Removal Efficiency Of Different Substrates Used In Vertical Flow Constructed Wetlands For Treating Domestic Wastewater

Constructed wetlands (CW) are widely used to purify wastewater in some developing countries. As filter substrates in such wetland, these substrates play important role on removal of pollutants from wastewater. Soil, sand and gravel are the conventional substrates used in constructed wetland presently, but the disadvantages of these substrates are lower purified efficiency, easily adsorbed saturation and low removal rates for N and P from domestic wastewater. In allusion to these, this experiment was designed and the substrates poles system using zeolite, peat composed of rotten mosses, vermiculite, shale and sand as substrates simulating actually vertical CW was constructed to treat domestic wastewater for studying the relation between the substrates and its depth, running mode, running time and wastewater treatment. On the basis of single factor test, the L₄(2³) orthogonal test was done using the two best substrates, hydraulic loadmg and hydraulic retention time as the factors. At last, the retexture of 5 substrates were analyzed.The simulating system was composed of influent system, substrate pole system and effluent system, including 10 substrate poles, which divided into two teams, 1.2m and 1.6m in height respectively, filling with zeolite, peat composed of rotten mosses, vermiculite, shale and sand. The sampling points of 1.2m poles lied at the depth of 20cm, 50cm, 100cm and which of 1.6m poles lied at the depth of 80cm, 120cm, 140cm.The system run under two different modes during the whole experiment period (80 days).31 Samples were taken from sampling points and influent once 8 days, and the concentration of COD,BOD5,TN,TP were measured, gaining 1240 data. The variance of 5 substrates and 6 depths was analyzed and regression equation was built by SAS (Statistical Analysis System) software. Then the comprehensive synthesis judgment was put up to choose the best combination of substrate and running mode to deal with wastewater. In the end, the L₄ (2³) orthogonal test of peat composed of rotten mosses and shale was done at the depth of 100cm, in which the levels of hydraulic loading were 0.3m/d, 0.4m/d and the levels of hydraulic retention time were 3.5h, 4.2h, using comprehensive equilibrium method to analyze 48 data. The including as follows:1. The order of excellence for removing COD from wastewater is vermiculite, zeolite, shale, peat composed of rotten mosses, sand, the order of excellence for removing BOD₅ is shale, vermiculite, peat composed of rotten mosses, zeolite, sand, and the order of excellence for removing TP is peat composed of rotten mosses, shale, sand, zeolite, vermiculite. The order of excellence for removing TN of different substrates is similar, only zeolite, vermiculite and shale be better in some sort. In terms of the different influence of 4 indexes, the synthetical order of excellence of 5 substrates is peat composed of rotten mosses, zeolite, shale, vermiculite and sand, and that zelite and shale are equal.2. Within 80cm, the removal rates of COD,BOD₅ of 5 substrates become higher with the increasing depth, while exceeding 80cm, this trend is not obvious. The depth of substrates has little effect on the removal of TN. Within 100cm, the removal rates of TP of 5 substrates become higher with the increasing depth, while exceeding 80cm, this trend is not obvious. So, for 4 indexes, the optimal depth of substrates is 100cm.3. When the hydraulic loading heightened and the hydraulic retention time shortened, the removal rates of COD of zeolite, peat composed of rotten mosses, vermiculite, shale and sand dropped 9.26%, 6.24%, 1.76%, 10.15%, 0.26%, respectively. The removal rates of BOD₅ of 5 substrates dropped 18.71%, 9.82%, 6.87%, 2.36%, 9.34%, respectively. And the removal rates of TP of 5 substrates dropped 3.99%, 1.69%, 5.53%, 22.19%, 22.65%, respectively. The removal rates of TN of zeolite, vermiculite dropped 2.56%, 19.78% respectively, while peat composed of rotten mosses, shale and sand heightened 19.81%, 1.04%, 18.16%, respectively. The increased hydraulic loading and curtate hydraulic retention time have much affection on these 4 indexes.4. The regression analysis was developed using substrates species, running mode, substrates depth, conditional temperature, running days as independent variables and the removal rates to COD,BOD₅,TN,TP as induced variables, which showed the most significant factors influencing the removal rate of COD is substrates species, depth and temperature, of BOD₅ is substrates species, of TN is substrates species and temperature, of TP is substrates species and running time.5. According to the orthogonal test, the optimal removal effect occurred when the hydraulic loading was 0.3m/d and the hydraulic retention time was 4.2h, using peat composed of rotten mosses as substrate.