Prediction of Strutive Formation Potential in EBPR Digested Sludges

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 (MgNH4PO4·6H2O – 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. NH4 +, Mg2+, PO4 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 (NH4 +), 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 - 780 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 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.

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 - MgNH4PO4.6H2O) 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(PO4 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 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.

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