Solving the City of Tacoma's High Effluent TSS Problem: Unexpected Causes of Deflocculation

The Central Treatment Plant for the City of Tacoma has had problems with elevated effluent TSS concentrations and turbidity during periods of low flow. After eliminating other possibilities, it was determined that ammonia return in the filtrate from the belt filter presses was the likely cause of deflocculation, as concentrations in the mixed liquor were found to increase from 20 mg/L-N to 90 mg/L-N during the day. Given that ammonia acts as a monovalent cation, it is likely that the high filtrate ammonia concentrations affected the ionic composition of the mixed liquor, increasing the monovalent to divalent cation ratio and leading to deflocculation and high effluent TSS concentrations. In order to test this, bench-scale protocols were developed. Results of continuous flow and batch experiments clearly showed that the filtrate caused an increase in effluent turbidity and TSS concentrations. Several modifications to the operation of the dewatering system at the Central Treatment Plant were recommended to reduce the impact of filtrate on the mixed liquor.