Study On The Technology And Mechanism Of Enhanced Floating Treatment Wetland To Treat Multi-source Rural Decentralized Runoff

In rural areas,sewage discharge is huge and the sources are scattered and complex,which becomes a pollution source of regional water environment.Research and application of suitable multi-source decentralized sewage treatment model and technology according to local conditions is the key to sewage treatment in rural areas of China.Floating treatment wetland（FTW）is a simple,low-cost and low-maintenance technology for in-situ ecological treatment of polluted water bodies.It can adapt to instantaneous changes of water depth and flow,and is not easy to block or backwater,also has the potential to increase habitat and aesthetic value with little environmental intervention.In this study,two kinds of enhanced FTW systems,integrated floating treatment wetland（IFTW）and floating treatment wetland-sediment microbial fuel cell（FTW-SMFC）were constructed to explore the treatment performance and mechanism of multi-source decentralized runoff.The functions of plants,substrates and bioelectrochemical processes in the conversion and removal of different forms of pollutants were determined to better reveal the mechanisms of pollutant removal and the relationship between the components in IFTW and FTW-SMFC,which lays a theoretical foundation for strengthening the practical application of enhanced FTW systems and the effective treatment of multi-source decentralized runoff.In this study,Vetiveria zizanioides（Vetiver）and crushed autoclaved aerated concrete（CAAC）were selected as plant and substrate in the enhanced FTW systems respectively.The kinetic experiments of nitrogen（N）and phosphorus（P）uptake show that Vetiver had excellent NH₄⁺-N and PO₄^3--P absorption capacity.The ecological stoichiometric internal stability experiments show that Vetiver could maintain its homeostasis and balance of N and P in the face of nutrient concentration changes,especially facing a wide range of P concentration changes,and had strong adaptability and more obvious advantages.The adsorption capacity and mechanism of four non-reactive phosphorus（NRP）model compounds（STPP,EDTMP,PBTC and AMP）on CAAC were investigated for the first time by using isotherm,kinetic and desorption experiment.The results show that CAAC adsorbed about 90%of NRP within 16 hours and the removal of NRP was chemisorption dominated involving a two-step pore diffusion process.The gradual hydrolysis of NRP to reactive phosphorus（RP）could affect the removal of NRP by CAAC.The maximum Langmuir adsorption of STPP on CAAC was 1.24 mg/g and this of EDTMP,PBTC and AMP was between 0.18 mg/g and 0.25 mg/g.Most NRP was irreversibly adsorbed on CAAC and the adsorption involved inner-sphere complexation.Using CAAC as FTW substrate can enhance the removal of RP and NRP in water.At the same time,CAAC has advantages in costs and environmental impact,which is suitable for the enhanced P removal from rural decentralized runoff.Four FTWs with different components were constructed and compared to analyze the performance and mechanism of IFTW in the treatment of multi-source decentralized runoff.The results show that IFTW can remove organic matter and N stably and efficiently.Plants and microorganisms cooperate to complete the transformation and removal of N.Deposition was the main way of total nitrogen（TN）removal in the FTW without living plants or substrates,but not the main way of N removal in IFTW.The plant tissue debris and root exudates may contribute to N deposition.Denitrification,N fixation and nitrification are the main microbial N removal processes in FTW.The abundance and diversity of microorganisms affected the N removal performance of the system,especially became the limiting factor of denitrification process.Plants can affect the abundance and composition of microorganisms,leading to more significant nitrification and N fixation of microorganisms in the planted system.Moreover,the high relative abundance of Chryseobacterium and Zoogloea may enhance N removal through denitrification and other processes.The conversion and removal of P in FTW mainly involves several important pathways:sedimentation,substrate sorption,plant uptake,hydrolysis,and microbial degradation.The removal of NRP mainly depends on the direct removal by sedimentation or the conversion into RP or at least a form more readily available for decomposition by hydrolysis and microbial degradation.RP removal mainly depends on sedimentation,substrate sorption,and biological assimilation.Plants and substrates could complement each other to improve the NRP conversion rate and RP removal rate while enhancing the stability of the FTW system and its adaptability to water quality changes.The P removal performance of plants at low P concentration was better than that of substrates.The proportion of P removed by plants through direct uptake was about 8%,but plants greatly affected the P removal performance of FTW by influencing sedimentation,physicochemical sorption,and plant-microorganism interaction.The specific enrichment and culling of microorganisms by plants resulted in the formation of specific rhizosphere microbial communities and promoted the removal of RP and NRP.The microorganisms responsible for P degradation in FTW mainly included Pseudomonas,Enterobacter,and Acidovorax.In addition,different NRP species affect the P removal performance of FTW.For STPP,which is easily hydrolyzed and adsorbed,plants provide continuous and stable P removal performance,while the rapid adsorption of NRP by substrates may increase the risk of subsequent P release.For phosphates（EDTMP and PBTC）,which are difficult to hydrolyze and adsorb,the combination of plants and substrates can ensure the removal of all forms of P and the stability of effluent water quality to a certain extent.For AMP that are easy to be used by microorganisms,plants have become the key to guarantee the P removal effect of FTW because of their excellent RP removal ability.Stubble temporarily reduced the efficiency of N and P removal,and this effect was more significant for P removal.By introducing SMFC into FTW to construct FTW-SMFC coupling system,the bio-electrochemical oxidation at the anode,the reduction reaction and plant hyperaccumulation at the cathode all promote the pollutant removal.In the competition between electrochemical process and plant uptake for N,electrochemical process is dominant.The electrochemical process increased the removal rates of TN and total phosphorus（TP）by 8.3-27.8%and 3.5-13.9%,respectively,and promoted the removal of P by enhancing the conversion of NRP to RP,providing 5.0-6.9%additional NRP conversion.Electrochemical process can directly remove NRP by promoting the hydrolysis,adsorption and precipitation at the anode or cathode,and indirectly improve the removal rate of NRP by promoting the absorption of RP by plants,improving the diversity and activity of microorganisms,and strengthening the mineralization of NRP by microorganisms.Furthermore,introducing plants into SMFC increased bioenergy output,while the presence of alkaline substrates inhibited the electrochemical performance of SMFC.SMFC promoted plant growth and nutrient uptake to a certain extent,and also accelerated the degradation of organic matter in sediments.The microbial community succession induced by plants was greater than that induced by electrode stimulation.IFTW and FTW-SMFC meet the needs of in-situ treatment of multi-source decentralized runoff in rural areas,and at the same time realize the enhancement of FTW and stable and efficient treatment of pollutants,providing theoretical support for the new treatment mode and technology of multi-source decentralized runoff,which is of great practical significance to popularize the enhanced FTW system and improve the in-situ treatment of multi-source decentralized runoff.