Research On The Quality Of Combined Sewer Overflows (CSO) And Its Effect With Preliminary Treatment

Urban runoff is a random and sudden event, characteristic of heavy pollution. Urban runoff and its resultant combined sewer overflows have been considered as important sources of water pollution. Nanfei River is a natural river, which originated in Changfeng County, being intercepted by Chuhe Canal and Dongpu Reservoir upstream, Nanfei River has become a heavily polluted water body receiving the continuous influx of various types of smelly sewage causing river siltation and river bed elevation, therefore, it is imperative to take measures to control water pollution in Nanfei River.This thesis originates from National Water Control Project, with an intent to improve water quality in Nanfei River. Research target is focused on water quality flowing through outfalls at QingchongⅠand QingchongⅡ, and continual samplings, measurements and analyses were taken. conclusions were drawn as follows:(1)Values of COD and SS concentration at QingchongⅠshare a substantially similar trend, both COD and SS concentration values vary sharply, with COD concentration in the range of 1806～88mg/L,SS concentration in the range of 2357～323mg/L, if untreated, it will cause a pollution in Nanfei River.(2)Water quality from QingchongⅠoutfall during dry weather is much worse than that during wet weather, especially in the early stages of the formation of runoff. In the rainy days, the COD value can be 1713mg/L, during dry weather only 169mg/L, mainly due to the season that rainwater washed the ground and stirred bottom sediments of pipe network.(3) CSOs from QingchongⅠindicate good results via free settlement by 30 minutes of free settlement, COD dropped by 1300mg/L, SS by 1700mg/L.(4)Significant removal efficiency of COD,SS were demonstrated by coagulation process at QingchongⅠoutfall and pollutants were removed in a short time(5) COD and SS concentration from QingchongⅡoverflows is more complicated and difficult to see that there is significant correlation between the two, and overall COD, SS change range is not too much.(6) there is a certain performance of free settlement about CSOs from Qingchong II,COD drop up to 475mg/L, SS drop up to 785mg/L in the maximum. Research shows that if the first rainstorm of year at QingchongⅡis well handled,pollution can be greatly reduced, thus subsequent CSOs quality can be kept equivalent of sewage level. using coagulation process water quality of CSOs at QingchongⅡindicates a good response via coagulation sedimentation, with COD, SS removal rate 91.0% and 51.8% respectively.(7)Concentration Values of soluble COD, NH3-N,TP,TKN are merely connected with conditions of pollution in dry weather at outfall of QingchongⅠ, not so much affected by washing-off of sediments in drainage network, and their overall concentration change curves are similar, increasing a bit and then in decline. The QingchongⅡshares the situation with QingchonⅠ, however, water quality of QingchongⅠvaries sharper than that of QingchongⅡ. By contrast, the situation at QingchongⅡis much better than that of QingchongⅠ.(8) The direct filtration treatment of sewage overflow that, as a measure of primary treatment, walnut shells as filter media can reduce the pollution loading to some extent, and can be combined to design effective device using walnut shells as filter media.