Removal Of Nitrogen And Phosphorus In Low-polluted Water By Multi-stage Pond-wetland

With the increase of population and the rapid development of social economy,Erhai is in the transitional stage from mild eutrophication to moderate eutrophication.At present,the discharge of high-concentration sewage in Erhai watershed has been generally controlled,but the discharge of low-polluted water from runoff into the lake has become a new difficulty in Erhai protection.In order to mitigate the impact of low-polluted water on Erhai,local government using multi-stage ecological pond-wetland?MEPW?technology to treat low-pollution water.By now,over 100 multi-stage ecological pond-wetland system?MEPWS?had built to reduce the nutrition load into Erhai.Eight MEPWS at five townships in the west of Erhai were selected in this study.By regularly sampling and monitoring the influent and effluent quality of each system and its sub-units,their purification effect on low-polluted water were analyzed.According to the structural characteristics of MEPWS,a typical system was selected to establish a model of nutrients reduction along the processing path.To verify the universal applicability of the model,the fitting values and measured values in another seven MEPWS were compared by t test.The following results provide the basis for the design,operation,maintenance and management of MEPWS.?1?In MEPWS,the inflow of??TN????NH₄⁺-N????NO₃^--N????TP?and?(PO₄^3--P)were in a range of 0.85³7.97?0.13²4.69?0.22¹5.05?0.09¹2.94?0.004⁵.504 mg/L,over the Environmental Quality Standard for Surface Water?GB3838-2002?class?,and the outflow were reduced to 0.18¹1.58?0.08⁷.58?0.06¹0.89?0.003².806?0.001¹.309 mg/L,the removal rate in these systems were13.85%⁸0.75%?22.62%⁸6.43%?0.4%⁹9.27%?37.51%⁹1.30%?0.60%⁹9.27%.The nitrogen and phosphorus nutrients of inflow had been effectively purified.The removal effect of nitrogen and phosphorus in MEPWS were different due to rain and dry season.The removal effect in dry season was better than that in rainy season.The intermittent shutdown in dry season was beneficial to the removal of nitrogen and phosphorus pollutants in the system.?2?A MEPWS located in Meiba Village,Xizhou Town were operated as 5-stage series and structured with“storage pond+surface flow wetland+ecological plant pond”,its effluent quality met the Environmental Quality Standard for Surface Water?GB 3838-2002?class?,the mean values of??TN?,??TP?were 1.77,0.05mg/L,respectively.It is found that the average removal rate of TN,TP were 57.75%and 67.43%,respectively,of which 45.35%of TN and 50.59%of TP were removed in the first two stage.A linear model,a quadratic polynomial model,and an exponential model were analyzed for the removal of nutrients in MEPWS.The model fitting results show that the exponential reduction model is more suitable to TN,and the linear reduction model is the best fitting model for??TP?reduction in MEPWS.The establishment of this model can provide a certain theoretical basis for the optimal design of reservoir-wetland system and the prediction of water quality.?3?The??TN?and??TP?purification models were used to fitted the other 7 multi-stage pond-wetland systems.It is found that??TN?reduction model is not universally applicable,and it only has better fitting results for D2 and D3 systems,but the fitting effect for other systems is poor.Due to the lack of data used for model validation,the results are likely to be random.However,??TP?purification models was showed good fit results,the fitted value and the measured value were basically R²>0.5,this model can be applied to predict the effluent of MEPWS in the west Erhai.