Prediction of Struvite 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+, PO43-) 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.