Both denitrification and biological phosphorus removal depend on the presence of readily biodegradable carbon. Primary sludge fermentation is one way of producing such substrates. Soluble products of sludge fermentation are mainly short-chain fatty acids with two to five carbon atoms (84% of dissolved COD). In considering molar degradation rates by denitrifying organisms in batch experiments, the acids can be divided into two groups: the preferentially degraded linear forms of C2 to C5 acids and the branched C4 and C5 acids. The non-identified fraction (16% COD) which might contain acids with more than five carbon atoms is also removed by the activated sludge either by degradation or adsorption. In batch experiments with anaerobic and aerobic cycles, the uptake rate for short-chain fatty acids by phosphorus accumulating organisms and the ratio of phosphorus release to substrate uptake are determined. Acetate and propionate are taken up much faster than the C4 and C5 acids, which show very similar rates. For the ratio of phosphorus release to substrate uptake the situation is more complicated. The acids with four carbon atoms show by far the highest values.

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