Heavy or prolonged rainfall can rapidly increase the flow in a combined sewer until the volume becomes too much for the sewer to carry and excess storm sewage is discharged to river or sea via relief “valves” known as combined sewer overflows (CSO’s).  Without CSO’s the overloaded sewers could flood properties and sewerage treatment works.  Although the discharge from CSO’s contain sewerage it is heavily diluted with rain water.  Nevertheless many of these discharges are today considered unacceptable.