Full Scale Experience with Phosphorus Removal in an Alternating System

Abstract

Nitrogen removal performed by the alternating mode of operation results in alternating aerobic, anoxic and anaerobic conditions during different operational sequences of the treatment process. Phosphorus release and phosphorus uptake have been observed under the anaerobic and aerobic conditions respectively. Phosphorus release is observed when oxygen, nitrate, and nitrite are absent in the bulk liquid. With oxygen or nitrate present re-uptake of phosphorus is observed. Therefore, information about the dynamic parameter variations in the reactors is considered important in order to optimize the effluent phosphorus quality. Operational results from two treatment plants show enhanced phosphorus removal: 8 mg P/l out of 11 mg P/l in the inlet is removed. Chemical analyses of sludge samples indicate that precipitation of a calcium-iron-phosphate is responsible for the enhanced phosphorus removal. The precipitate constitutes 12% of the total sludge mass. At one treatment plant nitrogen removal is combined with phosphorus removal by simultaneous precipitaion with ferrous sulphate. It is shown that even in this system phosphorus release and phosphorus uptake is observed, and knowledge about the dynamic parameter variation during the alternating mode of operation can be used to ensure a stable and low effluent phosphorus concentration.