Abstract：To study the effect of mixed carbon source concentration on nitrogen (N) and phosphorus (P) removal in the oxic/extended-idle process, four sequencing batch reactors were operated using sodium acetate and sodium propionate with 1:2 hybrid configured influent COD concentrations of 200, 400, 600 and 800mg/L respectively. The variations of intracellular polymers and the metabolic mechanism were investigated. The experimental results showed that with influent P and N concentrations of 12 and 30mg/L respectively, P and total nitrogen (TN) removal efficiencies were respectively promoted from 39.9% and 54.5% to 86.4% and 98.0% with the increase of influent COD from 200 to 800mg/L. The highest N, P removal per unit VSS [(4.31±0.08) and (6.15±0.22)mg/g] were both detected under influent COD of 400mg/L. P accumulating activity was enhanced when COD rose from 200 to 400mg/L, but the further increase of influent COD would result in deterioration of sludge settling, leading to a compromised biological N and P removal activity. Both aerobic phosphorus uptake and simultaneous nitrification and denitrification were driven by poly-β-hydoxyalkanoate (PHA), and the largest amount of PHA consumption per unit VSS was observed under influent COD of 400mg/L. The mixed carbon source concentration influenced microbial aerobic metabolism, which resulted in the difference of the accumulation/ conversion of intracellular polymers, and thus influenced N and P removal performance.
