Removal of Dissolved Organic Carbon in Sanitary Gravity Sewer

Utilization of dissolved organic matter in a 1.5 km section of a sanitary gravity concrete sewer with an inner diameter of 450 mm constructed on a slope of 0.0075 was studied. Continuous measurements of dissolved organic carbon (DOC), suspended solids, dissolved oxygen, and flow rate were performed along the sewer. About 14% of the DOC was removed in an 18-min retention time. A slower flow rate in the sewer would favor higher DOC removal efficiency because it offers a longer retention time. Oxygen was not a limiting factor as the dissolved oxygen level was at least 1 mg/L. Batch tests using raw sewage and either suspended solids or settled sediments yielded specific DOC rates of 1.3 mg and 2.6 mg DOC/mg dry wt/day for the sewage phase and sediment phase, respectively. Adenosine triphosphate content analysis of the suspended solids and sediment samples confirmed that both contained substantial amounts of active biomass. In the 1.5-km sewer system, it is estimated that 39.13 kg of DOC can be stabilized/day; the sewage phase contributes 40% while the sediment phase contributes 60%.