Study On The Performance Of Low COD/N Ratio Urban Wastewater In Cold Area By Modified A~2/O Process

Mudanjiang, located in the southeast of Heilongjiang Province, which is the second largest tributary of the Songhua River. According to the division of water functional areas, the river belongs to the class Ⅲ water. However，studies had shown that the days in which the concentration of ammonia did not reach the standard accounted for more than half of the year, mainly due to the increase in emissions of ammonia from point sources around the city, making the city sewage present the characteristics of low C/N ratio and the removal efficiency of nitrogen and phosphorus decrease especially at low temperatures. To solve this problem, in the study, subsidized by national12th five-year plan, a modified A2/O process was proposed with the additional aerobic sludge return and fillers. By the regulation of operating parameters, the optimal water conditions was obtained. Then the raw water flew into the anaerobic and anoxic in a certain proportion to improve nitrogen removal efficiency. At last, by means of high-throughput sequencing, result about microbial properties in the process was obtained.Experimental studies the startup of reactor at low temperature and the influence of contaminants removal under different conditions. The main designed parameters are as follows: The influent distributive ratio to pre-anoxic zone/anaerobic zone was1:9. The SRT was15d. The dissolved oxygen (DO) in anaerobic zone was less than0.2mg/L, anoxic zone was0.2~0.5mg/L, aerobic zone was2.5~3.5mg/L. The temperature was below12℃. Under these conditions, the optimum operating conditions was as follows: the pre-anoxic zone sludge recycle ratio was15%, aerobic zone sludge recycle ratio was50%, and the internal recycle ratio was250%. The concentration of COD, NH3-N, TN, TP was26.74mg/L,3.72mg/L,9.40mg/L,0.27mg/L, the removal ratio was85.42%,83.10%,65.21%,92.75%.The influence had been demonstrated which the adjustment of the sludge age to the nitrogen and phosphorus removal after adding the fillers. Results showed that the problem of organic matter competing between denitrifying bacteria and phosphorus accumulating bacteria can be solved, and when the SRT was shorten in the appropriate range, the efficiency of nitrogen and phosphorus removal is not significantly reduced. Compared to the traditional A2/O process, the optimum SRT was12d instead of15d, and the efficiency of phosphorus removal is improved, the removal ratio was92.75%，but the removal ratio of ammonia was still efficient.The influence also had been demonstrated under different distributive ratios to pre-anoxic zone/anaerobic zone, and the sludge age was changed four times in the order of3:1,3:2,1:1,1:2. Studies had shown that there was little impact on COD and ammonia nitrogen removal under different distributive ratios, when the water flowing into the anoxic was increased, accompanied by an increase of total nitrogen removal, the removal ratio of phosphorus was decreased, and the effluent concentration of TP was too high, so the optimum ratio of3:2was determined, at this time, the concentration of COD, NH3-N, TN, TP was36.89mg/L,4.55mg/L,8.88mg/L,0.32mg/L, the removal ratio was82.93%,81.03%,69.92%,91.70%.In this study, high-throughput sequencing methods was adopted to study the microbial properties, and the sludge samples taken from modified A2/O process at the low temperature, wastewater plant process at the low temperature, laboratory scale at the room temperature. Results on the microbial diversity and microbial differences indicated that as more types of carbon source, microbial diversity would be increased, and under room temperature conditions, microbial diversity and differences of each processing unit was similar, however, there are significant differences at the low temperature, the biomass in anaerobic was more abundant than aerobic, indicating the low temperature had a greater impact on the denitrifying bacteria, and PAO’s tolerance to low temperature was stronger than the nitrification bacteria. At the low temperature, in modified A2/O process the PAO’s dominant strain was Acinetobacter, and the main denitrifying bacteria belonged to Pseudomonas, then the dominant species of nitrifying bacteria was Nitrosomonas.The result of the experiment can give theoretical support to optimation of Mu Danjiang wasterwater plant, contribute to the reduction of pollutants in Mu Danjiang, make the water quality of Mu Danjiang basin better, improve people living standard. The research content aiming at the cold water of the low ratio of carbon to nitrogen had a certain universality and could give a technical idea to the experiment in the future.