PREDICTION OF STRUVITE FORMATION POTENTIAL IN EBPR DIGESTED SLUDGES

Transcription

1 PREDICTION OF STRUVITE FORMATION POTENTIAL IN EBPR DIGESTED SLUDGES Jacimaria R. Batista and Hyunju (Judy) Jeong University of Nevada Las Vegas 4505 Maryland Parkway, Las Vegas, NV ABSTRACT Low operating costs and less sludge generation associated with EBPR (Enhanced Biological Phosphorus Removal) have led existing wastewater treatment plants to retrofit their phosphorus removal systems and have encouraged new plants to incorporate EBPR into their design. However, in plants that use anaerobic digestion, the introduction of EBPR may result in the formation of scales of the mineral struvite (MgNH 4 PO 4 6H 2 O magnesium ammonium phosphate hexahydrate or MAP) because polyphosphate contained in phosphorus accumulating organisms (PAOs) can be released as orthophosphate when EBPR sludge is digested. If sufficient magnesium and ammonia are present to react with the orthophosphate, struvite can be formed. There have been several reports of struvite precipitation in sludge handling systems after introduction of EBPR and significant time and money have been expended to solve the problem. The present study addressed the prediction of struvite formation potential in plants where EBPR is introduced. The objective of the study is to demonstrate that mass balance computations, coupled with batch scale precipitation testing, can be used to predict the composition of digested sludge centrates and forecast the potential for struvite formation. Mass balance can predict the amounts of released phosphorus and ammonia present in the digester s centrate by balancing flows and concentrations of struvite constituents (i.e. NH 4 +, Mg 2+, PO 4 3- ) throughout the plant for current and future operating conditions. The potential compositions of the centrate can be used in batch testing to forecast the potential for struvite formation. In this research, mass balance computations were used to forecast the phosphorus and ammonia concentration in digested sludge centrates at the City of Las Vegas Water Pollution Control Facility (WPCF), located in Las Vegas, Nevada. The treatment capacity of the plant is being expanded to 90 MGD (million gallons a day) wastewater. Phosphorus removal of 30 MGD will be by EBPR while the remaining 60 MGD will be treated chemically with ferric chloride. To perform mass balances a detailed flow diagram of the plant was constructed and mass balances of critical parameters associated with the potential of struvite formation were performed. Mass balances of suspended solids (SS), total (TP) and orthophosphate (OP), ammonium (NH 4 + ), and magnesium (Mg) for the liquid and sludge streams were performed for the sludge digesters. It was found that most ammonia that enters the digesters is contributed by primary sludge and only 15.5% ~ 26% is due to BNR and nitrification sludges. The estimated ammonia concentration in the centrate varied from 750 mg/l mg/. The measured ammonia value in the digester centrate was 750 mg/l, indicating good agreement between measured and forecasted ammonia concentrations. OP concentrations, computed from OP measurements taken for 12 digesters from the WPCF, was found to be about 124 mg/l. The estimated OP 5185

2 concentrations in the digested sludge, assuming 50% phosphorus release, was found to vary from 121 mg/l-151 mg/l depending on the wastewater flowrate being treated biologically or chemically for phosphorus removal. The estimated concentration of orthophosphate is sufficient to stoichiometrically react with all the magnesium and ammonia present in the centrate to form struvite, in the alkaline ph range. Results of batch tests, using digester centrate, for the majority of OP levels tested, indicated that there is a potential for struvite formation. This potential was confirmed by X-ray diffraction of the precipitates formed in the batch tests. In summary, the results of this study indicate that mass balance calculations, coupled with bench scale precipitation tests can serve as a useful tool to wastewater treatment plants in predicting the potential of struvite formation when EBPR is introduced to the treatment train. The calculations are simple and can be performed using existing plant data and have the potential to avoid struvite formation. KEYWORDS Struvite, EBPR, Phosphorus Removal, Digested Sludge, Phosphorus Release. INTRODUCTION Enhanced biological phosphorous removal (EBPR) is becoming widely used as it produces effluents with very low phosphorus concentrations and also because chemical phosphorus removal with metal ions has several disadvantages. The disadvantages include increased sludge production, chemical costs, and chemical feed control requirements. EBPR enhances the ability of specific bacteria - PAO s (phosphorus accumulating organisms) to uptake phosphorus as a means to remove it from wastewater. The increased capacity for phosphorus uptake by PAO s is accomplished by submitting bacteria to alternate anaerobic and aerobic cycles. The bacteria release orthophosphate under anaerobic conditions and under aerobic conditions they uptake more orthophosphate than they released in the anaerobic zones. The sludge generated from EBPR contains, in general, 4 ~ 4.5% phosphorus on a dry weight basis which is about twice that of a normal sludge biomass (Rittmann and McCarty, 2001). Although EBPR can be used to remove 80 ~ 90% of influent phosphorus from wastewater without chemical addition, the sludge generated from this process has to be handled with care to avoid phosphorus release from the microbial cells (i.e sludge solids) during sludge digestion, conditioning, or dewatering. Phosphorus released during EBPR sludge handling can result in the formation of scales of the mineral struvite (magnesium ammonium phosphate hexahydrate - MgNH 4 PO 4.6H 2 O) inside digesters, in digested sludge pipelines, sludge supernatant system, or centrifuges. Struvite precipitation causes operational problems because it changes the capacity of plant s pumps and pipes. In the formation of struvite during anaerobic digestion, phosphorus is supplied by released orthophosphate(po 4 3- ) from the sludge solids. Ammonia comes from the degradation of nitrogenous material contained in the primary sludge. Magnesium originates from the degradation of organic material and poly-p hydrolysis (Wild et al., 1997, Jardin and Popel, 1996). In addition, when the plant is located in a region where the water is hard or in a coastal 5186

3 area, there also exist enough magnesium to promote struvite precipitation. Struvite precipitates when the needed concentrations of struvite constituents are satisfied and the ph is adequate. Struvite solubility decreases with increasing ph and therefore, in general, increasing ph causes struvite to precipitate. However, the solubility begins to increase reversely above a ph of 9 (Snoeyink and Jenkins, 1982; Borgerding (1972); and Booker et al.,1999). The formation of struvite scale during anaerobic digestion has been reported in wastewater treatment plants when biological phosphorus removal was introduced (Borgerding, 1972, Mohajit et al., 1989, Mamais et al, 1994, Ohlinger et al., 1998, Williams, 1999, Doyle et al, 2000, Doyle & Parsons, 2002). The Los Angeles Hyperion plant has suffered struvite scaling in pipes (Borgerding, 1972). To solve the problem, digested sludge was diluted to decrease the concentration of orthophosphate and pipes were cleaned with acid. In 1982, the diameter of the centrate discharging pipes decreased from 8 to 1.5 within 1 month in the Southeast Water Pollution Control Plant (SEWPCP) in San Francisco (Mamais et al., 1994). Unlike the Hyperion plant, SEWPCP added ferric chloride to precipitate phosphorus before centrifuging the sludge (Mamais et al., 1994). The Sacramento Regional Wastewater Treatment Plant (SRWTP) also observed struvite formation and replaced the piping system (Ohlinger et al., 1998). In the United Kingdom, several cases have been reported (Williams, 1999, Doyle et al., 2000). Therefore, wastewater treatment plants that retrofit or implement EBPR have to pay special attention to sludge handling and should be able to forecast the potential for struvite formation resulting from phosphorus release. The types of sludge and their relative amounts that are submitted to anaerobic digestion greatly influence the potential for struvite formation. The characteristics of sludges vary depending upon the unit processes involved in the plant s treatment train. Several scenarios of sludge input to anaerobic digesters are possible. In Figure 1, two potential scenarios for digester feed sludge are shown. Alternative 1 Alternative 2 RAW PRIMARY SLUDGE RAW PRIMARY SLUDGE w/o chemical precipitation TS : 5 ~ 9 % P : ~ 1.15 % of TS ANAEROBIC EPBR WAS w/ chemical precipitation TS : 5 ~ 9 % P : 1 ~ 2 % of TS ANAEROBIC DIGESTER EPBR WAS DIGESTER TS : 0.8 ~ 1.2 % TS : 0.8 ~ 1.2 % P : 8 ~ 12 % of TS P : 8 ~ 12 % of TS Activated sludge WAS TS : 0.8 ~ 1.2 % P : 2 ~ 3 % of TS Figure 1. Alternatives of sludge input to digesters In some plants, raw primary sludge and EBPR WAS (waste activated sludge) are digested together when the plant adopts EBPR process (Alternative 1). If the plant adopts both EBPR and 5187

4 chemical precipitation to remove phosphorus (Alternative 2), EBPR WAS, WAS, and chemical primary sludge may be input to digesters (Figure 1). The objective of this study was to demonstrate that mass balance computations can be used to predict the composition of digested sludge centrates and forecast the potential for struvite formation. Mass balance can predict the amounts of released phosphorus and ammonia present in the digester s centrate by balancing flows and concentrations of struvite constituents (i.e. NH 4 +, Mg 2+, PO 4 3- ) throughout the plant for current and future operating conditions. The potential compositions of the centrate can be used in batch testing to confirm the potential for struvite formation. MATERIALS AND METHODS The hypothesis guiding this study was that material balances can be used to predict the composition of digested sludge centrates due to the introduction of EBPR in wastewater treatment plant. To perform mass balance computations, two tasks should be completed. First, a detailed flow diagram of the plant has to be developed containing the actual locations of all flow streams. Second, mass balances of critical parameters associated with the potential of struvite formation have to be performed. Most parameters needed for the mass balances are available from plant operational data. However, some parameters may have to be obtained by sampling and analyzing the plant s flow streams. Mass balances of suspended solids (SS), total (TP) and orthophosphate (OP), ammonium (NH 4 + ), and magnesium (Mg) for the liquid and sludge streams are generally satisfactory to obtain the potential concentration of critical parameters in the digester. When these tasks are completed, the composition of the centrate stream and the potential for struvite formation can be determined. The mass balance approach discussed above was used to forecast the phosphorus concentration in digested sludge centrates at the City of Las Vegas Water Pollutant Control Facility (WPCF), in Las Vegas, NV. The plant treats 60 MGD of municipal wastewater and currently removes phosphorus by coagulation with ferric chloride (FeCl 3 ). The treatment capacity of the plant is being expanded to treat 90 MGD of wastewater. Phosphorus removal of 30 MGD will be by EBPR while the remaining 60 MGD will be treated chemically with ferric chloride. When EBPR is introduced, the anaerobic digesters will receive : (a) primary sludge containing iron phosphate (i.e resulting from FeCl 3 addition), (b) secondary sludge containing polyphosphate from the EBPR system, and (c) secondary sludge generated in the nitrification system. Development of Mass Balances To perform the mass balances, the wastewater treatment schematic for the Las Vegas WPCF was expressed through a process flow diagram (Figure 2). Basically, Water, sludge, and recycle streams were identified. Next, mass balance points were established on the diagram at locations where two or more streams are combined. Flowrates and concentrations of components of interest were obtained from plant operating data. Sampling and analyses were performed at specific points throughout the WPCF to fill out data gaps. Two operating scenarios were considered in the mass balances: 5188

5 FePO RAS RAS WAS CITY OF LAS VEGAS - WATER POLLUTION CONTROL FACILITY OP CONCENTRATION(mg/l) & MASS(lb/day) Figure 2. Process Flow Diagram for the Plant where EBPR has been Introduced showing A Mass balance for orthophosphate. Scenario 1- represents conditions that will prevail in the plant in the first 3-5 years after EBPR is introduced and consists of 30 MGD wastewater treated by EBPR and 30 MGD wastewater treated chemically with FeCl 3 addition (CPR); Scenario 2 represents conditions that will prevail in the plant in the last years of the design period and include 30 MGD wastewater treated by EBPR and 60 MGD wastewater treated chemically with FeCl 3 addition. To meet its total maximum daily loading (TMDL ) discharge requirements, WPCF must remove d 95 % of the phosphorus entering the plant. The percent removal needed in the future will be even higher given wastewater flows are increasing and the TMDL is likely to remain at its current level. Process Flow Diagram Description In the process flow diagram (Figure 2), basically, all the components of the plant from headworks to effluent discharge were identified. Water, sludge, and recycle streams were drawn as lines in blue, red, and green colors, respectively. Lastly, sampling points were established on the diagram at the end of every treatment unit and at places where two or more streams are combined. Influent to the WPCF was separated into two streams according to phosphorus removal process. In the flowstream of chemical phosphorus removal ferric chloride is added to remove orthophosphate and to enhance the settlement of primary solids. The stream treated with FeCl 3 is sent to primary sedimentation basins and the effluent from the nitrification basins enters the trickling filters for BOD removal and then the nitrification basins for ammonia oxidation. In the case of the biological phosphorus removal stream, ferric chloride is not added and the effluent from the primary sedimentation basins is sent to the Biological Nutrient Removal (BNR) basins. The effluents from the nitrification and BNR basins are combined and undergo filtration. The effluent from the filters is chlorinated and dechlorinated before discharge into the Las Vegas Wash. Thickened primary sludge, and thickened waste activated sludge (TWAS) from both 5189

6 BNR and nitrification basins are combined in the anaerobic digesters. The digested sludge is dewatered through centrifuging. The resulting sludge cake is disposed of in a landfill and the centrifuge centrate in recycled back to the headworks. In addition to the major operating streams, WPCF s flow diagrams contain several recycle streams including secondary sludges from the trickling filters, clarifiers, filter backwash, reuse water and centrate from sludge dewatering. Reuse water has been added to the centrifuge centrate since 2002 to avoid the formation of iron scales in the centrifuge piping. Mass Balance Calculation To perform mass balance calculations, flow rates and concentrations of components of interest were obtained from plant operating data. To fill out concentration data gaps, sampling and analyses were performed at 38 points throughout the WPCF. Sampling and analyses were performed as per Standard Methods (APHA, 1998). The data collected not only fill out concentration data gaps, but they also were used to validate results of the mass balance calculations. The mass balance calculations were run on the Excel software. RESULTS AND DISCUSSION Suspended solids balance is one of the most significant balances for the determination of struvite formation potential because it is during solids handling that struvite may be formed. Figure 3 depicts an example suspended solids balance for the WPCF plant prior to EBPR implementation. Table 1 shows the amounts of sludge computed in the mass balances for both scenarios under consideration. The results indicate that primary sludge will constitute the bulk of the sludge produced in the plant, and 78.13% for scenarios 1 and 2, respectively. EBPR and nitrification sludges combined for both scenarios make up only over 20% of the total sludge produced. However, while the primary sludge contains phosphate that is strongly bound to iron and is unlikely to be released in large amounts during digestion or sludge storage, the EBPR and nitrification sludge contains biomass phosphorus that can be easily released during digestion. Ammonia is a major component of struvite and it is generated in the digesters from the degradation of proteins. Table 2 shows the ammonia loading to the WPCF digesters for different types of sludge. Notice that most ammonia that enters the digesters is contributed by primary sludge and only 19% ~ 26% is due to BNR and nitrification sludges (Table 2). Table 3 shows the ammonia loading to the digesters from the different types of sludges. Notice that most ammonia that enters the digesters is contributed by primary sludge and only 15.5% ~ 26% is due to BNR and nitrification sludges (Table 2). The majority of the ammonia generated during digestion will end up in the digested sludge centrate. Table 3 shows the estimated amounts of ammonia generated in the digesters. Secondary solids reduction of 20% and a primary solids reduction of 56% were assumed for the digesters. Furthermore, it was considered that 0.8 moles of ammonia are generated for each mole of volatile solids (MW of 207 g and 80% volatile) reduced in the digesters. Those values were used in the mass balances to forecast the ammonia concentrations for the different scenarios. For scenarios 1 and 2 the estimated ammonia concentration was 750 mg/land 780 mg/l, respectively. The measured value in the centrate was 750 mg/l. Therefore, this result indicated that it is possible to use mass balance calculations to estimate the concentration of ammonia in centrate of digested sludge. 5190

7 SS by Fe Figure 3. Example Mass Balance of Solids in the Plant Before EBPR Implementation RAS CITY OF LAS VEGAS - WATER POLLUTION CONTROL FACILITY SS CONCENTRATION(mg/l) & MASS(lb/day) Table 1. Amounts of primary, EBPR, and nitrification sludges generated Primary EBPR Nitrification Total Scenarios Lbs/day Lbs/day Lbs/day Lbs/day (% of total) (% of total) (% of total) Scenario 1 90, , , ,917 EBPR = 30 (73.92) (17.25) (8.82) CPR = 30 Scenario 2 EBPR = 30 CPR = ,020 (78.13) 21,033.5 (10.8) 21,517 (11.06) 194,

8 Table 2. Ammonia Loading to Anaerobic digesters at WPCF Scenarios Scenario 1 EBPR = 30 CPR = 30 Scenario 2 EBPR = 30 CPR = 60 Primary Lbs/day (%) EBPR Lbs/day (%) Nitrification Lbs/day (%) TWAS Lbs/day 372 (73.96) 97 (19.28) 34 (6.8) (78.8) 97 (12.5) 67.5 (8.7) Table 3. Estimated NH 3 loading (lbs/day) generated during anaerobic digestion at the WPCF Assumes: 56% primary sludge reduction, 20% secondary sludge reduction with 80% volatile solids Released NH 3 Lbs/day Scenario 1 Scenario 2 Primary Secondary Primary Secondary TS VSS Digested VSS Released NH Entering NH 3 Lbs/day NH 3 after digestion Lbs/day Estimated ammonia concentration, mg/l The concentrations of magnesium, another major component of struvite, in the WPCF plant varied from about 36 mg/l in the plant influent to mg/l in the centrate from the dewatering centrifuges. The high Mg levels at WPCF is associated with the hard water from the Colorado River that is used as the water source for Las Vegas The total phosphorus and orthophosphate content of the primary and secondary sludges were calculated (Table 4) using the mass of solids generated in the system (Table 1) and the mass balance of phosphorus in the plant. The phosphorus content of the EBPR sludge was in average 3.4% on a dry weight basis while the primary sludge and nitrification sludges had P contents of 2.5% and 2.3%, respectively. However, these sludges have very different potential to release orthophosphate. Phosphate precipitated with FeCl 3 as FePO 4 is strongly bound and is unlikely to be released during digestion. Indeed, in the digesters, FePO 4 will be reduced to vivianite 5192

9 (Fe 3 (PO 4 ) 2 ) which is a very stable phosphate mineral. Conversely, the phosphorus contained in the biomass from EBPR and denitrification sludges is released during digestion and will end-up in the centrate. There are reports in the literature indicating that the amount of EBPRphosphorus which remains in soluble form may be as high as 54% of the EBPR-phosphorus and 38% of the total phosphorus brought into the digester (Jardin and Poepel, 1994). Table 4. Estimated TP and OP in sludge generated at WPCF for two flowrate scenarios Scenarios Flowrate (MGD) Primary BNR Nitrification TWAS Scenario 1 EBPR = 30 CPR = 30 TP (Lbs/day) OP (Lbs/day) % P in solids Scenario 2 EBPR = 30 CPR = 60 TP (Lbs/day) OP (Lbs/day) % P in solids OP concentrations, obtained from OP measurements taken for all 12 digesters from the WPCF, was found to vary from mg/l. Table 5 presents OP concentrations in the digested sludge centrate assuming different % releases of phosphate from the sludge. For example, in Scenario 2, a 10% release would generate P concentrations of 52.2 mg/l. Release of about 50% would cause the OP concentration to increase to 121mg-P/L (681.2bs/day). This concentration of orthophosphate is sufficient to stoichiometrically react with all the magnesium and ammonia present in the centrate to form struvite, in the alkaline ph range. Table 5. Estimated OP Concentration for Scenario 1(CPR = 30 MGD, EBPR = 30 MGD) % OP release Released OP (Lbs/day) OP entering the digesters (Lbs/day) EBPR WAS Nitrification WAS Primary sludge EBPR WAS Nitrification WAS OP in centrate Lbs/day mg/l

10 Table 6..Estimated OP concentration for Scenario 2 (CPR = 60 MGD, EBPR = 30 MGD) % OP release EBPR WAS Released OP Nitrification (Lbs/day) WAS Primary sludge OP entering EBPR WAS the digesters Nitrification (Lbs/day) WAS Lbs/day OP in centrate mg/l Notice that OP concentration in the centrate could vary from 52 mg/l to 396 mg/l, depending on the % released obtained. For a percent release of 50%, as suggested by (Jardin and Poepel, 1994), the OP concentrations in scenarios 1 and 2 were found to be mg/l, respectively. The measured P value in the digesters (i.e 124 mg/l) is of the same order of magnitude as the Op values computed by mass balances. The OP, ammonia, and magnesium levels found in the mass balance calculations were used to supplement the existing centrate at WPCF for batch struvite formation testing. The details of the batch tests are subject of a future publication. Nonetheless tests were performed for a range of ph values and with P concentrations varying from 50 to 400 mg/l. For the majority of OP levels tested, struvite was formed and its presence detected by X-ray diffraction (Figure 4). Figure 4. Precipitates from batch tests used to detect struvite by X-ray diffraction 5194

11 CONCLUSIONS The following conclusions can be drawn from this research: Mass balance calculations on WPCF digesters indicated that, at current operating conditions, phosphorus concentrations in the centrifuge centrate can vary from mg/l for 50% P release from the sludge. Potential orthophosphate concentrations in the centrifuge centrate, estimated from actual measurements within the anaerobic digesters, revealed that OP concentrate in the centrate of WPCF would vary from 87 mg P/L to 235 mg P/L. The estimated and measured values are of the same order of magnitude lending support to the estimated values via mass balances. 3. Ammonia levels in the digested sludge centrate, forecasted by mass balance were found to be very similar to the measured values in the centrate (i. e. 750 mg/l) 4. The expected orthophosphate concentration in the digester centrate is sufficient to stoichiometrically react with all the magnesium and ammonia present to form struvite, in the alkaline ph range. 5. Struvite was detected by X-Ray diffraction in most precipitates obtained in the batch precipitation test using the WPCF centrifuge centrate for varying orthophosphate concentrations. 6. It is possible to use mass balance to forecast potential concentrations of struvite constituents in the centrate of anaerobically digested sludge. Batch tests can be performed, using the forecasted centrate chemical composition to confirm the potential of struvite formation. ACKNOWLEDGEMENTS This research was funded jointly by the City of Las Vegas Water Pollution Control Facility and the State of Nevada Applied Research Initiative (ARI). Jacimaria R. Batista is an Associate Professor of Environmental Engineering at the University of Nevada Las Vegas. Hyunju (Judy) Jeong was a M.S. student in the Department of Civil and Environmental Engineering at the University of Nevada, Las Vegas. She is currently pursuing her Ph.D. Correspondence should be sent to Jacimaria R. Batista: 4505 Maryland Parkway, Department of Civil and Environmental Engineering, UNLV, Las Vegas, NV , jaci@ce.unlv.edu; phone (702) REFERENCES APHA (1998). Standard Methods for the Analysis of Water and Wastewater. Booker,N. A., Priestley, A. J. and Fraser, I. H. (1999). Struvite Formation in Wastewater Treatment Plants; Opportunities for Nutrient Recovery. Environmental Technology, 20, Borgerding, J. (1972). Phosphate Deposits in Digestion Systems.. Water Pollution Control Federation, 44, 5,

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