Intracellular carbon flow in phosphorus accumulating organisms from activated sludge systems

A biochemical model for the anaerobic and aerobic carbon flow in phosphorus accumulating organisms (PAO) is discussed. It is based on the assumption that for the anaerobic poly(hydroxyalcanoates) (PHA) storage the reduction power is delivered by glycogen and the energy is supplied with hydrolysis of glycogen or polyphosphate. These have to be regenerated under aerobic or anoxic conditions which requires energy and releases carbon dioxide. Batch experiments with activated sludge from a pilot plant fed with domestic wastewater were performed. 13C (in the form of acetate) was added and traced with solid state carbon NMR. The anaerobic formation of intracellular PHA from metabolised acetate together with degraded glycogen and the aerobic conversion of PHA to glycogen could be detected. The low ratio of acetate to carbohydrate conversion and the detection of poly(hydroxyvalerate) formed from glycogen shows that there is a substantial anaerobic breakdown of glycogen for energy production. The results from the NMR spectra also indicate that glycogen is degraded via the Entner-Doudoroff pathway. From the model it can be concluded that the maintenance of a glycogen and polyphosphate pool reduces the growth yield on substrate. This reduction depends on the type of substrate. It is estimated that for acetate there is a loss of biomass yield of about 12% as compared to strictly aerobic conditions. The model also shows that glycogen as well as polyphosphate may serve for anaerobic energy supply, however with acetate as the only carbon source, polyphosphate is essential for growth.