The response of activated sludge following exposure to various organic compounds is decribed. Batch systems simulating the various stages of biological nutrient removal processes were used to study the dependence of phosphate release, enhanced phosphate uptake and denitrification on the nature and level of organic substrate, and the presence or absence of nitrate. The phenomenon of phosphate release is shown to be controlled primarily by the nature of the substrate rather than the creation of an anaerobic state. Certain short-chain fatty acids or their salts, such as acetate and propionate, induce phosphate release even under anoxic or aerobic conditions but with compounds such as ethanol and glucose release occurs only after the onset of anaerobiosis. Given the necessary conditions, the time course of phosphate concentration in initially anoxic mixtures of phosphate-rich sludge and short-chain fatty acids is shown to proceed in three consecutive stages, comprising primary release, anoxic uptake and secondary release respectively. It is concluded that phosphate uptake and release occur simultaneously in the presence of fatty acids, which also render the best overall phosphate removal during aeration.

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