| In order to find the effective method to deal with the sewage and directly recharge it into groundwater. The removal effects of SWIS under different pollution loads were investigated by the simulated soil column experiments. The environmental effect of soil leachate and the land carrying capacity of system were evaluated by the groundwater standard and drinking water standard. Then, the optimized parameters were applied in field and used to verify results of soil column experiment, which provides a scientific basis for the engineering application of a SWIS in China. The main conclusions are summarized as follows:(1) In the soil column simulated experiments, the effluent total nitrogen (TN) and nitrate nitrogen (NO3-N) concentrations under pollution load of 600 kg N ha-1 y-1 were significantly lower than that under pollution load of 1200 kg N ha-1 y-1. The ratios of concentrations in the effluent, which satisfied III grade (≤20 mg L-1) of the groundwater standard were 28.6% and 96.6%, and reached the maximum international contaminant level of drinking water standard (≤10 mg L-1) were 3.6% and 96.6%, respectively, for high pollution load and low pollution load.(2) Adopting the soil leachate of annual average NO3--N concentration≤10 mg L-1 as the evaluation standard, the nitrogen of the land carrying capacity would ranged from 600 kg ha-1 y-1 to 1200 kg ha-1 y-1. Adopting the soil leachate of annual successive NO3-N concentration≤10 mg L-1 as the evaluation standard, the nitrogen of the land carrying capacity would be less than 600 kg ha-1 y-1.(3) There were no significant differences between the treatment with and without adding peat in deth in the concentrations of TN, ammonia nitrogen (NH4+-N), N03--N, total phosphorus (TP) and chemical oxyen demand (CODmn). It showed that the peat added had no promoting effect on nitrogen removal in the sixth year. So peat should be updated regularly in the practical engineering application.(4) Compared with bare land, the removal amounts and removal rates of the TN, TP and CODMn increased significantly in the system covered by grass. Under pollution load of 600 kg N ha-1 y-1, the contribution rates of turf on nitrogen and phosphorus removal were 88.6% and 79.8%, respectively. Average effluent concentration of TN in bare land was 53.7 mg L-, and the concentration was rised significantly in the 69th day. TN removal rate of TN in bare land was 92.9% for the original 69 days, while decreased to-12.1% during the 69th day-270th day. TN concentration in SWIS was 41.5 mg L-1, and the concentration was rised in the 180th day. The removal rate of TN in bare land was 99.7% for the original 180 days and decreased to 45.0% during the 180th day and the 270th day.(5) It showed that the superior performances of the SWIS for TP and CODMn in the two years’ experiments. The removal rates of TP and CODMn in SWIS were all above 99.8% and 95.6%. And the removal rates of TP and CODMn in bare land were up to 99.2% and 90.9%.(6) According to the results of soil column simulation test, the running parameters of the pilot SWIS were determined as follows:the annual pollution loads were 420 kg N ha-1 y-1 and 600 kg N ha-1 y-1, under the control of influent total nitrogen concentration of 40 mg L-1, the hydraulic loads were 3.5 mm d-1 and 5 mm d-1. According to these parameters, the hydraulic calculation was carried out and the design and construction of the project of pilot-scale experiments were completed.(7) The soil leachate concentrations of TN, NH4+-N, NO3--N, TP and CODMn,, in the first year in the pilot scale field subsurface infiltration system increased with the amount of nitrogen application. Adopting annual successive NO3--N concentration of soil leachate was not overstandarded as the evaluation standard, the NO3-N concentrations all satisfied the grade Ⅱ (≤ 5 mg L-1) of the groundwater standard by 200 kg N ha-1 y-1 of fertilizer and 420 kg N ha-1 y-1 of pollution load. The NO3--N concentration satisfied the international drinking water standard (10 mg L-1) by 600 kg N ha-1 y-1 of pollution load. |
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