Performance Evaluation And Mechanism Study Of Simulated Constructed Wetlands For The Treatment Of Mariculture Wastewater

In recent years,China’s mariculture industry has developed rapidly,and the discharge of mariculture wastewater has become a problem that cannot be ignored.However,the characteristics of mariculture wastewater with high salinity and low C/N increase the difficulty of its treatment.Constructed wetlands can remove and transform pollutants in wastewater through the synergistic effects of physics,chemistry,and biology,and have a broad application prospect in the treatment of mariculture wastewater because of their low cost,high efficiency,and easy maintenance.At present,there is less research on constructed wetlands in the field of seawater,and there is even less research on the mechanism of carbon,nitrogen and sulfur conversion in them.In this study,vertical flow constructed wetlands were constructed,and Bruguiera gymnorrhiza,which can adapt to the seawater wetland environment,was selected as the wetland plant.The efficacy of the constructed wetland in treating mariculture wastewater was evaluated,and the influence of sulfur on pollutant transformation mechanism was further analyzed in-depth.The main research results are as follows:(1)The simulated seawater constructed wetlands maintained a stable removal of conventional pollutants and low greenhouse gas emission fluxes,but the growth of wetland plants slowed down,the salt-tolerant bacteria in the seawater system ensured stable removal of pollutants.Regardless of the salinity and C/N conditions,the removal rates of chemical oxygen demand(COD)were more than 95%and total phosphorus(TP)were more than 80%for the four systems.Two systems with C/N of 6 removed more than 75%of total inorganic nitrogen(TIN),and the other two systems with C/N of 3 removed more than 70%of TIN.High salinity reduced the emission fluxes of three greenhouse gases(CH4,N2O,and CO2),but inhibited the growth of wetland plants.Salt-tolerant phyla such as Crenarchaeota,Actinobacteriota,Calditrichota,Halobacterota,etc.and salt-tolerant genera such as Acanthopleuribacter,Vibrio,Caldithrix,Calditrichaceae,etc.were the dominant species in the seawater constructed wetlands and ensured the stable removal of pollutants.(2)Sulfate reduction and sulfur oxidation reactions occurred in the sulfur-containing system,which significantly enhanced TIN removal efficiency at low C/N,reduced GHG emission flux and GWP,but adversely affected the growth of wetland plants.Sulfate reduction bacteria(SRB)and sulfur oxidizing bacteria(SOB)in the sulfur-containing system mediated the sulfur conversion process and were the dominant microbial communities.Regardless of the conditions,the four groups of systems achieved more than 95%removal of COD and more than 80%removal of TP.Under low C/N conditions,the removal of TIN was significantly higher in the sulfur-containing system(74.37 ± 3.20%)than that in the sulfur-free system(52.69 ±2.76%).The presence of elemental sulfur reduced the emission fluxes of three greenhouse gases.Sulfate reduction and sulfur oxidation reactions occurred within the sulfur-containing system.and high concentrations of organic matter promoted SO42-reduction and acid volatile sulfur(AVS)accumulation.S2-produced by sulfate reduction adversely affected the physiological condition of Bruguiera gymmorrhiza,with low concentrations leading to growth cessation and high concentrations leading to death.SRB,such as Desulfobulbus,Desulfofustis,and Desulfosarcina,and SOB,such as Sulfurimonas,Sulfurovum,Thioalbus,and Sedimenticola occupied an important position in the microbial network and played an important role in the pollutant removal process.