Increased polluting loads are discharged to the tidal Thames at times of heavy rainfall. This can lead to a rapid decline in dissolved-oxygen (DO) concentrations, which, on occasions, has resulted in fish mortality. The problem is caused by the cumulative effect of surface runoff, increased flows from sewage-treatment works, and storm-sewage discharges from the combined drainage of Inner London.
An experimental system of in-river oxygenation has been operated by Thames Water in order to alleviate the problem. This scheme demonstrated that the river oxygenation concept is viable. There was, however, a need to increase the operating reliability of the plant and to provide extra oxygenation capacity. Thames Water carried out a detailed investigation into the various options prior to the acquisition of a new, purpose-built, self-propelled, oxygenation vessel, which was delivered to the River Thames in October 1988.