Research On Advanced Treatment Process And Microflora Structural Characteristics Of O₃+MBSF For Secondary Effluent Of The WWTP

Nowadays, wastewater reclamation has raised great concern in modern society with the increasing percentage of wastewater treatment rate. However, due to the social progress, agriculture and industry development and the increasing living standard of the urban residents, trace contaminants such as heavy metals, pesticides and Ph ACs, etc. have been observed in the influent and effluent of the municipal wastewater system and posed a great threat on the wastewater recycling, as well as safety of the water environment and water ecology.In this paper, characteristics of secondary effluent from three WWTPs（A, B, C） which employs the ―Tri-channel oxidation ditch‖、―Anaerobic selection tank+modified Carrousel oxidation ditch‖ and ―Biological Aerated Filter‖ were respectively analyzed, the ecological risk of Ph ACs in secondary effluent has been evaluated, and finally the target WWTP and the advanced treatment process have been determined. Experimental research was conducted in A-WWPT and the quartz sand were modified through hydrophilic、hydrophobic and iron processes. The optimal dosage and reaction time of ozone were also determined, and the removal rates and mechanisms of conventional contaminants and trace substances were investigated. Besides, the microbial community structure characteristics of the biofilm in the biological sand filter were also studied.The removal rates of color, UV₂₅₄ and COD in the secondary effluent were 47.96%, 20.30% and 12.81%, respectively and BOD₅/COD was increased to greater than 0.4 by ozone under pre-oxidation time of 15 min and optimal dosage of 3 mg/L. The SEM results of the modified quartz sand filter material showed that the surface of hydrophilic-modified quartz sand had a reticular and stereochemical structure and larger specific surface area, whereas the surface of hydrophobic-modified quartz sand and the iron-modified ones formed certain kinds of deposits, which apparently exhibited a rough structure and increased porosity. Under the hydraulic load of 4.5m3/（m2·h）, the thickness of biofilm in BSF were among the average range of 47¹02μm which belonged to the super-thin and aerobic biofilm and exhibited high removal rate on ammonia and organic matter. Among the process of ―Ozone+normal BSF‖, ―Ozone+hydrophilically-modified BSF‖, ―Ozone+hydrophobically-modified BSF‖ and ―Ozone+iron-modified BSF‖, the ―Ozone+hydrophilically-modified BSF‖ and ―Ozone+iron-modified BSF‖ presented efficient removals of NH3-N, of which the average removal rates were 88.24% and 83.98% respectively. However in ―Ozone+normal BSF‖ and ―Ozone+hydrophobically-modified BSF‖, the average removal rates were 73.06% and 69.38%, respectively. The average removal rates of four processes on UV₂₅₄ were 23.02%, 27.38%, 29.39% and 26.46%, respectively, as well as on COD were 23.02%, 34.27%, 21.11% and 31.45%, respectively. The color and turbidity in the effluents from the four processes remained below 12 degree and 2.0 NTU, respectively.The analysis results of trace pollutants in the secondary effluent from the target A-WWTP showed that the average concentrations of Hg, Cd, Pb and Ni were 5.572μg/L, 0.707μg/L, 4.017μg/L and 10.312μg/L, respectively. In the combined process of ―Ozone+BSF‖, the removal of heavy metal mainly depends on biological flocculation and biological adsorption, while slight influence was shown by modified quartz sand on removal rate, with the removal rates of Hg, Cd, Pb and Ni in the range of 79% to 82%, 81% to 86%, 79% to 83% and 81% to 83%, respectively; Meanwhile, the average concentrations of DBP, BBP, DEHP and DNOP were 0.976μg/L, 1.039μg/L, 0.735μg/L and 7.103μg/L, respectively, which were removed rather efficiently by combined process of ―Ozone+hydrophilic-modified BSF‖, while the removal rates of DBP, BBP, DEHP and DNOP were 72.19%, 68.14%, 52.62%, and 69.30%, respectively. The analysis results also showed that different modified methods had little influence on the organic chlorine pesticide lindane（γ-HCH）, of which the average concentration was maintained at 0.112 μg/L, with removal rates of the four combined processes were 51.78%, 59.89%, 58.44% and 62.29%, respectively; The average concentrations of three pharmaceutical active substances such as ciprofloxacin, anhydroerythromycin and tetracycline dehydration were kept at 47.88ng/L, 525.17ng/L and 149.51ng/L, respectively; The removal rates of ciprofloxacin and tetracycline were unstable in ―hydrophobically-modified BSF‖ process, with the average removal rate of dehydration erythromycin was kept at 23.62%; The similar removals of tetracycline dehydration could be achieved in the combined processes of ozone with normal, hydrophilic and iron modified BSF, with average removal efficiencies of 46.56%, 46.59% and 46.61% respectively; The removal rate of ciprofloxacin in normal BSF combined process was unstable but 52.15% of anhydroerythromycin could be removed by this process; In ―hydrophilic modified BSF‖ process, the removal rates of ciprofloxacin and erythromycin dehydration were 90.44% and 34.41%, respectively; In ―iron-modified BSF‖ process, the removal rates of ciprofloxacin and erythromycin dehydration were 92.24% and 34.09%, respectively.The differences in characteristics of biofim microbial community coated on hydrophilic-modified, hydrophobic-modified, iron-modified quartz sand and normal quartz sand showed that hydrophilic, hydrophobic, iron changes and the surface roughness of the filter material increased. Under the same substrate and environment, the highest diversity of the bacteria was detected in the surface of iron-modified BSF through diversity index analysis. Through Mi Seq high-throughput analysis, 27 bacteria types were detected in the four samples, among which Proteobacteria was the dominant one.According to category, four kinds of bacteria which have the function of hydrolysis and resistance（Lysobacter, Enterococcus, Taoeshi sp., Sphingobacterium） were found in this current study. In normal quartz sand samples, Lysobacter and Taoeshi sp. had a higher abundance; In the hydrophilic-modified quartz sand samples, Nitrospira and Sphingobacterium had a higher abundance; In the hydrophobic-modified quartz sand samples, denitrifying bacteria had a higher abundance; In the iron-modified quartz sand samples, Nitrosomonas, phosphorus removing bacteria and Enterococcus had a higher abundance. According to function, Nitrosomonas was the dominant genus in AOB bacteria and Nitrospira was the dominant genus in NOB bacteria. While the denitrifying bacteria such as Thauera, Rhodobacter, Flavobacterium Comamonadaceae unclassified etc were found as the dominant genus in this system. Enterococcus was the dominant genus in phosphorus removing bacteria.