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PHOSPHORUS RELEASE IN AEROBIC SLUDGE DIGESTION

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Significant (60–80%) phosphorus (P) release has been reported during anaerobic digestion of BNR sludge. The objectives of this study are to examine the P release in aerobic sludge digestion and to better understand its governing mechanisms. In this study, P release was examined using the secondary sludge from both conventional and BNR processes. The experiments were carried out at room temperature (22 ± 2 °C), with or without automatic control of pH (4.5–7.8), and under three aeration schemes: fully aerobic (DO at 3–4 mg/L), low-DO (0.2–0.8 mg/L), and cyclic (with alternate on/off-aeration). The released P concentrations were 20–80 mg/L for the conventional sludge and 60–130 mg/L for the bio-P sludge. Higher P release also occurred at low pH (< 6.0). As for the effect of DO, fully aerobic digestion caused higher P release than the low-DO and cyclic operations. For better understanding, the solid-P in sludge was conceptually categorized into three forms: the inorganic P precipitates, the organic cellular P, and the poly-P in polyphosphate-accumulating organisms (PAOs). Dissolution of inorganic P precipitates is controlled by physical/chemical conditions, pH being the most important in this study. Lowering pH to 4 – 6 clearly promoted the release of inorganic P. Poly-P hydrolysis, on the other hand, was found regulated biologically (sensitive to occurrence of anaerobic conditions) and was insignificant in the glutaraldehyde-fixed sludge. P release from organic-P should correlate with the VS digestion, which lyses the cells and frees the P covalently bonded with the organic matters. The amounts of P released per unit VS digested, ΔP/ ΔVS, were therefore calculated for experiments with long periods of constant pH (to minimize interferences from dissolution/precipitation of inorganic-P). The results suggested that some poly-P was hydrolyzed and released accompanying the aerobic VS digestion, but at rates far lower than those under anaerobic conditions.
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Document Type: Research Article

Publication date: 2002-01-01

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