Utilization of oxygen in a sanitary gravity sewer

This paper studied oxygen utilization in a 1.5-km sanitary gravity sewer section having a slope of 0.0075. The sewer was built with a cement pipe in an inner diameter of 450 mm and collects campus sewage at a mean flow rate of 0.057 m3/s. To evaluate the change of dissolve oxygen (DO) in the sewer line, online DO monitorings at both the influent and effluent of the sewer section were conducted. Meanwhile, 24-h variations of sewage flow rate were also determined by Manning’s formula using an identified roughness value of 0.011. In addition, on-site measurement of the specific oxygen uptake rate (SOUR) of the suspended solids (SS) was conducted during a 24-h sampling period. To compare the SOUR values between the sewage phase and the sediment phase, potential SOUR of the sediment was also investigated. Further to the SOUR measurements, ATP contents in both phases were analyzed. It was found that flow rate and DO level in both the influent and effluent varied with time but that they were not correlated. The DO decreased progressively along the sewer line and the mean concentration difference between the influent and effluent was found constant at 3.1 mg/l throughout the entire sewer section. Such a constant reduction indicated a substantial oxygen demand of the sewer within a hydraulic retention time of 18 min. Although the SOUR in the sewage phase fluctuated in time, it was maintained at a level of 17.7 mg/g SS/h; while the potential SOUR in the sediment slightly decreased in the downstream of the sewer and the mean value was found to be 32 mg/g dry wt/h. It is clear that both the sewage and the sediment phases are able to uptake oxygen, however, the sediment may be more active in uptaking oxygen. The mean ATP contents of the SS and the sediment were analyzed to be 0.53 mg/g SS and 0.94 mg/g dry wt. of the sediment, indicating that the sediment phase contains more active biomass than the sewage phase. The above results demonstrate that the sediment phase plays a key role in the oxygen utilization in the sanitary gravity sewer. Oxygen budget analysis of the sewer section also confirmed this conclusion.