Study On Removal Of Nitrogen And Phosphorus In Domestic Wastewater By Subsurface Flow Constructed Wetland

With rural urbanization in rapid development of China,environmental pollution becomes increasingly serious,especially for direct discharge of domestic wastewater in small rural towns,which leads to heavy water eutrophication.It not only destroys the environment,but also hinders the social economic sustainable development.Owing to the shortage of water resource,at present removals of nitrogen and phosphorus in domestic wastewater are considered as a central focus of researchers' studies.Subsurface flow constructed wetland is a new ecological technology for wastewater treatment,since it has many advantages including lower capital expenditure,saving of operation expenditure,relative stable treatment effect,good effluent quality and powerful resistance to impact load.Therefore it is applied extensively in rural areas.However,it has a general disadvantage of unstable removal efficiency for nitrogen and phosphorus.The acidification technology and active sludge technology were combined with the subsurface constructed wetland system to improve nitrogen and phosphorus removal.The acidification-subsurface flow constructed wetland composite system was located in Southwest University,composed of sand basin,acidification pond and subsurface flow constructed wetland.The composite system was sampled weekly from October 2006 to September 2007.The active sludge-subsurface flow constructed wetland composite system was located in suburban of Chongqing,composed of pretreatment unit,activated sludge aeration unit and subsurface flow constructed wetland.The composite system was sampled from April 2008 to December 2008 every ten days.Laboratory simulation test was conducted using coal cinder,crushed stone and red brick detritus.And then a static simulation test and a dynamic simulation test were designed to examine the removal of phosphorus in domestic wastewater by three substrates and six mixed substrates respectively.The purpose of this research is to search for the appropriate substrate used for phosphorus removal of subsurface flow constructed wetland.Through monitoring the two composite systems and simulated constructed wetland system in laboratory,experimental data were analyzed and majr conclusions were as follows:1.The studied acidification-subsurface flow constructed wetland combined system could removal about 7%of total nitrogen and about 11%of total phosphorous. This was due to the effective reduce in organic loading the inflow wastewater by the acidification technology.In this research,the organic loading of inflow was relatively low,so the carbon source of denitrification was deficient in latter half wetland after treatment by acidification pond,leading to the low nitrogen removal.In addition,the substrate was nearly saturated due to the long term operation of this system,which resulted in the decrease of the phosphorus removal.Therefore,acidification technology can be set in front of subsurface flow constructed wetland system to decrease inflow organic loading when treated high concentration organic wastewater.When treated low concentration organic wastewater,filled rice husk can be used as carbon source in constructed wetland,or splitting a fraction of inflow wastewater into latter half part of wetland to faciliate the denitrification.2.The average concentration of AN and TN in active sludge-subsurface flow constructed wetland outflow reached the GB18918-2002 primary discharge standard. TP average concentration of this composite system outflow achieved the GB18918-2002 secondary discharge standard.Because substrate was saturated after the long term operation of this composite system,phosphorus removal rate was low, the average removal rate was about 11%;The prepositive active sludge technology can improve the water quality condition,enhance the wastewater inflow DO concentration level,provide favorable conditions for the occurrence of the nitrification and accelerate nitrogen removal efficiency of the whole system,the average removal rate of AN was nearly 100%,the average removal rate of TN was about 21%.3.The results showed that coal cinder had the best static adsorption efficiency for phosphorus,and adsorption capacity was 2.25 mg/kg,followed by coal cinder-red brick detritus mixture,red brick detritus,coal cinder-crushed stone mixture and crushed stone.The dynamic simulation test of phosphorus removal by constructed wetland showed that the average removal rate of TP from domestic wastewater by three step series coal cinder constructed wetlands was over 93%under hydraulic retention time of 24h,and the saturated adsorption capacity of coal cinder was 2.08 mg/kg.The coal cinder with high physicochemical adsorption capacity for phosphorus could be used as substrate in subsurface flow constructed wetland.