BELT FILTER PRESS PRODUCES SIMILAR PERFORMANCE TO CENTRIFUGATION: SIDE-BY-SIDE DEWATERING RESULTS FROM THREE ANAEROBIC DIGESTED BIOSOLIDS

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1 BELT FILTER PRESS PRODUCES SIMILAR PERFORMANCE TO CENTRIFUGATION: SIDE-BY-SIDE DEWATERING RESULTS FROM THREE ANAEROBIC DIGESTED BIOSOLIDS Mohammad Abu-Orf 1, Peter Nese 2, William Suchodolski 3 1 Biosolids Technology Leader, Metcalf and Eddy/AECOM, 1700 Market Street, Suite 1700, Philadelphia, PA ( mohammad.abu-orf@m-e.aecom.com) 2 CH2MHILL, at the time of conducting this project, Mr. Nese was the Project Manager for OCUA 3 Engineering Manager, Ocean County Utilities Authority, Bayville, NJ ABSTRACT The recent Biosolids Management Plant conducted by the Ocean County Utilities Authority (OCUA) recommended side-by-side pilot testing of belt filter press and centrifuge technology to select the best technology to dewater their anaerobically digested biosolids from the authority three plants. Initially, screening of available vendors for providing dewatering technologies and conducting the side-by-side piloting were conducted and two vendors were selected: 0.6 meter Wiklepress of Ashbrook and skid mounted ALDEC 406 centrifuge of Alfa Laval. A detailed pilot testing protocol for both technologies and guidance support documents including all necessary support from both vendors and the OCUA staff to achieve successful testing were finalized prior to conducting the testing. The testing protocol called for one week testing at each plant. The testing protocol was consistent for all plants. Side-by-side testing commenced on May 7 th and ended on May 25 th, The performance of each technology was measured through achieving consistent cake solids at the end of testing at each facility with a certain polymer dose and percent solids recovery higher than 95%. Based on the obtained cake solids with the used polymer dose, and the electricity consumed for dewatering during the consistent performance testing, the operating costs for each technology were estimated. The testing showed that the belt filter press technology produced similar performance to the centrifuge at all three plants and in some cases using lower polymer doses to achieve the same cake dryness. Based on estimated operating costs, BFP technology was recommended to dewatering biosolids at all three facilities. KEYWORDS Dewatering, pilot testing, Belt filter press, high solids, centrifuge 527

2 INTRODUCTION The Ocean County Utilities Authority (OCUA) operates three different wastewater treatment facilities ranging in size from 20 million gallons a day (MGD) to 32 MGD: The Northern, Central, and Southern Water Pollution Control Facilities (WPCF). The three plants combined produce approximately 9,000 dry tons per year of biosolids. The biosolids at Northern and Southern facilities are anaerobically digested, thickened and hauled to the Central WPCF where it is blended with their thickened anaerobically digested biosolids, dewatered and dried using Andritz rotary drum dryers to produce a pellet known as OCEANGRO. Dewatering the blended thickened biosolids at the CWPCF is done via three, 2-m Andritz belt filter presses with one press as a stand-by and achieves an average of 16% cake solids. The OCUA conducted biosolids master planning to evaluate the long term viability of the existing solids handling processes at the three treatment plants and evaluate future alternative for biosolids management. Results from the master planning indicate that modifications to the existing solids handling practices will be required to accommodate future growth. The Authority decided on a phased approach for implementing required expansions and upgrades in order to reduce capital and operating costs. The OCUA realizes the benefits form improved dewatering at the CWPCF and implementation of dewatering at the NWPCF and SWPCF to decrease operating cost and increase drying capacity. Based on the existing drying system capacity, it was determined that an averaged 21% cake solids must be achieved in order to avoid constructing a third dryer train. Details regarding this study are available in Laustsen et al. (2007). OCUA decided to conduct side-by-side pilot testing of both belt filter presses and centrifuge dewatering at all three plants. Based on the results of the pilot testing, the authority would select the dewatering technology to achieve cake solids grater than 21%, and determine the corresponding operating costs. Metcalf and Eddy, Inc/AECOM was selected for conducting the side-by-side pilot dewatering studies. In this study, the performance and operating costs of belt filter press and centrifuge technology was evaluated at the three treatment plants. This paper presents tasks conducted to successfully complete the side-by-side pilot dewatering including equipment selection, testing protocol, site visits and support needed from the vendor, and the authority personnel. This paper also presents the performance of BFP and centrifuge technology in terms of cake solids and solids recovery with polymer consumption. The operating costs for each technology, including polymer usage, estimated costs from producing cake solids, and energy usage at the three treatment plant are also presented in this paper. Finally, based on these results, a recommendation as to the type of technology to be implemented at each site is presented. EVALUATION OF DEWATERING TECHNOLOGY After review of available providers for centrifuges and BFP, three BFPs and three of centrifuge manufacturers were selected for detailed evaluation. The criteria for evaluation was agreed upon with OCUA before conducting the evaluation, and included: Existing installations for each dewatering equipment 528

3 Market reputability and quality of equipment Ability to provide equipment for piloting, piloting experience, and anticipated cost of piloting Equipment and support provided during piloting Amenability to conduct side-by-side testing within the established timeframe for project completion For BFP equipment, amenability to pilot test enzyme pretreatment The three centrifuge manufacturers evaluated include Andritz, Alfa Laval, and Westfalia. The three BFP manufacturer evaluated include Andritz, Siemens, and Ashbrook. Based on the evaluation criteria and availability of vendors for piloting, Alfa Laval skid mounted ALDEC 406 was selected for centrifuge piloting and the trailer mounted 0.6 m Winklepress of Ashbrook was selected for BFP piloting. DESCRIPTION OF THE PILOT TESTING PROTOCOL A well designed protocol is vital to obtaining information necessary for the equipment selection process and for future design of the selected dewatering equipment for each facility. This task was closely coordinated with the appointed personnel from each of the three WPCFs. Site visitations and meeting with appropriate persons from each site to establish the protocol were conducted. Since conditions and option may be unique to each facility, a separate testing protocol for each facility was provided. Separate protocols for BFP and centrifuge testing due to differences in operation parameters were also provided. Each vendor provided their own operating personnel and operations were monitored and supervised order to adhere to the agreed upon testing protocol. The sections below describe the main elements of the testing protocol. Side-by-side testing was performed at the three plants of OCUA over three consecutive weeks (one week per plant). The testing protocol called for one week testing at each plant starting from the Southern WPCF, moving to the Central WPCF and ending with the Northern WPCF. The testing protocol was consistent for all plants. All samples during testing were collected by the consultant staff, and laboratory analyses were conducted by OCUA internal laboratory personnel. Testing commenced on May 7 th and ended on May 25 th, Performance of both technologies was measured as the % solids cake produced from the plant s anaerobically digested biosolids. Percent cake solids content was measured against operating factors, including: Specific loading rate (lb solids per hour) and in the BFP case lbs per hour per meter width of the belt. Usually cake solids increase with lower loading rates, however, this requires higher capital investment for purchasing more equipment. Percent solids recovery (%TSS), which should be greater than 95%, it is well documented that solids recovery can be sacrificed to obtain higher cake solids. Polymer dose in lb active content/dry ton (DT). 529

4 Energy consumption (kw-h/dt solids) was measured for cost comparison. Other comparative information such as the ease of bringing up the equipment to a steady state after an operational parameter change or equipment upset, and required operator s attention was also recorded. Polymer Selection Polymer selection for each plant was the responsibility of the vendor. The vendor was responsible for providing the entire polymer required for the duration of the testing. Cost alone was not the determining factor for polymer selection. Alfa Laval opted to use a representative of PolyDyne for on-site jar testing for polymer selection for each plant s biosolids. At Ashbrook s request, OCUA sampled biosolids from each plant as it leaves the secondary digester to be tested for polymer selection at Ashbrook s laboratory for polymer selection during pilot testing. Sampling Protocol Sampling protocol required sample collection throughout the testing. All samples were tested using the OCUA facilities laboratory utilizing OCUA personnel. Table 1 shows the analysis conducted by the plant and the frequency of testing for cake, biosolids, polymer, filtrate and centrate. The vendors used their own testing equipment for establishing guidelines of performance, however only laboratory data was used testing and reporting purposes. These results were entered in the corresponding data sheets. Results that have been found to be in obvious mistake, were disregarded from the testing data, and adjusted accordingly if required. The OCUA laboratory provided plastic containers for sampling and a mechanism for pick-up of the collected sample containers. Laboratory persons pick up the sample containers twice a day, one half-way through the testing day and the second at the end of the day. Empty, cleaned and labeled bottles were returned to the testing site during the first sample pick up event. Table 1. Frequency and Analysis Performed by OCUA Laboratory for Each Set of Sample SAMPLE SAMPLING ANALYSES Feed* Every parameter change and following min of steady run depending on unit TS Cake Every parameter change and following min of steady run depending on unit TS Filtrate/ Every parameter change and following centrate min of steady run depending on unit TSS Polymer Every new batch of polymer only TS *TVS will be measured once a day Day One of the Pilot Testing The vendors were requested to arrive with their equipment at the Southern WPCF either Sunday afternoon between 2-5pm or Monday morning the week of testing. Each vendor operated their own equipment under the consultant supervision. An average period of three (3) hours was allowed to install the mobile unit on site. The services of an electrician and other maintenance 530

5 persons from OCUA were provided to provide the electrical, sludge and water connections required. Connections requirements for pilot testing were finalized at earlier stages between the authority and the vendor. OCUA provided dewatered cake conveyance system and dumpster(s) for the produced cake, and was responsible for emptying and disposing the cake solids. OCUA provided water, electricity, biosolids and access to drains for wash water and centrate/filtrate. The remainder of Day One was considered a pre-testing period where the dewatering equipment operator was optimizing the equipment setup and getting familiar with the biosolids and dewatering parameters. The centrifuge operator adjusted parameters such as pond dept, differential speed, torque, sludge flow, polymer feed rate and polymer concentration in order to obtain the range of proper dewatering through observation of the centrate quality. Following this testing the pond depth will be established, the range of differential speed, polymer dose and sludge flow will be verified. The BFP s operator adjusted parameters such as belt speed, sludge flow, polymer feed and concentration, belt pressure in order to obtain proper dewatering. Some sampling will be taken this day for confirmation of visual observation of dewatering. Sampling during Day One was conducted for confirmation of visual observation of dewatering performed. Days Two and Three of the Pilot Testing During these testings, operational parameters were tested and changed systemically allowing one parameter to be changed at a time. Between each adjustment of any operating parameter, a delay of minutes was used to assure the stability of the equipment prior to collecting samples. This constant pattern of operational and sampling procedures allowed maintaining a high level of precision in the results and laboratory data. Three types of testing were conducted during these days and discussed below. 1. Effect of Polymer Dose on Cake Solids and Determining Optimum Polymer Feed Range Four to five different polymer feeds (and thus doses) were tested at a constant, optimized biosolids flow rate and pond depth in the case of centrifuge and belt speed, belt pressure, and wash water (as determined from pre-testing). Based on Day One pre-testing observations, the polymer dosage was expanded beyond what is considered optimum, allowing lower and higher dosages to be practiced if required. Usually, testing start from the higher polymer feed rate and decrease in a stepwise manner. With each polymer dosage, the unit was allowed at least minutes of stabilization time and samples of unconditioned sludge and cake were collected for total solids measurements (%TS). Centrate/filtrate was collected for total solids and suspended solids measurements (%TS and %TSS). 2. Effect of Hydraulic and Solids Loading on Cake Solids At the optimum polymer dose determined earlier, and keeping other parameters constant, five different sludge flow rates were tested, two high and two low flow rates were used in addition to the near optimum flow rate. The objective of this testing is to test the performance of the dewatering device under higher loading conditions. In the case of the centrifuge, the differential speed and torque were adjusted based on the clarity of the centrate, and in the BFP case, the belt speed was adjusted to accommodate the biosolids flow rate. As before, with each flow rate, the 531

6 units were allowed ~ minutes of stabilization time and the different samples as discussed before were collected and analyzed. Day Four of the Pilot Testing Day Four was allocated for testing consistency of delivering cake solids and solids recovery for extended periods using operational parameters such as polymer dose, biosolids flow rate and other equipment setting parameters unchanged. Same samples as before will be collected every hour. This testing is considered performance testing using previously recognized operational parameters for the biosolids being testing. The testing results from this day were used in comparing the centrifuge and the BFP performance for the plant being tested, and was communicated with the vendors. Day Five of the Pilot Testing In this day, the vendor disassembled the pilot unit and transported to the next testing plant with the help of OCUA personnel, to be assembled the following Monday. PILOT TESTING REQUIREMENTS AND COORDINATION Requirements for the side-by-side pilot testing at each site were communicate, discussed and finalized with the dewatering vendors and OCUA appropriate staff from each treatment plant. Site visitations to each plant from Ashbrook persons for the BFP and Alfa Laval persons for the centrifuge were conducted in order to communicate and finalize testing requirements and support needed for successful pilot testing of the equipment. These requirements (staff, electricity, water, dumpsters, connection of dewatering trailer to the plant, filtrate/centrate drainage, etc,) were communicated to each plant through pilot testing guidance memorandum. Table 2 shows connection requirements for each pilot unit. Table 3 shows the division of responsibilities among the vendors, OCUA and the consultant during the pilot testing period. A point of contact person from each of the three facilities was responsible for all communications and pilot testing logistics. Table 2. Connection requirements for each pilot unit Requirements BFP/Ashbrook (1) Centrifuge/Alfa Laval (2) Equipment Size Trailer: 48 long, 8.6 wide, 13.3 high Skid: Length: 11.3 long, 6 wide, 8 high Power 480 volts at 100 Amps, 60 cycle/3 phase plus ground 400V. MIN/ at 100 Amps, 60 cycle/3 phase plus ground Water (potable or plant effluent if <30mg/LSS) 1 1/2" quick disconnect fitting, min 60psi 1 1/2" quick disconnect fitting, min 60psi Sludge Filtrate/Centrate 4" quick disconnect fittings 4" quick disconnect fittings 4" camlock connection 4" camlock connection Cake Container A 20 or 30 cubic yard dumpster for one day. No more 4.6 high A 20 or 30 cubic yard dumpster for one day 532

7 Sludge Conveyor Specs Discharge Height: 5 ft Discharge Height: 5 ft Distance from Skid: 12 ft (1) Trailer has sufficient hose and electrical cable to make connections within 50 ft only. Discharge filtrate and sludge feed connection will be made from the back of the trailer within 50 ft range (2) Skid has 50 foot electrical cable, and 50 Feet Suction/Discharge Hosing Table 3. Division of Responsibilities for Pilot Testing During Setup, testing and dismantling OCUA Consultant Vendor Setup and Connections Provide personnel for electrical, sludge and water connection Provide water, sludge and electricity Provide dumpster(s) for the sludge cake Provide access to drains for wash water, filtrate/centrate Oversee set-up and coordinate with OCUA Execute actual setup of the equipment Provide all needed polymer amount for the duration of the testing Testing Provide containers for sample collection on a continuous basis Provide containers labels Conduct total solids analysis on collected samples and provide tabulated results Empty dumpster and disposing the cake solids Disconnect Provide personnel to assist in disconnecting electricity, sludge and water Observe and assure that the testing adheres to the established protocol Complete daily data sheets during testing Collect samples and deliver labeled containers to OCUA labs for analysis Execute the testing and operate the pilot unit Test operating parameters based on established protocol Disconnect the pilot unit Transport unit to next site or departure 533

8 DEWATERING EQUIPMENT SETUP The first day of testing at each facility focused on proper setting of the equipment for delivery of biosolids to the dewatering devices and conveying the dewatered cake to the dumpster. Set up of the skid mounted Alfa Laval ALDEC 406 centrifuge and the polymer make-up and feeding system are shown in Figure 1. For this system, neat polymer solution as received from vendor was poured into a five-gallon container and allowed to flow by gravity to the polymer make up system. Dilution water, mainly tap water, is mixed with the polymer prior to conditioning the biosolids. The marked graduated cylinder shown in the figure was used to measure the polymer flow using a stop watch each time the polymer feed is changed. Figure 1. Skid Mounted Alfa Laval ALDEC 406 Centrifuge and Its Polymer Makeup System Figure 2 shows Ashbrook s trailer mounted Winklepress and the polymer makeup system. For the polymer system, either one gallon or two gallons (depending on the polymer needed to condition the sludge) of neat polymer was added to a 600 gallon mixing tank. Two tanks were available allowing continuous feed of polymer to the belt press. Regardless of the amount of polymer added, for every new batch made, a polymer sample was taken for total solids analysis and the measured TS was used in polymer feed calculations. The amount of polymer delivered in gallons per minute to dewatering was determined by using a stop watch to measure the time required to withdraw a certain volume of polymer from the tank. 534

9 Figure 2. Ashbrook s 0.6 m Trailer Mounted Winklepress and Its Polymer Makeup System OCUA s 6,000 gallon smooth-bore tank trailer shown in Figure 3 was used as an equalization tank for feeding both dewatering devices with biosolids. The biosolids from the secondary digester were pumped to the tank trailer slightly higher than the amount dewatered by both dewatering devices, thus allowing no stratification of the biosolids and delivering the same biosolids to both dewatering system. Figure 3. Tank Trailer and Hose for Delivering Biosolids to Both Dewatering Devices The dewatered biosolids from the BFP and centrifuge were conveyed to a container fabricated by the authority and then conveyed to the dumpster via a conveyor belt rented by the Authority. The container and conveyor belt to the dumpster was located between the BFP and Centrifuge. This setup is shown in Figure 4 below. 535

10 Figure 4. Cake Conveyance System from BFP and Centrifuge to the Dumpster EXAMPLE TESTING PROTOCOL RESULTS A large sum of data was collected during this side-by-side testing to be presented in this paper. The following sections represent examples of results only. Example Polymer Selection Results Four different polymers were screened for biosolids conditioning at the NWPCF prior to BFP dewatering at a constant biosolids feed and belt speed of 1.07 m/min with different polymer doses. The cake solids and percent solids recovery were measured for each polymer used. Based on the results presented in Table 4, the BFP operator selected polymer C-9555 due to the lower dose needed to obtain good dewatering as indicated by the cake solids and solids recovery. Polymer NE-1316 shows similar results at lower polymer doses and Polymer 7878FS40 appears to give the highest cake solids, but only at a very high dosage. Note that the solids content in the feed was consistent throughout the afternoon. Table 4. Polymer Selection Results from the NWPCF Polymer Type C FS40 NE-1316 C-9555 Solids Loading (lb/hr-m) Active Polymer Dose (lb/dt) % Cake Solids % Solids Recovery Example Polymer Dose Response Results Different polymer (7878FS40) feed rates were tested a constant biosolids flow rate of 47 gpm at the SWPCF with at a constant belt pressure of 260 psig and a constant belt speed of 1.26 m/min. The testing started from lower polymer feed and increasing gradually. The results are shown in Figure 5. Increasing the polymer dose from 21.6 to 41.7 lb/dt resulted in higher cake solids. However, higher polymer doses beyond 25 lb/dt resulted in only a slight increase in the cake 536

11 solids, but caused the percent solids recovery to fall below 95%. Thus from these results a polymer dose of about lb/dt is considered optimum, achieving cake solids of 25.1% and solids recovery of 98.3%. As testing progress, it was observed that the biosolids concentration started to become low leading to lower loading rate. Average loading rate during this polymer testing was about 770 lb/hr per meter of belt width. Note that a wide range of polymer dose (21-42 lb/dt) was tested with this biosolids ( ) 99 Cake %TS % Recovery Cake %TS % Recovery Poly Dose (lb/dt) Figure 5. Polymer Dose Response Testing Results at the SWPCF showing percent cake solids and solids recovery as a function of active polymer dose used. Example of Throughput Testing Table 5 shows results from testing the performance of the BFP at the CWPCF when increasing solids loading and using polymer 7878FS40 at a constant average active dose of about 30.2 lb/dt and with belt pressure of 160 psig. It can be observed that slightly higher than 20% cake solids can be achieved under these conditions with acceptable solids recovery. Increasing the solids loading rate from 368 to 1,180 lb/hr-m resulted in slight decrease in the cake solids. However, when increasing the polymer dose at the highest solids loading rate at 2:00 PM, the cake solids obtained were lower. Table 5. Belt Filter Press Throughput at the CWPCF, May Active Feed Feed TS Loading Cake Recovery Time Poly Dose (gpm) (%) (lb/hr-m) TS (%) (%) (lb/dt) Belt Speed (m/min) 09: : : : : : , : ,

12 Example of Performance Testing Under Consistent Operating Conditions Table 6 below shows the centrifuge performance testing under no changes in operation parameters. Average results from this testing as shown in the table were used in cost analysis for the centrifuge dewatering at the NWPCF. It is worthy to note during this day that the centrifuge operator appeared to be operating the centrifuge at a low polymer dose and at the edge of proper operation, which resulted in centrifuge washout at 9am, thus testing started mid-morning. Table 6. Centrifuge Consistent Operation Testing Results at the NWPCF, May Time Polymer Type Feed (gpm) Feed TS (%) Loading (lb/hr) Poly Dose (lb/dt) Cake TS (%) Recovery (%) 10:15 Mix :00 Mix :30 Mix :15 Mix :00 Mix :45 Mix :15 Mix Ave EXAMPLE COST ANALYSIS AT THE SWPCF Table 7 below summarizes the average performance results under consistent operating conditions from both dewatering devices at the SWPCF. These results were used to estimate operating costs for both dewatering device. Table 7. Average BFP and Centrifuge Performance Testing Results at the SWPCF Poly (lb/dt) Cake TS (%) Loading (lb/hr) Amps Measured Recovery (%) BFP Centrifuge The operating costs in terms of transportation and drying costs using either dewatering device while achieving the measured cake solids were estimated first from the previously published biosolids management plan report. These costs were then added to the estimated polymer and electricity costs for each dewatering device. The following sections describe the approach taken in estimating the cost from the drier cake and the costs due to polymer use and electricity consumption. 538

13 Operating Costs with Dewatering The Biosolids Master Plan report estimated operating costs when achieving dryer cakes at each of the three treatment facilities. However, these costs were estimated up to achieving 23% only and the results from pilot testing as shown in Table 6-7 indicated that dewatering devices can achieve higher cake solids. A curve fit of the projected operating costs including: 1) diesel cost during transportation, 2) natural gas and electricity costs during drying, and excluding electricity and polymer and labor costs incurred through dewatering at the SWPCF indicated that the data fits well a linear regression as shown from the equation below Operating Cost in $/Yr = 3,625, x Cake Solids in % R 2 = Accordingly, these operating costs when achieving ~23.8% using BFP dewatering can be estimated to be $121,390 per year, and operating costs when achieving 26.09% using centrifuge dewatering can be estimated to be $98,280 per year. Polymer Cost Assuming a polymer cost of about $2.0 per active pound and using the polymer doses in Table 6-7, the polymer cost is $33,870/year for the BFP and $57,240/year for the centrifuge for the current SWPCF biosolids production rate of 2.94 DT/day (value obtained from the Biosolids Master Plan report). Electricity Cost The amp draw during operation was measured for the BFP and centrifuge throughout the week. Table 8 below presents the average measured amps. In the absence of conceptual design to determine the number of required dewatering devices and the operating time to process the biosolids, the calculated loading rate for the BFP and centrifuge was used to estimate the electricity cost on a yearly basis as shown in Table 8. Table 8. Electricity Cost for the BFP and Centrifuge at the SWPCF Item BFP Centrifuge Amps, measured Volts kw Electricity Cost, $/kw-hr Consumption, $/hr Loading, dry lb/hr Unit Electrical Cost, $/dry lb processed Biosolids to be processed, DT/day Electricity cost, $/year 3,250 11,

14 Overall Cost Estimated operating costs from using the BFP and centrifuge without considering labor costs are shown in Table 9 below. The results showed that using BFP dewatering would result in a nominally lower total operating cost ($~9,000/Yr) despite the fact that drier cake solids was obtained with centrifugation. This is mainly due to polymer cost and electricity usage. It is noteworthy that during the dose response testing results previously shown in Figure 5, a higher cake solids of about 25% (closer to centrifuge results) could be achieved with the BFP when using a polymer dose of about 25 lb/dt or greater (similar to what was used with the centrifuge), which if used in the cost analysis will greatly favor the use of BFP technology at this facility. Table 9. Estimated Cost Comparison between BFP and Centrifuge Dewatering at SWPCF Dryer and Diesel Estimated Cake Polymer Electricity Operating Cost Operating TS (%) Cost ($/Yr) Cost ($/Yr) ($/Yr) Cost* ($/Yr) BFP ,390 33,870 3, ,510 Centrifuge ,280 57,240 11, ,395 * Excluding labor costs SUMMARY AND RECOMMENDATIONS Table 10 below summarizes the pilot testing results at each plant. As the table shows the BFP produced comparable performance to the centrifuge in terms of cake solids and recovery at the CWPCF and NWPCF with slightly less polymer dose. At the SWPCF the centrifuge outperformed the BFP in terms of cake solids, but with near doubling the required polymer dose. The reason for this performance difference at the SWPCF can be attributed to using low polymer dose during the performance testing of the BFP. Table 10. Side-By-Side Performance Results of BFP and Centrifuge at the Three Facilities under Consistent Operating Conditions. SWPCF CWPCF NWPCF Parameter BFP Centrifuge BFP Centrifuge BFP Centrifug e % Cake TS Poly Dose (lb active/ds) % Solids Recovery Amp Draw Throughput (lb/hr)* *Throughput in the BFP case is per meter of belt width Table 11 presents the estimated operation cost comparison summary for each technology at the three plants. The operation cost analysis used operating costs when dewatering at each facility per the biosolids management plan report (excluding the labor costs, and dewatering electricity and polymer costs); polymer cost to achieve the reported cake solids from the pilot testing; and electricity cost to process the desired biosolids from the pilot testing. Operator s attention to the technology during dewatering was not considered in the cost analysis at this point. It was 540

15 observed during piloting that more operator attention is needed for centrifuge operation compared to BFP operation. As can be seen from the table, using BFP technology would result in nominally lower operating costs at the SWPCF and much lower operating costs for the CWPCF and NWPCF as compared to centrifuge technology. Table 11. Estimated Operating Cost Comparison between BFP and Centrifuge at the Three Treatment Facilities. SWPCF CWPCF NWPCF Parameter BFP Centrifuge BFP Centrifuge BFP Centrifug e Diesel and Drying 121,390 98, , , , ,125 Costs* ($/Yr) Polymer Cost ($/Yr) 33,870 57, , , , ,575 Electricity Cost ($/Yr) 3,250 11,875 14,630 60,930 13,390 40,910 Total Operating Cost ($/Yr) 158, ,395 1,113,570 1,175, , ,610 * Biosolids Management Plan report Based on the results of side-by-side pilot testing of BFP and centrifuge equipment and the operating costs associated with each technology, it was recommended that OCUA use BFP technology for dewatering biosolids at all three facilities. It was also recommended that OCUA conduct an investigative study to examine the reasons behind the relatively poor dewaterability exhibited by digested biosolids at the CWPCF (using both technologies) compared to the dewaterability of digested biosolids at the NWPCF and SWPCF. Also, practicing polymer optimization methods are recommended when implementing BFP dewatering due to the high polymer dosage obtained from the pilot testing in both the CWPCF and the NWPCF. REFERENCES Laustsen, et al. Do Not Stop When You Are Ahead Ocean County Utilities Authority Critical Review of Current Biosolids Operation. WEF/AWWA Joint Residuals and Biosolids Management Conference