New Type Subsurface Flow Constructed Wetland For Treatment Of Rural Domestic Sewage And Optimization

Nansihu Lake as the storing lake and the water channel of South-to-North WaterTransfer East Route Project, the water quality there is the key to the success. But for along time, due to lacking of environmental protection facilities, as well as lackingeffective management, most of the domestic sewage was directly or indirectlydischarged into the lake, causing the lake water pollution. Therefore, effectivetreatment of the domestic sewage must be used to guarantee the water quality in ruralareas. Constructed wetlands water purification technology has the advantages of lessinvestment, low energy consumption and strong buffering capacity, so it’s one of thebest choices for dealing with rural domestic sewage in China.The removing of nitrogen in constructed wetlands mainly depends onammonification, nitrification and denitrification. Nitrification is occurred in aerobicenvironments, while denitrification is in anaerobic conditions. Duing to its structurallimitations, conventional subsurface flow constructed wetlands can not provideaerobic and anaerobic environment at the same time. TN removal of a single type ofconstructed wetlands is often not high.To deal with the problem above, our research team has designed the aerobic/anaerobic multi-level series subsurface flow constructed wetlands water purificationtechnology, and the technology has succeed in laboratory test phase. In this study, apilot plant system has been built, and the removal efficiency of NH4+-N, TN, COD arestudied to verify whether the water purification technology can applied in real life. Atthe same time, the price、the adsorption properties and the amount of biofilm of threekinds of constructed wetlands matrix were compared, radial oxygen loss of four kindsof wetland plant were also carried out a comparison，aims to provide theoreticalsupport to the optimization of substrate and plant for new type subsurface flowconstructed wetlands, and provide a realistic basis for popularization and application of constructed wetland in the vast rural areas.Main results are as follows:First, the research of water purifying effect with new type subsurface flowconstructed wetlandsCOD removal mainly occurs in the aerobic section which is at the front ofconstructed wetland, COD removal rate of up to90%in summer, and it is also higherthan55%during autumn; higher ammonia nitrogen removal rate occurs in the aerobicsection which is at the rear of constructed wetland, ammonia nitrogen removal rate ismaintained at above60%; total nitrogen removal rate is maintained at above50%,sometimes it can get up to68%. The results are close to the laboratory test data,indicating that the new type purification process also has good treatment effect in thereality.Second, the comparison and studying on several wetland matrixesThe comparison of volcano stone, zeolite, gravel reveals that volcano stone bulkdensity is minimum, only693kg·m^-3, packing density of gravel and zeolite is abouttwice that of the volcanic rock; The adsorption kinetics experiment showed that theadsorption of ammonia nitrogen in the three types of matrix surface to reachequilibrium after6hours; after nonlinear fitting of adsorption isotherm experimentaldata, it is shows that volcanic rock and gravel in line with the Langmuir isothermequation, while the zeolite is more in line with the Freundlich isotherm equation; Byanalysis of the relevant parameters of the equation show that that the theoreticalsaturation of the zeolite on the ammonia nitrogen adsorption capacity is the largest,followed by volcanic rock, gravel worst; biofilms amount experiments show thatwhen the volume is fixed, zeolite’s biofilm is the largest.Third, the comparison and studying on several wetland plantsAfter detecting the oxygen saturation at the root surface along the root, it showsthat the maximum oxygen saturation occurred in the basal region at about1/4of thetotal root length and values decreased towards the root tip for both plants; Toinvestigate the oxygen gradient as well as the extension of the oxygen diffusion zone,vertical oxygen profiles from the root surface into the agar were measured, the results revealed an oxygen diffusion zone around the roots of500μm thickness for both plant;the ROL（radial oxygen loss） ability of four wetland plants is: iris>Arundodonax>cattail>reed.