Upgrading a Municipal Activated Sludge Plant for High-Rate Biological Nutrient Removal

Abstract

A full-scale activated sludge plant was modified for high-rate BNR operation at minimal cost. Three different processes (A/0+, A2/0+ and UCT) were studied at BSRTs varying from 4 to 14 days and HRTs of 2 to 6 hours. Onset of BPR was almost immediate without addition of special seed. Good phosphorus removal was obtained with all three processes except for periods of extremely high influent flows or high nitrate recycle to the anaerobic zone. However, because of high process phosphorus loadings resulting from high influent concentrations and sludge processing recycles, effluent TP concentrations were typically in excess of 2 mg/L. Nitrogen removal was excellent when nitrification could be maintained and nitrate recycle was sufficiently high. All processes yielded excellent nitrogen removals during warm weather, but complete nitrification could not be maintained during winter conditions. Implementation of a statewide phosphate detergent ban resulted in a 40 per cent decrease in the influent wastewater concentrations. Following the ban, the UCT process was shown to be capable of consistently meeting the Virginia nutrient discharge standards of 10 mg/L TN and 2 mg/L TP during the growing season.