Study On The Treatment Of Polluted River Using By Constructed Wetlands Planted With Iris Sibirica In Low-temperature Season

Compared with the traditional sewage treatment plant, the constructed wetland(CW) system is a reasonable option for treating not only domestic wastewater, butalso polluted river or lake water, owing to the little reliance on energy inputs, lowerinvestment and operation cost, fewer operation and maintenance requirements, andgood ecological landscape effect. However, Aquatic plants easily wither and go intodormancy in winter when temperature is extremely low at the northern areas. Little isknown about nitrogen and phosphorus uptake by plants, heavy metals uptake byplants and its role in removing nutrient and heavy metals in CWs. Therefore,investigating the performance of CWs planted with overwintering plants and selectingout which plants will support an effective CW over the winter period are important.In this paper, microcosmic subsurface vertical flow constructed wetlands (MVFCWs)were planted with Iris Sibirica and the treatment performance and related removalmechanism of treating the simulated polluted river water were studied.(1) MVFCWs were planted with Iris Sibirica for treating simulated polluted riverwater and evaluated for nutrient removal, plant growth characteristics and plantnutrient uptake under different nutrient concentrations. Treatment performancesindicated that the highest average TN, NH4+-N, TP, soluble reactive phosphorus(SRP), and COD removal efficiencies were obtained in MVFCW units with mediumnutrient (10-16mg/L TN,7-10mg/L NH4+-N,1.8-2.5mg/L TP, and80-120mg/LCOD); the removal efficiencies were45.8±15.4%,62.1±8.8%,57.7±8.3%,59.1±10.1%, and39.3±12.1%, respectively. MVFCW units with low nutrient (5-8mg/LTN,3.5-5mg/L NH4+-N,0.9-1.25mg/L TP, and40-60mg/L COD) exhibited theworst treatment effects. Plant nutrient uptake in the different MVFCW units rangedfrom19.86%to50.19%of N removal and from13.19%to22.32%of P removal atthe end of experiment. The N and P accumulation ability of the below-ground part ofplants were better than that of the above-ground part.(2) MVFCWs were planted with Iris Sibirica for treating simulated heavy metal Cdpolluted river water. In this section, Cd removal capability, plant biomass, absorption and accumulation capacity of plant, substrate adsorption and the effect onmicroorganism and enzyme activity of root zone matrix under different nutrientconcentrations were studied. The experimental results indicated that all the treatmentunits showed better Cd removal capability, and the removal efficiencies reached over90%. The best removal efficiency was attained in the treatment unit of2mg/L; theaverage removal efficiency was97.4%. Besides, the Cd removal efficiency of plantedwetland treatment unit was higher than unplanted wetland treatment unit. Iris Sibiricademonstrated higher tolerance to heavy metal Cd and accumulation capacity in thelow temperature season. The Cd accumulation by plant was most in the treatment unitof8mg/L, it reached98.4mg in the below-ground part and higher than that of theabove-ground, which was12.2mg. However, the removal process of Cd by plantaccumulation was insignificant, it accounted for about3%of Cd removal. Thepredominant removal process of Cd in constructed wetland was substrate adsorption,which accounted for about50%of Cd removal. Through the research on the effects ofdifferent Cd concentrations on total cultivable microbial population and enzymeactivities, it was found that the root zoon substrate microbial populations and enzymeactivities in the planted wetland were higher than those in unplanted unit, and theywere affected by high Cd concentration (8mg/L) with an adverse effect. While theywere insignificant affected by low Cd concentration (low2mg/L), which even had acertain positive role.Through the work of this article, it was found that Iris Sibirica demonstrated highnutrient uptake and accumulation capacity of heavy metal in the low temperatureseason, and to a certain extent, it could improve the treatment ability of the CW andsupport an effective CW over the winter period. Therefore, Iris sibirica can beconsidered an effective overwintering plant selection in CWs for restoring pollutedriver water and potentially be suitable for generalizing elsewhere, especially at thenorthern areas over the winter period.