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) org anism cultures of activated sludge by means of acetate supplementation of t he feed. BEPR was strongly exhibited in these systems. One was used as a te st system (to which alum was dosed), while the other served as control (no chemical addition). Additional removal due to chemical precipitation was me asured as the difference in system P removal between the two systems. The e xtent of P release in the anaerobic reactors of the two systems was compare d 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 pho sphate limitation. However, the degree of inhibition was relatively low, ra nging 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 addit ional P removal of the order of 3 to 7 mgP/l over periods of one to three s ludge ages per experimental period. Sustained operation of the BEPR mechani sm in the presence of alum was possible ever a continuous period of 7 sludg e ages.