Study On Enhanced Nitrogen Removal In Subsurface Wastewater Infiltration System(SWIS) With High-hydraulic Loading

As an ecological treatment process for decentralized domestic sewage, the subsurface wastewater infiltration system(SWIS) has the advantage of non-power or micro-power drive, low construction and operation cost, non-secondary-pollution, compared with the conventional wastewater treatment methods. However, there remain some problems with low hydraulic loading rate and nitrogen removal, which limit the improvement of the SWIS. Therefore, on the premise of high-hydraulic loading, this paper concentrated on the nitrogen removal effects of the SWIS, taking the enhanced nitrogen removal as the goal. The substrate and construction were the breakthrough points and the operation parameters were also mainly studied. Finally, contrasted by the removal of nitrogen, the project was demonstrated and verified to dispose the swine wastewater or other wastewater. The main conclusions are:(1) The substrate for enhancing nitrogen removal effects with high hydraulic loading was designed. Based on the brown soil, slag, activated sludge, cinder, sand and other traditional substrate, the bentonite was added in the inflow layer and the rice husk and flyash were added in the infiltration layer, to enhance the nitrogen removal with high hydraulic loading. By the contrast experiment, when the hydraulic loading was 25cm/d, the results showed that, the SWISs had higher removal rate of COD and TP, the SWIS added bentonite + rice husk had better removal rate in NH4+-N and TN, and the removal rate was increased by 12.44% and 4.07%,respectively, compared with the SWIS without bentonite and rice husk. The isothermal absorption experiment of the bentonite and the cinder and their mixture(1:3) showed, the NH4+-N adsorption properties of bentionite was stronger than the cinder, the NH4+-N saturated adsorption capacity of bentionite was twice than the bentienite; the TP adsorption properties of cinder was higher than bentionite,but their saturated adsorption capacity were all less than 400 mg/kg; The TP saturated adsorption capacity of bentonite + cinder was 1454.80 mg/kg, which indicate that the bentonite and cinder had synergistic effect and could extend 2.78 times of the working lifespan.(2) A set of SWIS with double layers NiiMi process was designed, and was filled with by the substrate included soil, cinder, active sluge, sand, benonite and rice husk. When the hydraulic loading was 25cm/d, compared with the traditional NiiMi process, the double layers NiiMi process could extend the hydraulic retention time(HRT), construct an aerobiotic or anaerobic microenvironment, so as to enhance the nitrogen removal. The results showed, the COD, TP, NH4+-N, TN of double layers NiiMi process were increased by 4.59%, 2.41%, 5.16%, 7.74%, respectively.(3) To improve the nitrogen removal by optimizing the operation parameters of double layers NiiMi process. The simulated SWIS operated continually in different operation cycles and pollution loading with high hydraulic loading, the results showed that, when the operation cycle was 12 h and the pollution loading(COD) was 130-140.05mg/L, the average concentration of NH4+-N and TN in effluent was 6.85 and 11.84mg/L, respectively. The COD, TP, TN of the effluent met the first level A criteria specified in of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant(GB18918-2002), when the NH4+-N was below the first level B. The results indicated the substrate modification, the construction modification and operation parameters optimizing in this paper were demonstrated effectively.(4) The SWIS was applied in the treatment of high concentration wastewater with nitrogen, swine wastewater was taken for consideration. The results showed that the SWIS had a good performance in the swine wastewater treatment. The average removal rates of COD, TP, NH4+-N and TN in the effluent were 90.75%、93.86%、69.24% and 64.79%, respectively. The effluent quality met the level B for the standard of Wastewater Quality Standards for Discharge to Municipal Sewers(GB CJ-343-2010). The results show the SWIS can dispose the high concentration wastewater with nitrogen and the SWIS application of space can be expanded.