| Centralized wastewater treatment plant(CWWTP)is widely used in large cities.However,it is not only costly in both construction and operation,but generates large amounts of sludge which must be properly disposed of,and releases odorous compounds to the surrounding environment. Further,in countryside,the use of CWWTP is largely limited by the difficulties in wastewater collection.Thus,there is an urgent need for the development of cost-effective and environment friendly decentralized wastewater treatment technologies.Subsurface infiltration wastewater treatment system has the characteristics of(1)very low cost of operation;(2)realeasing little smells;(3)free of noise;(4)slightly influenced by climate. Moreover,its surface ground can be used as vegetation land,parking lot,etc.Therefore,it is a low-cost system that can be built in urban and separate colonies and,thus,has been widely used abroad.In this study,an experimental system of septic system for domestic wastewater treatment was set up.Under different hydraulic loading rates,daily effluent flow rate was recorded and water samples were taken for analysis of COD,NH4+ -N,NO3--N,total nitrogen and total phosphorus concentrations.Major findings are as follows:(1)It can improve the organic content consequently enhance the removal rate of total nitrogen by means of adding composted leaves to the system.(2)With a hydraulic loading rate of 2.4cm/d,the system reached the mature state after 25 days.The average removal efficiencies was 91.7%for COD,99%for NH4+-N,70.5%for total nitrogen and 96.3%for total phosphorus;With a hydraulic loading rate of 1.8cm/d,the average removal efficiencies was 92.4%for COD,99.2%for NH4+-N,65.7%for total nitrogen and 97%for total phosphorus;With a hydraulic loading rate of 1.3cm/d,the average removal efficiencies was 93.6%for COD,99.2%for NH4+-N,74.4%for total nitrogen and 97.1%for total phosphorus. (3)Under different hydraulic loading rates of 2.4cm/d,1.8cm/d and 1.3cm/d,it was found that when the loading rate decreased,the system removal efficiency increased for COD, changed little for NH4+-N and total phosphorus,and influenced greatly for total nitrogen.(4)The results of testing the effluent concentration of COD,NH4+-N,NO3-N,total nitrogen,total phosphorus each two hours one day under three hydraulic loading rates of 2.4cm/d,1.8cm/d,1.3cm/d shows the effluent concentration of COD changed greatly and the trend was decreased under a hydraulic loading rates of 2.4cm/d,while that of 1.8cm/d,1.3cm/d changed slightly.And it didn't have significant laws on NH4+-N,NO3-N,total nitrogen,total phosphorus.Furthermore,the removal rates of COD,NH4+-N,total nitrogen,total phosphorus increased while hydraulic loading rate decreased.(5)The results of testing the soil temperature in the range of 15-30℃shows that this range of temperature didn't impact obviously to the removal rate of COD,NH4+-N,total nitrogen, total phosphorus.(6)Nitrification and denitrification prosesses are the main path for nitrogen removal in a subsurface infiltration system.Phosphorus removal depends on adsorption and chemical precipitation.In this research,the denitrification process was slow,while the nitrification process was well proceeded.So speeding denitrification process was the key to enhance nitrogen removal rate in the loess soil.
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