It is important to draw a distinction between treated and untreated sewerage discharges. Treated effluent discharges from sewage treatment works and some intermittent discharges from overflows during wet weather are measured in cubic metres. Discharges consented by the Environment Agency are made to the Thames by three water companies: Thames Water (inland and estuary); Anglian Water and Southern Water (estuary only).
Thames Water reports it has discharged an estimated 4,800 million cubic metres of sewage effluent to the Thames in the last five years. Of this, 4,500 million cubic metres receives secondary treatment. Some of this effluent also receives more stringent treatment at Abingdon, Little Marlow and Windsor's sewage treatment works, which discharge to the freshwater section of the Thames. Approximately, 100 million cubic metres will have been treated storm sewage from treatment works, and the remaining 200 million cubic metres as untreated storm sewage (sewage and rainwater run-off). It is estimated that about 30 per cent. of storm sewage is discharged unmeasured.
The consented daily dry weather effluent flows, which receive secondary treatment prior to discharge, from three sewage works operated by Anglian Water are 95,514 cubic metres. As it is generally accepted that average daily flows are 1.25 times the dry weather flow, it is estimated that 218 million cubic metres of treated effluent has been discharged from these works (Canvey Island, Southend and Tilbury) in the last five years.
Southern Water discharges effluent after it has received secondary treatment from two sites (Gravesend and Northfleet). In the last five years (2000–04) the sewage treatment works at Gravesend has discharged approximately 19 million cubic metres of treated effluent. Since April 2001, when flow monitors were installed at the Northfleet sewage works approximately 12.5 million cubic metres of treated effluent has been discharge to the end of 2004.
Storm sewage is discharged to the Thames at three sites at times of unusually heavy rainfall when the capacity of sewerage system is exceeded. The consents for these storm overflows do not require Southern Water to monitor the volume of discharges that occur.