Biofilm in the sediment phase of a sanitary gravity sewer

Microbial activity of the sediment phase in a 1.5-km-long concrete sewer section with a cement pipe in a 540-mm diameter was investigated in this paper. SEM examinations and elementary composition analyses of the sediment samples have identified the presence of a biofilm layer at the sediment surface. Bacterial counting results with a DNA-staining technique have revealed that the amount of bacteria in this layer was 2.1×10^11 cell g^-1 dry wt, which is close to that of activated sludge. ATP content in the sewer biofilm was found relatively high, demonstrating that the sewer biofilm is active. Throughout the entire 1.5-km sewer section, the biofilm activity was maintained at almost the same level. Lab-scale sediment oxygen uptake flux (SOUF) tests showed that the shear flow velocity above the sediment phase linearly increases the SOUF, which of the potential value was determined to be 32 g O2 m^-2 day-1 at an estimated shear flow velocity of 0.055 m s^-1 at 25°C in the sewer line, provided that the mean flow velocity was 1.5 m s^-1, and the mean water depth was 220 mm. Such a high SOUF value further endorsed the existence of the active sewer biofilm.