The Optimization Of Biological And Ecological Combination Technology Of Rural Domestic Wastewater Treatment And Demonstration Project

Taihu basin is an area under strict pollution control;Due to the characteristics of aquatic environment and the complicated socioeconomic development situation of Tiaoxi River,the inlet of Taihu Lake area,a single biological or ecological wastewater treatment technology cannot make wastewater effluent meet the environmental requirements.And the combination of biological and ecological technology can enhance the reduction of major pollutants of rural domestic wastewater.The forepart biological treatment is mainly responsible for removal of some nutrients and organic matters,then the ecological treatment is intended for enhancing denitrification and phosphorus removal.The combination gives full play to their strengths to improve water quality.In this paper,based on status of the rural domestic wastewater of Tiaoxi River,the inlet of Taihu Lake area,a biological and ecological combination technology of“Anoxic/Oxic-constructed wetland-riparian area-aftificial floating island”pilot-scale treatment system（AOWRF system）was developed.The main technical parameters（dissolved oxygen,packing type and reflow ratio）were optimized to determine the optimum process parameters of the“Anoxic/Oxic”combination system.Simultaneously,the constructed wetlands,riparian areas and the artificial floating islands with different filling methods and configuration were studied in terms of its efficiency in main pollutants removal,combined with the contribution rates of plants absorption to nitrogen and phosphorus removal,packing and the microorganism structure in rhizosphere were analyzed in this system;Finally,based on“AOWRF system”,a demonstration project of"Anoxic/Oxic-constructed wetland-riparian area"to treat rural domestic wastewater was constructed in Yujia Tian village in Huzhou city,Zhejiang Province and the economic costs were estimated.The research main findings are as follows:（1）The results showed that the performance of Anoxic/Oxic system on removal of total nitrogen（TN）,ammonia nitrogen（NH₄⁺-N）,total phosphorus（TP）and Chemical Oxygen Demand(COD_Cr)were the best under the conditions of reflow ratio（1:1）+braided packing+dissolved oxygen(2.50 mg L^-1)combination,and the removal rates were 34.58%,51.31%,31.21%and 48.16%,respectively.There was no obvious improvement in major pollutants removal from domestic wastewater with low C/N ratio even after reflow ratio.Only the"Anoxic/Oxic"combined treatment system cannot meet the<Standards for pollutant discharge in urban sewage treatment plants>（GB 18918-2002）in level B.（2）Under the conditions of hydraulic retention time（HRT）of 56 h and hydraulic loading rate（HLR）of 0.3 m³/（m²·d）,the influent TN,NH₄⁺-N,TP and COD_Cr concentrations of constructed wetlands were 24.83⁵9.63 mg L^-1,15.65⁵2.55mg L^-1,3.04⁸.22 mg L^-1,and 47.42¹11.52 mg L^-1,respectively.The average removal rates of TN,NH₄⁺-N,TP and COD_Crr in the constructed wetland（W2）with ceramsite（upper filling height was 45 cm）and zeolite powder（lower filling height was 15 cm）could reach 64.97%,81.80%,38.22%and 32.45%,respectively in winter.This might be due to the fact that the removal of nitrogen（N）from the wastewater mainly depended on the adsorption of zeolite powder.Absorption of calamus during the prosperous period contributed only 0.45%and 0.34%to the removal of N and phosphorus（P）from wastewater,respectively.However,the rhizosphere microbe were rich in diversity and were dominated by Proteobacteria,Bacteroidetes,and Firmicutes,and bacteria with N and P removal capacity accounted for a large proportion（42.16%）,which also confirmed the synergistic effect of rhizosphere microbes.The characteristics of microbial community structure in fillers were similar to the rhizosphere microorganisms,and the abundance of the microbes in W2 were relatively higher.Based on this,it was postulated that the optimal performance of W2on major pollutants removal was related to these miroorganisms.（3）Under the conditions of HLR=0.09 m³/（m²·d）,the influent TN,NH₄⁺-N,TP,and COD_Cr concentrations of riparian areas were 8.90²4.80 mg L^-1,4.65²0.47 mg L^-1,0.89³.85 mg L^-11 and 26.72⁷2.75 mg L^-1,respectively.The average removal rates of TN,NH₄⁺-N,TP,and COD_Crr of riparian area（R1）with ceramsite（upper filling height was 15 cm）and zeolite powder（lower filling height was 5 cm）could reach 35.26%,62.16%,44.53%and 31.20%,respectively.Absorption of the perennial ryegrass during the prosperous period contributed 23.54%and 22.65%separately to the removal of N and P from wastewater,and the rhizosphere microorganisms were dominated by Planctomycetes（19.08%）and Proteobacteria（36.59%）,which strengthened N and P removal by ryegrass.Microbes affiliated to Proteobacteria and Bacteroidetes were most widely distributed（53.44%and 14.20%,respectively）in the filling’s surface microbial population of riparian area（R1）,and bacteria with capacity of N and P removal accounting for 1.10%in natural soil and the riparian area（R1）filled with ceramsite is relatively greater than that of the other two riparian zones filled with humus and ceramsite（or zeolite powder）（accounting for 0.90%in R2 and0.68%in R3）.Based on this,it was postulated that the optimal performance of R1on major pollutants removal is related to these miroorganisms.（4）Under the conditions of HRT=8 d and HLR=0.09 m³/（m²·d）,the influent TN,NH₄⁺-N,TP and COD_Cr concentrations of artificial floating islands were 6.40¹9.69mg L^-1,2.61⁸.73 mg L^-1,2.43⁶.58 mg L^-1,1.39².52 mg L^-1 and 25.95⁹1.17 mg L^-1,respectively.The removal rates of TN,nitrate nitrogen（NO₃^--N）,NH₄⁺-N,TP and COD_Cr for the artificial floating island（F3）with PVC pipe as floating body,straw mat and felt mat as carrier,and water-inner-cycling as enhanced purification measure were54.57%⁷8.67%,85.19%⁹6.67%,31.68%⁶3.55%,60.15%⁸0.51%and10.67%³3.26%,respectively.The maximum contribution rates of the cress absorption to N and P removal were 2.17%and 2.69%,respectively.However,the rhizosphere microbes were rich in diversity,and were dominated by Proteobacteria and Bacteroidetes,and bacteria with the capacity of N and P removal accounting65.62%in microbes,confirming the synergistic effect of rhizosphere microbial communities.Additionally,the proportion of Proteobacteria in the microbes on the surface of the straw mat accounting up to 91.45%in the artificial floating island（F3）.Bacteria with capacity of N and P removal separately accounting 58.30%and 57.10%in the microbial communities on the surface of straw mat and felt mat.Based on this,it was hypothesized that the optimal performance of major pollutants removal from artificial floating island F3 was related to these miroorganisms.（5）The rural domestic wastewater after the treatment of“Anoxic/Oxic”system in the“Anoxic/Oxic-constructed wetland-riparian area”demonstration project still could not meet the“Discharge standard of pollutants for rural sewage treatment facilities”（Zhejiang Local Standard DB 33/973-2015）;however,after further treatment with constructed wetland and riparian area,the effluent COD_Cr,NH₄⁺-N,and TP concentrations were 12.34²1.54 mg L^-1,1.62⁴.57 mg L^-1 and 0.23⁰.51 mg L^-1 respectively,the quality of effluent was significantly improved.It has been estimated that the operating cost of this demonstration project was 1.61 yuan/m³.This mode has certain reference and demonstration effect for the treatment of domestic wastewater in the rural areas of Tiaoxi River basin.