Treatment Of Highly Polluted River Water In Different Types Of Constructed Wetlands In Low-temperature Seasons

As a wastewater treatment facility, constructed wetland （CW） has thecharacteristics of low investment and energy consumption. CW also shows higherremoval efficiencies of the organic and the nitrogen and phosphorous which aredifferently removed in the conventional wastewater treatment facilities.As theprocessing object of this study, the Zaohe River in the city of Xi’an is the most pollutedtributary of the Weihe River and characterized by highly polluted urban river inwater-stressed areas. In order to apply the ecological idea of wastewater treatment basedon local conditions, the local materials and phragmites australis in the watershed of theZaohe River were used as substrate and hydrophyte in six different constructed wetlandswhich were constructed to study the treatment efficiencies of highly polluted river waterin low-temperature seasons.These six constructed wetlands are: phragmitesaustralis-graval horizontal subsurface flow wetland （H-1）, gravel horizontal subsurfaceflow wetland （H-2）, phragmites australis-gravel vertical subsurface flow wetland （V-1）,phragmites australis-slag vertical subsurface flow wetland （V-2）, phragmitesaustralis-river sand surface flow wetland （F-1）, phragmites australis-slag surface flowwetland （F-2）. The impact of different substrates, wetland types, plants, temperature andpollution loading on the treatment efficiency of highly polluted urban river wereanalyzed as well. Reliable treatment processes and parameters of constructed wetland tomanage river pollution were provided in this study. Meanwhile it has a significantpractical meaning for constructing river ecological system.The experiment lasts for ten months, the first two months were commissioning stage of the reactors, and the rest eight months were stable-operation stage. Thewastewater was continuous injected the reactors and the hydraulic loading was kept at0.05m3/m2d. The pollutant concentrations of the influent and the effluent weremonitored, while the decontamination effect of plants in constructed wetland and thetreatment efficiencies of three different types of wetlands and two different substrateswere analyzed as well; Through the different research combinations of substrate andwetland types were studied in order to get the best choice of CW process and measuresof decontamination efficiency improving for constructed wetland.The results show:（1） In the low temperature seasons, the contribution of plants is negligible in theremoval of pollutants in constructed wetland, and the major removal is carried out bythe constructed wetland bed.（2） When the materials near the Zaohe River bank used as the filler in the wetlands,the removal rates on NH₄⁺N、TP、PO₄^3--P and COD of the slag vertical flow wetlandV-2unit were higher than the gravel V-2unit for2.59%,7.36%,13.31%,34.28%and5.63%, respectively. However, the differences of removal rates were very insignificanton TOC and SS. The removal rates on TN, NH₄⁺N, TP, PO43-P, COD and TOC of theslag surface flow wetland F-2unit were higher than river sand F-1unit for5.96%,8.85%,3.74%,13.36%,4.48%and10.91%, respectively.（3） The nitrogen removal efficiencies of the three types wetlands were low underthe experiment condition, and the order of the efficiency was surface flow> verticalflow> horizontal flow; However, the vertical flow have the best efficient on theremoval of TP, PO₄^3--P, COD and SS.（4） In the low temperature seasons, the nitrogen removal rate of horizontalsubsurface flow wetland, vertical subsurface flow wetland and surface flow wetlandwere significantly affected by temperature, which is more obvious when the temperatureis low; but the effect of temperature on the removal efficiency of phosphorus andorganic matters in the wetland was insignificant.（5） Under the experimental conditions, the nitrogen removal ability of three typesof construted wetland were increasing at first and then decreasing as the pollutionloading increasing; And with the increase of pollution loading, the phosphorus removal ability of each type of wetlands were first decreasing and then increasing; Thesubsurface wetland presents a better ability to resist loading fluctuation.（6） In the low-temperture seasons, the surface flow-vertical flow wetland appliedwith the slag near the Zaohe River bank as the substrate material to treat highly pollutedurban river are highly recommended.