

1.

NOTES:

00nf002d.dgn 10-JUL-2006 11:55:14

GRAVITY

BELT

THICKENER


EXHIBIT 2

ALTERNATIVE 2 - WAS THICKENING

GRAVITY BELT THICKENER

NEW

DIGESTER

NO.3



18

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


• 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.

PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

3.5% SOLIDS

ALTERNATE

DISCHARGE TO

THICKENING

FILTER

BACKWASH

EQUALIZATION

BASIN

FILTER

BACKWASH

SOLIDS PUMP

STATION

TO DRYING BEDS

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2TWAS

PUMPS

0.8% SOLIDS


IN DIGESTER.

1.

NOTES:

00nf003d.dgn 10-JUL-2006 11:55:46

CENTRIFUGE

THICKENING


EXHIBIT 3


CENTRIFUGE THICKENER

NEW

DIGESTER

NO.3



20

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


• 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.

PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

3.5% SOLIDS

ALTERNATE

DISCHARGE TO

THICKENING

FILTER

BACKWASH

EQUALIZATION

BASIN

FILTER

BACKWASH

SOLIDS PUMP

STATION

TO DRYING BEDS

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2TWAS

PUMPS

0.8% SOLIDS


IN DIGESTER.

1.

NOTES:

00nf004d.dgn 10-JUL-2006 11:56:06

ROTARY

DRUM

CONCENTRATOR

EXHIBIT 4


ROTARY DRUM CONCENTRATOR


NEW

DIGESTER

NO.3



22

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.



23

4. Placing primary sludge over the gravity belt thickener would trigger odor control of the room exhaust, driving up the capital cost of the gravity belt thickener building.

For these reasons it is recommended that this alternative not be considered further.

PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

ALTERNATE

DISCHARGE TO

THICKENING

TO DRYING BEDS

FILTER

BACKWASH

SOLIDS PUMP

STATION

FILTER

BACKWASH

EQUALIZATION

BASIN

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

GRAVITY

BELT

THICKENER

BLENDING

TANK

TWAS

PUMPS

3.5%

SOLIDS

.08%

SOLIDS

BLEND TANK PROVIDES STORAGE

OF PRIMARY SLUDGE FOR NON-

THICKENING HOURS.

MORE ODORS AT GBT.

HIGHEST SOLIDS TO DIGESTER.


IN DIGESTER.

NO NET DIGESTER CAPACITY.

NOTES:

1.

2.

3.

4.

5.

00nf005d.dgn 10-JUL-2006 11:57:20 City of Tracy - WWTP Expansion Phase B

NEW

DIGESTER

NO.3

EXHIBIT 5

ALTERNATIVE 5

CO-THICKENING OF PS AND WAS

GRAVITY BELT THICKENER



25

Alternative 6 – Co-Thickening of PS and WAS with Gravity Thickener Gravity thickening has been used for concentrating raw solids for more than 50 years and is common in many wastewater treatment facilities. Solids removed during primary treatment are fed continuously to the gravity thickener, where they initially aggregate in a sedimentation zone and then become compressed by the pressure of overlying solids in a thickening zone. Displaced water flows upward through channels in the solid matrix to a zone of clear liquid, where it is drawn off into launders and returned to the liquid primary or secondary treatment process. The concentrated biosolids are collected and removed from the bottom of the gravity thickener and pumped to stabilization and/or dewatering processes. Most gravity thickeners are circular in plan view.

Design criteria for gravity thickeners are dependent upon the type and settling characteristics of the biosolids to be thickened. Typically, gravity thickeners are used to concentrate primary solids from primary clarification. Primary solids tend to be larger, denser, and easier to separate from water than solids produced in subsequent biological treatment processes. They can, however, be combined to thicken biological solids from secondary treatment processes (WAS), thermally conditioned solids, solids from tertiary treatment processes, or a variety of blended biosolids. Because biological solids do not settle well, gravity thickener loading rates and performance vary with different types of solids.

Some wastewater treatment facilities add polymer, alum, ferric chloride or other coagulant aids to the solids to improve flocculation and settling characteristics in gravity thickeners. While coagulant aids can improve solids capture, they generally have little effect on increasing underflow solids concentration.

An important consideration in the successful operation of gravity thickeners is the prevention of septic or anaerobic (without air) conditions. Anaerobic conditions cause severe odors and generate gases that prevent solids from settling properly. To prevent this condition, treatment plant effluent, which is highly aerobic, is usually added to the thickener (dilution water). Secondary effluent is normally blended with the solids feed to accomplish this objective. Limiting the time that solids remain in the thickener is also an important consideration in preventing excessive biological degradation. During warmer weather, the average solids retention time is usually reduced to prevent anaerobic conditions from developing. Because odors can be a significant concern with gravity thickeners, it is not uncommon for gravity thickeners to be covered. Foul air can be extracted from under the covers and treated by chemical scrubbers, biofilters, or other odor control devices. In addition, chlorine or ferric chloride is frequently used to enhance settling and reduce odors.

The major advantages of gravity thickeners are:

• High solids storage capabilities • Low level of operational skill required • Low operation and maintenance costs • Proven process with extensive experience, although poor performance on WAS The major disadvantages are:

• Requires a relatively large land area • Can be a contributor to odors



26

• For some types of solids, results can be erratic (especially with WAS)

Alternative 6 is shown schematically in Exhibit 6. For this Alternative both primary sludge at 3.5 percent solids and waste activated sludge at 0.8 percent solids are pumped to a gravity thickener on a 24-hour per day operation. The solids are thickened to 4.5 percent solids. 4.5 percent solids is about the maximum solids concentrations that can be pumped and is accomplished with very short suction lines from the bottom of the gravity thickener to the thickened sludge pumps.

For this alternative the gravity thickener are sized based on the limiting solids, which in this case would be the WAS solids. For gravity thickening operation it is recommended that two gravity thickeners are sized to handle the maximum month solids loading in order to provide a level of redundancy. At a design loading of 400 gpd/sf for WAS solids, the required diameter for two equal size thickeners is 25 feet. This alternative would require new polymer feed facilities consisting of a polymer tote, neat polymer mixing, polymer feed, and polymer dilution facilities. The gravity thickener would also require a new thickened sludge pump station with new TWAS pumps located below grade and between the gravity thickeners.

With this thickening alternative the existing 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 would meet all of the process performance criteria with respect to thickening and digestion. Gravity thickeners are a reliable way to thicken both primary and secondary sludge solids. However, from a digester perspective this alternative provides no additional benefit over thickening only the WAS sludge. Several other disadvantages include:

1. A large footprint is required for two new gravity thickeners.

2. Primary sludge must be pumped twice prior to getting to the anaerobic digesters.

3. No existing infrastructure could be used. The gravity thickeners would require all new facilities including a TWAS pump station which would add considerable expense.

4. The gravity thickeners would need to be covered and the exhaust air from them would need to be scrubbed for odors.

5. Placing primary sludge over the gravity belt thickener would trigger odor control of the room exhaust, driving up the capital cost of the gravity belt thickener building.


PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

THICKENED

PRIMARY

SCUM

ALTERNATE

DISCHARGE TO

THICKENING

TO DRYING BEDS

FILTER

BACKWASH

SOLIDS PUMP

STATION

FILTER

BACKWASH

EQUALIZATION

BASIN

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

.08%

SOLIDS


GRAVITY

THICKENER

PUMP STATION

GRAVITY

THICKENER

3.5% SOLIDS

MORE ODOR CONTROL AT

THICKENER.

NO NET DIGESTER CAPACITY.

POTENTIAL FLOATING SLUDGE

FROM NITRIFIED GAS.

REQUIRES MAKING IMPROVEMENTS

IN DIGESTER.

4.

3.

2.

1.

NOTES:

NEW

DIGESTER

NO.3

EXHIBIT 6

ALTERNATIVE 6

CO-THICKENING OF PS AND WAS

GRAVITY THICKENER



28

Biosolids Stabilization Alternatives Anaerobic digestion facilities are used to stabilize raw primary sludge and waste activated sludge. The City has expressed a preference for retaining the existing digestion facilities. All of the Alternatives developed, with the exception of Alternative 13 (discussed later), incineration, assume digestion facilities are retained for stabilization.

• Alternatives 7 –1 2, Thickening, Anaerobic Digestion, Dewatering, and Disposal

• Alternative 13, Co-Thickening with Incineration

Alternatives 7 – 12, Digestion Three main factors will influence the capacity of the existing anaerobic digestion facilities.

1. Solids Retention Time (SRT): The required SRT under maximum month conditions to achieve volatile solids reduction is 20 days.

2. Volatile Solids Loading: The maximum recommended volatile solids loading rate is 0.11 lb VSS/cf/day.

3. Digester Mixing: The ability of the digester to mix the incoming solids and provide a higher total effective volume. The current digester mixing system uses compressed digester gas for mixing.

Thickening solids prior to digestion removes unnecessary water from the sludge, provides more sludge retention time for stabilization, reduces digester volume requirements and associated costs, and requires less energy for heating and mixing. Having a thicker sludge also increases the volatile solids loading in the digester and brings the SRT and volatile solids loading criteria closer together so that one criterion does not control prematurely over the other criteria. Increasing the sludge feed concentration will therefore maximize the use of the existing digester volume and will maximize the use of existing infrastructure. This saves capital costs associated with unnecessary construction of digester volume.

Effective digester mixing is the key to maintaining a high net effective volume available for digestion. As noted above, increasing the solids feed concentration to the digester will also increase the SRT and associated capacity. However, as solids are increasingly thickened and fed to the digester, the mixing system becomes more important because it is more difficult to mix a thicker sludge within the digester. It is estimated that the existing gas mixing system provides the digesters with not more than an 80 percent effective volume. With the existing mixing system, a thicker feed solids concentration will reduce the effective digester volume even further. In add-on, it was noted earlier that the existing gas mixing system is operational and maintenance intensive with several components of the system inadequate and failing.

The Phase 1B Predesign assumed that at a minimum, new thickening facilities would be constructed for the Phase 2 expansion to increase WAS solids concentrations from 3.5 to 6.0 percent. Without these improvements the digesters would not have adequate solids retention time to treat the Phase 2 sludge loads. At the Phase 1B flows and loads without a thickening project, the digesters are at their maximum month capacity. Therefore a thickening project is necessary to meet the Phase 2 capacity objectives.



29

There was also concern over the ability of the digesters to properly mix the thicker feed solids resulting from a thickening project in Phase 2. However, based on staff recommendations at the time no project was identified for digester mixing. Assuming that current digester mixing system is retained for Phase 2 flows and loads, and the net effective volume is reduced from 80 percent to 75 percent, the digesters will still have slight capacity deficit. If a digester mixing project is implemented and the net effective volume can be increased to 95 percent, then the existing two digesters would have adequate capacity to meet the Phase 3 flow and load requirements. A new digester would be required to meet the Phase 4 flow and loads and to provide process reliability. A digester mixing project is therefore recommended to increase the existing digester capacity, maximize the City’s investment in existing infrastructure, resolve the current condition assessment deficiencies, and to delay the capital cost of constructing a new digester until Phase 4.

Alternatives for improving digester mixing include 1) new digester gas mixing system, 2) external draft tube mixers, and 3) external pump mixing. Plant staff has expressed a preference for external pump mixing. Gas mixing systems are known to be highly operationally and maintenance intensive to sustain and are typically less effective than draft tube or external mixing. Draft tube mixers are not suitable for floating cover digesters. Therefore, an external pump mixing system that recalculates the digester contents and distributes the contents through a series of nozzles mounted internally on the digester floor is recommended. Chopper pumps, horizontal screw centrifugal pumps, or recessed impeller pumps, depending on the amount of grit in the digesters, are used for recirculation. This type of system produces extremely effective mixing with both horizontal and vertical mixing of the digester contents.

Digester No. 1 has a fixed cover and the floating cover on Digester No. 2 has completely failed and will require replacement immediately. Digester No. 1 cover should be inspected every three years. For the purpose of this analysis, it is assumed that Digester No. 1 cover will be replaced. There is also some minor concrete damage at the overflow structures that will need repair.



30

Dewatering Alternatives Biosolids conditioning and dewatering (volume reduction) processes produce a dewatered product. The conditioning process must be coordinated with conditioning procedures used with a thickening process if applicable. Stabilization processes, such as the processes discussed earlier, would also be used. The conditioning and dewatering processes generally considered are listed below in Table 4.

TABLE 4 Biosolids Conditioning and Dewatering Process Alternatives Used to Produce a Dewatered Product

Evaluation of Biosolids Management Alternatives, City of Tracy, CA

Biosolids Conditioning Alternatives Biosolids Dewatering Alternatives

Chemical Conditioning (polymer) Belt Filter Press

Thermal Conditioning Centrifuge

Other Processes Recessed Plate Filter Press

• Elutriation Screw Press/Rotary Press

• freeze-thaw Vacuum Belt Filters

• Bacteria Other Processes

• electricity • Drying beds • solvent extraction • tube filters • ultrasonic • Cyclones • screens • electro-osmosis

Biosolids Conditioning Chemical conditioning with polymers is the most commonly used conditioning process for conditioning biosolids prior to mechanical dewatering processes. Chemical conditioning and dewatering can produce a biosolids product dry enough for landfill disposal.

Thermal conditioning combined with dewatering has been used to pretreat the biosolids before thermal oxidation or landfill disposal. Thermal conditioning can produce biosolids with higher solids content than chemical conditioning, without increasing the bulk of the biosolids. It is also effective on difficult-to-dewater biosolids. However, thermal conditioning is more complex, requiring highly skilled operational personnel, and has a higher capital, operation and maintenance cost than chemical conditioning for most types of biosolids. High capital costs generally limit thermal conditioning to large wastewater treatment plants. Also, thermal conditioning produces gas and high-strength liquid sidestreams that require further treatment.

Thermal conditioning releases a substantial portion of the organic nitrogen from the biosolids. The nitrogen is removed in the dewatering sidestream. For incineration, this is beneficial as it lowers the nitrogen oxide emissions from the incineration exhaust gases. For landfill disposal, this is also beneficial, as it reduces the leachate generation rate and the risk



31

of nitrate contamination of the ground water. For fertilizer production, release of nitrogen is not beneficial, as it lowers the nitrogen content and value of the biosolids as a fertilizer.

Elutriation has been used in the past, but is not as commonly found today. Elutriation involves washing the biosolids with water (i.e. plant effluent) to remove alkalinity and fines from the biosolids. Elutriation is generally used in combination with chemical conditioning to reduce chemical requirements. Elutriation is not commonly used because the elutriate that is recycled back to the wastewater treatment system can degrade the final effluent quality.

Natural freeze-thaw conditioning is used in many small facilities in the colder climate areas of North America. This process requires large land areas and is generally limited to populations less than 20,000. Mechanical freeze-thaw systems are at the development stage and have not been proven at a full-scale facility.

Other biosolids conditioning processes, such as those involving bacteria, electricity, solvent extraction and ultrasound, have not been proven to be viable at a full-scale facility.

Chemical conditioning with polymers is the preferred conditioning process in terms of the desirable criteria to utilize the biosolids on land. Polymers do not reduce the nitrogen content of the biosolids or change the characteristics of the final product.

Biosolids Dewatering There are a variety of biosolids dewatering options that may be considered for reducing the volume of biosolids and concentrating the solids. Biosolids dewatering processes are generally used following stabilization/pathogen reduction processes. For Tracy all but one of the alternatives uses anaerobic digestion for the stabilization process. Biosolids dewatering may be used to process raw unstabilized biosolids in some specific cases, such as prior to incineration and composting. The incineration alternative includes both thickening and dewatering of raw sludge prior to incineration.

The selection of the most appropriate dewatering method should be based on a consideration of subsequent handling processes and on the ultimate biosolids utilization/disposal method selected. It is generally desirable to achieve as high dry solids content as possible in the dewatering stage in order to reduce the volume and total mass of biosolids material. Most applications require the dewatered biosolids between 20 and 35-percent dry solids concentrations. Lower solids concentration could be acceptable for agricultural or forested land applications in some specific cases; however, the lower costs generally do not outweigh the benefits of adaptability to other utilization/disposal alternatives.

Based on discussions with City of Tracy staff, the following dewatering alternatives have been selected for evaluation. All of these dewatering alternatives coupled with anaerobic digestion will result in a Class B biosolids under normal operation. Modifications to the drying beds in conjunction with certain testing procedures may result in a Class A biosolids product and will be discussed in subsequent sections. A schematic for each alternative is shown in Exhibits 7 through 9. A solids balance for each alternative is shown in Exhibit 15. Each alternative is described in detail below.



32

• Alternative 7 – Sludge Drying Bed Dewatering

• Alternative 8 - Belt Filter Press Dewatering

• Alternative 9 – Centrifuge Dewatering

Alternative 7 – Sludge Drying Bed Dewatering Sludge drying beds are used in many small treatment facilities in North America. The beds are generally limited to small communities with populations under 20,000 because of the large land requirements and the potential for odor. Climatic conditions such as high precipitation also affect the suitability of the beds.

The advantages of a Sand Drying Bed are:

• Where elaborate lining and leachate control is not necessary and where land is available, capital cost is low for small plants

• Low requirement for operator attention and skill • Low electric power consumption • Low sensitivity to biosolids variability • Low polymer consumption • Moderate to high dry cake solids contents

The disadvantages of a Sand Drying Bed are:

• Lack of rational design approach for sound economic analysis • Large land requirement • Stabilized biosolids requirement • Impact of climatic effects on design • High visibility to general public • Labor-intensive biosolids removal • Permitting and groundwater contamination concerns • High fuel and equipment costs for bed cleaning systems • Real or perceived odor and visual nuisances • Effectiveness is weather dependent

As part of the Phase IB expansion project four of the existing sixteen sand drying beds were removed from service in order to provide room to expand secondary treatment facilities. This significantly reduced the sludge handling capacity of the system. One of the existing drying beds will also be dedicated to filter backwash solids. This was done to avoid mixing the alum sludge solids with the digested solids, which may impact the ability of the drying beds to achieve Class A biosolids. Furthermore, with the underdrain system in poor condition, dewatering capacity has been further reduced.

Alternative 7 consists of dewatering anaerobically digested sludge similar to the current operations. The alternative is shown schematically in Exhibit 7. For this alternative, the following additions and enhancements to the existing drying beds are recommended.

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

LAND

DISPOSAL

BIOSOLIDS

STORAGE

EXISTING

DRYING

BEDS

MODIFIED

OPEN

STORAGE

ONSITE

CLASS B

TO AERATION

BASINS

DEWATERING

FILTRATE

1-3% SOLIDS

EXHIBIT 7

ALTERNATIVE 7 - BIOSOLIDS

DEWATERING SLUDGE DRYING BEDS

NEW

DRYING

BEDS

DEWATERING / DISPOSAL

NEW

DIGESTER

NO.3




34

1. Replace the existing four sand drying beds demolished in the Phase 1B project with four new paved sludge drying beds of the same size to be located either to the north of the existing beds or on adjacent City owned property.

2. Add an additional five new paved drying beds to meet the Phase 4 flow and loads. The drying beds should be the same size as the existing drying beds and located either to the north of the existing beds or on adjacent City owned property.

3. Four of the existing twelve sand drying beds will be paved as part of the Phase 1B Expansion Project. Pave the remaining existing eight sand drying beds. This will require removal of the sludge material, regarding of the drying bed floor, and placement of base coarse material for the asphalt.

4. Incorporate a ramp section in each drying bed to enhance drainage and allow operators to push dried sludge up the ramp and store for further stabilization.

5. Modify the existing drying bed decant system to include weir gates. Place the gates on the east side of drying beds to avoid westerly winds from blowing surface scum and solids into the decant area. Place the gates in the northeast corner of each of the south row of drying beds and in the southeast corner of the north row of drying beds. In these locations the gates will be accessible from the walkway between the north and south sets of drying beds. The operators should be able to operate the gates without entering the drying beds.

6. Install new decant piping at both new and existing drying beds and connect to the new dewatering filtrate drain system constructed as part of the Phase 1B project.

Modifications to drying beds would be completed in several phases. The first phase would include the modifications to the existing drying beds. Paving the existing drying beds and improving the decanting system should substantially increase the capacity of the existing beds. The increase in capacity is difficult to ascertain because drying bed capacity is related to the type of drying bed, the site conditions, and the weather. This phased approach would allow an accurate assessment of the existing capacity prior to investing in new construction.

The second phase would consist of constructing new drying beds on adjacent City property. These drying beds could be constructed to provide more capacity if it is found that the modified beds are not adequate. They could also be constructed to assist in the production of a Class A biosolids. It is recommended that these drying beds be constructed using a “just in time” approach. This approach saves capital by deferring investment and maximizing the use of the existing infrastructure.

This alternative provides the City with the maximum flexibility for both construction and for operation. In addition, this process has historically provided a Class A biosolids product based on testing, for these reasons it is recommended that this alternative be considered further.



35

Alternative 8 – Belt Filter Press Dewatering Belt filter presses (BFPs) are a commonly used type of equipment for dewatering biosolids. Figure shows a diagram of a typical belt filter press and the table shows typical performance of BFPs with different types of biosolids.

Figure 6 – Typical Belt Filter Press

Type of Biosolids

Dry Feed Solids (%)

Loading Per Meter Belt Width

X Dry Polymer (g/kg Dry Solids)

Cake Solids x

x x L/s Kg/h x Typical Range

Raw primary (P) 3-7 1.9-3.2 360-550 1-4 28 26-32

Waste activated Sludge (WAS)

1-4 0.6-2.5 45-180 3-10 15 12-20

P + WAS (50:50) 3-6 1.3-3.2 180-320 2-8 23 20-28

P + WAS (40:60) 3-6 1.3-3.2 180-320 2-10 20 18-25

P + Trickling Filter (TF)

3-6 1.3-3.2 180-320 2-8 25 28-30

Anaerobically digested

P 3-7 1.9-3.2 360-550 2-5 28 24-30

WAS 3-4 0.6-2.5 45-135 4-10 15 12-20

P + WAS 3-6 1.3-3.2 180-320 3-8 22 20-25

Aerobically digested

P + WAS, unthickened

1-3 0.6-3.2 135-225 2-8 16 12-20

P + WAS (50:50),

4-8 0.6-3.2 135-225 2-8 18 12-25



36

thickened

Oxygen activated

WAS 1-3 0.6-2.5 90-180 4-10 18 15-23

a Polymer needs based on high molecular weight polymer (100% strength, dry basis) b Ratio is based on dry solids for the primary and WAS c L/s x 15.85 = gpm d kg/h x 2.205 = lb/hr e g/kg x 2.0 = lb/ton * Biosolids types similar to some of those considered for the WWTP

The advantages of a Belt Filter Press are:

• Relatively low capital cost • Relatively low power consumption • High solids capture with minimum polymer requirements • Continuous feed • Moderate cake solids concentration • Moderate throughout capabilities versus space requirement • Open design provides good visual control capability for process performance

The disadvantages of a Belt Filter Press are:

• Housekeeping – open design does not allow containment during process upsets, • Moderate operator attention requirements; larger installations require continuous

operator attention • Odor potential • Downtime • Sensitive to incoming feed characteristics

Alternative 8 is shown schematically in Exhibit 8. For this alternative primary sludge and waste activated sludge would be anaerobic ally digested and the digested sludge would be pumped to a blend tank for storage and equalization. Biosolids at 2.7 percent solids would be fed to the belt filter presses with new belt filter press feed pumps. The biosolids would be dewatered with belt filter presses to approximately 18-22 percent solids. Two new 2 meter belt filter presses would be required, with one unit being completely redundant. A maximum hydraulic loading of 700 gpm per unit was used for sizing. 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 belt filter presses would also require a new conveyor to collect the dewatered solids and convey them to a new solids load out facility. Alternatively, the solids could be stored at the existing drying beds and further dried to achieve Class A biosolids. This alternative has several major disadvantages including:

1. The new belt filter presses would require a large building to house them, thus driving up cost.



37

2. A high polymer dosage is required for effective dewatering. This is a chemical cost the City does not currently have.

3. The City has previously had negative experience with belt filter presses and plant staff is not comfortable with this technology.

For these reasons the City does not wish to pursue this alternative further.

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

LAND

DISPOSAL

BIOSOLIDS

STORAGE

OPEN

STORAGE

AT DRYING

BEDS

1-3%

SOLIDS

BLEND TANK

DEWATERING DISPOSAL

BELT

FILTER

PRESS

CLASS B

18-22% SOLIDS

POLYMER

EXHIBIT 8


DEWATERING BELT FILTER PRESS


NEW

DIGESTER

NO.3

DEWATERING

FILTRATE TO

AERATION

BASINS



39

Alternative 9 – Centrifuge Dewatering The centrifuge operation involves the application of centrifugal force to a liquid biosolids stream, which accelerates the separation of the liquid and solid fractions. The process involves both clarification of the centrate stream and compaction of the biosolids.

The style of centrifuge equipment typically used to thicken and dewater municipal biosolids is the solid bowl conveyor centrifuge. Figure shows a cut-away view of a solid bowl centrifuge. This centrifuge unit operates with a continuous feed and discharge similar to the belt filter press. The biosolids, which are 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. The bowl is driven by a motor at speeds ranging from 2,000 to 3,000 rpm. This spinning action creates the centrifugal forces required to concentrate the biosolids 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 biosolids towards the conical solids discharge. The centrate water is discharged over the weir plates at the opposite end of the centrifuge.

Figure 7 – Cut-Away View of a Centrifuge

Typical biosolids concentrations for conventional centrifuges are similar to those achieved with a belt filter press. Recently, technical advancements have developed what is referred to in the industry as a “high solids” centrifuge. These machines combined with a squeezing action, and can produce biosolids cakes in the 28 to 40-percent dry solids range for most of the biosolids types. Typical performance data for different biosolids types is included in Table 5.



40

TABLE 5 TYPICAL PERFORMANCE DATA FOR A CONVENTIONAL SOLIDS BOWL CENTRIFUGE

Biosolids Feed Solids Concentration (% solids)

Average Cake Solids Concentration (% solids)1

Dry Polymer Required g/kg Feed Solids (lb/ton)

Recovery Based on Centrate Solids (%)

Raw primary 5-8 25-36 28-36 0.5-2.5 (1-5) 0 90-95 70-90

Anaerobically digested primary2 2-5 9-12 28-35 30-35 3-5 (6-10) 0 98+ 65-80

25-30 0.5-1.5 (1-3) 82-92

Anaerobically digested primary irradiated at 400 krad

2-5 29-35 3-5 (6-10) 95+

Waste activated 0.5-3.0 8-12 5-8 (10-15) 85-90

Anaerobically digested waste activated2

1.3 8-10 1.5-3 (3-6) 90-95

Thermally conditioned: Primary + waste activated Primary + trickling filter 9-14 13-15 7-10

35-40 29-35 35-40 30-35

0 0.5-2 (1-4) 0 1-2 (2-4)

75-85 90-95 60-70 98+

High lime 10-12 30-50 0 90-95

Raw primary + waste activated 4-5 18-25 1.5-3.5 (3-7) 90-95

Anaerobically digested: (primary + waste activated)2

2-4 4-7 15-18 17-21 3.5-5 (7-10) 2-4 (4-8)

90-95 90-95

Anaerobically digested (primary + waste activated + trickling filter

1.5-2.5 18-23 14-16 1-2.5 (2-5) 6-8 (12-

15) 85-90 85-90

Combined sewer overflow treatment sludge

highly variable

1 Assumes skimming of cake 2 Biosolids types similar to some of those seen at facilities in the Eastern Area



41

The advantages of a centrifuge are:

• Contained process minimizes housekeeping and odor considerations • Continuous operation provides flexible control capability for process performance • Moderate or high cake solids concentration • Relatively small area requirements • Moderate to high throughput capabilities versus space requirements • Low operator attention requirements • High solids capture

The disadvantages are:

• Relatively high capital cost • Relatively high power requirements • Moderate to high polymer requirements • High operating speeds

Alternative 9 is shown schematically in Exhibit 9. For this alternative primary sludge and waste activated sludge would be anaerobic ally digested and the digested sludge would be pumped to a blend tank for storage and equalization. Biosolids at 2.7 percent solids would be fed to the centrifuges with new centrifuge feed pumps. The biosolids would be dewatered with centrifuges to approximately 28-32 percent solids. Three new centrifuges would be required, with one unit being completely redundant. A maximum hydraulic loading of 125 gpm per unit was used for sizing. 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 new conveyor to collect the dewatered solids and convey them to a new solids load out facility. Alternatively, the solids could be stored at the existing drying beds and further dried to achieve Class A biosolids. This alternative has several major advantages over belt filter presses including:

1. The centrifuges would require a smaller building footprint than belt filter presses, thus having lower building costs and HVAC requirements.

2. Centrifuges produce significantly higher solids concentrations than belt filter presses, reducing solids handling and disposal costs and require smaller load out facilities.

3. Centrifuges have lower odor emissions than belt filter presses.

4. Centrifuges have low maintenance requirements than belt filter presses. This is because the centrifuges operate continuously not requiring frequent shut downs and cleanings.

Due to the reasons provided above, Plant staff has identified Centrifuge dewatering as their preferred mechanical dewatering approach and would like to consider this technology for further consideration.

LAND

DISPOSAL

BIOSOLIDS

STORAGE

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

OPEN

STORAGE

AT DRYING

BEDS

1-3%

SOLIDS

BLEND TANK

DEWATERING DISPOSAL

CLASS B

28-32% SOLIDS

POLYMER

CENTRIFUGE

DEWATERING

NEW

DIGESTER

NO.3


DEWATERING

FILTRATE TO

AERATION

BASINS

EXHIBIT 9


DEWATERING



43

Class A Biosolids During the February 7, 2006 meeting, discussions were held with plant staff regarding biosolids management alternatives. In that meeting City staff noted meeting alternatives such as composting, stabilization, and incineration. It was decided that the City did not have any interest in pursuing incineration or lime stabilization due to air quality permitting, worker safety concerns, and public perceptions issues. The City did not have any interest in composting options because they are essentially windrow composting in the sludge drying beds. The City did not have any interest in thermal-heat drying because the existing drying beds provide a cost savings alternative to drying the biosolids.

A general industry trend involves a move towards the production of Class A biosolids which, coupled with low metals concentrations allows a publicly owned treatment works plant (POTW) to produce an exceptional quality (EQ) product with a reduced regulatory/monitoring burden. EQ biosolids typically have improved public acceptance over non-EQ and Class B products. EPA notes that no significant public controversies have arisen around programs that manage biosolids as Class A or EQ products. Some of the most common reasons in support of Class A biosolids include:

• Improved public perception, particularly if EQ status is achieved. • Increased number and types of application sites available for Class A products (i.e.,

biosolids could be more readily used on rangeland, turf farms, and in the nursery industry).

• Reduced site management burden for Class A products. • Concern over increased future regulatory burden for Class B biosolids.

In some U.S. locations, such as the Central Valley of California, counties or other local jurisdictions have implemented land use or public health regulations intended to control or discourage the import of biosolids from other areas. Regulations have involved fees imposed on biosolids land application sites, increased treatment requirements beyond the EPA 503 Regulations, and site management restrictions.

Class A Biosolids TABLE 6 Class A Biosolids Evaluation of Biosolids Management Alternatives

Treatment Biosolids Criteria

Thermal Treatment Processes When the biosolids are used or packaged, they must meet:

• Fecal Coliform density < 1000 MPN/gram of total dry solids, OR

• Salmonella bacteria density <3 MPN/4 grams of total dry solids

Biosolids must be held at the following time/temperature relationship



44

TABLE 6 Class A Biosolids Evaluation of Biosolids Management Alternatives


• Biosolids with ≥7% Total Solids

Biosolids must be ≥50°C for ≥20 minutes1

Biosolids must be ≥50°C for ≥15 seconds2

Time-Temperature Relationship: D = 131,700,000/(100.14t)

• Biosolids with <7% Total Solids:

Biosolids heated: 15 seconds to 30 minutes

Time-Temperature3 Relationship: D = 131,700,000/(100.14t)

• Biosolids with ≥7% Total Solids

Biosolids must be ≥50°C for ≥30 minutes

Time-Temperature Relationship3: D = 50,070,000/(100.14t)

Alkaline Stabilization Process

(high pH-high temperature processes)

When the biosolids are used or packaged, they must meet:

• Fecal Coliform density ,1000 MPN/gram of total dry solids, OR


Solids must be held at or above a pH of 12 for 72 hours

• Temperature must be >52°C for 12 hours. Following this period, the solids must be air-dried to achieve a total solids concentration of >50%.

Other Known Treatment Processes When the biosolids are used or packaged, they must meet:



Solids must meet, at a minimum

• Enteric virus density: <1 PFU4/4 grams of



45



total dry solids

• Viable helminthes ova density: <1/4 grams of total dry solids

• Monitoring the operating parameters until process demonstrates adequate reduction of pathogens.

New or Innovative Processes When the biosolids are used or packaged, they must meet:



Solids must meet, at a minimum

• Enteric virus density: <1 PFU4/4 grams of total dry solids

• Viable helminthes ova density: <1/4 grams of total dry solids

• Testing of each batch of the biosolids that are used or disposed.

Process to Further Reduce Pathogens (PFRP) When the biosolids are used or packaged, they must meet:



Must operate according to the criteria of one of the defined Processes to Further Reduce Pathogens.

Process Equivalent to a PFRP When the biosolids are used or packaged, they must meet:



Solids must be treated in an equivalent process to one



46


Treatment Biosolids Criteria of the defined PFRP methods.

1 Sludge not heated by warm gases or immiscible liquid. 2 Sludge heated by warm gases or immiscible liquid. 3 D = time in days; t = temperature in degrees Celsius. 4 PFU = Plaque Forming Unit

As mentioned, the use of a PFRP method can achieve a Class A biosolids production. There are seven PFRPs, namely composting, heat drying, heat treatment, thermophilic aerobic digestion, beta ray irradiation, gamma ray irradiation, and pasteurization.

Vector Attraction Reduction Requirements Biosolids that contain high levels of volatile solids are subject to uncontrolled decomposition, and attraction of disease carrying vectors (e.g., insects, rodents, birds, etc.) which may transmit pathogens back to the human population. For this reason, land application of biosolids must also satisfy vector attraction reduction (VAR) requirements. VAR requirements vary based on treatment processes. For unrestricted use of Class A biosolids, requirements for any one of the first five processes in the following table must be satisfied.

TABLE 7 Vector Attraction Reduction Requirements Evaluation of Biosolids Management Alternatives

Process Criteria1

Anaerobic Digestion 1. Volatile solids must be reduced by a minimum of 38%, OR

2. Perform bench-scale tests demonstrating that if after anaerobic digestion of the biosolids for an additional 40 days at a temperature between 30° and 37°C, the volatile solids in the biosolids are reduced by <17% from the beginning to the end of the bench test.

Aerobic Digestion 3. Volatile solids must be reduced by a minimum of 38%, OR

4. Aerobically digested biosolids with ≤2% solids are considered to have achieved VAR if, in the laboratory, after 30 days of aerobic digestion in a batch test at 20°C, volatile solids are reduced by



47

TABLE 7 Vector Attraction Reduction Requirements Evaluation of Biosolids Management Alternatives

Process Criteria1

<15%2

Composting 5. A minimum retention time of 14 days at 40°C is required for aerobic processes. The average solids temperature must exceed 45°C.

Alkaline Stabilization 6. Sufficient alkali must be added to raise the pH to 12 or higher for a 2-hour period. For an additional 22 hours without further alkali addition, the solids must remain at pH 11.5 or higher.

Thermal Drying 7. The-percent solids of sludge not containing unstabilized primary treatment solids shall be a minimum of 75% based on the moisture content and total solids prior to mixing with other materials.

8. The-percent solids of sludge containing unstabilized primary treatment solids shall be a minimum of 90$ based on the moisture content and total solids prior to mixing with other materials.

Subsurface Injection 9. Within one hour of subsurface biosolids injection no significant amount of biosolids should remain on the surface. For Class A biosolids, injection must occur within 8 hours after discharge from the pathogen treatment process.

Incorporation 10. Surface applied biosolids must be incorporated within 6 hours after land application. For Class A biosolids, application must occur within 8 hours after discharge from the pathogen treatment process.

1 The numbers corresponds to the criteria requirements identified in the 503 regulations. 2 This test only applicable to liquid aerobically digested biosolids.



48

Class A Biosolids Alternatives As discussed previously the City would like to move towards a more sustainable biosolids approach producing a Class A biosolids product if the economics make sense. To accomplish this objective, three alternatives are being considered as shown below. A schematic for each alternative is shown in Exhibits 10 through 12. A solids balance for each alternative is shown in Exhibit 16. Each alternative is described in detail below. Each alternative is discussed in more detail in subsequent sections.

• Alternative 10 – Sludge Drying Beds - Class A Biosolids

• Alternative 11 – Centrifuge Dewatering and Lime Stabilization - Class A Biosolids

• Alternative 12 – Centrifuge Dewatering and Heat Drying - Class A Biosolids

Alternative 10 would utilize existing and new infrastructure to achieve Class A biosolids. Alternatives 11 and 12 would require mechanical dewatering to achieve Class A biosolids. As discussed in the previous sections the only desirable mechanical dewatering alternative identified was Centrifuge dewatering. Therefore, for purposes of discussion and facility sizing, Centrifuge dewatering was assumed for both of these alternatives.

Alternative 10 – Sludge Drying Beds – Class A Biosolids Sludge drying beds are currently used at the Tracy WWTP. Through testing the City has shown that the existing sludge drying beds can produce Class A biosolids. Reusing the drying beds and confirming that the biosolids produced continue to meet Class A standards through testing would be the most cost effective Class A biosolids alternative. Longer detention times in the drying beds to achieve Class A biosolids would require additional drying beds over simple dewatering; however, this Alternative would require very little capital expenditure over and above that previously identified. In addition, the plant staff is familiar this mode of operation. No additional drying bed improvements over those previously discussed for dewatering would be required.

Alternative 10 is shown schematically in Exhibit 10. For this alternative primary sludge and waste activated sludge would be anaerobic ally digested and the digested sludge would be pumped to a blend tank for storage and equalization. Biosolids at 2.5 percent solids would be batch fed to new and existing drying beds with the existing feed pumps. The biosolids would be dewatered in the drying beds to approximately 28-30 percent solids. A minimum of six new drying beds would be anticipated to achieve Class A sludge.

Alternative 11 – Thermal Drying - Class A Biosolids Thermal drying processes are generally used to produce a dried product. Dewatering processes, such as Centrifuge dewatering discussed earlier, would also be used. Stabilization processes may be used, but are not necessary. Figure shows a flow schematic of a direct heat drying system. The drying processes considered are listed below in Table 8.

Methane gas produced by the anaerobic digestion process could be used to replace some or all of the natural gas required in the thermal drying process.

CLASS A

BIOSOLIDS

STORAGE

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

1-3% SOLIDSEXISTING

DRYING

BEDS

MODIFIED

OPEN

STORAGE

ONSITE

TO AERATION

BASINS

DEWATERING

FILTRATE

LAND

DISPOSAL

OR

COMMERCIAL

USE


NEW

DRYING

BEDS

NEW

DIGESTER

NO.3

EXHIBIT 10

ALTERNATIVE 10 - CLASS A BIOSOLIDS

SLUDGE DRYING BEDS

DEWATERING / DISPOSAL



50

Figure 8 – Flow Schematic of a Direct Heat Drying System

TABLE 8 THERMAL DRYING PROCESS ALTERNATIVES USED TO PRODUCE A DRIED PRODUCT Thermal Drying Process Alternatives

x Direct (convective) Dryers

__ Rotary Dryers Flash Dryers

x Indirect (conductive) Dryers

x Other Processes

____ _

Radiant Dielectric Microwave Carver Greenfield Process (multi-effect evaporation) Solvent extraction

Direct (Convective) Dryers Direct, convective-heat dryers include rotary dryers and spray or flash dryers. Rotary dryers are the most common. Rotary dryers are used at several facilities in the U.S., including Cobb County, GA; Boston (Deer Island), MA; New York, NY; Hagerstown, MD; Largo, FL; and Tampa, FL.



51

Manufacturers of drying systems have developed methods of improving the thermal efficiencies of drying systems. For example, Swiss Combi Inc. continuously recycles the sweep gases used to evaporate the water. The sweep gases are indirectly superheated, passed through the dryer, and then cooled to condense water to allow the gases to be recycled.

The principal advantages of direct dryers are:

• High heat transfer rates due to direct contact of the drying medium with the biosolids, thereby decreasing the residence time of the biosolids within the dryer

• Flexibility of temperature control achievable by varying the flow and/or temperature of the hot gas over the biosolids

The principal disadvantages of direct dryers are:

• Potential for combustion and explosions of the biosolids material in the dryer • Thermal inefficiency due to high sensible heat loss in the stack gases • The large volume of off-gas requiring treatment for dust entrainment and odors

The disadvantages can be overcome by recycling a portion of the exhaust air, condensing and scrubbing the exhaust air, and then burning the non-condensable off-gases after scrubbing.

Flash dryers are used at two facilities in Houston, TX. The operation and maintenance of flash drying facilities is relatively complex. Also, dust from the process is extremely abrasive and can create explosive conditions.

Indirect (Conductive) Dryers Several different types of indirect dryers are available, including heated agitation equipment, such as the hollow disk, paddle and helical screw dryers, and also drum type dryers with jacketed walls for the heat medium, as well as vertical tray dryers.

Hollow disk dryers have been successfully used in a full-scale biosolids drying demonstration study for Seattle (Metro), WA. A full-scale facility in Buffalo, New York, was used to thermally dry sludge solids that had been dewatered to 16 to 18-percent on a BFP, to 35-percent prior to incineration, in order to provide an autogenous feed for the incineration process. A full-scale facility was operated at the Hyperion WWTP in Los Angeles, CA for several years. YWC Inc.’s thin film and paddle-type indirect dryers have been used for drying biosolids in Europe.

Vertical tray dryers have been used in Baltimore and Toronto. They are arranged in a vertical insulated vessel with several trays forming separate drying compartments. There is a vertical rotating shaft with arms at each compartment. Hot oil is circulated through each tray, biosolids are fed into the top compartment and the material travels horizontally due to the action of the rotating arms and drops to the compartment below. Dry material is discharged from the bottom.



52

There are several advantages to indirect drying:

• Minimal volumes of off-gas are produced when compared to direct drying; a relatively low flow rate of purge gas (if any) is required to discharge the vapor resulting from the evaporated liquid

• Dust entrainment in the exhaust air is minimized when compared to direct dryers because the heating medium does not contact the biosolids

• The atmosphere inside the dryer is inherently inert, minimizing the potential explosive and fire hazards

• A higher thermal efficiency can be achieved • A variety of thermal media can be used including gas, oil, and steam.

The disadvantages of indirect drying are:

• Higher costs for providing a thermal source such as steam, hot water or hot oil (if such a source is not readily available)

• Heat transfer surfaces could become fouled if not cleaned regularly • Indirect drying produces a dusty product with relatively fine particles compared to a

direct dried product

Other Drying Processes Other drying processes, such as radiant drying, dielectric drying, and microwave drying, have high capital costs and have not been successfully used for municipal sewage biosolids drying.

Solvent extraction was evaluated by the cities of Seattle, WA and Los Angeles, CA where it was determined that the process was not cost effective. No full-scale facilities using the process have been constructed.

The Carver-Greenfield process has been used primarily in the food and agricultural industries. The process is used to dry biosolids at two facilities in Japan and was installed but never worked properly at the Los Angeles, CA Hyperion plant. The system at the Hyperion plant was plagued by operating problems and never reached its design capacity, prior to shutdown.

Alternative 11 is shown schematically in Exhibit 11. For this alternative primary sludge and waste activated sludge would be anaerobic ally digested and the digested sludge at 2.5 percent solids would be pumped to Centrifuges for dewatering. Dewatering would provide biosolids at 28-32 percent solids to a thermal indirect heat dryer. The solids would then be dried to 90% solids meeting the vector attraction reduction requirements. The dried solids would typically be stored in load out towers with nitrogen gas to prevent combustion of the stored material. The dried material could be spread on land or sold commercially as fertilizer.

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

BLEND TANK

DEWATERING DISPOSAL

CLASS ALAND

DISPOSAL

POLYMER

BIOSOLIDS

STORAGE

CLASS B

28-32%

SOLIDS

90%

SOLIDS

BAGGING/

DISPOSAL

CENTRIFUGE

DEWATERING

NEW

DIGESTER

NO.3


1-3%

SOLIDS

DEWATERING

FILTRATE TO

AERATION

BASINS

INDIRECT

HEAT

DRYING

TOWER

STORAGE

WITH NITROGEN

EXHIBIT 11


CENTRIFUGE DEWATERING

AND HEAT DRYING



54

Although thermal drying is quickly becoming more popular, this technology is not recommended for the Tracy WWTF. None of the existing plant infrastructure could be utilized to achieve the Class A product and the capital expenditure cost would be extremely high.

Alternative 12 – Lime Stabilization - Class A BiosolidsAlkaline stabilization of biosolids has been a practical stabilization method for many years. The basic approach is to elevate the pH of the biosolids by the addition of one of the several materials containing lime, either as calcium oxide (CaO - quicklime) or calcium hydroxide (Ca[OH]2 - hydrated lime).

Essentially, any alkaline material with sufficient alkalinity can be used. Certain methods of using cement kiln dust (CKD) for alkaline stabilization are, however, covered by patents. No known patents apply to the use of other alkaline sources, such as fly ash, quicklime and hydrated lime.

Alkaline stabilization can be used to stabilize a liquid (pre-dewatering or pre-lime) or dewatered sludge (post-dewatering or post-lime). Figure shows a schematic of a post-dewatering alkaline stabilization system. In most cases, the alkaline material is used following dewatering. Lime requirements vary from 10 to 50-percent of the sludge dry solids weight, depending on several factors as discussed below. Adding calcium oxide (CaO) or quicklime generates high pH values. It also generates high temperatures exceeding 55qC (131qF), when added to dewatered biosolids, which destroys pathogens. Addition of lime to biosolids and the high temperatures also volatilizes ammonia, amines and other odorous compounds. Care must be taken in the design of these systems to prevent odors.

The advantages of the alkaline stabilization process are:

• Simplicity of operation • Organic nitrogen content of biosolids is not significantly reduced • High pH reduces pathogens and the odor potential of the final biosolids product; high

temperatures generated when quicklime is used in post-dewatering stabilization also destroys additional pathogens

The disadvantages of the alkaline stabilization process are:

• High operating cost due to chemical consumption • Difficult to handle chemicals • Volatile solids are not oxidized; therefore, there is a risk of odors redeveloping • Ammonia generation during processing requires odor control • Biosolids products with a high pH may have restricted uses • The dry mass and volume of the biosolids may be increased considerably



55

Figure 9 – Alkaline Stabilization Schematic

Post Dewatering (Post-Lime) Lime Stabilization The selection of a suitable alkaline product dosage to achieve and maintain an elevated pH depends on a variety of factors, including:

• Chemical characteristics of the material used as the alkaline source • Chemical characteristics of the biosolids, including both organic and inorganic

constituents • Physical characteristics of the biosolids, including moisture content and viscosity • Adequacy and speed of mixing the biosolids and the alkaline material • Length of time high pH is to be maintained

The involvement of these variables has led to a stabilization process where no rational method has been developed that predicts the alkaline dose required to meet a given treatment objective.

Process Objectives Alkaline stabilization depends on maintaining the pH at a high enough level for a sufficient period of time to inactivate the microorganism population of the biosolids and prevent odors from re-developing. Experience has shown that stabilization objectives are met by maintaining a pH of 12 or more for at least 2 hours. To meet these criteria, previous studies have found the pH should be raised to 12.5 and maintained for at least 30 minutes, as the pH typically decreases slowly during and after the stabilization process. Stabilization by this process halts or substantially retards the microbial reactions that can otherwise lead to odor production and vector attraction. The process can also inactivate viruses, bacteria and other microorganisms that are present.



56

Process Variations Several variations in the design and operation of alkaline stabilization processes are available. Alkaline stabilization can be used to stabilize raw solids or further stabilize digested biosolids. Both proprietary and non-proprietary processes are available. The most common post-dewatering alkaline stabilization processes currently in use are summarized below. The advantages and disadvantages of each process are summarized in Table 9.

TABLE 9 SUMMARY OF POST-DEWATERING ALKALINE STABILIZATION PROCESSES Process Advantages Disadvantages

Conventional Lime Stabilization x Low capital cost x Class b biosolids

x Flexible operation x Potential pH instability

x Product handling concerns

Modified Lime Stabilization x Flexible operation x High chemical requirements

x Class A Biosolids x High operating costs

Advanced Alkaline stabilization (N-Viro) x Class A Biosolids x High chemical

requirements

x Stable pH/high CCE x High operating costs

x Good produce handling characteristics x Large increase in

solids mass

x Revenue from sale as liming agent x Two chemicals added

En-Vessel Pasteurization (RDP) x Class A Biosolids x Potential pH instability

x Low operating costs x Product handling concerns

x Low chemical requirements

x Flexible operation

Biofix (Bio Gro) x Flexible operation x High chemical requirements

x Class A or B biosolids x High operating costs

Bioset Process x Low area requirements x High capital costs

x Class A biosolids x Two chemicals added



57

x Good odor control x

Potential pH instability

Conventional Lime Stabilization (US EPA Class B) Conventional lime stabilization consists of mixing quicklime or hydrated lime with raw (or partially digested) dewatered solids to achieve a pH of 12 or greater for a minimum of two hours. This conventional process is classified as a PSRP (Class B) process by the EPA. The system generally includes a lime storage silo(s), a mixer, a sludge storage bin(s), metering screws and conveyors.

Modified Lime Stabilization (US EPA Class A) Modified lime stabilization is similar to conventional lime stabilization except additional quicklime is added to raise the temperature of the biosolids to greater than 70°C. An insulated reactor may be provided to maintain a minimum temperature of 70°C for 30 minutes. The process is classified as a PFRP (Class A) process by the EPA.

Advanced Alkaline Stabilization (N-Viro International Corp.) The Advanced Alkaline Stabilization with Subsequent Accelerated Drying (AASSAD) process is a variation of lime stabilization and drying processes. The process involves mixing cement kiln dust (CKD) and quicklime with raw (or digested) dewatered solids. Sufficient calcium oxide (in the form of CKD or quicklime) is added to raise the temperatures to a range of 52-62°C. The mixture is then air dried in windrows or thermally dried in one-pass rotary dryers. “Heat curing”, by maintaining temperatures at a minimum of 52°C for 12 hours, is provided after mixing when air drying is used, or after thermal drying when thermal drying is used.

CKD, a by-product of the cement manufacturing industry, may be used as a partial substitute for quicklime. CKD can be relatively inexpensive source of alkaline material as cement manufacturing plants generally dispose of this material in a landfill.

Like other lime stabilization processes, ammonia is generated by the alkaline addition. The use of thermal drying allows easier containment and collection of odors when drying is required. N-Viro currently has about 30 installations worldwide. The largest is at Middlesex, NJ. The process, patented by N-Viro International Corporation (Toledo, OH), is classified by the EPA as an PFRP (Class A) process.

En-Vessel Pasteurization (RDP Technologies Inc.) En-Vessel Pasteurization is a variation of the modified lime stabilization process. Quicklime is mixed with dewatered solids to raise the pH to greater than 12. Supplemental heat is added to the biosolids in the blender by electrical heating elements, rather than quicklime, to raise the temperature to about 72°C. The mixture is conveyed to an insulated pasteurization reactor, which maintains the solids at a minimum temperature of 70°C for 30 minutes.



58

The process, patented by RDP Technologies Inc. (Norristown, PA), produces a Class A biosolids by meeting the PFRP pasteurization criteria.

Biofix (Bio-Gro Division, Wheelabrator Water Technologies Inc.) Biofix is similar to the modified lime stabilization process. Quicklime is mixed with dewatered solids in a blender. The quicklime dosage can be varied to produce either a Class A or Class B product.

The Biofix process is marketed by the Bio Gro Division of Wheelabrator Water Technologies Inc.

Bioset Process The Bioset process is similar to RDP’s En-vessel Pasteurization process. Quicklime is mixed with dewatered solids to raise the pH to greater than 12. Sulfamic acid, quicklime and sludge are blended together. An acid-base chemical reaction raises the temperature to 150 to 200°F (65 to 93°C). The mixture is conveyed by a modified progressive cavity pump through a long pipe referred to as the “pressure zone”. Pressures of 15 to 20 psig are achieved in the pressure zone.

Bioset, Inc. has one full-scale installation in Houston, TX. The process, patented by Bioset, Inc. (Houston, TX), produces a Class A biosolids by meeting the PFRP pasteurization Alternative 12 is shown schematically in Exhibit 12. For this alternative primary sludge and waste activated sludge would be anaerobic ally digested and the digested sludge at 2.5 percent solids would be pumped to Centrifuges for dewatering. Dewatering would provide biosolids at 28-32 percent solids. An En-Vessel Pasteurization process would add heat and lime to the dewatered biosolids producing a class A product. The process must heat the biosolids to 169 degrees F for 30 minutes and adequate lime must be added to raise the pH to 12 for 2 hours and must maintain a pH of 11.5 for an additional 22 hours. The product would then be stored in the existing sludge drying bed area prior to disposal.

This particular lime stabilization processes have high chemical costs. Typically lime dosage is approximately 30%-40% on a dry weight basis. In addition, the RDP process also has a relatively high operational cost because it must heat the sludge to 169 degrees F. Identifying a market for the final product may also prove to be very difficult in the area as its uses are may be restricted due to its high pH. In addition, none of the existing plant infrastructure could be utilized to achieve the Class A product and the capital expenditure cost would be extremely high. For these reasons this technology is not recommended for further consideration.

BIOSOLIDS

STORAGE

EXISTING

DIGESTER

NO.1

EXISTING

DIGESTER

NO.2

LAND

DISPOSAL

OR

COMMERCIAL

USE

OPEN

STORAGE

AT DRYING

BEDS

1-3%

SOLIDS

BLEND TANK

DEWATERING DISPOSAL

CLASS A

POLYMER

LIME

STABILIZATION

(RDP)

TIME AND

TEMPERATURE

CENTRIFUGE

DEWATERING35-40%

SOLIDS


NEW

DIGESTER

NO.3

EXHIBIT 12


CENTRIFUGE DEWATERING

AND LIME STABILIZATION

28-32%

SOLIDSDEWATERING

FILTRATE TO

AERATION

BASINS



60

Alternative 13 – Co-Thickening with Incineration Incineration evaporates water and burns all the organic substances present in the wastewater solids. Combustion reactions are exothermic and release large amounts of energy as heat. The initial moisture content of the solids and the organic content determine whether additional fuel is required to support combustion. With sufficient dewatering, combustion usually is self-sustaining, except for the initial warm-up and for heat control.

Wastewater solids incineration has been practiced since the early part of the 20th century. Availability of cheap energy, limited capability of wastewater solids dewatering equipment, and minimal or nonexistent air pollution requirements all led to the selection of incineration as a practical and inexpensive method of wastewater solids disposal. However, increasing concern for air quality and experience with solids from more advanced treatment processes, which are more difficult to dewater and require more energy to evaporate the excess water, considerably dampened the enthusiasm for incineration. These problems, coupled with rising energy costs, increasing quantities of wastewater solids, and limited resources, have led to the development of improved dewatering methods and more energy efficient incineration equipment and systems.

Incineration offers significant advantages over other final use options; it reduces the volume of solids to a compact residue consisting of about 5 to 15-percent of the original volume, and it eliminates some potential environmental problems by destroying pathogens and degrading many toxic organic chemicals. Metals are not degraded but are concentrated in the ash and in particulate matter entrained in the exhaust gases generated by the process. Pollution control devices are needed to prevent degradation of air quality. Heat energy can be captured and reused.

Over 80-percent of the identified operating municipal wastewater solids incinerators are of the MHF design (). The MHF was originally developed for mineral ore roasting nearly a century ago. The air-cooled variation has been used to incinerate wastewater solids since the 1930s. About 15-percent of the identified operating municipal wastewater solids incinerators are FBIs. The FBI () was developed for catalyst recovery in oil refining by the Standard Oil Development Company in the early 1900s. The first FBI used for incineration of municipal wastewater solids was installed in 1962.



61

Figure 10 – Typical Schematic Showing MHF and Auxiliary Equipment

Figure 11 – Typical Schematic Showing Hot Windbox FBI and Auxiliary Equipment

Multiple Hearth Furnace The MHF is a vertical, cylindrical, refractory-lined steel shell containing a series of horizontal refractory hearths, one above the other. A central shaft, hollow to allow the passage of cooling air through it, runs the height of the furnace and rotates within the furnace at roughly one revolution per minute carrying the rabble arms above each hearth with it. There are two or four rabble arms per hearth. Each arm contains rabble teeth or plows that rake the dewatered cake spirally across the hearth as the arm rotates above each hearth. Dewatered cake typically is fed at the periphery of the top hearth and raked toward the center, where they drop to the hearth below. On the second hearth, the solids are raked outward, to holes in the periphery. The alternating drop hole locations on each hearth and the countercurrent flow of rising exhaust gases and descending solids provide contact between the hot combustion gases and the solids feed, to ensure complete combustion.

The MHF can be divided into four zones during incineration (). The first zone, which consists of the upper hearths, is the drying zone, where most of the water is evaporated. The



62

second zone, generally consisting of the central hearths, is the combustion zone, where temperatures reach 1,400qF to 1,700qF. The third zone is the fixed carbon burning zone, which oxidizes the carbon to carbon dioxide. The fourth is the cooling zone. Excess air of 100 to 125-percent must be provided to ensure adequate burnout of solids cake.

Some 20-percent of the ash is airborne, and gas cleaning equipment must be provided for its capture. Occasional odor problems may require installation of afterburning equipment. The ash is collected in a dry form.

Figure 12 – Typical MHF Cross Section

Fluidized Bed Incinerator The FBI is a steel, refractory-lined chamber with a bed supporting plate separating the windbox from the reactor and supporting a bed of sand (). Air is heated and injected into the sand bed. At a temperature of 600qF the air will generate sufficient turbulence within the sand bed to produce fluidization; that is, the sand will appear to act as a fluid. The bed is maintained at 1,500qF for incineration. Wastewater solids are injected within the reactor and as soon as they contact the sand/air bed, water flashes from the solids particles and oxidation begins. Ash residual from combustion is airborne and exits from the reactor in the flue gas. It is removed from the gas stream by high-energy scrubbing equipment downstream on the incinerator exit. The ash is collected as wet ash slurry that must be dewatered. The exit gas temperature is normally in the range of 1,400qF to 1,600qF.



63

Figure 13 – Typical FBI Cross Section

In the typical FBI, the fluidizing air passes through a heat exchanger prior to injection into the combustion chamber. This arrangement is known as a hot windbox. The air preheater at the incinerator exit will heat the fluidizing air to a maximum of 1,000qF while reducing the exiting flue gas temperature by 500qF to 700qF.

The FBI is a simple piece of equipment with no moving parts. Furthermore, only one major item of air moving equipment is normally required: the fluidizing air blower (or forced draft fan). The large amount of sand within the incinerator is an effective heat sink. The incinerator can be shut down with minimal heat loss—the sand will retain enough heat to allow startup after a weekend shutdown with the need for only 1 or 2 hours of preheating. Because of the intimate mixing of air and solids in the fluid sand bed, excess air requirements are low (usually around 40-percent). Supplementary fuel is required for startup, reheat, and, depending on the properties of the solids, for incineration.

There are advantages and disadvantage for each type of incineration furnace. Some of the common advantages include:

• Maximum solids reduction • Possible energy recovery • Pathogens eliminated • Stable, odorless ash



64

The disadvantages include:

• High operation and maintenance costs • High capital costs • Ash may be hazardous due to metal leachability • Air pollution control requirements can be prohibitive • Maximum possible dewatering is essential to reduce cost of evaporating excess water • Negative public perception A key element of any incineration device is permitting. Both the Clean Air Act Amendments and the biosolids regulations impact incineration and govern its performance and use.

This is the only alternative considered that does not utilize digestion for stabilization. That is because it would not make sense to stabilize the raw sludge and remove its high BTU value prior to incineration, which is of value in the incineration process. Alternative 13 is shown schematically in Exhibit 13. Similar to Alternative 5 for thickening primary and secondary sludge are pumped to a blend tank where they are mixed. Both primary and secondary sludge are then pumped to a gravity belt thickener and thickened to approximately 6.0 percent solids. It is recommended that incinerators operate 24 hours per day for most efficient operation, so this would require two new 1.0 meter gravity belt thickeners, with one unit being completely redundant. 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.

Thickened primary sludge and TWAS would then be further dewatered using centrifuges to obtain as dry a product as possible. Centrifuges would be sized to provide approximately 27 percent solids. These solids would then be fed to the incinerator on a 24 hour operation. The ash removed from the incinerators would be disposed of in a landfill provided the ash does not contain excessive metals. Although solids stabilization and disposal costs would be lower, the capital cost to construct this facility would be excessive. Several other major disadvantages include:

1. With this alternative no existing thickening, stabilization, or dewatering infrastructure would be utilized. This would not fare well with public perception.

2. It would be difficult if not impossible for the City to obtain an air quality permit for the incinerator discharge.

3. Operation of this facility would be complex, require significant training, and require 24-hour staffing.

4. The polymer usage would be extremely high for both thickening and dewatering operations in an effort to remove as much water as possible and avoid wasting energy to evaporate excess water.


PRIMARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.2

SECONDARY

CLARIFIER

NO.1

SECONDARY

CLARIFIER

NO.3

PRIMARY

SLUDGE

PUMP

STATIONPRIMARY

CLARIFIER NO.3

PRIMARY

CLARIFIER NO.4

WAS

PUMP

STATION

WAS

PUMP

STATION

SECONDARY

SCUM

21

5 6 7

3 4

8

TERTIARY

FILTERS

FILTER

BACKWASH

SOLIDS PUMP

STATION

FILTER

BACKWASH

EQUALIZATION

BASINTO

LANDFILL

GRAVITY

BELT

THICKENER

BLENDING

TANK

CENTRIFUGE

DEWATERING

(BELT PRESSES)

INCINERATION ASH

THICKENED

PRIMARY

SCUM

TWAS

PUMPS

EXHIBIT 13

ALTERNATIVE 13

CO-THICKENING AND INCINERATION


3.5% SOLIDS

.08%

SOLIDS

1% SOLIDS



66

Alternatives Selected for Further Consideration Upon critical review of the thirteen alternatives, the following alternatives were selected for further consideration:

Thickening Alternatives: • Alternative 2 - WAS Thickening with GBT – WAS at the Tracy WWTP would be

thickened with gravity belt thickeners. For this alternative the existing DAFT units would be replaced with two 1.5-meter GBTs. Thickened solids to the digesters would be increased to approximately 6-percent solids. A new GBT building, polymer feed facility, and TWAS pumps would be provided.

• Alternative 3 – WAS Thickening with Centrifuge – WAS at the Tracy WWTP would be thickened with centrifuges. For this alternative the existing DAFT units would be replaced with three 125 gpm centrifuges. Thickened solids to the digesters would be increased to approximately 6 percent solids. A new Centrifuge building, polymer feed facility, and TWAS pumps would be provided.

Biosolids Stabilization Alternatives: • Alternatives 1 through 12 – Anaerobic Digestion – The use of anaerobic digestion is

recommended for continued stabilization of primary and waste activated sludge. Recommended improvements to the existing digester complex includes new external mixing facilities, new aluminum floating covers, some concrete repair, and new gas conditioning system. This would allow the City to burn digester gas in there new boiler and save operational cost. A new digester is required for Phase 4 flow and loads and to provide process reliability.

Biosolids Dewatering Alternatives: • Alternative 7 - Sludge Drying Beds – Anaerobic ally digested sludge at the Tracy

WWTP would be dewatered in sludge drying beds. The existing 8 drying beds that were not modified in the Phase 1B project would be improved with paving. All 12 existing drying beds would be modified with improved decanting gates. Four new drying beds would be constructed to replace the four beds that were demolished for the Phase 1 and Phase 1B construction project. Five additional new drying beds would be added to accommodate the Phase 4 flow and loads and to expand dewatering capacity, for a total of 21 drying beds. With the new drying beds it is estimated that digested sludge could be dewatered to approximately 28 percent solids in 45 days. New drying beds could be constructed to the north of the existing drying beds or on adjacent City property.

• Alternative 9 – Centrifuge Dewatering - Anaerobically digested sludge at the Tracy WWTP would be dewatered with centrifuges. For this alternative two new 125 gpm centrifuges would be required with one centrifuge completely redundant. Dewatered solids would be increased to approximately 28 percent solids. A new Centrifuge building, polymer feed facility, and load out facility would be provided.



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Class A Biosolids Alternatives: • Sludge drying beds – Additional detention time in the existing modified and

potential new sludge drying beds could be used to provide a Class A biosolids product at very little additional capital expenditure. It is estimated that providing an additional two drying beds and increasing the drying bed detention time to 60 days would allow the City to continue to meet a Class biosolids product through testing.

Alternatives Costs A preliminary cost estimate has been developed for the alternatives selected for further evaluation. Alternative costs are shown in Exhibit 17. Each Alternative cost includes the following common parameters:

• Contractor mobilization and bonding at 10 percent

• Contractor overhead and profit at 15 percent

• Contingency at 30 percent

• City Administration at 5 percent

• Engineering at 10 percent

• Services during construction at 12 percent

Recommended Plan Thickening The recommended thickening approach for the Tracy WWTF is WAS thickening with a gravity belt thickener, Alternative 2. This is the lowest capital cost alternative having an estimated capital cost of $4.18M as shown in Exhibit 17. This alternative will also have lower operation and maintenance cost than the centrifuge alternative that has an estimated capital cost of $5.17M as shown in Exhibit 18.

Stabilization The recommended stabilization approach for the Tracy WWTF is to continue with anaerobic digestion. This approach requires modifications to the existing digesters to improve performance and defer the cost of the third digester until Phase 4. The estimated total cost to modify the existing two anaerobic digesters is $3.5M as shown in Exhibit 19. The estimated total project cost to construct a new digester to meet the Phase 4 flow and loads is $5.86M as shown in Exhibit 20.

Dewatering The recommended dewatering approach for the Tracy WWTF is to continue the current approach utilizing sludge drying beds. This approach requires modifications to the existing 12 drying beds and the construction of nine new drying beds. As discussed previously, this



68

alternative can be implemented in a phased approach. The estimated total project cost to modify the existing twelve drying beds and to construct nine new drying beds is $6.5M as show in Exhibit 21. This is a lower cost alternative compared with centrifuge dewatering at an estimated cost of $8.0M shown in Exhibit 22. In addition, it has the added benefit of utilizing much of the existing infrastructure. It also defers capital construction costs using a “just in time” building approach. This alternative will also have the lowest operation and maintenance costs.

Class A Biosolids Retaining the existing sludge drying beds also provides the City with a way to achieve Class A biosolids if desired. This has historically been accomplished through testing verification. It is estimated that by increasing the detention time in the drying beds from 45 to 60 days the City would be more able to achieve Class A biosolids. This would require the addition of two more drying beds in Phase 4 over that required for dewatering. Testing should be performed to verify actual detention times necessary to meet Class A requirements. Estimated cost for two additional drying beds is $842,000 as shown in Exhibit 23.

Tracy WWTF Solids Master PlanDewateringAlternative 9 - Centrifuge Dewatering

Item Quantity Units Unit Cost Total Cost

Yard Piping 1 LS $150,000 $150,000Excavation 178 CY $20 $3,556Structural Backfill 89 CY $30 $2,667Building Concrete Slab 89 CY $500 $44,444Masonry Centrifuge and Polymer Building (50x80) 4000 SF $200 $800,000Digester Transfer Pumps 2 EA $50,000 $100,000Sludge Blend Tank 240000 GAL $1.3 $300,000Sludge Blend Tank Install 1 LS $90,000 $90,000Centrifuge Feed Pumps 2 EA $45,000 $90,000Polymer Feed Unit 1 EA $80,000 $80,000Polymer Feed Pump 2 EA $15,000 $30,000125 gpm Centrifuges 3 EA $200,000 $600,000Loadout Facilities 1 LS $200,000 $200,000HVAC Equipment 1 LS $100,000 $100,000Mechanical Allowance 1 LS $250,000 $250,000Electrical 25% $710,167I&C 10% $284,067Mobilization and Bonding 10% $383,490Contractor Overhead and Profit 15% $632,759Construction Subtotal $4,851,149Contingency 30% $1,455,344.55Total Estimate Construction Cost $6,306,493

Engineering 10% $630,649.31Services During Construction 12% $756,779.17City Administration 5% $315,324.65Total Estimated Project Cost $8,009,246

EXHIBIT 22CENTRIFUGE DEWATERING

Tracy WWTF Solids Master PlanDewateringAlternative 7 - Drying Bed Dewatering


Modified Drying BedsYard Piping - 8 inch 1200 LF $80 $96,000Structural Backfill 3926 CY $30 $117,778Asphalt Pavement 206064 SF $3.5 $721,224Decanting Gates 12 EA $40,000 $480,000New Drying BedsPerimeter Wall 291 CY $500 $145,417Structural Backfill 4417 CY $30 $132,500Yard Piping - 8 inch 900 LF $80 $72,000Asphalt Pavement 231822 SF $3.5 $811,377Decanting Gates 9 EA $40,000 $360,000Mechanical Allowance 1 LS $75,000 $75,000Electrical 2% $60,226I&C 2% $60,226Mobilization and Bonding 10% $313,175Contractor Overhead and Profit 15% $516,738Construction Subtotal $3,961,660Contingency 30% $1,188,498.09Total Estimate Construction Cost $5,150,158


EXHIBIT 21DRYING BED DEWATERING

Tracy WWTF Solids Master PlanStabilizationNew Anaerobic Digester


New Digester Excavation 1051 CY $20 $21,017New Digester Structural Backfill 420 CY $30 $12,610New Digester Footings 209 CY $500 $104,720New Digester Base Slab 491 CY $700 $343,612New Digester Walls 436 CY $900 $392,699New Digester Mixing System 1 LS $300,000 $300,000New Digester Heat Exchanger 1 EA $40,000 $40,000New Digester Sludge Transfer Pump 2 EA $25,000 $50,000New Digester Floating Cover 3 EA $250,000 $750,000Mechanical Allowance 1 LS $250,000 $250,000Electrical 18% $407,638I&C 6% $135,879Mobilization and Bonding 10% $280,817Contractor Overhead and Profit 15% $463,349Construction Subtotal $3,552,341Contingency 30% $1,065,702.39Total Estimate Construction Cost $4,618,044


EXHIBIT 20NEW ANAEROBIC DIGESTER

Tracy WWTF Solids Master PlanStabilizationModified Anaerobic Digesters


Existing Digester Concrete Repair 2 EA $50,000 $100,000New Digester Mixing System 2 EA $300,000 $600,000New Digester Floating Cover 1 EA $500,000 $500,000Gas Conditioning System 1 LS $150,000 $150,000Mechanical Allowance 1 LS $100,000 $100,000Electrical 10% $145,000I&C 6% $87,000Mobilization and Bonding 10% $168,200Contractor Overhead and Profit 15% $277,530Construction Subtotal $2,127,730Contingency 30% $638,319.00Total Estimate Construction Cost $2,766,049


EXHIBIT 19MODIFIED ANAEROBIC DIGESTER

Tracy WWTF Solids Master PlanThickeningAlternative 2 - WAS Thickening With GBT


Yard Piping 1 LS $200,000 $200,000Excavation 150 CY $20 $3,000Structural Backfill 75 CY $30 $2,250Building Concrete Slab 100 CY $700 $70,000Masonry Centrifuge and Polymer Building (40x60) 2400 SF $200 $480,000Polymer Feed Unit 1 EA $80,000 $80,000Polymer Feed Pump 2 EA $15,000 $30,000125 gpm Centrifuges 3 EA $200,000 $600,000Sludge Conveyor - 25' 1 EA $50,000 $50,000TWAS Pumps at 50 gpm each 2 EA $35,000 $70,000HVAC Equipment 1 LS $75,000 $75,000Mechanical Allowance 1 LS $175,000 $175,000Electrical 25% $458,813I&C 10% $183,525Mobilization and Bonding 10% $247,759Contractor Overhead and Profit 15% $408,802Construction Subtotal $3,134,148Contingency 30% $940,244.46Total Estimate Construction Cost $4,074,393


EXHIBIT 18WAS WITH GRAVITY BELT THICKENER

Tracy WWTF Solids Master PlanThickeningAlternative 2 - WAS Thickening With GBT


Yard Piping 1 LS $200,000 $200,000Excavation 150 CY $20 $3,000Structural Backfill 75 CY $30 $2,250Building Concrete Slab 100 CY $700 $70,000Masonry GBT and Polymer Building (40x60) 2400 SF $200 $480,000Polymer Feed Unit 1 EA $60,000 $60,000Polymer Feed Pump 2 EA $15,000 $30,0001.5 Meter Gravity Belt Thickeners 2 EA $175,000 $350,000Sludge Conveyor - 25' 1 EA $50,000 $50,000TWAS Pumps at 50 gpm each 2 EA $35,000 $70,000HVAC Equipment 1 LS $75,000 $75,000Mechanical Allowance 1 LS $150,000 $150,000Electrical 20% $308,050I&C 10% $154,025Mobilization and Bonding 10% $200,233Contractor Overhead and Profit 15% $330,384Construction Subtotal $2,532,941Contingency 30% $759,882.34Total Estimate Construction Cost $3,292,823


EXHIBIT 17GRAVITY BELT THICKENER COST

Drying Beds Lime Stabilization Sludge DryingClass A Alt 10 Class A Alt 11 Class A Alt 12

Sludge FeedSolids Content 2.7 % 28.0% % 28.0% %TSS 22,430 lbs/d 21,310 lbs/d 21,310 lbs/dTSS na cy/day 43 cy/day 43 cy/day

Sludge Drying Bed PerformanceSolids throughput 8,186,950 lb per yearHydraulic Throughput 35,888,332 MG per yearWater Removed by Decant 45% %Solids Concentration after Decant 4.87% %Annual Precipitation 0.84 ft/yearAnnual Pan Evaporation 8.12 ft/yearEstimated Drying Bed Size 610,000 sfSolids Target Concentration 40% %Actual Drying Bed Size 610,148 sfDrying Bed Detention Time 60 daysEffective Drying Bed Area, each 27,878 sfSize Based on Solids 22 unitsSize Based on Hydraulics 23 unitsSelect Size 23 units

Lime StabilizationSolids Content 28% %Solids Feed TSS 21,310 lbs/dSilo Capacity 30 tonsSilo Operating Capacity 25 tonsLime Usage at 0.35lb/lb 7459 lb/daySilo Refill Frequency 7 daysCentrifuge Feed Rate 1308 lb/hrLime Feed Rate 7.6 lb/min

Lime Usage at 0.50lb/lb 10655 lb/daySilo Refill Frequency 5 daysBFP Solids Loading 1308 lb/m/hrLime Feed Rate 10.9 lb/min

RDP DischargeTotal Solids at 35% lime addition 28,769 lbs/dEstimated Solids Content 35% %Volume at 65 lb/cf 46.8 cy/dTotal Solids at 50% lime addition 31,965 lbs/dEstimated Solids Content 35% %Volume at 65 lb/cf 52.0 cy/d

Sludge Storage - Lime Stabilization3 Months Storage at 35% Lime 4215 cyRequired Storage Volume 113809 cfStorage Height 3 ftLength of Storage 200 ftWidth of Storage 106 ftVolume Calculation 63600 cf

Dryer DischargeTotal Solids 21,310 lbs/dWet Tons 38.1 tons/dayEstimated Solids Content 90% %Volume at 45 lb/cf 19.5 cy/dWet Tons 11.8 tons/day

Sludge Storage - Dehydration30 Day Storage 585 cyRequired Storage Volume 15785 cfStorage Height 30 ftStorage Diameter 16 ftStorage Volume 6032 cfNumber of Towers 3 ftTotal Storage 18096 ft

EXHIBIT 16CLASS A ALTERNATIVES

Drying Beds Belt Filter Press CentrifugeDewaterubg Alt 7 Dewaterubg Alt 8 Thickening Alt 9

Total Digested SludgePercent Solids 2.7 % 2.7 % 2.7 %Flow 0.10 mgd 0.10 mgd 0.10 mgdFlow 68 gpm 68 gpm 68 gpmTSS 22430 lbs/d 22430 lbs/d 22430 lbs/d

Blend TankDays of Storage 2.0 days 2.0 days 2.0 daysDaily Sludge Volume Required 98,324 gal 98,324 gal 98,324 galTotal Storage Volume 235,978 gal 235,978 gal 235,978 gal

Dewatering Feed SolidsDays Per Week of Operation 7 days 5 days 5 daysHours per day of Operation 24 hr 16 hr 24 hrFlow 0.10 mgd 0.21 mgd 0.14 mgdFlow 68 gpm 144 gpm 96 gpmTSS 22430 lbs/day 47103 lbs/day 31402 lbs/day

Dewatering Performance Drying Beds Belt Filter Press CentrifugeSolids throughput 8,186,950 lb per year 700 lbs/hr/m na lbs/hr/unitHydraulic Throughput 35,888,332 MG per year 75 gpm/m 125 gpm/unitWater Removed by Decant 45% %Solids Concentration after Decant 4.873% %Annual Precipitation 0.842 ft/yearAnnual Pan Evaporation 8.120 ft/yearEstimated Drying Bed Size 574,000 sfActual Drying Bed Size 573,921 sfDrying Bed Detention Time 45 daysEffective Drying Bed Area, each 27,878 sfSize Based on Solids 21 units 2.8 m na unitsSize Based on Hydraulics 17 units 1.9 m 0.8 unitsSelect Size 21 units 3.0 m 1.0 unitsHour of operation/day Solids na hrs/d 15.0 hrs/d na hrs/dHour of operation/day hydraulics na hrs/d 10.2 hrs/d 18.4 hrs/dActual Feed Rate na gpm 144 gpm 96 gpm

Dewatered SludgeSolids Capture 90% % 95% % 95% %Solids Content 28.0% % 18.0% % 28.0% %TSS 20,190 lbs/d 21,310 lbs/d 21,310 lbs/dTSS 41 cy/day 67 cy/day 43 cy/day

Average Polymer UsageAverage Polymer Dosage 0 lb/Ton 12 lb/Ton 15 lb/TonAveragePolymer Usage 0 lb/day 135 lb/day 168 lb/dayMax Polymer Feed Concentration 3500 mg/L 3500 mg/L 3500 mg/LWeight of Solution 8.5 lb/gal 8.5 lb/gal 8.5 lb/galMaximum Polymer Feed Rate 0.0 gpm 3.1 gpm 3.9 gpm

EXHIBIT 15DEWATERING ALTERNATIVES

Primary SludgePercent Solids 3.5% % 3.5% % 3.5% % 3.5% % 3.5% % 3.5% %Flow 0.06 mgd 0.06 mgd 0.06 mgd 0.06 mgd 0.06 mgd 0.06 mgdFlow 44 gpm 44 gpm 44 gpm 44 gpm 44 gpm 44 gpmTSS 18650 lbs/d 18650 lbs/d 18650 lbs/d 18650 lbs/d 18650 lbs/d 18650 lbs/d

WASPercent Solids 0.8% % 0.8% % 0.8% % 0.8% % 0.8% % 0.8% %Flow 0.27 mgd 0.27 mgd 0.27 mgd 0.27 mgd 0.27 mgd 0.27 mgdFlow 189 gpm 189 gpm 189 gpm 189 gpm 189 gpm 189 gpmTSS 18135 lbs/d 18135 lbs/d 18135 lbs/d 18135 lbs/d 18135 lbs/d 18135 lbs/d

Primary Sludge Thickening NO NO NO NOWAS Sludge Thickening YES YES YES YES

Total Sludge to ThickenerPercent Solids 0.8% % 0.8% % 0.8% % 0.8% % 1.3% % 1.3% %Flow 0.27 mgd 0.27 mgd 0.27 mgd 0.27 mgd 0.34 mgd 0.34 mgdFlow 189 gpm 189 gpm 189 gpm 189 gpm 234 gpm 234 gpmTSS 18135 lbs/d 18135 lbs/d 18135 lbs/d 18135 lbs/d 36785 lbs/d 36785 lbs/d

Blend TankDays of Storage na na na naDaily Sludge Volume Required na na na na 1 days naTotal Storage Volume na na na na 335,699 gal na

402,839 gal na

Thickening Feed SolidsDays Per Week of Operation 7 days 7 days 7 days 7 days 7 days 7 daysFlow 0.272 mgd 0.272 mgd 0.272 mgd 0.272 mgd 0.336 mgd 0.336 mgdFlow 189 gpm 189 gpm 189 gpm 189 gpm 234 gpm 234 gpmTSS 18135 lbs/d 18135 lbs/d 18135 lbs/d 18135 lbs/d 36785 lbs/d 36785 lbs/d

Thickening Performance DAFT Gravity Belt Centrifuge Drum Thickener Gravity Belt Gravity ThickenerSolids throughput 2.6 lbs/hr/sf 1100 lbs/hr/m na lbs/hr/unit 3000 lbs/hr/unit 1600 lbs/hr/m 16 lb/day/sfHydraulic Throughput 1.5 gpm/sf 220 gpm/m 125 gpm/unit 120 gpm/unit 220 gpm/m 200 gpd/sfSize Based on Solids 291 sf 0.7 m na units 0.3 units 1.0 m 38 ftSize Based on Hydraulics 126 sf 0.9 m 1.5 units 1.6 units 1.1 m 33 ftSelect Size 300 sf 1.5 m 2.0 units 2.0 units 2.0 m 40.0 ftHour of operation/day Solids 23.3 hrs/d 11.0 hrs/d na hrs/d 6.0 hrs/d 11.5 hrs/d na hrs/dHour of operation/day hydraulics 10.1 hrs/d 13.8 hrs/d 18.2 hrs/d 18.9 hrs/d 12.7 hrs/d na hrs/dHour per day of Operation 24.0 hrs/d 16.0 hrs/d 24.0 hrs/d 24.0 hrs/d 16.0 hrs/d 24.0 hrs/dSolids Capacity 780 lb/hr 1650 lb/hr na lb/hr 6000 lb/hr 3200 lb/hr 20,944 lb/hrHydraulic Capacity 450 gpm 330 gpm 250 gpm 240 gpm 440 gpm 349 gpmActual Feed Rate 189 gpm 283 gpm 189 gpm 189 gpm 350 gpm 233 gpm

Thickened SludgeSolids Capture 90% % 95% % 95% % 90% % 95% % 90% %Solids Content 3.5% % 6.0% % 6.0% % 4.5% % 6.0% % 4.5% %

DAFT Cent Rotary Co-Thicken with GBTCo-Thicken with Gravity

ThickenerThickening Alt 1 Thickening Alt 2 Thickening Alt 3 Thickening Alt 4 Thickening Alt 5 Thickening Alt 6

GBT

EXHIBIT 14THICKENING ALTERNATIVES

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GBT

Flow 0.06 mgd 0.03 mgd 0.03 mgd 0.04 gpm 0.07 gpm 0.09 gpmFlow 39 gpm 24 gpm 24 gpm 30 gpm 49 gpm 61 gpmTSS 16,320 lbs/d 17,230 lbs/d 17,230 lbs/d 16,320 lbs/d 34,950 lbs/d 33,110 lbs/d

Average Polymer UsageAverage Polymer Dosage 10 lb/Ton 6 lb/Ton 12 lb/Ton 10 lb/Ton 6 lb/Ton 6 lb/TonAveragePolymer Usage 91 lb/day 54 lb/day 109 lb/day 91 lb/day 110 lb/day 110 lb/dayMax Polymer Feed Concentration 3500 mg/L 3500 mg/L 3500 mg/L 3500 mg/L 3500 mg/L 3500 mg/LWeight of Solution 8.5 lb/gal 8.5 lb/gal 8.5 lb/gal 8.5 lb/gal 8.5 lb/gal 8.5 lb/galMaximum Polymer Feed Rate 2.1 gpm 1.3 gpm 2.5 gpm 2.1 gpm 2.6 gpm 2.6 gpm

Digester FeedPrimary Sludge

Percent Solids 3.5% % 3.5% % 3.5% % 3.5% % 6.0% % 4.5% %Flow 0.06 mgd 0.06 mgd 0.06 mgd 0.06 mgd 0.04 mgd 0.04 mgdFlow 44 gpm 44 gpm 44 gpm 44 gpm 25 gpm 31 gpmVSS 13988 lbs/d 13988 lbs/d 13988 lbs/d 13988 lbs/d 13288 12589TSS 18650 lbs/d 18650 lbs/d 18650 lbs/d 18650 lbs/d 17718 lbs/d 16785 lbs/d

WASPercent Solids 3.5% % 6.0% % 6.0% % 4.5% % 6.0% % 4.5% %Flow 0.06 mgd 0.03 mgd 0.03 mgd 0.04 mgd 0.03 mgd 0.04 mgdFlow 39 gpm 24 gpm 24 gpm 30 gpm 24 gpm 30 gpmVSS 12240 lbs/d 12923 lbs/d 12923 lbs/d 12240 lbs/d 12921 lbs/d 12241 lbs/dTSS 16320 lbs/d 17230 lbs/d 17230 lbs/d 16320 lbs/d 17228 lbs/d 16322 lbs/d

Digester Feed SludgePercent Solids 3.5% % 4.4% % 4.4% % 3.9% % 6.0% % 4.5% %Flow 0.12 mgd 0.10 mgd 0.10 mgd 0.11 mgd 0.07 mgd 0.09 mgdFlow 83 gpm 68 gpm 68 gpm 75 gpm 49 gpm 61 gpmVSS 26228 lbs/d 26910 lbs/d 26910 lbs/d 26228 lbs/d 26209 lbs/d 24830 lbs/dTSS 34970 lbs/d 35880 lbs/d 35880 lbs/d 34970 lbs/d 34946 lbs/d 33107 lbs/d

VSS Distruction 0.50 % 0.50 % 0.50 % 0.50 % 0.50 % 0.50 %VSS 26,228 lbs/d 26,910 lbs/d 26,910 lbs/d 26,228 lbs/d 26,209 lbs/d 24,830 lbs/dInert Solids 8,743 lbs/d 8,970 lbs/d 8,970 lbs/d 8,743 lbs/d 8,736 lbs/d 8,277 lbs/dResidual VSS 13,110 lbs/d 13,460 lbs/d 13,460 lbs/d 13,110 lbs/d 13,100 lbs/d 12,410 lbs/dResidual TSS 21,853 lbs/d 22,430 lbs/d 22,430 lbs/d 21,853 lbs/d 21,836 lbs/d 20,687 lbs/dBOD reduction 100% % 100% % 100% % 100% % 100% % 100% %Digester Gas cf of gas/lbVSS dest 13 cf/#vss 13 cf/#vss 13 cf/#vss 13 cf/#vss 13 cf/#vss 13 cf/#vssDigester Gas 170,530 cf/d 174,850 cf/d 174,850 cf/d 170,530 cf/d 170,420 cf/d 161,460 cf/d

VSS Loading 0.11 lbs/cf/d 0.11 lbs/cf/d 0.11 lbs/cf/d 0.11 lbs/cf/d 0.11 lbs/cf/d 0.11 lbs/cf/dRequired HRT 20 days 20 days 20 days 20 days 20 days 20 daysSize of Digestion Based on Solids 238,432 cf 244,636 cf 244,636 cf 238,432 cf 238,266 cf 225,726 cfSize of Digestion Based on Solids 1.80 Mg 1.80 Mg 1.80 Mg 1.80 Mg 1.80 Mg 1.70 MgSize of Digestion Based on HRT 2.40 Mg 1.97 Mg 1.97 Mg 2.15 Mg 1.40 Mg 1.76 MgRequired Digestion 2.40 Mg 1.97 Mg 1.97 Mg 2.15 Mg 1.80 Mg 1.76 Mg


PAGE 2 OF 3



GBT

Total Number of Digesters 3 # 3 # 3 # 3 # 3 # 3 #Dia 75 ft 75 ft 75 ft 75 ft 75 ft 75 ftSidewall 25 ft 25 ft 25 ft 25 ft 25 ft 25 ftVolume Each 110,447 cf 110,447 cf 110,447 cf 110,447 cf 110,447 cf 110,447 cfCone Volume Each 8,836 cf 8,836 cf 8,836 cf 8,836 cf 8,836 cf 8,836 cfTotal Volume Each 119,282 cf 119,282 cf 119,282 cf 119,282 cf 119,282 cf 119,282 cfTotal Volume Each 0.89 mg 0.89 mg 0.89 mg 0.89 mg 0.89 mg 0.89 mgEffective Volume Each 95% % 95% % 95% % 95% % 95% % 95% %Net Effective Volume Each 0.85 mg 0.85 mg 0.85 mg 0.85 mg 0.85 mg 0.85 mgTotal Effective Volume 2.54 mg 2.54 mg 2.54 mg 2.54 mg 2.54 mg 2.54 mgHDT All in Service 21.2 days 25.9 days 25.9 days 23.7 days 36.4 days 28.8 days

Solids Conc 2.2 % 2.7 % 2.7 % 2.4 % 3.8 % 2.8 %Flow 0.12 mgd 0.10 mgd 0.10 mgd 0.11 mgd 0.07 mgd 0.09 mgdTSS 21,853 lbs/d 22,430 lbs/d 22,430 lbs/d 21,853 lbs/d 21,836 lbs/d 20,687 lbs/d


PAGE 3 OF 3

Tracy WWTF Solids Master PlanClass AAlternative 10 - Drying Beds


Perimeter Wall 65 CY $500 $32,315Structural Backfill 981 CY $30 $29,444Yard Piping - 8 inch 200 LF $80 $16,000Asphalt Pavement 51516 SF $3.5 $180,306Decanting Gates 2 EA $40,000 $80,000Mechanical Allowance 1 LS $50,000 $50,000Electrical 2% $7,761I&C 2% $7,761Mobilization and Bonding 10% $40,359Contractor Overhead and Profit 15% $66,592Construction Subtotal $510,539Contingency 30% $153,161.60Total Estimate Construction Cost $663,700

Engineering 10% $66,370.03Services During Construction 12% $79,644.03City Administration 5% $33,185.01Total Estimated Project Cost $842,899

EXHIBIT 23DRYING BEDS

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