Page 2 The digesters are operated in series as a high rate system, meaning the primary digester is operated as high rate fermentation process with heating and mixing. The secondary digester is used to concentrate digested solids and currently is not heated or mixed. Digested sludge is conveyed from Digester 2 to the storage lagoons. Basic characteristics of the anaerobic digesters are summarized in Table 1. Table 1. Existing Anaerobic Digesters Units Design Value Digester No. 1 Diameter Feet 55 Side Water Depth Feet 33 Volume Cubic Feet 78,400 Volume Gallons 586,000 Mixing Gas mixing provided, but not currently used (a) Cover Floating Heating Equipped with heating, but not currently used. Process Secondary Digester Digester No. 2 Diameter Feet 50 Side Water Depth Feet 35 Volume Cubic Feet 68,700 Volume Gallons 514,000 Mixing Central Draft Tube Cover Fixed Heating Heated Process High Rate, Complete Mix (Primary Digester) (a) The gas mixing system in Digester 1 will be replaced with a mechanical mixing as part of the City s current Repair and Replacement Project. This project will be complete before the WWTP Improvements Project is initiated. Currently digested sludge is discharged to one of three existing sludge storage lagoons, each with an area of approximately one acre. Lagoon total depth is 12 feet, which includes 2 feet of freeboard, making the maximum operating depth 10 feet. According to the original construction drawings for these cells, they were designed for a VSS loading of 20 lb /day/1000 ft 2 (Typical design range is 20-50 lb /day/1000 ft 2 ).
Page 3 Continuing the current method of sludge dewatering is not considered feasible for the following reasons: The RWQCB has indicated that is a potential that the current dewatering practice will contaminate groundwater, and that continuation of the current practice cannot be justified; and The future digested combined primary and secondary sludge is expected to be more difficult to dewater than the current digested primary sludge, and as a result sludge that is only air-dried sludge would have to be spread over a much larger area in the future. The lagoons are periodically dewatered and the solids are air dried for subsequent disposal. The biosolids that are produced meet the requirements of Class B biosolids as regulated by the EPA Part 503 Rules. Historically biosolids were land applied to the overland flow slopes, but the current practice is to dispose of biosolids at the nearby Yolo County Landfill. Recommendations of Prior Investigations 2008 Preliminary Design Report City of Davis WWTP Improvements Project In 2008, Carollo Engineers completed a report titled: Preliminary Design Report for the City of Davis Water Pollution Control Plant Improvement Project. This report recommended construction of a third digester with a fixed-cover, pump-mixing, and with a 0.51 MG capacity. The report also recommended installation of piping to allow operation of all digesters in parallel, or two in parallel with the third in series, and that a replacement boiler be provided to heat the three digesters and the expanded operations/maintenance buildings. (Carollo Engineers, 2008). The 2008 Preliminary Design Report also recommended that Waste Activated Sludge (WAS) be thickened in gravity belt thickeners (GBTs) (2 meter belts), and that one duty and one standby thickening unit be provided. Finally, the 2008 Preliminary Design Report recommended a sludge dewatering system consisting of 0.3 MG batch tank, 2 belt presses (1 duty and 1 standby), and 4 acres of paved solar drying beds. 2010 Summary Report City of Davis Wastewater Planning Charrette In October 2009, Edward Schroeder and George Tchobanoglous (both professors emeritus from the Department of Civil Engineering, at the University of California, Davis) organized and conducted a wastewater planning Charrette to develop a cost-effective wastewater management plan for the City. The findings of the Charrette were presented in a report that was submitted to the City in January 2010 (Schroeder and Tchobanoglous, 2010). The report states The Panel concluded that current digester capacity is more than adequate for the new plant. The Charrette Report recommendations also state: Sludge from the primary and secondary sedimentation tanks, and the disk filters will be combined with flows from the oxidation ponds and thickened in the DAF.
Page 4 Finally, the Charrette Report stated: While sludge processing volume and costs will increase with construction of the new facility, continuing the method of management now employed may prove, on further evaluation, to be a cost effective option. Reduced Influent Loads During Non-Student Periods In Davis, WWTP influent flows and loads are generally reduced in the summer when the majority of college students leave town (i.e., the Non-Student Period). July through September are the lowest flow and load months, as shown in Figure 1, which indicates average monthly influent BOD and TSS loads relative to annual average loads for 2006 through 2011. From July through September, influent BOD and TSS loads are about 90 percent of annual average loads and about 80 percent of maximum month loads. The City plans to take advantage of the fact that lower loads occur during this period, and will plan to do digester maintenance when the influent loads are the lowest. Influent BOD Load Influent TSS Load 110% Percent of Annual Average Load 105% 100% 95% 90% 85% 80% Figure 1. Davis WWTP Seasonal Load Variations (2006-2011 Influent Data)
Page 5 Regulations Applicable to the Disposal of Biosolids in Landfills Federal Regulations If the City continues its current practice for co-disposing solids in the landfill, pathogen and vector reduction requirements in Subpart D of Part 503 of EPA s regulations will not apply. Co-disposal landfilling involves combining wastewater solids with municipal solid waste and placing the mixture in a permitted landfill. The design and operation of co-disposal landfills is regulated by EPA under Subpart I of 40 CFR, Part 258, Criteria for Municipal Solid Waste Landfills. Standards set forth in the Part 258 regulations address general requirements, pollutant limits, management practices, operational pathogens, vector attraction, monitoring, recordkeeping, and reporting requirements. State Regulations The design and operation of municipal landfills in California is regulated by Title 27 of the California Code of Regulations. The California Integrated Waste Management Board issues Solid Waste Facility Permits to operating landfills within the State. Sewage sludge (biosolids) may only be disposed of in landfills that have Solid Waste Facility Permits which allow the facility to receive sewage sludge. Local Regulations In addition to Federal and State regulations, each landfill may adopt requirements that are applicable to biosolids that are disposed of at the landfill. Typically landfills require a minimum solids concentration; however, some landfills also require that biosolids meet Class B biosolids as established by the EPA Part 503 Rules. While individual landfills may require that biosolids that are received meet Class B requirements, Federal and State regulations do not require this level of treatment for biosolids that are disposed of in permitted landfills. West Yost Associates staff contacted three landfills in the vicinity of the Davis WPCF that are permitted to receive sewage sludge to determine treatment requirements. The Solid Waste Permits of these facilities were also reviewed to determine the facilities estimated closure date. This information is summarized in Table 2.
Page 6 Table 2. Treatment Requirements of Selected Landfills in Vicinity of Davis WWTP that are Permitted to Receive Sewage Sludge Facility Yolo County Landfill (contact: Jeff Kieffer) Potrero Hills Landfill (Solono County) contact: Joe Lynch) Sacramento County Landfill (a) Treatment Requirements >20% solids content >50% solids content >50% solids content Estimated Closure Date 01/01/2081 02/14/2048 01/01/2064 Comments Can only receive biosolids during dry weather, Monday through Friday before 9 A.M. Can accept up to 250 tons/day for use as alternative daily cover Only accepted on Tuesday or Thursday between 9:00 AM - 3:00 PM Tipping Fee, dollars/wet ton $12 if suitable for daily cover (a) $43.50 if unsuitable for daily cover $22-25 (fee decreases as volume increases) Jeff Kieffer stated that biosolids can be used for daily cover unless testing indicates that CA Title 22 metals concentrations are too high, and that high metals concentration have not been detected in biosolids received from either the Davis or Woodland WWTP. $48 PROJECT CRITERIA Minimum Requirements A filtrate storage basin should be provided to allow associated loads to the secondary process to be equalized (i.e. returned off-peak). There are a number of technologies available to thicken waste activated sludge. Some of these technologies have extensive operating track records, thus performance, service life, and maintenance requirements are well established, while other technologies are relatively new or may not be well-suited for medium size wastewater treatment plants. Potentially acceptable technologies include: Gravity Belt Thickener Dissolved Air Flotation Thickener Suspended Air Flotation Thickener Rotary Drum Thickener Screw Thickener Centrifuge Thickener
Page 7 Mechanical sludge dewatering technologies are similar to thickening technologies in that there are a number of technologies available to dewater anaerobically digested sludge. Some of these technologies have extensive operating track records and performance, service life, and maintenance requirements are well established, while other technologies are relatively new and may not be well-suited for medium size wastewater treatment plants. Potentially acceptable technologies include: Belt Filter Press Rotary Fan Press Dewatering Screw Press Centrifuge Performance Criteria The thickening unit (or units) should produce six percent total solids (TS) thickened waste activated sludge (TWAS) while operating continuously and unattended. The units shall include the instrumentation and controls necessary to automate and monitor sludge and polymer feed, equipment operation, and TWAS pumping. Dewatering unit (or units) should be capable of producing 15 percent TS. Design Criteria Digester Loading Allowances A minimum digestion SRT of 15 days is required to produce Class B biosolids in conformance with 40 CFR Part 503. This level of biosolids stabilization is required by EPA for land-applied biosolids, not for biosolids that are disposed of in a permitted landfill. As indicated previously in this TM, landfills in the vicinity of the Davis WWTP require that biosolids be treated to achieve a minimum solids content but do not require that biosolids be treated to the meet Class B criteria. The Wastewater Residuals Stabilization Manual of Practice (MOP FD-9) (WEF, 1995) states: Typical SRTs for high rate digestion at mesophilic temperatures range from 15 to 20 days. A minimum 15-day SRT is normally maintained for stability, ease of control, to account for the accumulation of grit and scum and to allow for imperfect mixing. The minimum SRT for high rate mesophilic digestion is 10 days to ensure that the necessary microorganisms are being produced at the same rate as they are removed from the system each day.
Page 8 Digester solids loading rates (including both primary solids and TWAS) should ensure the following solids treatment criteria are achieved: The maximum-month SRT should be 15 days with both digesters in service; An SRT between 10 and 15 days is acceptable with one digester in service, provided that: Digester 1 is mechanically mixed; and Digester 2 is taken out of service and cleaned before Digester 1 is cleaned (Digester 1 has about 14 percent greater volume than Digester 2, and this cleaning sequence would insure that the full volume of Digester 2 is available when operating alone). Similarly to the above discussion, the Manual of Practice 8: Design of Municipal Wastewater Treatment Plants (WEF, 1998) states: Typical design sustained-peak VSS loading rates are between 0.12 to 0.16 lb VSS/day/cubic-foot (ft 3 ) digester volume. 0.2 lb/day/ft 3 digester volume is a typical maximum short-term VSS loading rate. Digester solids loading rates (including both primary solids and TWAS) should ensure the following solids treatment criteria are achieved: The maximum-month VSS loading rate should be 0.15 lb/day/ft 3 digester volume with both digesters in service. A VSS loading rate between 0.15 and 0.20 lb/day/ft 3 digester volume is acceptable with one digester in service, provided that: Digester 1 is mixed; and Digester 2 is taken out of service and cleaned before Digester 1 is cleaned. WAS Thickening and Digested Solids Dewatering Processes WAS thickening facilities should be designed to meet the following criteria: 1. Produce a TWAS total solids concentration greater than or equal to six percent. 2. Total capacity of the thickening equipment should be adequate to process the volume of biosolids generated under the Maximum Month Load Condition over a four-week period where units are operated (including time needed for preparation, start-up, and shut-down) 6 hours/day, 5 days/week. 3. The firm capacity of the pumping facilities used to convey TWAS should be based on 1.5x the anticipated maximum month solids production rates with the largest unit out of service.
Page 9 4. A redundant thickening unit must be provided, however, the redundant thickening and dewatering units may be same unit if this unit is capable of satisfying both thickening and dewatering performance criteria. 5. A minimum of two concrete-lined liquid digested sludge storage lagoon should be provided that are capable of storing at least seven (7) days sludge production. 6. A minimum of two digested sludge storage lagoons should be provided. 7. The filtrate storage basin should provide two (2) days of dewatering filtrate/centrate storage. Dewatered Sludge Storage A solar drying/storage area should be provided that: 1. The capacity shall be adequate to store six months of sludge production that has been dewatered to 15 percent solids content, based on the annual average solids generation rate. 2. Accommodate all precipitation falling on the dewatered sludge solar drying/storage facility during a 1-in-100, 48-Hour Rain Event while maintaining a minimum of a 2- foot freeboard. 3. Accommodate all precipitation falling on the dewatered sludge solar drying/storage facility during a 1-in-100 Rainfall Year without exceeding an allowable combined stormwater runoff return flow rate of 8% of the ADWF (6.0 mgd). Demonstration Requirements 1. Demonstrate compliance with minimum successful performance history requirements (specified in the RFP). 2. Estimate average and maximum energy consumption and polymer use. REFERENCE PROJECT This section provides the information regarding the Reference Project: A solids balance evaluation for the Reference Project; An evaluation of the existing digestion capacity to meet the Design Criteria listed above given the results of the solids balance; A discussion of the selection of a solids thickening technology for the Reference Project; A discussion of the selection of a solids dewatering technology for the Reference Project; A summary of the size of the solids handling units, given the results of the solids balance.
Page 10 Solids Balance Solids balances have been developed for the Reference Project to evaluate the range of primary and secondary solids loadings anticipated at the 6 million gallon per day (mgd) Average Dry Weather Flow (ADWF) design condition. The solids balances account for influent and effluent TSS and VSS loads at each major liquid and solid treatment unit of the Reference Project. These solids balances are presented as the following figures: Figure 2: Annual Average Load Condition Solids Balance Figure 3: Full Population Maximum Month Load Condition Solids Balance Figure 4: Non-Student Period Maximum Month Load Condition Solids Balance For these solids balances, where possible, VSS loads have been calculated directly, as described below, and then VSS-to-TSS ratios calculated and presented, for reference, from the VSS and TSS values. However, for several treatment units, a VSS-to-TSS ratio must first be assumed to calculate VSS loads. Any VSS-to-TSS ratios that have been assumed, and the basis for each assumption, are detailed by treatment unit in Table 3. Other major assumptions used in the solids balance calculations, as well as the bases for those assumptions, are summarized by treatment unit in Table 4. Treatment Unit Headworks/ Grit Removal Primary Sedimentation Secondary Treatment Cloth Disk Filtration WAS Thickening Biosolids Storage Lagoon Biosolids Dewatering (a) Table 3. Assumed VSS-to-TSS Ratios for WWTP Improvements Project Reference Project Solids Balance Parameter (a) Assumed Value, percent Basis Average of calculated VSS-to-TSS ratios for paired Raw Influent 91 VSS and TSS data points from influent sampling for 7/31/12-8/6/12 and 10/29/12-11/11/12. Grit 10 Typical value Average of calculated VSS-to-TSS ratios for paired Primary Effluent 91 VSS and TSS data points from primary effluent sampling for 7/31/12-8/6/12 and 10/29/12-11/11/12. Mixed Liquor Bioreactor Effluent Return Activated Sludge Equal to WAS VSS-to-TSS ratio predicted by Secondary Effluent Varies BioWin secondary process model results for the Reference Project Tertiary Effluent Thickened Solids Lagoon Effluent Dewatered Biosolids Varies Equal to calculated digested solids VSS-to-TSS ratios for the Reference Project For solids balance parameters not detailed in this table, VSS-to-TSS ratios are calculated directly from VSS and TSS values, as indicated on the solids balance figures.
Table 4. Major Solids Balance Assumptions and Calculations (Reference Project) Treatment Unit Parameter Assumed Value Basis Flow 6.6 mgd Annual Average Flow (AAF) provided in Table 4 of the Basis of Design Development TM. Headworks/ Raw Influent BOD 5, NH 4 -N, and TSS Grit Removal Loads Varies As detailed in Table 8 of the Basis of Design Development TM. Solids Removal 1 percent Typical grit removal unit performance. Primary Sedimentation Secondary Biological Process Secondary Clarification Tertiary Filtration BOD 5 30 percent Average WWTP primary clarifier performance for Feb.-Aug. 2012, excluding uncharacteristic data for Jul. 23-30 and negative values, as summarized in Load Table 9 of the the Basis of Design Development TM. (That table presents a BOD 5 removal value of 31.0 percent and a TSS removal value of 65 Removals NH 4 -N 1 percent percent, which have been rounded to the nearest 5 percentage points for the current analysis. Primary influent and effluent NH 4 -N data is not available TSS 65 percent for the same period, so a value was not calculated for NH 4 -N. The assumed value here is relatively small.) Primary Solids: Solids Content 4 percent Typical value per discussion with City staff TSS Concentration 3,000 mg/l MLSS Design Criteria for secondary process Mixed Liquor VSS Concentration Varies Calculated from the MLSS Design Criteria concentration and the assumed VSS-to-TSS ratio (see Table 3) TSS and VSS Mass Varies Calculated from TSS and VSS concentrations and total reactor volume Secondary TSS Concentration 3,000 mg/l MLSS Design Criteria for secondary process Reactor VSS Concentration Varies Calculated from the MLSS Design Criteria concentration and the assumed VSS-to-TSS ratio (see Table 3) Effluent Flow Varies Sum of Secondary Effluent flow, WAS flow, and RAS flow Solids Production WAS, TSS, and VSS Loads Varies Values predicted by BioWin secondary process model. See Special Design Study TM #3 Secondary Process MCRT Varies TSS mass in the reactor divided by the combined WAS TSS loads and secondary effluent TSS loads Flow Varies Calculated from the loads predicted by BioWIN secondary process model and an assumed WAS solids content of 0.5%. WAS TSS and VSS Concentrations Varies Calculated from the loads predicted by BioWIN secondary process model and predicted WAS flow. RAS Flow Varies Calculated as 0.5 times the combined influent flow. TSS and VSS Loads Varies Calculated from WAS concentrations and RAS flow. Flow Varies Difference between primary effluent flow and WAS flow. Secondary BOD 5 Concentration 5 mg/l Effluent NH 4 -N Concentration 1 mg/l Typical values expected for the Reference Project design. Tertiary Effluent TSS Concentration BOD 5 Concentration NH 4 -N Concentration 5 mg/l 2 mg/l 1 mg/l Typical values expected for the Reference Project design. TSS Concentration 2 mg/l Solids Capture Rate 90 percent Project Criterion for the WAS thickening process. WAS Thickening Thickened Solids: Solids Content 6 percent Project Criterion for WAS Thickening unit Solids Digesting VSS Reduction 40 percent Typical anaerobic digestion value (WEF, 1998) Biosolids Storage Lagoon Biosolids Dewatering Lagoon Effluent No change from influent Simplifying assumption Solids Capture Rate 95 percent Project Criterion for the dewatering process. Dewatered Solids: Solids Content 20 percent Conservative assumption (note that the Project Criterion for the dewatering process is 15%) BOD 5 Load Varies with flow Load determined based on flow and assumed BOD concentration of 2,000 mg/l. Dewatering Filtrate Load determined based on flow and assumed NH NH 4 -N Load Varies with flow 4 -N concentration of 800 mg/l per Figure 16-2 of Process Design Manual for Sludge Treatment and Disposal (EPA, 1979).
Last saved: 6/10/13 3:54 PM Charles Hardy; N:\Clients\011 Davis\00-12-42 WWTP Design-Build\ENGR\Solids Handling\[Davis-Solids Balance Figure2.xlsm]Maximum Monthly Return Activated Secondary Sludge (RAS) Grit Primary Biological Process Secondary (i) 263,370 Headworks Removal Sedimentation Reactor Volume = 2.7 Mgal Clarification (i) 208,360 (c) 79% 3.5 mgd Cloth Disk Filtration Filter Return Backwash 170 Flow 0 4.5% 170 140 82% (c) 310,000 gpd Raw Combined Primary Influent Influent Influent Removal (e) Primary Effluent Secondary Process Secondary Effluent Tertiary Effluent 11,100 11,790 Solids 11,790 30% 8,250 141 mg/l 3,000 mg/l Bioreactor 290 5 mg/l (j) 120 2mg/L (j) 1,720 1,930 Removal 1,930 1% 1,910 33 mg/l 67,550 lb Effluent 60 1 mg/l (j) 60 1mg/L (j) 11,500 12,800 0.50% (b) Mixed 12,740 65% 4,460 76 mg/l 2,370 mg/l 262,750 290 5 mg/l (j) 120 2mg/L Liquor (j) 10,470 11,490 11,480 4,060 70 mg/l 53,370 lb 207,570 230 90 91% (a) 7.02 mgd 90% (c) 91% (d) 79% (c) 79% (d) 79% (g) 79% (d) 6.60 mgd 7.02 mgd 6.99 mgd MCRT = 9.8 days 10.50 mgd 6.90 mgd 6.90 mgd To Grit Classifier and Disposal Primary Solids Waste Activated Sludge 3,540 (WAS) Grit 20 (g) 6,620 9,000 mg/l 60 8,280 <-- 8,280 Secondary (g) 5,240 7,120 mg/l 6 Primary 7,390 <-- 7,390 Solids (k) 79% (c) 10% (d) Solids (f) 89% (c) 0.9% 88,200 gpd 4% 24,820 gpd (h) (h) To CCBs, then 3W or Receiving Water Digester Gas Generation 1.12 Assumes densitiy of gat is eqal to 0.86 tunes that of air (1.204) 5612.91 kg/d WAS Solids Biosolid Biosolids Thickening Digesting Storage Lagoon Dewatering Dewatered Thickened Solids (k) 6% Digested Solids (m) 3.1% Dewatered Solids (l) 20% Biosolids 8,930 6,910 77% (d) 5,350 gpd Assume no change in lagoon. (a) Average of VSS-to-TSS ratios for influent samples for periods 7/31/12-8/6/12 and 10/29/12-11/11/12. (b) Assumed solids removal through headworks and grit tank. (c) Ratio calculated from VSS and TSS load values shown. (i) Based on WAS concentrations and calcluated RAS flow rate (50% of combined influent). Dewatering (j) Typical effluent quality assumed. Solids Primary + Solids Filtrate (k) Design solids content and capture rate values assumed. Thickening Capture Thickened Thickened Digested Lagoon Capture 520 (o) (l) Solids content conservatively assumed to be higher than design rate of 15%. Filtrate Rate (k) Solids Solids VSS Solids Effluent Rate (k) 210 (p) (m) No flow reduction assumed through digestion. Digested solids content calculated from flow and TSS values. 660 90% 5,960 14,240 Reduction (n) 9,400 9,400 95% 470 (n) Typical anaerobic digestion VSS reduction assumed (WEF, 1998). 520 4,720 12,110 40% 7,270 7,270 360 (o) Based on assumed filtrate BOD concentration of 2,000 mg/l. 76,290 gpd 79% (d) 85% (c) 77% (c) 77% (d) 31,380 gpd (p) Based on assumed filtrate NH 4-N concentration of 800 mg/l per Figure 16-2 of EPA Process Design Manual 11,910 gpd 36,730 gpd 36,730 gpd (m) 36,730 gpd for Sludge Treatment and Disposal, 1979. NOTES: (d) Assumed VSS:TSS ratio. (e) Assumed removals based on WWTP primary clarifier data for February through August 2012. (f) Primary solids content based on discussions with City staff. (g) Based on value from BioWin secondary process model. (h) Based on WAS loads predicted by BioWin secondary process model, and flow at an assumed 0.9% solids content. Not to Scale LEGEND: 1,000 BOD Value (lb/day, unless noted otherwise) Solids flow through liquid treatment 1,000 NH 4 -N Value (lb/day, unless noted otherwise) Solids flow through solids treatment 1,000 TSS Value (lb/day, unless noted otherwise) Solids flow recycle streams 1,000 VSS Value (lb/day, unless noted otherwise) Solids flow external to mass balance (grit disposal) 1,000 Assumed VSS percentage of TSS 1,000 Flow (units as indicated) 1,000 Values used for iterative process FIGURE 2 WWTP Secondary and Tertiary Improvements Project ANNUAL AVERAGE LOAD CONDITION SOLIDS MASS BALANCE (6.6 MGD ANNUAL AVERAGE FLOW CONDITION)
Last saved: 6/10/13 3:54 PM Charles Hardy; N:\Clients\011 Davis\00-12-42 WWTP Design-Build\ENGR\Solids Handling\[Davis-Solids Balance Figure2.xlsm]Maximum Monthly Secondary Return Activated Sludge (RAS) (i) Grit Primary Biological Process Secondary 264,830 Cloth Disk (i) Headworks Removal Sedimentation Reactor Volume = 2.7 Mgal Clarification 212,740 (c) 80% 3.5 mgd Filtration Filter Return Backwash 170 Flow 0 4.5% 170 130 76% (c) 310,000 gpd Raw Combined Primary Influent Influent Influent Removal (e) Primary Effluent Secondary Process Secondary Effluent Tertiary Effluent 14,400 15,230 Solids 15,230 30% 10,660 182 mg/l 3,000 mg/l Bioreactor 290 5 mg/l (j) 120 2mg/L (j) 2,240 2,510 Removal 2,510 1% 2,480 42 mg/l 67,550 lb Effluent 60 1 mg/l (j) 60 1mg/L (j) Mixed 13,800 15,490 0.50% (b) 15,410 65% 5,390 92 mg/l 2,410 mg/l 264,070 290 5 mg/l (j) 120 2mg/L Liquor (j) 12,560 13,900 13,890 4,900 84 mg/l 54,270 lb 212,140 230 100 91% (a) 7.06 mgd 90% (c) 91% (d) 80% (c) 80% (d) 80% (g) 80% (d) 6.60 mgd 7.06 mgd 7.03 mgd MCRT = 7.1 days 10.55 mgd 6.90 mgd 6.90 mgd Primary Solids Waste Activated Sludge 4,570 (WAS) Grit 30 (g) 9,250 9,000 mg/l (h) 80 10,020 <-- 10,020 Secondary (g) 7,430 7,230 mg/l (h) 8 Primary 8,990 <-- 8,990 Solids (p) 80% (c) To Grit Classifier 10% (d) Solids (f) 90% (c) 0.9% 123,230 gpd To CCBs, then 3W and Disposal 4% 30,040 gpd or Receiving Water Digester Gas Generation 1.12 Assumes densitiy of gat is eqal to 0.86 tunes that of air (1.204) 5612.91 kg/d NOTES: Average of VSS-to-TSS ratios for influent samples for periods 7/31/12-8/6/12 and 10/29/12-11/11/12. (b) Assumed solids removal through headworks and grit tank. WAS Solids Biosolid Biosolids (c) Ratio calculated from VSS and TSS load values shown. Thickening Digesting Storage Lagoon Dewatering Dewatered (d) Assumed VSS:TSS ratio. Thickened Solids (k) 6% Digested Solids (m) 3.1% Dewatered Solids (l) 20% Biosolids 11,480 8,940 78% (d) 6,880 gpd (e) Assumed removals based on WWTP primary clarifier data for February through August 2012. Primary solids content based on discussions with City staff. Based on value from BioWin secondary process model. (h) Based on WAS loads predicted by BioWin secondary process model, and flow at an assumed 0.9% solids content. Assume no change in lagoon. (i) Based on WAS concentrations and calcluated RAS flow rate (50% of combined influent). Dewatering (j) Typical effluent quality assumed. Solids Primary + Solids Filtrate (k) Design solids content and capture rate values assumed. Thickening Capture Thickened Thickened Digested Lagoon Capture 660 (o) (l) Solids content conservatively assumed to be higher than design rate of 15%. Filtrate Rate (k) Solids Solids VSS Solids Effluent Rate (k) 270 (p) (m) No flow reduction assumed through digestion. Digested solids content calculated from flow and TSS values. 920 90% 8,330 18,350 Reduction (n) 12,080 12,080 95% 600 (n) Typical anaerobic digestion VSS reduction assumed (WEF, 1998). 740 6,690 15,680 40% 9,410 9,410 470 (o) Based on assumed filtrate BOD concentration of 2,000 mg/l. 106,580 gpd 80% (d) 85% (c) 78% (c) 78% (d) 39,810 gpd (p) Based on assumed filtrate NH 4-N concentration of 800 mg/l per Figure 16-2 of EPA Process Design Manual 16,650 gpd 46,690 gpd 46,690 gpd (m) 46,690 gpd for Sludge Treatment and Disposal, 1979. Not to Scale LEGEND: 1,000 BOD Value (lb/day, unless noted otherwise) Solids flow through liquid treatment 1,000 NH 4 -N Value (lb/day, unless noted otherwise) Solids flow through solids treatment 1,000 TSS Value (lb/day, unless noted otherwise) Solids flow recycle streams 1,000 VSS Value (lb/day, unless noted otherwise) Solids flow external to mass balance (grit disposal) 1,000 Assumed VSS percentage of TSS 1,000 Flow (units as indicated) 1,000 Values used for iterative process FIGURE 3 WWTP Secondary and Tertiary Improvements Project FULL POPULATION MAXIMUM MONTH LOAD CONDITION SOLIDS MASS BALANCE (6.6 MGD ANNUAL AVERAGE FLOW CONDITION)
Last saved: 6/10/13 3:54 PM Charles Hardy; N:\Clients\011 Davis\00-12-42 WWTP Design-Build\ENGR\Solids Handling\[Davis-Solids Balance Figure2.xlsm]Maximum Monthly Return Activated Secondary Sludge (RAS) Grit Primary Biological Process Headworks Removal Sedimentation Reactor Volume = 2.7 Mgal Clarification (i) 210,860 (c) 80% 3.5 mgd (i) Secondary 263,940 Cloth Disk Filtration Filter Return Backwash 180 Flow 10 4.5% 180 140 78% (c) 310,000 gpd Raw Combined Primary Influent Influent Influent Removal (e) Primary Effluent Secondary Process Secondary Effluent Tertiary Effluent 12,200 12,970 Solids 12,970 30% 9,080 155 mg/l 3,000 mg/l Bioreactor 290 5 mg/l (j) 110 2mg/L (j) 2,060 2,300 Removal 2,300 1% 2,280 39 mg/l 67,550 lb Effluent 60 1 mg/l (j) 50 1mg/L (j) 12,700 14,170 0.50% (b) Mixed 14,100 65% 4,940 85 mg/l 2,400 mg/l 263,250 290 5 mg/l (j) 110 2mg/L Liquor (j) 11,560 12,720 12,710 4,500 77 mg/l 54,040 lb 210,600 230 90 91% (a) 7.03 mgd 90% (c) 91% (d) 80% (c) 80% (d) 80% (g) 80% (d) 6.60 mgd 7.03 mgd 7.01 mgd MCRT = 8.6 days 10.52 mgd 6.90 mgd 6.59 mgd Primary Solids Waste Activated Sludge 3,890 (WAS) Grit 20 (g) 7,600 9,000 mg/l (h) 70 9,160 <-- 9,160 Secondary (g) 6,070 7,190 mg/l (h) 7 Primary 8,210 <-- 8,210 Solids (p) 80% (c) To Grit Classifier 10% (d) Solids (f) 90% (c) 0.9% 101,250 gpd To CCBs, then 3W and Disposal 4% 27,460 gpd or Receiving Water Digester Gas Generation 1.12 Assumes densitiy of gat is eqal to 0.86 tunes that of air (1.204) 5612.91 kg/d NOTES: Average of VSS-to-TSS ratios for influent samples for periods 7/31/12-8/6/12 and 10/29/12-11/11/12. (b) Assumed solids removal through headworks and grit tank. WAS Solids Biosolid Biosolids (c) Ratio calculated from VSS and TSS load values shown. Thickening Digesting Storage Lagoon Dewatering Dewatered (d) Assumed VSS:TSS ratio. Thickened Solids (k) 6% Digested Solids (m) 3.1% Dewatered Solids (l) 20% Biosolids 10,000 7,790 78% (d) 6,000 gpd (e) Assumed removals based on WWTP primary clarifier data for February through August 2012. Primary solids content based on discussions with City staff. Based on value from BioWin secondary process model. Based on WAS loads predicted by BioWin secondary process model, and flow at an assumed 0.9% solids content. Assume no change in lagoon. (i) Based on WAS concentrations and calcluated RAS flow rate (50% of combined influent). Dewatering (j) Typical effluent quality assumed. Solids Primary + Solids Filtrate (k) Design solids content and capture rate values assumed. Thickening Capture Thickened Thickened Digested Lagoon Capture 590 (o) (l) Solids content conservatively assumed to be higher than design rate of 15%. Filtrate Rate (k) Solids Solids VSS Solids Effluent Rate (k) 230 (p) (m) No flow reduction assumed through digestion. Digested solids content calculated from flow and TSS values. 760 90% 6,840 16,000 Reduction (n) 10,530 10,530 95% 530 (n) Typical anaerobic digestion VSS reduction assumed (WEF, 1998). 610 5,460 13,670 40% 8,200 8,200 410 (o) Based on assumed filtrate BOD concentration of 2,000 mg/l. 87,580 gpd 80% (d) 85% (c) 78% (c) 78% (d) 35,130 gpd (p) Based on assumed filtrate NH 4-N concentration of 800 mg/l per Figure 16-2 of EPA Process Design Manual 13,670 gpd 41,130 gpd 41,130 gpd (m) 41,130 gpd for Sludge Treatment and Disposal, 1979. Not to Scale LEGEND: 1,000 BOD Value (lb/day, unless noted otherwise) Solids flow through liquid treatment 1,000 NH 4 -N Value (lb/day, unless noted otherwise) Solids flow through solids treatment 1,000 TSS Value (lb/day, unless noted otherwise) Solids flow recycle streams 1,000 VSS Value (lb/day, unless noted otherwise) Solids flow external to mass balance (grit disposal) 1,000 Assumed VSS percentage of TSS 1,000 Flow (units as indicated) 1,000 Values used for iterative process FIGURE 4 WWTP Secondary and Tertiary Improvements Project NON-STUDENT PERIOD MAXIMUM MONTH LOAD CONDITION SOLIDS MASS BALANCE (6.6 MGD ANNUAL AVERAGE FLOW CONDITION)
Page 15 Other major calculations used for the solids balances are summarized as follows: Combined influent flows and loads are calculated from the raw influent flows and loads, which are defined as described in Table 4, combined with the calculated flows and loads for the recycle streams (tertiary filter backwash, WAS thickening filtrate, and biosolids dewatering filtrate). Primary effluent concentrations are calculated from primary effluent loads and the combined influent flow (which is assumed to be equal to the primary effluent flow). For primary solids, thickened WAS, and dewatered biosolids, flow is calculated from the solids content value assumed and the calculated TSS loads. The remaining flows are calculated as the sum of or difference between the process influent and effluent flows. Clarified effluent and tertiary effluent loads for BOD, NH 4 -N, and TSS are calculated from the respective effluent concentrations and flows presented in Table 4.. Evaluation of Existing Anaerobic Digestion Capacity The solids balance was used to define the solids loadings to the digesters under the following three load conditions: Annual Average, Full Population Maximum Month, and Non-Student Period Maximum Month. The calculated primary and secondary BOD, TSS, and VSS loads from the solids balance calculations are summarized in Table 5.
Page 16 Table 5. Reference Project Primary and Secondary Solids Digester Design Loads Parameter Annual Average Load Condition Full Population Maximum Month Non-Student Period Maximum Month Flow, gallons per day Primary Solids 24,820 30,040 27,460 Secondary Solids 88,200 123,230 101,250 Thickened Secondary Solids 11,910 16,650 13,670 Total Feed to Digester 36,730 46,690 41,130 TSS Load, lb/day Primary Solids 8,280 10,020 9,160 Secondary Solids 6,620 9,250 7,600 Thickened Secondary Solids 5,960 8,330 6,840 Total Feed to Digester 14,240 18,350 16,000 VSS Load, lb/day Primary Solids 7,390 8,990 8,210 Secondary Solids 5,240 7,430 6,070 Thickened Secondary Solids 4,720 6,690 5,460 Total Feed to Digester 12,110 15,680 13,670 To evaluate the ability of the existing anaerobic digesters to meet the Project Criteria under the Reference Project, SRTs and VSS Loading Rates under the load conditions of interest need to be calculated. Accordingly, SRTs and VSS Loading Rates have been calculated for the two existing digesters operating under Average Annual and Full Population Maximum Month Load Conditions, based on the projected solids loads shown in Table 5. In addition, SRTs and VSS Loading Rates have been calculated for only Digester 2 operating under annual average load conditions and the Non-Student Period Maximum Load Condition, based on the projected solids loads from Table 5. These calculated SRTs and VSS Loading Rates are summarized in Table 6. Table 6. Reference Project Digester SRT and VSS Loading Rates Load Condition Digesters In Service SRT, days VSS Loading, lb/day/ft 3 Full Population Maximum Month 24 0.11 1 and 2 30 0.08 Annual Average 14 0.18 2 Non-Student Maximum Month 12 0.20
Page 17 The capacity of the existing digesters is adequate to meet the Design Criteria, as indicated by the values in Table 6. Specifically, the following statements can be made with respect to the Design Criteria: The projected SRT with both digesters in service under both the Full Population Maximum Month and Annual Average Load Conditions will exceed the minimum 15-day criterion. The projected SRT with only Digester 2 in service under the Non-Student Period Maximum Month and Annual Average Load Conditions will exceed the minimum 10-day criterion. The projected VSS Loading Rate with both digesters in service under the Full Population Maximum Month and Annual Average Load Conditions is well below the maximum 0.15 lb/day/ft 3 criterion. The projected VSS Loading Rate with only Digester 2 in service under the Full Population Maximum Month and Annual Average Load Conditions does not exceed the maximum 0.20 lb/day/ft 3 criterion. Therefore, neither projected SRTs nor VSS loading rates are expected to be problematic with both digesters in service, providing the digesters are operated in parallel as complete-mix, high-rate single phase, mesophilic units. With only one digester in service and under Non-Student Period Maximum Month Loading Conditions, the SRT may be as low as 12 days and the VSS loading rates may be as high as 0.20 lb/day/ft 3 digester volume, but these levels are not considered problematic providing: Biosolids are disposed in a manner that does not require Class B treatment; Both Digesters are mechanically mixed; Digesters are cleaned during the Non-Student Period in the same or consecutive years; and Digester 2 is taken out-of-service and cleaned before Digester 1. Solids Thickening Technologies Gravity Belt Thickeners (GBTs) Description. GBTs are manufactured by many companies. Prominent, experienced manufacturers include Ashbrook Simon-Hartley, Arus-Andritz, and Komline Sanderson. Dilute sludge (typically 0.5 percent to 1.0 percent) is introduced at the feed end of a horizontal filter belt. As the slurry makes its way down the moving belt free water drains through the porous belt. The solids are continuously turned, encouraging the drainage of more water. Sludge is discharged at the end of the horizontal filter belt as a pumpable thickened sludge. The GBT can be an open frame design or can be enclosed and connected to an exhaust fan to remove odorous compounds released during the dewatering process. The enclosure also minimizes aerosol dispersion caused by the pressurized sprays used to clean the belts. Enclosures are particularly useful when BFPs are installed inside buildings.
Page 18 Performance and Operations. General GBT process performance and operations considerations are summarized on Figure 5. Track Record. GBTs have an excellent record of durability and WAS thickening performance and have been used for sludge dewatering for years at WWTPs throughout the country. GBTs typically require periodic checks by plant staff and greater operator attention to maintain stable operation, however instrumentation and controls can be provided to automate and monitor sludge and polymer feed, equipment operation, and TWAS pumping. Dissolved Air Flotation Thickeners (DAFTs) Description. DAFTs are manufactured by many companies, including WesTech, Ovivo, and Siemens. DAFTS thicken sludge by dissolving air in the diluted sludge under pressure and then releasing the air at atmospheric pressure in a flotation tank. The dissolved air is released in the form of tiny bubbles. The bubbles adhere to the suspended matter, causing the sludge to float to the surface and form a froth layer which is then removed by a skimmer. Performance and Operations. General DAFT process performance and operations considerations are summarized on Figure 6.
Sludge Thickening Technologies Gravity Belt Thickener (GBTs) Figure 5 Prominent Manufacturers Experience Process Performance TWAS Concentration Polymer Usage Capture Operating Speed Wash Water Consumption Power Requirements Maintenance Considerations Operations Considerations Level of Operator Attention Level of Automation Operator Exposure to Pathogens, Aerosols, & Odors Noise Generation Required Ancillary Processes Ashbrook Simon Hartley, Arus Andritz, Komline Sanderson, others Established, well proved technology with numerous installations. To 8% TS (w/polymer addition) 8 20 lbs/dt 90 98 %, with polymer addition 1 3 rpm 100 gallons per operating hour 2 meter (300 gpm) 3HP 3 meter (450 gpm) 5 HP Long track record, reliable bearing and belt designs. Fine tuning of sludge feed rate and polymer dose required to adjust to changing sludge characteristics. TWAS pumping may require periodic inspection. Can be automated and monitored through SCADA system, parameters typically adjusted are feed rate and polymer dose. Low, if equipped with enclosure and exhaust fans. High Flocculation Tank, Polymer Feed System, Cake Conveyance System
Sludge Thickening Technologies Dissolved Floatation Thickeners (DAFTs) Prominent Manufacturers Experience Process Performance TWAS Concentration Without polymer addition With polymer addition Polymer Usage Capture Operating Speed Power Requirements Maintenance Considerations Operations Considerations Level of Operator Attention Level of Automation WesTech, Ovivo, Siemens, others Established, well proved technology with numerous installations. Up to 4% TS Up to 6% TS 8 20 lbs/dt 95 98%, with polymer addition 2 rpm Significant (e.g. 60 hp) Long track record establishes known maintenance requirements Stable process that is typically run continuously. Can be automated and monitored through SCADA system. Figure 6 Operator Exposure to Pathogens, Aerosols, & Odors Noise Generation Required Ancillary Processes Low Moderate Polymer Feed System, Pressurization Tank and Recycle Pump System, TWAS pumping, pressurized air (60 80 psig) supply.
Page 21 Track Record. DAFTs have an excellent record of durability and WAS thickening performance and have been used for sludge dewatering for years at WWTPs throughout the country. Although both DAFTs and GBTs have been utilized for WAS thickening for many years, DAFTs have been used for a longer time. If solids loadings are kept below approximately 2 lb/sf/hr, DAFTs lend themselves to continuous, unattended operation. Rotary Drum Thickeners (RDTs) Description. RDTs are manufactured by Parkson, Vulcan, Ashbrook Simon Hartley, and other well-known wastewater equipment manufacturers. A rotary drum thickener, similar to a gravity belt thickener, achieves solid-liquid separation by coagulation and flocculation of solids and drainage of free water through a rotating porous media. The porous media typically consists of a drum with wedge wires, perforations, or stainless steel mesh screen. The thickener consists of an internally fed rotary drum with an internal screw, which is used to transport the thickened sludge out of the drum; the drum rotates on trunnion wheels and is driven by a variable speed drive. Sludge is usually polymer conditioned and mixed in a flocculation tank prior to thickening. The conditioned sludge is then fed directly to the interior of the drum via piping to one end of the drum. As the drum rotates free water passes through the drum perforations into a collection trough, leaving thickened sludge inside the drum that is discharged by the internal screw at the opposite end into a hopper. A continuous fixed spray bar extends along the entire length of the drum to clean and prevent blinding of the screen. Performance and Operations. General RDT process performance and operations considerations are summarized on Figure 7. Track Record. RDTs are an established, well-proved technology with numerous installations. Parkson alone has over 200 installations dating back over 20 years. Vulcan lists 47 installations. Thickening Screw Thickener Description. Huber manufactures a screw thickener which appears to be derived from their screw press technology, the latter used for dewatering digested sludge. A screw thickener is somewhat similar to a RDT in that it accomplishes thickening by coagulation and flocculation of solids and drainage of free water through a porous media. The porous media is a wedge-screen basket. A screw, slowly rotating with variable speed, conveys the sludge gently upward through the inclined basket. Water drains through the basket. The screw flights are provided with a brush for continuous internal cleaning of the wedge section basket. Periodically the wedge section basket is also cleaned with spray water from the outside. Spray bars rotate around the basket, but within the machine. The screw pushes the thickened sludge to the upper end of the wedge section basket where it drops through a chute into the thickened sludge pump that conveys it to further treatment. Performance and Operations. General GBT process performance and operations considerations are summarized on Figure 8. Track Record. Screw thickeners are a relatively new technology, but there are a fair number of existing installations.
Sludge Thickening Technologies Rotary Drum/ Rotary Disk Thickeners Figure 7 Prominent Manufacturers Experience Process Performance TWAS Concentration Polymer Usage Capture Operating Speed Wash Water Consumption Power Requirements Maintenance Considerations Operations Considerations Level of Operator Attention Level of Automation Operator Exposure to Pathogens, Aerosols, and Odors Noise Generation Required Ancillary Processes Parkson, Vulcan, Ashbrook Simon Hartley, Huber others Established, well proved technology with numerous installations. To 8% TS (w/polymer addition) 8 20 lbs/dt 90 98 %, with polymer addition Variable speed, 2 10 rpm 20 30 gpm For 400 gpm unit, 8 hp. Mechanical simplicity minimizes maintenance. Low. Simple process. Grease trunnion wheels on startup. Can be automated and monitored through SCADA system, parameters typically adjusted are feed rate and polymer dose. Minimal, if equipped with full enclosure. Very Low Flocculation Tank, Polymer Feed System, Cake Conveyance System
Sludge Thickening Technologies Screw Press Figure 8 Description Screw Thickener Prominent Manufacturers Huber Experience Newer technology. 14 installations reported by Huber, including Lake Arrowhead, Goleta, Dinuba, and Oakdale in CA Process Performance TWAS Concentration To 8% TS (w/polymer addition) Polymer Usage 8 20 lbs/dt Capture Greater than 95 % Operating Speed Approximately 1 2 rpm Wash Water Consumption 38 gpm @ 73 psi Power Requirements For 350 gpm unit, approximately 5 HP for drive, 0.25 HP for stirrer drive, and 0.25 HP for spray bar drive. Operations Considerations Level of Operator Attention Can run continuously unattended. Level of Automation Fully automated Operator Exposure to Pathogens, Minimal, fully enclosure. Aerosols, and Odors Noise Generation Very low Required Ancillary Processes Flocculation Tank, Polymer Feed System, Cake Conveyance System Other Comments Newer technology but fair number of installations.
Page 24 Centrifuges Centrifuges have been used for many years at WWTPs throughout the country to both thicken WAS and dewater digested sludge. They are a well-proved thickening process that is more commonly used at large WWTPs. Centrifuges are discussed in greater detail in Section 5 of this TM. Reference Project Solids Thickening Technology The Reference Project includes three (3) gravity belt thickener (GBTs) units (two duty and one standby) such as the AquaBelt, manufactured by Ashbrook Simon Hartley or equivalent unit manufactured by Andritz. Each unit is fitted with 2-meter wide belts. The GBTs are sized such that the two duty units are capable of thickening the volume of biosolids generated under the Maximum Month Load Condition when operated 40 hours per week and based on a WAS flow of 500 gpm. The GBTs are designed to take WAS at 0.5 to 1.0 percent solids, and produce a thickened sludge with at least 6 percent solids with a solids capture rate of not less than 90 percent. The GBTs are supported by a dedicated polymer dosing facility. Polymer consumption to achieve 6 percent TWAS does not exceed 10 lb/dt of WAS processed. Solids Dewatering Technologies Belt Filter Presses (BFP) Description. BFPs are manufactured by many manufacturers. Prominent, experienced manufacturers include Ashbrook Simon-Hartley, Arus-Andritz, and Komline Sanderson. The feed sludge to be dewatered is introduced from a hopper between two filter cloths (supported by perforated belts) which pass through a convoluted arrangement of rollers. As the belts are fed through the rollers, water is squeezed out of the sludge. When the belts pass through the final pair of rollers in the process, the filter cloths are separated and the filter cake is scraped off into a suitable container. The BFP can be enclosed and connected to an exhaust fan to remove hydrogen sulfide and other odorous compounds released during the dewatering process. The enclosure also minimizes aerosol dispersion caused by the pressurized sprays used to clean the belts. Enclosures are particularly useful when BFPs are installed inside buildings. Performance and Operations. General BFP process performance and operations considerations are summarized on Figure 9. Track Record. BFPs have an excellent record of durability and performance on a wide variety of anaerobically digested municipal wastewater sludges and have been used for sludge dewatering for years at WWTPs throughout the country. BFPs are a mature, well-proved technology against which the newer technologies should be measured. BFPs do require periodic checks during operation by plant staff and arguably more initial startup and shutdown time in a given shift.
Sludge Dewatering Technologies Belt Press Figure 9 Prominent Manufacturers Experience Cake Concentration Polymer Usage Operating Speed Wash Water Consumption Power Requirements Maintenance Considerations Level of Automation Operator Exposure to Pathogens, Aerosols, and Odors Noise Generation Required Ancillary Processes Ashbrook Simon Hartley, Arus Andritz, Komline Sanderson, others Established, well proved technology with numerous installations. 15 20% TS 10 20 lbs/dt 1 5 rpm 40 gpm/meter belt width @85 psi Long track record establishes known maintenance requirements. Reliable bearing and belt designs. Not typical. Operator attention required at start up and shutdown, as well as periodically throughout the run. Low, if equipped with enclosure and exhaust fans; high if not enclosed. Moderate Flocculation Tank, Polymer Feed System, Cake Conveyance System
Page 26 Rotary Fan Presses Description. Rotary fan presses (RFPs) are manufactured by two companies Fournier Industries (Fournier) and Prime Solutions Inc. (Prime Solutions). The principle of operation is relatively simple. The thickened sludge from the lagoon is conditioned with a dose of polymer in a flocculation tank that is separate from the RFP. This promotes flocculation and coagulation of the solids. The solids are then pumped into the RFP, which consists of a series of hollow discs that enclose stainless steel screens. As the filtrate passes through the screens, a cake begins to form inside the screens. The screens are in constant, slow rotation (approximately 0.5-3 revolutions per minute) and act to slowly move the sludge towards the outlet. During this process, water drains through the screens. The outlet port is pressure controlled, acting to squeeze the sludge to remove more water just prior to discharge. Performance and Operations. General RFP process performance and operations considerations are summarized on Figure 10. Track Record. While the technology was developed in the late 1980 s by the predecessor to Fournier, the rotary press has been used in the U.S. wastewater applications only since 1999. Until fairly recently, neither Fournier nor Prime Solutions had municipal installations in California or elsewhere on the West Coast. WWTPs contacted by telephone reported a high degree of satisfaction with both the mechanical function and the process performance of the RFP. The ease of operation was a consistent and notable topic of discussion with these plant operators. The manufacturer s follow-on service was considered very good and prompt. Dewatering Screw Presses Description. The two prominent companies that manufacture screw presses for dewatering municipal sludge are Fukoku Kogyo Company (FKC) and Huber Technology, Inc. Both manufacturers produce screw presses that are of the same fundamental design with some minor differences and features. A screw press consists of an auger-shaped conveyor that is mounted inside a cone-shaped screen. Liquid sludge enters the press at the feed end and is moved through the press by a stainless steel rotating screw. As the sludge is advanced, filtrate flows out through the cone-shaped perforated screen. The frictional forces at the sludge-screen interface coupled with increased pressure caused by the gradual reduction in the screen diameter produces the dewatered sludge cake. The dewatered sludge cake is discharged from the press on to (or into) a conveyor or directly into a dumpster or truck. The screws are fitted with brushes for internal cleaning of the screen. Periodically the screen basket is also cleaned with spray water from the outside. Spray bars rotate around the basket, but within the enclosure of the press. Performance and Operations. General dewatering screw press process performance and operations considerations are summarized on Figure 11.
Sludge Dewatering Technologies Fan Press Figure 10 Prominent Manufacturers Fournier, Prime Solutions Experience Newer technology. Fournier lists 24 installations dating from year 2000. Cake Concentration 15 20% TS Polymer Usage 10 20 lbs/dt Operating Speed 1 3 rpm Wash Water Consumption 100 gallons per operating hour Power Requirements 6 channel unit has 20 HP motor drive. Maintenance Considerations Short time on market and relatively small number of installations makes long term track record unclear, however, installations report few maintenance issues. Level of Operator Attention Fine tuning of sludge feed rate and polymer dose during the operating period may be necessary to adjust to changing sludge characteristics during the course of the day. Level of Automation Can be automated and monitored through SCADA system. Operator Exposure to Low Pathogens, Aerosols, and Odors Noise Generation Low Required Ancillary Flocculation Tank, Polymer Feed Processes System, Cake Conveyance System
Sludge Dewatering Technologies Screw Press Figure 11 Prominent Manufacturers FKC (Fukoku), Huber Experience Newer technology. FKC lists 38 installations dating from year 2001. Huber lists 47 installations dating from year 2006. Cake Concentration 15 20% TS Polymer Usage 10 20 lbs/dt Operating Speed 0.04 0.12 rpm Wash Water Consumption Power Requirements 80 gpm unit requires 7.5 HP motor drive. Maintenance Considerations Relatively small number of installations makes long term track record unclear, however, installations contacted by phone report few maintenance issues. Level of Operator Attention Fine tuning of sludge feed rate and polymer dose during the operating period may be necessary to adjust to changing sludge characteristics during the course of the day. Level of Automation Can be automated and monitored through SCADA system. Operator Exposure to Low Pathogens, Aerosols, and Odors Noise Generation Required Ancillary Processes Low Flocculation Tank, Polymer Feed System, Cake Conveyance System.
Page 29 Track Record. Since the screw press technology is relatively new to the wastewater industry, the number of completed and operational installations by either company is rather limited. FKC has approximately 20 operating installations while Huber has six installations. A phone survey of a portion of these installations was undertaken to obtain an indication of operational performance and overall track record. In general, WWTPs report good experience with the screw presses and operating data provided by these facilities confirmed the manufacturer s performance claims. Very few maintenance problems were reported (isolated instances of bearing replacements and seal repairs) although a high number of maintenance issues would not be expected for the short time these facilities have been in operation. Centrifuges Description. Solid bowl decanter centrifuges for dewatering anaerobically digested sludge are manufactured by the same companies that produce similar centrifuges for WAS thickening. These manufacturers include Westfalia Separator, Alfa Laval, Arus Andritz, and a number of other well-known companies. A centrifuge dewaters sludge by using centrifugal create solid/liquid separation. This force is created in a conical-cylinder bowl that rotates at high speed (2500-3500 rpm). The sludge particles are pressed against the bowl and conveyed out of the centrifuge by a screw that rotates at a slightly different speed than the bowl (a few rpms). Performance and Operations. General dewatering centrifuge process performance and operations considerations are summarized on Figure 12. Track Record. Centrifuge dewatering of digested sludge is a mature, well-proved process. They are installed at numerous WWTPs throughout the U.S. Reference Project Solids Dewatering Technology The Reference Project includes two (2) belt filter presses (BFP) units such as the Klampress as manufactured by Ashbrook Simon Hartley or equivalent unit manufactured by Andritz. Each unit is fitted with 2-meter wide belts. The BFPs are sized such that the two units are capable of dewatering the volume of biosolids generated under the Maximum Month Load Condition over a four-week period where units are operated (including time needed for preparation, start-up, and shut-down) 6 hours/day, 5 days/week. The BFPs are designed to take digested biosolids at 2 to 3 percent solids, and produce a dewatered biosolids product with at least 15 percent solids content with a solids capture rate of not less than 95 percent.
Sludge Dewatering Technologies Centrifuge Figure 12 Prominent Manufacturers Experience Cake Concentration Polymer Usage Operating Speed Power Requirements Maintenance Considerations Level of Operator Attention Level of Automation Operator Exposure to Pathogens, Aerosols, and Odors Westfalia Separator, Alfa Laval, Andritz, others Established, well proved technology 18 22% TS 10 20 lbs/dt 3500 rpm 200 500 HP, depending on size Long track record establishes known maintenance requirements. Reliable bearing and seal designs. Scroll requires re surfacing every 10,000 to 15,000 operating hours, potentially requiring a second redundant unit. Fine tuning of sludge feed rate and polymer dose during the operating period may be necessary. Can be automated and monitored through SCADA system. Parameters typically adjusted are feed rate, polymer dose, sludge pool depth, and differential speed between scroll and bowl. Low Noise Generation High Required Ancillary Processes Flocculation Tank, Polymer Feed System, Cake Conveyance System