BNR MBR for Low Level Phosphorus – The Little Things Matter

The City of Coeur d'Alene, ID conducted an 18 months 50,000 gal/d demonstration pilot to determine the validity of a BNR MBR to reliably produce effluent total phosphorus concentration of less than 0.05 mg/L; with or without chemical addition. The primary challenge with such low limits, for any technology, is that it does not allow any bad days. Brief excursions were observed on a regular basis; undoing otherwise excellent performance. Extensive troubleshooting eventually revealed that these excursion were related to maintenance cleans during which the membrane tank is isolated and not aerated. This paper examines these excursions and provides recommendations on how to eliminate them. In addition the pilot results showed that the biological phosphorus removal process had a higher soluble nonreactive phosphorus fraction (0.02 mg/L) compared to a chemical removal membrane process (0.01 mg/L).