Nitrate-rich Agricultural Runoff Treatment By Plant-sulfur Mixotrophic Denitrification Process And Its Mechanism

Agricultural runoff is one of the main nitrogen pollution sources for lake waterbody.The plant-sulfur mixotrophic denitrification process(PSMD)was put forward for treatment of the high-nitrate(accounted for 80%of total inorganic nitrogen)agricultural runoff with low COD/TN(C/N)ratio in free water surface constructed wetland mesocosms under static conditions,in which twelve ordinary plants around Erhai litoral zone and sulfur were selected as electron donors for heterotrophic and autotrophic denitrification respectively.The plant heterotrophic denitrification process(PHD)was also constructed and compared with PSMD in wetland mesocosms under static conditions to reveal its mechanism.The main results were shown below:(1)The experiment operation period was divided into three phases based on the releasing pattern of plant organic carbon and 5.0 C/N ratio,i.e.,quick-release Phase ?(0-6 d,C/N ratio>5.0),stable-release Phase ?(7-117 d,C/N ratio>5.0)and declined-release Phase ?(118-273 d,C/N ratio<5.0).(2)Quick accumulation of NH4+-N,NO2--N,TP and COD were observed at Phase ? in twelve PHD mesocosms,which could be successfully removed and utilized by microorganisms in stable-release Phase ? and declined-release Phase ?.COD and NO3--N removal efficiency and denitrification rates of each mesocosm reached the maximum in Phase I,and kept at high COD concentration,NO3--N removal efficiency(47.2-95.3%)and denitrification rates(1.1-1.5 mg/L d)in stable-release Phase II.Vallisneria,Zizania caduciflora,Hydrilla verticillata,Potamogeton malaianus and Water hyacinth mesocosms had the better denitrification performance than others in the twelve mesocosms.During declined-release Phase III,COD decreased gradually which caused the decline of nitrate removal efficiency(28.2-78.7%)and denitrification rates(0.4-1.3 mg/L d).However,better denitrification performance was achieved in Cyperus papyrus,Phragmites australis,Typha latifolia,Vallisneria and Pontederia cordata mesocosms compared with other PHD mesocosms.(3)Similar accumulation and removal trends of NH4+-N,NO2--N,TP and COD were obtained in twelve PSMD mesocosms as compared with those in PHD mesocosms.The COD concentration,NO3--N removal efficiency and denitrification rate of each mesocosm also reached the maximum in Phase I,and kept at high COD concentration,NO3-N removal efficiency(51.5-97.5%)and denitrification rate(1.1-1.5 mg/L d)in stable-release Phase II.Hydrilla verticillata,Myriophyllum propinquum,Potamogeton malaianus,Phragmites australis,Zizania caduciflora,Vallisneria and Water hyacinth mesocosms had the better denitrification performance than others in the twelve mixotrophic mesocosms.During declined-release Phase III,COD decreased gradually which did not cause the decline of nitrate removal efficiency(54.2-96.7%)and denitrification rates(1.1-1.5 mg/L d)due to supplementary function of sulfur autotrophic denitrification.Better denitrification performance was achieved in Pontederia cordata,Phragmites australis,Typha latifolia,Cyperus papyrus,Zizania caduciflora,Water hyacinth and Vallisneria mesocosms.pH in each PSMD mesocosm was in the range of 6.5-9.0 without significant differences(P>0.05).pH decreased in declined-release? demonstrated that sulfur autotrophic denitrification dominated the nitrate removal gradually.After 58d's operation,the fast increase of sulfate concentration also illustrated that sulfur autotrophic denitrification ability increased gradually with the decrease of the plant-carbon supply.(4)As for all PHD and PSMD mesocosms,the copy numbers of nitrate reductase(narG),nitrite reductase(nirS)and nitrous oxide reductase(nosZ)genes in mixture from litter bags were all obviously higher than those in soil from mescosms bottom,which showed that the growth and attachment of denitrificans could benefit from large surface areas and released organic carbon of the plant carbon.And copy numbers of narG and nirS genes were both obviously higher than that of nosZ gene.The copy numbers of narG and nirS genes in PSMD mesocosms were higher than that in PHD mesocosms(except for narG in Potamogeton maackianus,nirS in Myriophyllum propinquum,Potamogeton maackianus and Cyperus papyrus).Among twelve mesocosms,the copy numbers of nosZ genes in Water hyacinth,Hydrilla verticillata,Myriophyllum propinquum,Potamogeton malaianus,Phragmites australis mixotrophic denitrification mesocosms were higher than that in respective heterotrophic denitrification mesocosms.(5)Take Vallisneria-sulfur mixotrophic denitrification(VSMD)mesocosms and Vallisneria heterotrophic denitrification(VHD)mesocosms as the examples for comparison between PSMD and PHD mesocosms,more effective and stable denitrification performance was achieved in VSMD mesocosms than that in VHD mesocosms.Their effluent COD,NH4+-N and NO2--N could meet the Class ? standard of Enviromental quality standards for surface water(GB3838-2002)during stable-release Phase ? and declined-release Phase III if the experiment was conducted in batch mode.Total copy numbers of narG and nirS genes in VSMD mesocosms were higher than that in VHD mesocosms,while lower for nosZ genes.(6)The results showed that the PSMD process was feasible for enhancement of nitrogen removal from agricultural runoff.The synergistic nitrogen removal could be obtained by combination of plant heterotrophic denitrification and sulfur autotrophic denitrification in PSMD process,which had more effective denitrification performance than PHD and possessed significant research and potential application value.Considering the availability of each plant,Phragmites australis,Zizania caduciflora and Vallisneria were recommended as the preferred plant carbon source for real engineering application of both PHD and PSMD process.