| China is a mountainous country, and mountain area accounts for two thirds of land area, a higher proportion in the southwest, which is 97.46% in Sichuan and Chongqing area. The particularity of mountain area landform leads to sensitivity and vulnerability of ecological environment, with the quantity and quality of the water source fluctuating largely and non-point pollution being outstanding.In our previous research, the test that biological-ecological restoration of joint measures to supply water to the river system had been carried out, in the course of which discovering that the non-point pollution impact in rainy season affected the stability of restoration.On the basis of reducing the impact load of biological-ecological restoration and protecting water source to supply the river, considering engineering design elements, according to ecological diversity theory, the Multi-stage filtration system was built to abate agricultural runoff pollution in this paper; based on the investigation of agricultural nonpoint pollution and runoff monitoring in the characteristic catchment areas, the efficiency and characteristics of the system for agricultural runoff pollution treatment were studied, and the influences of pollutant load, hydraulic load, vegetation and other factors on the pollutant removal rate were investigated.The runoff monitoring of characteristic catchment areas showed: in the non-rainy season, the concentration of CODMn,TN and TP in runoff pollution were lower; in the rainy season, the CODMn, TN and TP went beyond III class of Surface Water Environmental Quality Standard (GB3838-2002), even exceeded V class in some time. The pollutant changed a lot with time, however, its loss was basically consistent to the time of rainfall peak and agricultural fertilization.The result of the Multi-stage filtration system showed that it was well adaptive to variation of inflow quality and quantity and better removal effect for SS, CODMn, NH4 +-N, TN and TP, with the average removal rates were 80 %, 35%, 78%, 37%, 48% respectively. The concentration of various pollutants decreased gradually along the way with the increase of stages in filtration system, the removal rate reaching maximum at the last stage.The removal rates of TN, NH4 +-N, CODMn increased steadily not linearly with the increase of stages, but the SS, TP removal rate of increase in the system, there is no significant slow down the trend along system. The regression analysis on inlet and outlet water load of system showed that removals of SS, CODMn and TP were at the highest level in the first stage, removals of TN, NH4 +-N were at the highest level in the second stage and in the third stage there was still a better removal efficiency of NH4 +-N ,SS even when the inlet water load was low.The analysis on N, P removal characteristics of system showed that N was removed mainly in particulate state during the first stage, mainly removed in dissolved state during the second and third stage; the removal efficiency of particulate and dissolved P were both better during the first stage, P was removed mainly in particulate state during the second stage, and in the third stage, in the case of removal of P was low,particle states and dissolved states have equal contribution to removal of P.The analysis on effect factors of water treatment showed that removals of SS, PN, PP, CODMn and DTN significantly increased, but NH4 +-N, DTP removal rate increased not markedly; the removal rates of SS, CODMn, PN, NH4 +-N and DTN similar with the hydraulic load. In conditions with low, average hydraulic load, the removal rates were similarly or increasing a little; with high hydraulic load, the removals of SS, CODMn, PN, NH4 +-N and DTN decreased significantly; DTP removal rate did not change with the hydraulic loading.CODMn, NH4 +-N and DTP removal rates increased with pollution load increasing, however,pollution load had no significant effect on the removals of the SS and DTN.
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