Up until recent times, rivers through cities have always been used as open sewers, relying on fast flowing river water to dilute and transport relatively small amounts of sewage away. This system worked well for small towns where sewage quantities were small and drinking water was not collected downstream.

The necessity of keeping sewage apart from drinking water was only fully understood 150 years ago, when devastating epidemics of Cholera were commonplace in London. Eventually it was discovered that the epidemics were found to cluster around particular supplies of drinking water that were being contaminated by sewage - some supplies were still drawn from the Thames. By closing off the affected supplies, or preventing the sewage leaks, the incidence of Cholera fell dramatically.

In 1850s Joseph Bazalgette was finally given the go-ahead for a gigantic engineering project to divert the sewage into three interceptor sewers to pump the sewage down to the treatment works at Crosness. The project took decades to complete, but resulted in the almost complete eradication of Cholera from the London population.

We still use these same sewers today. They were designed with a 60% over-capacity to allow for storm drainage. However, due to the increase in sewage supply from modern day London, this extra capacity has been filled, and so it has to be supplemented by permitting regular discharges into the river at peak times. The capacities of the sewage treatment works have also been increased, but not in pace with the rising population. They are now quite inadequate and resort to discharging massive amounts of untreated sewage sludge into the river at peak times (see aims).