Design Criteria for interception Sewers for St. Louis

Preliminary to the sizing of the sewers, pumping stations, and treatment plants of the Metropolitan St. Louis Sewer District's Mississippi River Pollution Abateme Project, it was necessary to adopt a design criterion for the interceptors on the combined sewers. The amount of pollutional load spilled to the river during storm water runoff was computed for interceptors of three different capacities when functioning under rainfall conditions corresponding to those experienced during 1955. Pollution was measured in terms of equivalent hours of sewage flow. It was found that with interceptors having a capacity of 1.0 times peak sewage flow, 3.1% of the total yearly sewage flow would be spilled during storm water runoff. Furthermore, the efficiency of the overall project after primary treatment would be approximately 0.25% less with interceptor capacity of 1.0 times peak sewage flow than with interceptor capacity of 1.25 times peak sewage flow, whereas the increase in cost of the project, using the larger interceptors would have been on the order of $2,800,000. The smaller size interceptor was adopted.