Phosphorus Removal in Extended Aeration Systems by Chemical Clarification

Sodium aluminate and alum were added to an extended aeration unit for phosphorus removal. Sodium aluminate was added at the end of the aeration basin whereas alum was added into the influent well to the clarifier. These studies indicated that 23 mg/l sodium aluminate and 50 mg/l alum could reduce total phosphorus in the effluent to 0.5 mg/l as P or less. However, the optimum ${\rm Al}\colon {\rm P}_{\text{filt}.}$ weight ratio for sodium aluminate was 1.8:1 while alum required only 1:1. Also biochemical oxygen demand and suspended solids removal improved. During sodium aluminate addition, nitrification provided some measure of pH control by consuming much of the alkalinity introduced by caustic aluminate. Total sludge production ranged between 80 and 116 ${\rm g}/{\rm m}^{3}$ (670 and 970 lb/mil gal) which was two to three times the normal sludge production rate. Percent mixed liquor volatile suspended solids decreased to the 50 to 65% range. Mean values of sludge volume index were reduced from 62 before chemical addition to 52 and 54 during sodium aluminate and alum addition respectively.