Whereas biokinetic parameters are known to depend on temperature, it is usually assumed that stoichiometry and wastewater composition do not depend on process temperature. With the aid of laboratory experiments with a Swiss wastewater of predominantly domestic origin it is demonstrated that the amount of inert, non-biodegradable organic material increases with decreasing process temperature. Based on changes in the composition of activated sludge, it is concluded that the inert material must originally be associated with a chemically reduced, slowly biodegradable, lipophilic fraction of the activated sludge, which contains little nitrogen.

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