[External carbon addition control strategies of step-feed A/O process]

To obtain high quality drainage, external carbon source (ethanol) was used to enhance denitrification when low COD/N sewage was fed to a pilot-scale step-feed A/O system. To use carbon source sufficiently, five different external carbon addition control strategieswere proposed based on different control parameters, ORP and on-line nitrate, and dosing positions, D3 and D4. Control strategies were evaluated on investment, treatment efficiency and operation cost basis. The results showed that for the short hydraulic retention time in anoxic zone, the control parameter could not reach the set-point when control strategy I and II were applied. As a result, excessive feeding of external carbon source was occurred and the average COD concentrations in D4 were 192.8 mg/L and 158.9 mg/L for the insufficient usage of carbon. In this condition, effluent TN concentrations were 17.42 mg/L and 19.04 mg/L and the external carbon dosage rates were 92 mL/(m3 x d) and 84 mL/(m3 x d), respectively. Control strategies III, IV and V controlled the external carbon dosage in D3 and D4 simultaneously. The results indicated that the strategies had strong resistance to peak loadings and reduced the effluent TN concentration effectively. The average effluent TN concentrations, 7.30 mg/L, 8.2 mg/L and 7.49 mg/L and the external carbon dosage rates, 29 mL/(m3 x d), 45 mL(m3 x d) and 27 mL/(m3 x d), were achieved respectively. Finally, based on the amount of on-line sensors, reliability of treatment efficiency and cost of operation, the control strategies proposed in this paper were evaluated together. Compared to the other control strategies, the suggested external carbon addition strategy III was testified to be an optimal control strategy, which was characterized by good dynamic qualities, strong resistance to peak loadings, quick responding to loadings, and the lower external carbon addition amount.