Mechanism Of Pollutant Transformation And Biological Metabolization In Urban Sewer Systems

The urban sewer system is an important component of urban water infrastructure,and undertakes the mission of residential and industrial wastewater and rainwater collection and transportation.Due to the significant effect of sewage quality variation on the inflow in wastewater treatment plants,the design parameter of water quality makes a big difference with the actual inflow.Thus,the phenomena of pollutant transformation and transport in urban sewer system gradually attract people's attention in recent years.However,due to the poor understand on physical and biological reactions in sewer systems,the mechanism of pollutant transformation has not been verified.Based on this,the major functional areas in urban sewer systems were firstly divided to sewage-biofim-sediment in this study,and according to the characteristics of physical and biological reactions,the model of pollutant transformation and transport pathways between sewage and sediment was established.And then,the diversity of dominant functional microbial communities in different areas of biofilm and sediment was analysed,and the change of biological metabolization along the sewer system was revealed.Based on the effect of environmental factors,the co-variation mechanism of microbial communities metabolization and pollutant transformation was determined.The results provide theoretical and application basis for optimizing of the desine of sewer system and operation of wastewater treatment plant.The main results was summarized as follows:The form and concentration of carbon,nitrogen and phosphorus pollutant changed significantly along the sewer systems.The concentration of diverse pollutant decreased in sewage and enriched in sediment,while the macromolecule organics degraded to micromolecular organics in both sewage and sediment.The results were caused by three pollutant exchange pathways between sewage and sediment which were called physical pollutant deposition(PPD),biological transformation adsorption(BTA)and biological transformation release(BTR).The PPD was the dominant pollutant exchange pathway in sewers and the BTR was the dominant biological reaction which affected the pollutant structures in sewage(especially resulting in the accumulation of volatile fatty acids),and consequently,changed the biodegradability of sewage.The dominant microorganism of fermentation bacteria(FB),hydrogen-producing acetogen(HPA),sulphate-reducing bacteria(SRB)and methanogenic archaea(MA)were changed significantly from the beginning to the rear of sewer.The distribution of those functional microorganism could induce the substrate transformation and then lead to the accumulation of micromolecule organics(i.e.,acetic acid,propionic acid and amino acid).However,the substrate transformation induced by these microbes was affected by the environmental factors such as oxidation-reduction potential,pH and dissolved oxygen.These environmental factors changed along sewer resulted in the variation of dominant bioreactions,which FB was enriched in the beginning of the sewer,while SRB and MA were found in the rear.Thus,the coupling mechanism between microbial community distribution and organics metabolization along the sewer was discovered,therefore,the dominant bioreaction changed from fermentation to methane production along the sewer.The fermentative bacteria play an important role in all regions of the sewer sediment,and it could induce the accumulation of easily degradable substrates for diverse pollutant transformation in sewer sediment.Owing to the variation of ORP values in different longitudinal profiles and depths of the sediment,dominant functional microbial communities were found existing in distinct regions.Methanogens and sulfate-reducing bacteria exhibited depth-dependent reproduction characteristics,which might led to the emission of methane and sulfide in the sediment of the middle and deep layer,respectively.Nitrifying and polyphosphate-accumulating bacteria could hardly survive in the sediment,which indicated that the variation of nitrogen and phosphorus contaminants might be mainly caused by the physical particulate matter deposition from sewage.