Long-term Operation Efficiency Of Surface Flow Constructed Wetland For Treatment Of Slightly Polluted River Water

Malodorous rivers' remediation,as well as slightly-polluted rivers' water quality improvement has been the focus of environment protection project and the sponge city construction,which will definitely show a promising future.To do such,one creative and prospective method is to enhance the ecological service function of urban public space,which has a profound influence on cities' spatial contexts and textures,and can effectively interconnect with green ecological infrastructures.An engineering experiment of constructed wetlands,located in Kunshan Culture&Art Center,Kunshan City,Jiangsu Province,China,is a best practice of such idea as above mentioned.The wetlands,consisting of three surface flow constructed wetlands in series,can function as a purification system for a lateral river.The surface areas of the wetlands are 500,370 and 270 m2,respectively,with a water depth ranging from 0.1 m to 1.0 m.Plants in the wetlands include iris,Thalia,reed,lotus,myriophyllum etc.This paper carried out a 18-month continuous monitoring programme based on the wetland project,explored its water purification efficiencies and some influencing factors(HRT,water temperature and external carbon sources),gauged the substrate enzyme activities and plants'growth.The main results were as follows.(1)The average removal rates of SS,NH3-N,NO3-N,TN,TP and CODMn were 34.2%,38.4%,22.3%,29.1%,35.6%,31.5%,respectively.SS,NH3-N,and TP were maily treated in the first and second wetlands,while NO3-N and TN were in the second and third wetlands,with a relatively uniform distribution of CODMn's removal loading in the wetlands.Water purification efficiencies under rainfall events are,to a certain extent,stable concerning the non-significance of pollutants' removal rates.(2)Analysis showed that in order to ensure the effluent compliance rates and improve the removal rates of NO3-N and TN,the hydraulic retention time(HRT)of wetlands should be set to 3-7 d.Remarkable positive correlations between the pollutants removal rates and water temperatures were found,with the fact that higher pollutants' removal rates in summer and fall,and lower in spring and winter.Besides,external carbon sources could promote the removal rates of NO3-N,TN and CODMn to a certain degree,but had little effect on NH3-N's removal.(3)Substrate enzyme activities would increase in summer and autumn,and be inverse in winter and spring.Correlations between dehydrogenase activity and the removal rates of SS,NH3-N,TP and NO3-N were significant,meaning it could be an indicator of microbial activities in the wetlands to some extent.The relevances between pollutants removal rates and activities of urease,acid phosphatase and catalase were found unremarkable.(4)Plants with relatively faster growth include Typha orientalis Presl,Iris pseudacorus L.,Acorus calamus L.,Pontederia cordata L.,Alisma plantago-aquatica Linn.and Scirpus validus Vahl.Plants with larger aboveground biomass include Thalia dealbata Fraser,Phragmites australias Trin.,Sagittaria sagittifolia and Pontederia cordata L..The correlation analysis showed no significance between TN and the heights of Iris pseudacorus L./Acorus calamus L.,with the same phenomenon between CODMn and the biomass of Sagittaria sagittifolia.The other plants' heights and(or)aboveground biomass showed remarkable relevances with pollutants' removal.This field experiments show that the operation efficiency of tested wetland is stably and reliably,and that it is feasible to transform the traditional hardened city square into constructed wetlands and give them water purification function.