Nitrification in rapid sand filter: Phosphate limitation at low temperatures

In winter 1995-1996 the RSF of AWS River-Lake Waterworks suffered from ammonium breakthrough. Research indicated that the nitrification was limited by stagnated growth of the nitrifying bacteria due to low phosphate concentrations at low temperatures (0-3°C). Phosphoric acid was dosed prior to the RSF (dose 35-50 μg PO₄ ^3- -P.I^-1) to stimulate bacteria growth. Two weeks after starting the addition, ammonium removal in the RSF had recovered. AWS is conducting research on alternatives for production capacity extension. Several alternatives involve the construction of an additional RSF prior to the reservoir. Pilot experiments on RSF prior to the reservoir show that complete nitrification in these additional RSF is possible. The phosphate concentration should amount at least 10 μg PO₄ ^3- -P.I^-1. A model has been developed to predict nitrification in RSF. The model confirms the results from both the River-Lake Waterworks' RSF as from the pilot RSF. The model is used for process control and scenario studies.