Study On Contaminant Removal Mechanism And Efficiency Of Constructed Rapid Infiltration System

Traditional sewage land treating systems have ubiquitous problems of lowerhydraulic load rates and lower wastewater treating capabilities per unit area. To solvethe problem, based on the summarization of all kinds of land treating systems,constructed rapid infiltration sewage treating system (CRI for short) was developed.During the development course, rapid infiltration land treating system and constructedwetland system were mainly used for references. By learning from their advantagesand offsetting their weaknesses, the CRI system, a new wastewater treatmenttechnique with its special characteristics was developed step by step. But study of CRIis just in the initial stage in China, and many of its aspects need to be further studied.In this study, through pilot experiment and practical projects, running impact of theexisting CRI system, microorganism distribution in the system, and pretreatmentsystem were analyzed systematically, and infiltration structure and filter medium wereoptimized.Constructed rapid infiltration system has been built in Shenzhen, which isrunning normally and stably with high outlet water quality and high capability ofbearing shock load. Outlet quality indexes, such as COD, BOD, SS, NH4-N can reachthe A criterion, standard I in Contaminant Emission Standard for Civil SewageTreatment Works, and can be ensured in long-term running. However, TN, TP can notmeasure up the standards.All kinds of microorganisms distribute widely in the infiltration tank, however,the amount and space distribution is different. In amount, aerobic bacteria is twoclasses higher than Actinomyces, fungi and anaerobic bacteria. In space distribution,they all reduce in amount from the surface layer to the bottom layer gradually, butwith different forms. Analysis on microorganism amount in the surface layer andwater purification results showed that amount of aerobic bacteria has remarkablerelativity with removal rate of COD and BOD and organic matters are mainlydecomposed by aerobic bacteria. Amount of nitrobacteria and nitrococcus hasremarkable contribution for NH4+-N removal. It also has some relativity with TNremoval rate, but the relativity is not so remarkable because the denitrification is notfully processed. Amount of denitrifying bacteria has remarkable relativity with TNremoval rate.When the infiltration tank is running, water logging period is shorter, which doesnot profit the function of denitrifying bacteria. So NO-3 –N is higher in the outlet andTN removal is not so perfect. Saturated layer laid at the bottom provides anaerobiccondition for denitrifying bacteria growing with a long-term water logging status. Inthe saturated layer, denitrifying bacteria reduce NO-3 –N and TN by deoxidizing NO-3–N to gaseity-N, so that the removal result of TN is improved. Practical work showedthat a certain interfusion ritio of scrap iron and charcoal uniformly in the filtermedium can enhance the removal result of TP effectively with the high sorption ofcharcoal and micro-electro analysis of scrap iron. Field experiments showed thatcomposite filter medium improve 10% of the TP removal rate.Pretreatment process is very important for the normal and stable running of thewhole system, which reduces the contaminant concentration into the rapid infiltrationtank, enhances the hydraulic load of the system, reduces the land area of the system,and prevents the block-up phenomenon. After technically compare, Coagulation andsedimentation process is an ideal pretreatment process, which can be the technicalcombination of CRI. It can effectively remove the SS in sewage and the phosphoruswith chemically, which offsets the weakness of lower TP removal rate of rapidinfiltration.Although CRI technique has a lot of advantages, as a newly developed technique,it still needs further study. Problems like running in winter, effective removal of N andP, treatment of high concentration wastewater need to be solved. It is suggested thatsystemic study be strengthened, techniques be consummated, outlet water quality beimproved, and application extension be developed. Then it will contribute more inutilization of sewage resource and economization of water resource.