The earliest sewers were built for collection and disposal of stormwater and, for convenience, emptied into the nearest watercourse. In later years, domestic or sanitary wastewater was put into these large storm drains, automatically converting them into combined sewers. Subsequently, combined sewers came into widespread use in communities because they represented a lower investment than the construction of separate storm and sanitary sewers. This was overcome by designing overflow structures called CSO regulators at selected points in the sewer system. In designing the sanitary-sewerage system's sewers and WWTPs, nominal allowances generally are made for stormwater and groundwater infiltration.
Infiltration occurs due to joints and cracks in piping, which increases with pipe age.
This problem of excessive flow entry is compounded by inappropriate and unauthorized stormwater connections, groundwater inflows and blockages. Reliefs in the separate sanitary system have beenused as an immediate and low-cost solution for these excessive flows.