The use of simultaneous chemical precipitation in modified activated sludge systems exhibiting biological excess phosphate removal Part 5: Experimental periods using a ferrous-ferric chloride blend

Abstract

A blend of ferrous chloride and ferric chloride (FeCl2-FeCl3) was simultaneously dosed into an activated sludge system at pilot scale in order to test the effect on biological P removal. Additional removal due to chemical precipitation was measured as the difference in system P removal between parallel test and control systems. Both systems strongly exhibited biological excess P removal (BEPR). The extent of Prelease in the anaerobic reactors of the two systems was compared by mass balance, as one indicator of the relative “magnitude” of BEPR. Phosphorus fractionation of the mixed liquor also served as an indicator of the biological and chemical mechanisms. Evidence was found that the BEPR mechanism is partially inhibited by simultaneous FeCl2-FeCl3 addition, even in the absence of effluent phosphate limitation. However, the degree of inhibition was relatively low, ranging from 3 to 25% (approximately) for Fe doses in the range ca. 10 to 20 mg/l as Fe, with an average system P removal of 14 to 18 mgP/l in the control.

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