| In recent years, water pollution and eutrophication of lakes is becoming more and more serious for rapid economic development and advancing industrialization, urbanization and agricultural modernization. And the enrichment of amounts of nitrogen and phosphorus is an important reason leading to eutrophication. The effluent of WWTP(Waste Water Treatment Plant) has the characteristics of large effluent and high nitrogen and phosphorus concentrations, which further exacerbates eutrophication when discharged into lakes, so it is imperative to study advanced treatment for the tail water of urban sewage treatment plant.This paper constructed a multistage ecological system as advanced treatment for the effluent of WWTP, which was constructed of ecological pond, wetland, surface flow wetland and submerged plant pond, with characteristics of easy management, low cost of running and powerful practicability. This experiment took the pond water as the processing object, analyzing the pollutant removal efficiency of different processes, and preferred optimized multistage ecological system. Discussing the functions of each ecological unit and the removal efficiency of pollutants under the optimum process; On this basis, with the village sewage treatment tail water in Lucun Wuxi as treatment object, exploring the enhanced removal to nitrogen and phosphorus by different wetland plants and different hydraulic loading rate. At last, according to the efficient and proper allocation matrix plant hydraulic load conditions, exploring carbon sources addition on the removal efficiency of sewage treatment plant tail water enhancement under low carbon-nitrogen ratio water, the main conclusions are as follows:(1) Under the conditions of water temperature 6℃-29.5℃ and hydraulic loading rate 0.24m3/(m2·d), the optimized process is "ecological pond+subsurface flow wetland+secondary surface flow wetland+aquatic plant pond". The removal rate of COD, NH4+-N, TN and TP for the pond water were 67.1%,40.4%,41.1% and 35.7% respectively, and the effluent concentration were 20-30mg/L,2.5-3.0mg/L,3-4.2mg/L and 0.21-0.28mg/L respectively.(2) When the water temperature and the hydraulic loading rate were 1℃-18℃ and 0.24m3/(m2·d) respectively, the TN average removal rate of the different substrates and plant configuration system was 45.6% and effluent concentration was 6-7mg/L; the TP average removal rate was 53.5% and the effluent concentration 0.05-0.15mg/L. In order to strengthen the sewage treatment plant tail water remove effect, the best substrate of wetland was the aerated concrete block, and wetland plant Canna and orris were arbitrary.(3) When the water temperature was 1℃-18℃ and the hydraulic load was reduced from 0.30m3/(m2·d) to 0.18m3/(m2·d), the removal rate of TN and TP in the wastewater treatment plant of the multistage ecological unit system increased from 39.4% and 56.2% to 47.2% and 58.6% respectively. It can be seen that reducing the hydraulic load and prolonging the residence time of the sewage treatment plant was of great significance to improve the removal efficiency of pollutants in the tail water of sewage treatment plant.(4) Organic carbon source is the main factor affecting the denitrification and also the necessary material and energy sources needed by denitrification bacteria. The TN removal efficiency of the combined system increased from 45.6% to 58.4% when the C/N value of the simulated water was increased from 2 to 4.1. The results show that the addition of organic carbon source can effectively improve the efficiency of the nitrogen pollutants remove in wetland.In summary, When the water temperature and the hydraulic loading rate were 1℃-18℃ and 0.24m3/(m2·d) respectively, taking "ecological pond+subsurface flow wetland+secondary surface flow wetland+aquatic plant pond" as advanced treatment for the effluent of WWTP can effectively remove the pollutants such as organic matter, nitrogen and phosphorus, simultaneously improve the water quality by adding carbon source. The application of the multistage ecological system will effectively improve the water quality of the water environment. |
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