A methodology to reduce the phosphorus content of biosolids produced by enhanced biological phosphorus removal treatment plants was studied. The process consists of mixing phosphaterich waste activated sludge (WAS) with primary sludge, or the supernatant from a primary sludge gravity
thickener, under anaerobic conditions to induce the release of phosphate from the WAS. The solubilized phosphate could then be chemically sequestered and removed from the biosolids. Bench-scale phosphate release experiments were conducted with sludge from the Nine Springs Wastewater Treatment
plant (Madison, WI) at different mixing ratios. A WAS to primary sludge (or supernatant) mixing ratio of 1:1 (by volume) resulted in the highest phosphate release in the batch tests. For experiments with less than 50% WAS (by volume), phosphate release was limited by polyphosphate availability.
When the mixture contained more than 50% WAS, phosphate release was limited by the volatile fatty acids availability. A mathematical model was developed to simulate the main biochemical reactions taking place during anaerobic phosphate release. The model included the activity of polyphosphate-accumulating,
denitrifying, and fermenting organisms, and was calibrated using the bench-scale phosphate release experiments. From the modeling results, it was estimated that the biosolids P content could be reduced from 5% to 3.9% if a completely mixed reactor was used, or to 3.4%
is a plug flow reactor was selected. Furthermore, with acetate addition to the phosphate release reactor, it was estimated that the biosolids P content could be further reduced to a minimum of 2.5%, which represented the complete hydrolysis of the polyphosphate present in the WAS.
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