| There exists a problem that sewage treatment facilities of Songhua River regions have poor treatment performance in winter frozen period and it badly needs to be resolved. In this paper, chemical coagulation and two-stage biological aerated filter (BAF) combined process was employed to treat the municipal wastewater at low temperature in Songhua River regions. Decontamination efficiencies by the combined process in small and pilot scales were studied and their optimal operation parameters were determined, respectively. Finally, the trial projects of the combined process were established on the basis of preceding experimental results in small and pilot scales.In the small scale study, removal efficiencies of COD, SCOD, TOC, TP, SS, NH4+-N and TN were selected as the assessment indicators in static chemical coagulation tests. Four kinds of inorganic coagulants such as ferric sulfate, ferrous sulfate, ferric chloride and aluminum sulfate were used to investigate their respective treatment efficiency on the municipal sewage at low temperature and aluminum sulfate was confirmed to have best performance. The optimal operation parameters for chemical coagulation at low temperature (6-10℃) were:pH 6.5-7.0 and dosage of aluminum sulfate 25-30 mg/L, under which the removal efficiencies of TP, SS, TOC, COD, SCOD, TN, NH4+-N were 89.6%,97.3%,49.7%,47.1%,25.3%,7.1%,8.2%, respectively.The performances of anaerobic and oxic phase of the two-stage BAF on wastewater treatment at room and low temperatures were investigated, respectively. Factors influencing the removal effects of the reactors such as backwash interval, backwash pattern and intensity were also studied. The experimental results showed that the removal performances of two-stage BAF at room temperature were excellent. When the filtering velocity was 0.33-1 m/h, the removal efficiencies of COD and NH4+-N were 84% and 95%, respectively. However, at low temperature, the removal values of COD and NH4+-N were only 60% and 47%, respectively. TN removal could be enhanced from 35% to 60% when methanol was used as the plus carbon, and TN removal value had some improvement with an increase in the dosage of methanol.In the pilot scale study, the combined process of chemical coagulation and two-stage BAF was adopted to treat the municipal wastewater, particularly the removal efficiencies of organic pollutants, nitrogen and phosphorus at low temperature by the combined process were investigated. The wastewater was collected from Dongcheng sewage treatment plant in Daqing city and the average concentrations of COD, NH4+-N, TN and TP were 450 mg/L,45 mg/L,60 mg/L and 6 mg/L, respectively. Experimental results showed that biofilms in the two-stage BAF could be formed completely within 25 days. When the temperature was above 10℃, the removal efficiencies of COD, NH4+-N, TN and TP by sole two-stage BAF were 94.3%,97.5%,77.71% and 95.6%, respectively. While the temperature lowered to 6-10℃, the combined process of chemical coagulation and two-stage BAF was applied to maintain the treating efficiency of wastewater. When the dosage of aluminum sulfate as the coagulant was 25-30 mg/L, COD, NH4+-N, TN and TP of the effluent were 45.2 mg/L, 3.26 mg/L,12.56 mg/L and 0.43 mg/L, respectively.After more than a year of commissioning, operation and research, the treatment effect of trial project remained basically stable. The average concentrations of COD, NH4+-N and TP of the effluent were 41.2 mg/L,2.91 mg/L and 0.46 mg/L, respectively, which all meet Level A of "Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant"(GB18918-2002).
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