Anaerobic metabolism of bacteria performing enhanced biological phosphate removal

Enhanced biological phosphate removal (EBPR) is an established activated sludge process although many of the fundamental metabolic mechanisms are still poorly understood. Therefore, the stoichiometry and enzymatic reactions of the anaerobic phase of this process were studied in a laboratory reactor with acetate as organic substrate. Enzyme assays showed that acetate activation is performed by acetyl-CoA synthetase. Results of 13C-NMR measurements after feeding 13C-labeled acetate indicated that glycogen is degraded via the Entner–Doudoroff pathway. Energy is supplied by glycolysis, hydrolysis of polyphosphate and probably also by hydrolysis of pyrophosphate and the efflux of MgHPO4. The ratio of phosphate released to acetate taken up is variable and apparently dependent on the contents of polyphosphate and glycogen. A biochemical model is proposed explaining the experimental results in terms of carbon, redox, and energy balances. Anaerobic operation of an incomplete tricarboxylic acid cycle (TCA) is proposed to explain the generation of extra reducing equivalents.