Performance of Offline Ponds and Wetlands Combined Systems on the Source Water Quality Protection in the Stream Networks of Yangtze River Delta in China

Large-scale offline pond/wetland combined systems (about 110 hm2) were built to treat the micro-polluted river water and the purified water (daily 0.25 million tonnes) was directly supplied to the largest drinking water plant in Jiaxing City. The systems became the largest drinking water source treatment facility in China. Ponds and plant-bed/ditch systems were the major structural units of the systems. They were connected together as integrity at a multiple level, and could treat about 10 per cent of the discharge across the river source water. The outlet water quality maintained relatively stable regardless of the inlet water conditions, which reflected the strong buffering capacity of the systems for most pollutants. The whole water quality at the outlet of the systems was improved by one level in class (from IV to III) according to the China national environmental quality standards for surface water. This case study provided a successful model for purifying the contaminated river water offline in the stream networks of Yangtze River delta in China.