ABSTRACT
This chapter deals with the special characteristics of combined sewers, and in particular with combined sewer overflows (CSOs). It discusses how flows are accommodated in combined sewers. The chapter describes the specific role of CSOs and discusses how and to what extent pollution is controlled using this system. The essential features of combined sewers are that they carry both wastewater and stormwater in the same pipe. The main function of a CSO is hydraulic: to take an inflow and divide it into two outflows, one to the wastewater treatment plant (WTP). If the surface of the flow passing through the CSO is below the crest of the weir, flow continues to the WTP only. Until 1970, the traditional CSO setting had been 6 × dry weather flow (6DWF) in United Kingdom. There is a continuing trend to monitor CSOs, in particular the major ones, to ensure and demonstrate compliance with the Urban Waste Water Treatment Directive.
