Characterization of microflora and transformation of organic matters in urban sewer system

• The chemical structure of organics was changed in the sewer via microbes.
• Most of fermentation products tended to be converted into acetic acid in the sewer.
• Fermentative bacteria were beneficial in improving the biodegradability of organics.
• The transformation mechanism of organic matters in the sewer system was proposed.

A study was conducted using a pilot sewer system consisting of 35 sequential sections, totalling 1200 m of gravity pipe. Urban sewage flowed into the sewer system at a constant flow rate until it reached physical and microbiological steady states. Microflora in the biofilm that attached to the inner surface along the pipe length were analysed. The organic compositions in both the liquid and gaseous phases of the sewer system were monitored. The results showed that typical fermentation bacteria, such as bacteroidetes and bacillus, were abundant in the system, indicating that the anoxic environment (DO = 0.3 mg/L) was suitable for fermentative bacterial growth. This resulted in a substantial reduction of the chemical oxygen demand (COD) along the pipe length and an increase of the biodegradable oxygen demand/chemical oxygen demand (BOD/COD) ratio from 0.68 at the beginning of the sewer system to 0.84 at the end of the sewer system; this was an indication of a transformation of organic matters from less-biodegradable to more-biodegradable products. Via molecular weight (MW) analysis, it was further identified that the larger organic molecules (MW > 10,000 Da) were transformed into products with smaller molecular weights. Regarding the fermentation products, the concentrations of the volatile fatty acids (VFAs) increased dramatically in the initial 600-m sections and then remained constant for the later sections except for the end section of the sewer; acetic acid was found to be the primary product of the VFAs. Gaseous carbon dioxide (CO2) and methane (CH4) were found to increase along the length of the sewer system, whereas the concentrations of ethanol, lactic acid, and hydrogen (H2) were high at the beginning of the sewer and then decreased in the rear sections of the sewer system. It could thus be concluded that in an urban wastewater sewer system, fermentative microflora could perform important roles in contributing to organic matter removal and/or improving the biodegradability of organic matter.

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