Optimization And Simulation Of Multi-stage AO Process Treating Low C/N Ratio Sewage

The C/N ratio of urban sewage in most cities of southern China is generally low.The multi-stage AO process can make full use of the carbon source by introducing the influent at multiple points,thereby reducing the dosage of the external carbon source.However,there are few studies on the optimal operating parameters of this process for treating low C/N ratio wastewater.To this end,this research aimed to make full use of the influent carbon source and studied the optimal operating parameters of the multi-stage AO process.On this basis,the carbon source types and dosing position as well as their dosages were optimized,and the process control measures based on the model prediction were proposed.To make full use of the carbon source in the influent water,the optimization of the operating parameters for the multi-stage AO process was carried out through a series of lab-scale tests.The results showed that under the condition of influent C/N ratio of about 3.6:1,the optimal operating parameters of the process were:the influent flow distribution ratios of the first and second stage were 7:3,the corresponding nitrification liquid reflux ratios were both 100%,the anoxic-aerobic volume ratio was 0.8:1,and the sludge reflux ratio was 60%.In this case,the influent carbon source was fully utilized,facilitating the satisfaction of the first-class A discharge standard for effluent COD and NH₄⁺-N.However,the average concentrations of effluent TN and TP were 19.0 and 2.73 mg/L,respectively,which could not meet the discharge standard.To further improve the effluent quality,the external carbon sources were added under the conditions mentioned above,and the effects of ethanol,acetic acid and sodium acetate on denitrification were studied.The results showed that acetic acid had the best effect,and the optimized dosage was 40 mg/L in anaerobic tank and 7 mg/L in the front section of the anoxic tank II.The multi-stage AO process had a stable treatment effect,and the effluent COD,NH₄⁺-N,TN and TP concentrations were 19.5,0.52,14.11 and 0.43 mg/L,respectively,meeting the first-class A discharge standard.In order to ensure the stable operation of the process under the large fluctuation of influent C/N ratio,the control measures of process operation were studied based on the mathematical model of the multi-level AO process.The results showed that the model could predict well the quality of the effluent water according to some variables such as influent water quality and process parameters.When the water quality fluctuated,the process parameter variables in the adjustment model could be optimized.In the case of external carbon sources,the influent COD concentration could be also adjusted.These control measures could ensure that the water quality meets the standard.