Research On The Efficiency Of Rotating Biological Contactors Combined With Constructed Wetlands On Purifying Polluted River Water

Social development was restricted and people life quality was decreased due to river pollution,which has gradually become one of the main environmental problems.As two common wastewater treatment technologies,biofilm process and ecological treatment technology have been gradually applied to the treatment of polluted river water.The problems,such as energy consumption and tight land use,will be caused when using one of them alone.However,the combination of biological and ecological processes can not only save operating costs,but also ensure the removal effect of pollutants and achieve good ecological environment benefits.As one of the representative technologies of biofilm process,the rotating biological contactors（RBCs）has the characteristics of high efficiency and easy operation and management,which has an excellent application prospect in the polluted river water treatment.Constructed wetland（CW）,used as a typical ecological treatment technology,has been a research hotspot in water environment remediation due to their low operating cost and good ecological effect.The study was conducted in the Liangtan River Wetland Project,the purification efficiency was studied,and the functions and effects of biological and ecological parts in the combined process were verified.The emission characteristics of greenhouse gases for the combined system was revealed using static chamber method.The spatiotemporal evolution characteristics of microbial community structure were clarified using high-throughput sequencing technique.Furthermore,the investment,operating costs and comprehensive benefits were evaluated based on Liangtan River Wetland Project.The main results of this study are as follows:（1）The effluent concentration of NH₄⁺–N,TN,TP,and COD of combined process could attain GradeⅤstandard base on Environmental Quality Standards for Surface Water in China（GB3838-2002）,and the average removal efficiencies were 95.09%,42.85%,37.67%and 50.08%,respectively.The purification effect in warm season was better than that in cold season.The trends of the p H and DO,fluctuated greatly along the direction of the water flow,were shown and had similar trends in the warm and cold seasons,and the electrical conductivity and turbidity decreased along the path.However,the water temperature had no obvious change.Although the concentration of NH₄⁺–N,TN,TP,and COD were fluctuant along the path,the trend of decrease was presented.The NH₄⁺–N was removed mainly in the RBCs,however,the NO₃^-–N,TN,TP and COD were mainly degraded in the CWs.The complementary functions of nitrogen conversion and pollutant removal were shown in RBCs and CWs,which ensured the good pollutant purification effect.（2）The removal rate constants of NH₄⁺–N,TN,TP,and COD in RBCs were 26.6 d^-1、0.1 d^-1、1.5 d^-1,and 2.6 d^-1 in the warm season,respectively.Meanwhile,the values were 2.0 d^-1、0.7 d^-1、0.5 d^-1,and 0.6 d^-1 in CWs,respectively.Compared with the warm season,the removal rate constants of NH₄⁺–N and TP in the RBCs were decreased in the cold season,and the values of TN and COD were decreased in CWs.The direct assimilation by plants accounted for 46.6%of N reduction and 24.4%of P reduction,however,the removal efficiency of NO₃^-–N,was restricted due to plant harvest,resulting in the downward trend of TN removal rate constant.（3）During the test period,although the inflow loading rate of NH₄⁺–N fluctuated greatly,the excellent purification effect was maintained.The purification effect of TN,TP,and COD was affected dramatically by inflow loading rate.For the TN,TP and COD,the proper ranges of inflow loading rate were 1.50～2.50 g/（m²·d）,0.15～0.40 g/（m²·d）and15～20 g/（m²·d）,respectively.（4）In the warm season,the N₂O,CH₄ and CO₂ fluxes of RBCs were 0.89,7.05 and36.63×10³ mg/（m³·d）,meanwhile,the N₂O,CH₄ and CO₂ fluxes of CWs system were0.45～3.60 mg/（m²·d）,4.06～189.89 mg/（m²·d）and 5.99×10³～18.64×10³ mg/（m²·d）,respectively.Compared with the warm season,the N₂O,CH₄,and CO₂ fluxes and cumulative emissions of each cell in the cold season decreased to different degrees.The comprehensive global warming potential（GWP）of the system in warm season was obtained with the value of 51.05×10⁴ g/d,while it decreased to 24.48×10⁴ g/d in cold season.Most of GWP was contributed by surface flow wetlands,and the main contributor was CO₂.（5）In warm and cold seasons,the bacterial community diversity and richness indexes of CWs were higher than those of RBCs,and the diversity and richness of the bacterial community along the direction of the water flow increased firstly and then decreased.The difference of each cell was obtained in the same season,however,the high microbial community similarity of same cell was acquired in different seasons.The predominant phyla were Proteobacteria,Chloroflexi,Actinobacteria,Acidobacteria,Bacteroidetes,Firmicutes and Nitrospirae.The temperature variation had little effect on the bacterial community of each cell.The predominant functional bacteria genera were detected as Kouleothrix,Nitrospira,Nitrosomonas,Hyphomicrobium,Bacillus,Comamonas,Dechloromonas,Thiobacillus,Anaerolineaceae,Sphingomonas,Rhodobacter,and Pseudomonas.The differences of pollutant removal and greenhouse gas emission were shown due to the relative abundance variation of functional bacteria.（6）The investment and operating costs of the Liangtan River Wetland Project were382.07 million RMB and 1.27 million yuan/y,respectively.Furthermore,the Project,possessed enormous comprehensive benefits comprised economic,ecological and social benefits of 65.45×10⁴,8.92×10⁶ and 37.04×10⁴ yuan/y,played an important role in material production,pollution purification,carbon sequestration and oxygen release,hydrological adjustment,habitat maintenance,recreation and ecotourism,scientific research and education,employment service.