Enhanced Nitrogen And Phosphorus Removal By Combining Biological Dephosphataion And Endogenous Denitrification In Constructed Wetland

Due to the high construction and maintenance costs of traditional wastewater treatment plants and the limited comprehensive local financial capacity,a large amount of untreated domestic sewage,livestock and aquaculture wastewater from rural area directly discharged into rivers and lakes,leading to intensified eutrophication of receiving water bodies,and cause a series of ecological and environmental problems such as deterioration of water quality,transparency decreasing,and disappearance of aquatic plants.Thus,exploring efficient,stable and affordable technologies to treat wastewater from rural area has become urgent practical significance to promote the green development and construction of new countryside in China.Compared to traditional wastewater treatment plants,constructed wetlands（CWs）are low-cost,efficient,easy-operation and environment-friendly wastewater treatment systems.Such systems are especially suitable for treating decentralized domestic sewage,livestock and aquaculture wastewater from rural areas.In conventional CWs,the main pathways of nitrogen and phosphorus removal are couple nitrification-denitrification and substrates adsorption,respectively.Due to the limit of oxygen,carbon sources and sorption capacity,the capacities and efficiencies are quite limited regard to nitrogen and phosphorus removal.This work combines CW system and anaerobic/anoxic/aerobic process,to assess the feasibility of achieving nitrogen and phosphorus removal through endogenous denitrification（ED）and denitrifying phosphorus removal（DPR）in CWs.The main research contents and results concluded during a 217-day period are as follows:（1）A polyphosphate-accumulating organism（PAO）and glycogen-accumulating organisms（GAO）enriched biofilm was successfully established in the CW by integrating anaerobic/anoxic/aerobic operation and periodic PO₄^3--P release strategy.This strategy enabled a simultaneously enhanced nitrogen and phosphorus removal performance even using normal gravel as a substrate.With influent COD/N ratio of 8,the total nitrogen and total phosphorus removal efficiencies were 80.96%and 83.12%,respectively.High total nitrogen（80.86%）and total phosphorus removal（87.84%）efficiencies were maintained even under carbon limiting condition（influent COD/N=4）.（2）In the anaerobic stage,PAOs and GAOs can convert more than 80%of the influent carbon source into endogenous carbon,which effectively reduce the oxygen demand for the subsequent nitrification process.Then,PAOs mainly use nitrate and nitrite as electron acceptors and endogenous carbon as electron donors to conduct endogenous denitrification and denitrifying dephosphatation,which simultaneously reduce the carbon source and oxygen demands in the biological nitrogen and phosphorus removal process.（3）Based on stoichiometric analysis,DPAOs and DGAOs competed NO₃^--N and carbon sources when the influent COD/N was high.However,nitrite was the main electron acceptor for DPR under low influent COD/N of 4,and DGAOs were the main organisms providing nitrite to DPAOs,suggesting effective nitrogen and phosphorus removal under carbon sources limited condition might be the results of the coordination of DGAOs and DPAOs in DPR and ED processes.（4）High-throughput sequencing results showed that the influent COD/N ratio can significantly affect the relative abundance of functional microorganisms（PAOs,GAOs and OHOs）in the SNPR-CW system.When the influent COD/N ratio was 8,the relative abundance of PAOs,GAOs and OHOs were 17.08%,19.15%and 4.21%,respectively,and the main PAO genus was Dechloromonas which mainly using nitrate as electron acceptor,the main GAO genus was Candidatus＿Competibacter.When the influent COD/N ratio reduced to 4,the relative abundance of PAOs,GAOs and OHOs were 0.53%,39.59%and 11.64%,respectively,and the Defluviicoccus（identified as DGAO）which can only reduce nitrate to nitrite enhanced,while the abundance of Dechloromonas（identified as DPAO）which can only use NO₃^--N as electron acceptor decreased.