Study On Wastewater Treatment In Intermittent-aerated Vertical Subsurface Constructed Wetland

Water shortage and water pollution has become a global problem, and it restricts the economic and social development seriously. Especially in many rural areas, due to economic and technological backwardness and lack of effective management, most domestic sewage is discharged directly, leading to serious non-point source pollution. There are kinds of sewage treatment processes, but because of high investment, high operating costs and secondary pollution, it is necessary to study and develop new decentralized sewage treatment processes. Because of the advantage of low investment, low infrastructure and operating costs, effective pollutant removal, easy maintenance and management, constructed wetland wastewater treatment technology was widely applied. But in most conventional constructed wetland, the removal rate of total nitrogen (TN) remained quite low, it is mainly due to the lack of favorable (i.e., alternate aerobic/anaerobic) conditions for nitrification and denitrification.In the study, the performance of intermittent aerated vertical subsurface constructed wetland (VFCWs) for treating simulated domestic wastewater was evaluated. Firstly, the optimum aeration time and aeration rate were determined, and the long-term wastewater purification efficiency in non-aerated and intermittent aerated VFCWs were studied. Then, the nitrogen removal efficiencies in non-aerated and intermittent aerated VFCWs under different operational conditions, i.e. substrate, feeding mode, carbon nitrogen ratio (C/N ratio), influent strength, were systemically studied. Finally, the microbial population and community structure in intermittent aerated VFCW was detected by molecular bio-techniques to analysis the pollutant removal mechanisms in the way of microbial population structure. The main research conclusions are as follows:(1) Based on the non-aerated VFCW, effect of intermittent aeration on pollutants removal efficiencies were studied. The COD、NH4+-N and TN removal rates with different aeration time and aeration rate showed that the optimum aeration time and aeration rate were4h/d and1.0L/min. Under this condition, compared with blank and non-aerated VFCWs, the intermittent aerated VFCWs exhibited much higher removal efficiencies in terms of COD、NH4+-N and TN, which were95.6%,96.1%and85.3%, respectively.(2) The non-aerated and intermittent aerated VFCWs under different operational conditions, i.e. substrate, feeding mode, C/N ratio, influent strength, were systemically studied, and the results showed that no significant differences were found between gravel and ceramsite wetlands. Step feeding technology could effectively improve the denitrification process because of the additional carbon source supply in the middle of one cycle, and improve TN removal efficiency, which were90.39%. When the influent C/N ratio increased from2.5to20, the COD. NH4+-N and TN removal rates in non-aerated VFCW decreased from84.63%to63.47%,32.18%to18.89%,33.99%to23.33%, respectively; while in intermittent aerated VFCW, increasing C/N ratio did not affect the removal efficiencies of COD and NH4+-N, which were all above95%, but the TN removal efficiency would increase when the C/N ratio increased from2.5to20, which increased from24.25%to89.65%. In non-aerated VFCW, with the influent strength increased, the COD, NH4+-N and TN removal rates decreased from86.30%to62.68%, NH4+-N49.30%to27.59%,43.33%to27.06%, respectively; while in intermittent aerated VFCW, increasing influent strength did not affect the removal efficiencies of COD, NH4+-N and TN, which were all above95%except TN (more than85%).(3) The microbial population and community structure in non-aerated and intermittent aerated VFCWs were analyzed by fluorescence in situ hybridization (FISH) and PCR-DGGE, and the results showed that compared to non-aerated VFCW, intermittent aeration could largely improve the number of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB)(which were more than45.32%and33.65%of the total bacteria, respectively), and also enrich the abundance and diversity of the wetlands’microbial population.