Influence of Step-Feed Ratio on the Nitrogen Removal Efficiency of Two-Stage AO Process in the Treatment of Municipal Sewage with a Low C/N Ratio

Abstract

The present study was envisaged to develop a step-feed two-stage AO process for the removal of nitrogen via biological methods in conformity with regulations for the discharge of low-C/N-ratio municipal wastewater. The carbon source was preserved by setting a preliminary anoxic tank and reducing the content of dissolved oxygen in the effluent of oxic tank 1. The effect of step-feed ratio on the removal rate of nitrogen was investigated. The results showed that the COD removal rate was not significantly affected by the step-feed ratio, and COD concentrations in the effluent were reduced to lower than 30 mg/L in all the samples. The removal rate of the ammonia nitrogen and total nitrogen were obviously affected by the step-feed ratio. When the step-feed ratio of anoxic tank 1 was set to 70, 80, and 90%, the concentration of the ammonia nitrogen and total nitrogen in the effluent was 1.49, 17.57, 0.52, 13.24, 0.31, and 10.28 mg/L, respectively, and the removal rates reached 96.60, 63.58, 98.77, 72.17, 99.27, and 78.74%, respectively. The removal rate of ammonia nitrogen and total nitrogen exhibited an increase with an increase in the step-feed ratio of anoxic tank 1. The removal rate of total phosphorus in the step-feed two-stage AO process was as low as 34.72%. When polyaluminium chloride (PAC) was added to the pipeline connecting oxic tank 2 and secondary sedimentation tank, the concentration of total phosphorus in the effluent was reduced to lower than 0.5 mg/L. The quality of the municipal wastewater with a low C/N ratio could attain the 1A-level discharge standards after the treatment with a combination of step-feed two-stage AO process and chemical phosphorus-eliminating process.