Study On Purification Effect Of Composite Tidal Flow Constructed Wetland On Black And Odorous Water In Southern Rural Areas

With the continuous improvement of social and economic level and rural productivity,the ecological environment is increasingly unbalanced,resulting in the increasingly frequent phenomenon of black and odorous water bodies,and the construction of ecological civilization countryside is facing severe challenges.Constructed wetland has gradually become a common technology for rural sewage treatment due to its advantages of less secondary pollution,low construction and maintenance costs.However,the single-structure constructed wetland cannot meet the emission requirements for the treatment of black and odorous water in rural areas.Therefore,how to construct a more economical and efficient constructed wetland to improve the water quality of black and odorous water in rural areas is the urgent task of rural sewage treatment.According to the water quality characteristics of black and odorous water in rural areas,a composite tidal flow constructed wetland（HTFCW）composed of two sets of tidal flow constructed wetlands A and B)was designed and constructed by using anthracite and zeolite as the main matrix filler.Parameters such as pollution load,hydraulic retention time（HRT）,temperature and plants were selected to investigate their effects on the removal efficiency of black and odorous water in rural areas.The concentration and morphology of pollutants along the route under the optimal operating conditions were analyzed to further explore the migration and transformation of pollutants in constructed wetlands.It is expected to provide a theoretical basis and engineering reference for the treatment and promotion of composite tidal flow constructed wetlands in rural black and odorous water.The main research results and conclusions are as follows:1.The HTFCW system with anthracite and zeolite as substrates and water celery as wetland plants had good purification effect on black and odorous water bodies in rural areas of South China,which significantly improved the oxygen environment inside the wetland,and showed high removal effects on nitrogen,phosphorus and COD.The removal rate of COD in the wetland system increased with the increase of pollution load,reaching 97.73%.The TN removal rate of the system was more than 80%,and the highest was 99.53%.The increase of pollution load led to a slight decrease in TN removal efficiency.The pollution load has a significant impact on TP removal,and it increases first and then decreases with the increase of pollution load.2.The removal rate of COD first increased and then decreased with the increase of HRT.The highest removal rates were 85.65%,96.94%and 96.64%at 4 h,4 h and6 h under light,medium and heavy pollution loads,respectively.The extension of HRT had a reaction on TP removal rate,and the removal efficiency reached the maximum at 2 h,corresponding to 76.63%,91.78%and 93.28%of the three pollution loads,respectively.When HRT was 4 h,the comprehensive removal efficiency of the wetland system was the best.The effluent quality of the wetland system was stable in winter,but the pollutant removal rates were lower than those in summer.The COD,TN and TP decreased by 7.47%,8.99%,4.59%,6.20%,11.90%,13.17%and46.49%,14.95%,17.64%,respectively.For COD and TP,the increase of pollution load can offset the negative effect of low temperature to a certain extent.Water celery showed good adaptability under large pollution load.The uptake of nitrogen and phosphorus by roots,stems and leaves were 11.97 mg·g^-1,9.17 mg·g^-1,10.24 mg·g^-1 and 3.00 mg·g-1,3.76 mg·g-1,2.16 mg·g-1,respectively.Each mu of water celery could bring about 1-24 thousand yuan of income,and remove about 48.52-77.64 Kg of nitrogen and 14.75-23.60 Kg of phosphorus.At the same time,it has high economic value and environmental value.3.By analyzing the changes of pollutant concentration and morphology along the route,nitrogen in the system was completed by traditional nitrification and denitrification and simultaneous nitrification and denitrification.In the upper part of red soil layer and matrix layer in the wetland system,the removal rate of ammonia nitrogen adsorption and degradation accounts for 89.25%of the whole system.Simultaneous nitrification and denitrification occurred in red soil layer,and denitrification mainly occurred in device B.COD decreased rapidly in red soil layer,and the removal rate accounted for 45.72%of the system removal rate.Due to the lack of dissolved oxygen and low biodegradability,the removal effect was poor,and the removal rate of unit B was only 11.43%of the system COD removal.TP decreased fastest in the red soil layer,mainly due to the strong chemical adsorption of red soil,while in other parts it was mainly physical adsorption and biological adsorption.The correlation analysis between DO concentration and pollutant removal rate showed that COD,TN,NH4+-N removal rate was positively correlated with DO concentration,and TP removal rate was not correlated with DO concentration.