| Decentralized rural domestic sewage became key and difficult points for rural domestic sewage treatment in Taihu basin for its large proportion of totol sewage discharge and dispersion. Anaerobic baffled reactor + baffled flow constructed wetland(ABR + BFCW) combined process was suitable for treating decentralized rural domestic sewage in Taihu basin, for its great removal effect of organism, strong anti shock loading capability, low cost and stable running, convenient maintenance and landscape. While the poor nitrogen and phosphorus removal performance of domestic sewage in winter, limiting the technology used in Taihu basin. In this study, through improving the nitrogen and phosphorus adsorption capacity of substrates, and optimizing the efficacy of ABR, the removal performance and stability of constructed wetland would be improved, and the engineering applicability of the combined process would be enhanced in Taihu basin which had high emission requirements. Main contents and results were as follows:(1) Screening and modifying of suitable substrates to improve phosphorus removal capability of constructed wetlands: Limestone, zeolite and slag were selected from10 types of substrates with higher phosphorus adsorption capacity in terms of removal effect, costs and sources. The three suitable substrates were modified by acid(alkali), aluminum or calcium solutions to enhance their phosphorus removal capability. The results showed that the nitrogen and phosphorus adsorption capacity of slag, zeolite and limestone would be best modified by 2 mol/L Al Cl3, 3 mol/L Na OH and 2 mol/L Al Cl3, 0.5 mol/L Al Cl3. The nitrogen and phosphorus adsorption capability of selected substrates modified by aluminum solution were stronger than other modified substrates, and Al-P was easily absorbed to plants and aluminum solution used widely. Therefore, aluminum solution was suggested to modify the substrates in constructed wetlands.(2) Study on optimization of pretreatment for ABR reaction: The improved configuration of ABR enhanced the interception capability of SS. The efficacy and ammonification effect of ABR were significant correlation with its hydraulic conditions. While the hydraulic retention time(HRT) of ABR was shorter and organic volume load(OLR) was higher, the hydraulic condition of ABR was better and the efficiency was higher. When the HRT of ABR was 12 h, the treatment efficiency and ammonification effect of the improved ABR was best.(3) Study on the hydraulic efficiency of BFCWs under different operation conditions: The result showed that the BFCW, which consisted of the substrates with smooth and regular surface, and larger particle size, had lower effective volume ratio and hydraulic efficiency in the same HLR. The trend increased with the increase of HLR. Plant root led to the decrease of the BFCW hydraulic efficiency. The plant roots were wide-distributed and sparse when the substrates had strong nitrogen and phosphorus adsorption ability, which led the decline of hydraulic efficiency lower, The modified steel slag wetland had stable hydraulic efficiency for its alkaline environment.(4) Study on the efficacy of BFCWs: Substrate type had no significant effect on organism removal of BFCW. The organism and nitrogen removal rate of BFCWs increased first and then decreased when the inlet organism and nitrogen surface loading increased, and phosphorus removal increased with the increase of inlet phosphorus surface loading in the same HLR. The phosphorus and nitrogen removal and anti shock loading capability of BFCWs was positively related with the adsorption capability of substrates. The nitrogen removal effect: modified zeolite wetland > zeolite wetland > ceramsite wetland > modified steel slag wetland. The phosphorus removal effect: modified steel slag wetland > modified zeolite wetland > ceramsite wetland > zeolite wetland. Plants enhanced the efficacy and anti shock loading capability of BFCWs. The organism and nitrogen removal of planted BFCWs were close in summer and in autumn, while the phosphorus removal effect was better in autumn.The phosphorus and nitrogen removal effect of modified zeolite wetland were not significantly decreased, but the fluctuations of outlet concentration increased.The outlet TN concentration was lower than 5.0mg/L stablely in modified zeolite wetland. The nitrogen removal effect of modified steel slag wetland was relative with water temperature, the removal effect was higher when the water temperature was higher, and the outlet TN concentration of modified steel slag wetland was unstable. The modified steel slag did not achieve the desired effect. The phosphorus removal capability of zeolite was bad in application, while the modified zeolite wetland showed great phosphorus and nitrogen removal performance. The HLR of modified zeolite wetland is recommended to lower than 152L·m-2·d-1 in application, in order to ensure the outlet quality in the modified zeolite wetland after long-time operation.(5) Study on the approach of nitrogen and phosphorus removal in modified zeolite wetland: The study showed that substrate type had greater influence on the nitrogen and phosphorus removal of wetland than plants, and oxygen exudation had greater influence on the nitrogen removal than plant-uptake. In the operation, the nitrogen and phosphorus removal quantity of modified zeolite wetland were 1.8% and 1 times higher than that of zeolite wetland. The interception capacity of substrates on nitrogen: modified zeolite > zeolite > large particle ceramsite > small particle ceramsite. The interception capacity of substrates on phosphorus: modified zeolite > small particle ceramsite > zeolite > large particle ceramsite. The down-flow compartment in wetland was easy to form the preferential flow. Therefore, the length of down-flow compartment in BFCW was recommended to shorten to 0.2~0.3m. The modified zeolite wetland mainly removed nitrogen and phosphorus by substrate adsorption, and the main fraction of modified zeolite was Ca-P. The oxygen-secretion and absorption of plants stabilized the water quality of the effluent. The substrate adsorption was main nitrification removal pathway in the front side of the wetland, and nitrification and denitrification was main nitrification removal pathway in the end side of the wetland. The nitrogen and phosphorus adsorption capacity during the pilot test were much higher than that of static test. The optimization of phosphorus adsorption capacity for modified zeolite was achieved under the synergy of multiple pathways. The effect of configuration and plant roots were the main reasons for the difference of nitrogen and phosphorus interception quantity. Effect of substrate type on plant uptake mainly reflected on the biomass increment of plant. Plant-uptake had greater contribution on the nitrogen and phosphorus removal of the wetland, when the substrate nitrogen and phosphorus adsorption capability was poor. Nitrification intensity led the seasonal fluctuation of nitrogen removal effect and stability in modified zeolite wetland, and the low nitrification intensity in the front of wetland was related to the strong adsorption of NH4+-N with the modified zeolite. |
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