The experimental results and material balance analysis in this paper revealed the regularity of poly-hydroxy alkanoates (PHA) and total phosphorus (TP) metabolism in a continuous-flow single-sludge wastewater treatment system under different main anoxic section oxidation-reduction potential (ORPan) conditions. We also evaluated the effectiveness of the operation control parameters of ORPan as the continuous-flow single-sludge sewage treatment system from the aspect of the reaction mechanism. Using a programmable logic controller (PLC) automatic control system to take the circulating flow in nitrification as the controlled variable based on the feedback control structure, an experimental study was carried out under the condition of ORPan setting value of -143mV, -123mV, -105mV, -95mV, -72 mV and -57mV, respectively, with other operational design parameters remaining unchanged. Influent water quality of chemical oxygen demand/total nitrogen (COD/TN) was 5.0 +/- 0.6. The results showed that when ORPan was set at -95mV, the maximum values of PHA synthesis and storage rate, PHA degradation rate, phosphorus release rate and phosphorus absorption rate in anaerobic and pre-anoxic segments were 82.34, 7.90, 47.31, 14.27, 1.50 and 8.52mg/ (L<bold>h</bold>), respectively. According to the metabolic mechanism of PHA and TP, ORPan was further proved to be the operation control parameter of the continuous-flow single-sludge sewage treatment system, and when the COD/TN value was 5.0 +/- 0.6, the optimal setting value was -95mV.
