Development of a Natural Treatment System for Stream Water Purification: Mechanisms and Environmental Impacts Evaluation

摘要

A constructed wetland type natural treatment system was built in southern Taiwan for stream water purification and natural habitat creation. The system influent (influent rate = 1,170 m(3)/d) was from the local stream containing secondary wastewater from hog farms. The system included a fully vegetated free-water surface basin, followed by an open-water pond, a second fully vegetated free water surface basin, and an eco-pond with isolated islands for natural habitat creation. The hydraulic loading rate, hydraulic retention time, water depth, and total volume of the wetland system were 0.27 m/d, 7.5 d, 0.64 m, and 7,800 m(3), respectively. In this study, surface water, sediment, and ground-water samples were collected and analyzed quarterly for each basin during the two-year investigation period. The results show that the overall removal efficiencies for biochemical oxygen demand, total nitrogen, total phosphorus, and total coliform were 71, 85, 82, and 75%, respectively. The calculated first-order decay rates for organics and nutrients ranged from 0.25 (total nitrogen) to 0.15/d (ammonia nitrogen). Thus, the system had a significant effect on water quality improvement and could remove most of the pollutants from influents through natural treatment mechanisms. The results show that the increased effluent suspended solid concentrations correlated with the increased chlorophyll a in the system, and the growth of wetland plants was an effective method for algal control. The observed high ammoniaitrate removal indicates that nitrification and denitrification processes occurred simultaneously. An increase in pollutant concentrations in the last basin was observed because the last basin was used as a wildlife habitat. Thus, natural habitat creation might conflict with the water quality protection issue. Groundwater recharge from the wetland basins resulted in the deterioration of the downgradient groundwater quality. Contaminant adsorption onto the sediment particles was also observed. Frequent groundwater and sediment monitoring is a necessity to assess the impacts of pollutant accumulation in the ecosystem. The results from the ecological investigation show that increased biodiversity was observed. The natural treatment system has become a successful multifunction ecosystem, which could be used for stream water purification and natural habitat creation. (C) 2015 American Society of Civil Engineers.