A Struvite Control and Phosphorus Removal Process for Centrate: Full-Scale Testing

The North End Water Pollution Control Centre (NEWPCC) in Manitoba, Canada serves a population of approximately 380,000. The dry weather flow to the NEWPCC is approximately 175 ML/d (46 MGD). The plant uses anaerobic digestion and centrifuge dewatering in its solids treatment train. The centrate stream comprises approximately 20 percent of the plant's phosphorus load. The plant suffers from severe struvite (magnesium ammonium phosphate) deposition: centrifuges and centrate piping have to be regularly cleaned. The Province requires that the plant control the phosphorus load recycled in the centrate: the total phosphorus (TP) load must not exceed 119 kg/d, as determined by a thirty day rolling average. In order to meet the requirements of the Province, and to control struvite, a metal salt will be dosed into the digested sludge, upstream of the centrifuges. Prior to design of a metal salt dosing system, a full-scale trial was initiated. Two potential metal salts were identified; ferric chloride (FeCl3) and aluminum sulphate or alum (Al2(SO4)3·14H2O).Examination of struvite crystals in the digested sludge indicated that nearly complete disappearance of crystals occurred at all dosing rates used in these trials. The trials demonstrated that alum and ferric chloride were highly effective in meeting the removal objectives and controlling struvite crystal formation in the centrate.