The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal Part 3: Experimental periods using alum

An investigation was conducted into the effect of simultaneous alum dosing on biological phosphonrus(P) removal in an activated sludge system at pilot scale. Two continuous flow (36 Ud) activated sludge pilot plants were used to develop "semi-enhanced" biological excess phosphorus removal (BEPR) organism cultures of activated sludge by means of acetate supplementation of the feed. BEPR was strongly exhibited in these systems. One was used as a test system (to which alum was dosed), while the other served as control (no chemical addition). Additional removal due to chemical precipitation was measured as the difference in system P removal between the two systems. The extent of P release in the anaerobic reactors of the two systems was compared by mass balance, as one indicator of the "magnitude" of BEPR in the test system. Phosphorus fractionation of the-mixed liquor suspended solids also served as an indicator of the respective biological and chemical mechanisms. Evidence was found that the BEPR mechanism is partially inhibited in the presence of simultaneous alum addition, even in the absence of effluent phosphate limitation. However, the degree of inhibition was relatively low, ranging 15 to 25% (approximately) for alum doses in the, range ca, 5 to 9 mg/l as Al (50 to 100 mg/l as dry alum), at a system P removal of 20 to 30 mgP/l in the control. Alum dosing in this range as sufficient to produce additional P removal of the order of 3 to 7 mgP/l over periods of one to three sludge ages per experimental period. Sustained operation of the BEPR mechanism in the presence of alum was possible ever a continuous period of 7 sludge ages.