The Research On Eco-Purification Of River Mouth Water Pullutants In The Taihu Lake Drainage Basin

Taihu Lake is one of the Great Lakes in China, whose water quality was mainly mesotrophic and was good enough to meeting drinking water resource standard from1950’s to1980’s; but from1980’s to the middle of1990’s, Taihu Lake was polluted by the organic matter and its water nutrition condition raised2grades, and now it is mainly middle entrophication, and heavy entrophication happens in some area. The Estuary of Taihu Lake has become one of the largest contributors and victims to the pollution load. Currently, the main technologies for estuary restoration are embodied technology, biotechnology and ecological restoration, etc. Among the ecological restoration techniques, the ecological slope protection, the ecological floating bed and constructed wetland are regarded as the best choice of good purification with low cost, and there have been many theoretical studies and applications at home and abroad.Focusing on the purification of estuarine combined pollutants, the author chose Yapu estuary, the inflow river of Zhushan Bay in the north of Taihu Lake, as the object to measure the pollutants and build a Demonstration project by field work and determining technical scheme. This paper analyzed the changes of CODMn, TP and TN in Yapu Port, and found the TN concentration always to be at bad Grade Ⅴ based on10-month monitoring Yapu Port, from estuary to its upstream1.8km, and this TN situation is the bottleneck of improving the water quality. After identifying the purification technology of ecologic floating bed-constructed wetland-aquatic pond, two and half month was cost to build these facilities, and the affiliated ecological slope protection and ecological floating bed for small-scale research were built. From the research, it is found that the ecologic slope protection lowered the index of CODMn, TP and TN in the surface runoff. At the flow rate of12m/L, the influent average concentration of CODMn and TP raised from Grade Ⅳ and Grade Ⅴ respectively to Grade Ⅲ, and at the flow rate of8m3/L, the influent average concentration of TN raised from bad Grade Ⅴ to Grade Ⅴ. It can be a widely used measurement on controlling estuary pollutants; and that the ecological floating bed well purified the Element P and Element N in the water. In the pilot plant of constructed wetland, the average rate of efficiency of CODMn of1#～4#is35%、31%、34%and35%respectively, and the TP’s is57%,58%、61%and54%respectively and the TN’s is44%、46%、46%and41%respectively. At the beginning period of constructed wetland running, the concentration of CODMn declined with the operation of constructed wetland, the average removing rate is50%, it is found that zeolite and ceramisite were good for the decline of concentration of CODMn and TP; while at the time of Blue-green algae bloom, the efficiency of constructed wetland degraded because of the high concentration of organic matter, and the inside eco-system became disordered, and it did not recovery after10days. The temperature was decreased at the end of November2009, so the water condition changed, and the concentration of Nitrate Nitrogen was greatly increased under the same influent concentration of CODMn, this change caused the lack of carbon source for denitrification and overstandard of total effluent Nitrogen. Besides, the efficiency of constructed wetland degraded with the reduce of hydraulic remain time. After5-month operation of the Demonstration Project, the CODMn, TP and TN were well treated, and the average efficiency of CODMn, TP and TN is44%,72%and49%respectively. Although the purification efficiency degraded at the time of Blue-green algae bloom(August and September), it basically recovered by its self purification one month later.The ecological slope protection, the ecological floating bed and constructed wetland are good for the river water purification, and they can stabilize the content of contaminants if they are designed to be an ecological purification system. There would be higher efficiency and more reasonable results if the technology and monitoring method are optimized, for example, in ecological slope protection monitoring, giving preference to slope protection with composite medium, which has better efficiency; in ecological floating bed monitoring, designing the structure and adopting more scientific and intuitive monitoring method to raise efficiency and get better results. Besides, the efficiency of treating pollutants can be improved by increasing pretreatment in constructed wetland and adopting the method of recycling the effluent and adding carbon source according to the seasons.