The Estuary Pretank System Purification Into The Dianchi Lake, The River Demonstration Project

The non-point source pollution in the Dianchi Lake area becomes another outstanding problem after point source pollution has been controlled. The non-point source pollution also has become one of key pollution sources for water eutrophication in the Dianchi Lake. Because of the eutrophication of the Dianchi Lake, the high concentration of nitrogen and phosphorus and the water quality deterioration seriously restricted the development of the society and economy, and the improvement of environment. It is difficult to control non-pollution source from the source due to river's several characters that non-point pollution is predominant such as large drainage area, many and complex pollution sources, uncertainty and space-time mobility. It is an effective way to reduce river pollutants for the non-point pollution control after establishing the pre-dam system at the bottom of the river way.A demonstration project, pre-dam system, which consists of pre-dam and estuarine lake shoreline wetland, was constructed at the estuary of the Dongdahe River. The pre-dam has a total area of 64380m2 and total cubage of 89290 m3. An area of 44741m2 lake shoreline wetland, located in both side of the river way, was in the Dongdahe River lakeshore. The removal efficiency of COD, TN, TP and SS in the inlet water are studied in clear days and rainy days. Also the efficiency of purifying river water flowing into the Dianchi Lake by the wetland is analyzed. Finally, the current of water in pre-dam is investigated in the rainstorm period. The main research results are as follows.(1) The inflow discharge of the Dongdahe River is relatively small in clear days. The high concentration water of Dianchi Lake will diffuse and recharge to the area of low concentration water of pre-dam because the outlets of pre-dam are linking together. The concentration of pollutants at outlets are high than those of inlet, even higher than those of other sites in the pre-dam. So the purification effect of the pre-dam is difficult to assess.(2) The inflow discharge of the Dongdahe River is relatively large in rainy days. The inlet water amount of Dianchi Lake water becomes small due to the relatively large discharge of inlet water discharge. The purification effect on the pollutants for the Dongdahe River water can be reflected. Compared the pollutants concentration of inlet water and the minimum values in different function areas, analysed the purification effect. It shows that the pre-dam has removal efficiency on the river water, and the maximum removal rate of COD, TN, TP and SS can reach at 13.83%,9.15%,32.29% and 72.58% respectively.(3) The west side wetland has a good removal efficiency on river water when it arrives in the normal running condition and then the removal rate of COD, TN, TP and SS can reach at 58.36%,41.48%,40.37%,90.56% respectively. The season factor has great influence on the purification effect of the estuarine wetland in Dongdahe River lake shore. It is a key element to harvest the wetland plants and cleanup the plant residues when those plants withered, otherwise the removal rate would be decrease. Then the removal rate of COD, TN, TP and SS would less than 40% or 30%,22%,30% and 29% respectively. The estuarine lake shoreline wetland is a beneficial component part for the pre-dam system and is the supplement and perfection to operate the pre-dam steadily. The estuarine lake shoreline wetland plays a beneficial guarantee role to purify the river water.(4) The results of numerical simulation show that the direction of current in pr-dam is forward flow during 25 hours along the rainstorm. The current direction shows that the flow field can help to make the water in pre-dam effuse and prevent the Dianchi Lake water from recharging. The inlet water discharge and water level of the lake decide the flow field form of the pre-dam. The wind field has a little effect on the flow field. It can be concluded that the smaller flow velocity can weaken water's process and diffusion ability and increase the recharging amount of the Dianchi Lake water, and then has disadvantage for the pre-dam to play its role to purify the river water.