Experimental Study On AOA Expanded Bed Combined Biofilters For Municipal Wastewater Treatment

In this paper,AOA expanded bed combined biological filters process was used to remove nitrogen and phosphorus in urban sewage.The process consists of three stage biological filters:Downflow suspended media pretreatment and denitrification filter（A）、Upflow suspended media aerated expanded bed filter（O）、Downflow suspended filter material denitrification and phosphorous removal filter（A）.The suspended filter material BAF has the advantages of simple operation,low energy consumption and little possibility of blockage.AOA expanded bed combined biological filters process can be applied to the renovation project of the existing small-scale wastewater treatment plant.It can also be built into small integrated sewage treatment equipment.In this study,the effect of biological nitrogen and phosphorus removal by AOA expanded bed combined biological filter process on domestic sewage treatment was investigated,which provides reliable operational parameters for engineering design.The experiment first studied on the biofilm culturing feature of AOA expanded bed combined biological filters process,then analyzed the removal law of pollutants along the filter layer when the filters were in steady operation,determined the backwashing method of filter at all levels,studied the influencing factors of nitrogen removal efficiency in AOA expanded bed combined biological filter process.Finally,determined the dosage of FeCl₃.The test results are as follows:（1）The experiments were performed on a staged natural biofilm start-up.After 27 days of biofilm culture,the removal rate of COD_Cr can reach more than 85%,the removal rate of NH₄⁺-N can reach above 90%,and the removal rate of TN can reach 60%.（2）By detecting and analyzing the changes of contaminants when them flow through the filters,it is known that the first step--Downflow suspended media pretreatment and denitrification filter,can be both pretreatment and denitrification,which is effective in removal COD_Cr,NO₃^--N and SS;the second step--Upflow aerated filter,has high efficiency in nitrification,NH₄⁺-N of the effluent is less than 5 mg/L;the third step--Downflow suspended filter material denitrification and phosphorous removal filter,which has a heavy DO and low-concentration organics,has a weak effect on denitrification but can intercept some suspended solids without any carbon resource.During the test,the concentrations of COD_Cr,NH₄⁺-N,TN,SS could achieve‘urban sewage treatment plant pollutant discharge standard’（GB18918-2002）A in first grade.（3）Under the test conditions,with the increase of influent hydraulic load,the concentration of COD_Cr,NH₄⁺-N and TN decreased.The recovery ability of the system was strong when it was subject to the large water shock.When the reflux ratio was 150%²00%,the system can achieve the best nitrogen removal effect.When increased the gas water ratio of aerobic filter,the effluent concentration of COD_Cr and NH₄⁺-N decreased gradually,and the removal efficiency of TN increased first and then decreased.When the water temperature dropped to below 12℃,the effluent concentration of COD_Cr and NH₄⁺-N can still reach the emission standard of A in first grade,but the effluent concentration of TN can only reach the emission standard of B in first grade.By changing the operation parameters and applying the carbon source before the third stage denitrifying filter column,the effluent TN concentration can be lower than 15 mg/L.（4）During the low temperature period,chemical phosphorus removal was carried out by multi-point dosing of FeCl₃.The dosage before the first stage filter column was 40 mg/L,and the dosage before the third stage filter column was 30 mg/L,can guarantee the effluent TP<0.5 mg/L.