Nitrogen And Phosphorus Removal By Surface Flow Constructed Wetland And Enhanced Floating Treatment Wetland From Low-polluted Water

Low-polluted water is one of the main nitrogen and phosphorus pollution sources for surface water.It could lead to water quality deterioration and intensify water eutrophication when directly discharged into surface water without appropriate treatment.Surface flow constructed wetland(SFCW)and enhanced floating treatment wetland(EFTW)are two ecological restoration techniques widely applied to water treatment,their treatment performance has been proved in purifying low-polluted water body and they have similar application situations.However,it is still controversial which kind of technique is more effective in nitrogen and phosphorus removal.Therefore,in this study,SFCW and EFTW planted with Iris pseudacorus were employed to remove N and P from simulated effluent of wastewater treatment plants,which was a representative type of low-polluted water.In the 15-month operation,the removal efficiencies of N and P by SFCW and EFTW were compared.The effects of influent nitrogen forms and water temperature on N and P purification performance were investigated.The contribution of plant uptake and sediment adsorption to N and P removal was studied,the composition and diversity of microbial community were analyzed,and the balance models of N and P in SFCW and EFTW systems were established.Besides,the feasibility of iron scrap addition to EFTW system aiming to improve N and P removal performance was proved.This could provide support for selection and application of ecological restoration techniques in low-polluted water treatment.The results showed that:(1)SFCW showed better treatment performance in low-polluted water purification,and it was more resistant to seasonal and temperature changes when compared to EFTW.When hydraulic retention time(HRT)was 3 d,influent TN was 14.81±0.87 mg/L and influent TP was 0.39±0.02 mg/L,both SFCW and EFTW had highest total nitrogen(TN)removal efficiencies in summer,which were 86.03±3.39 % and 83.87±4.37 % respectively.Increasing the proportion of ammonia nitrogen(NH4+-N)in the influent could improve TN removal performance in two ecological systems.While Low temperature(<15 ?)could inhibit nitrogen removal capability.The annual removal efficiencies of total phosphorus(TP)were 86.16±11.07 % and 72.25±13.32 % respectively in SFCW and EFTW.Dissolved oxygen(DO)was the main factor influencing P removal rates in two systems.(2)Nitrification-denitrification was the main pathway for N removal from SFCW and EFTW in low-polluted water treatment.The contribution of plant uptake and sediment adsorption to reduce nitrogen was much lower than that of microbial denitrification.Higher relative abundance of denitrifiers in SFCW system was conducive to more TN reduction.The main pathway for P removal was plant uptake in SFCW yet sediment adsorption in EFTW.The contribution of microorganism to reduce TP was relatively small.Since SFCW system had larger biomass of Iris pseudacorus,plant uptake contributed more to N and P removal in SFCW than in EFTW.(3)As a new type of filler,iron scrap could effectively enhance N and P purification in EFTW system,through stronger chemical reduction,adsorption,precipitation,and denitrification process.Under the conditions of HRT=3 d,influent TN=15.31±0.66 mg/L and influent TP =0.39±0.02 mg/L,the TN removal efficiency increased from 77.02±3.73 % to 92.37±2.64 % after iron scrap was added in EFTW within one month.And the TP removal efficiency increased from 85.75±4.62 % to 93.16±1.62 %.