MODELING THE IMPACT OF ANAEROBIC DIGESTION ON THE OVERALL PERFORMANCE OF BIOLOGICAL NUTRIENT REMOVAL WASTEWATER TREATMENT PLANTS

This paper illustrates the importance of modeling digester process behavior on predicting the overall performance of a BNR process. Of particular importance is the impact of ammonia and phosphorus recycled from digested solids handling back to the liquid train. The analysis uses the General Activated Sludge – Digestion Model (ASDM) that was developed and implemented in BioWin™ 2.1. Key aspects of the model include the consideration of ammonia and phosphorus release during hydrolysis and decay in the digester, and the inclusion of precipitation reactions for the formation of struvite and calcium phosphates. Simulation cases indicated that a significant amount of the nitrogen and phosphorus released in the digester may be incorporated in the precipitates, resulting in a substantial reduction in ammonia and phosphate concentrations in solids processing recycles such as solids dewatering filtrates. Published data from the York River, Virginia WWTP BNR demonstration study were used to illustrate that the recycled phosphate can have a significant impact on biological phosphorus removal performance, and that a comprehensive activated sludge anaerobic digestion model can allow a plant-wide simulation to examine this impact.