The Succession Of Microbial Community In Urban Sewer Networks And Its Effects On Wastewater Quality

Taking Xi’an city wastewater as experimental water in this paper, the formation law of of biochemical products, structures of biofilm and changes of microflora were studied through a 1200 m long pilot scale urban sewer networks. The succession law of microbial community and the effects of microorganism on sewage quality were further evaluated, which provide a theoretical basis for the degradation of pollutants in sewer networks.The experiment was consisted of 40 mm diameter PVC pipe and conducted at room temperature(25°C) in a controlled environment that kept the dissolved oxygen(DO) equal to 0.3 ± 0.05mg/L, During system operation, the flow rate was controlled to be equal to 0.6m/s by adjusting the pipe slope and wastewater fullness degree to 5‰ and 0.6, respectively. The results showed that TCOD、NO3-N、NH3-N and SO42- content were changed during sewage transportation, among which TCOD、NO3-N and SO42-content were reduced from 370mg/L、0.45mg/L and 38mg/L to 251.0mg/L, 0.11mg/L and 25.3mg/L and NH3-N content was increased from 35.8mg/ L to 42.9mg/L respectively, which was related to the action of microorganisms in sewer networks.The results of SEM showed the biofilm structure changed visiablely along the length of the sewer, the initial structure with flat surface became gradually loose along the length, with coccus, filamentous bacteria, streptococcus, bacillus etc in surfaces of biofilms and filamentous bacteria, bacillus winding in the biofilm seen. The results of PCR-DGGE showed that 10 dominant bands corresponding to main microbes were Aeromonas、proteobacterium、Trichococcus、Desulfomicrobium、Desulfobulbus 、 Sedimentibacter 、 spirochete 、 Firmicutes. The high-throughput sequencing results further showed that bacterial abundance and diversity increased along the length of the sewer. The absolute dominant bacteria phylum such as proteobacteria, firmicutes and bacteroidetes could be find in sewer networks, which their relative abundance kept the trend of increased, decreased and constant along the length of sewer nentworks respectively. Streptococcus, enterobacter clostridium etc were the main fermentation bacteria. Genusbacillus, Smithbacteria etc were hydrogen-producing acetogenic bacteria. Acetobacter and Ruminococcus are two kinds of homoacetogenicbacteria. Desulfurization, desulfovibrio, desulfurization were mainly sulfate-reducing bacteria; Methylococcus was methanogens.The effects of microorganisms on sewage quality were further analyzed and evaluated. The results showed that protein and humic substances content were reduced, while the blue shift occurred for protein and the red shift occurred for humic substances, which indicated that the chemical structure of these substances could be changed. Molecular weight(MW) analysis showed high MW(proteins, etc) and some medium MW organics(humic substances) were converted into small MW organics(VFA, etc). Studies in the biodegradability of the wastewater showed that microbial in sewer networks changed the composition of organics in water, hydrolysised complex organics into simple small organic molecules, transformed unsaturated organics into saturated substances, converted nonbiodegradable and biodegradable organics gradually to organics which is easy to be utilized by microorganisms, consequently, the biodegradability of the wastewater was improved.