Clay Lane Water Treatment Works, owned and operated by Three Valleys Water (part of the Vivendi Group), is located in Watford in the South East of England. The raw water supplying the works is taken from eight chalk borehole sources in the surrounding area and treatment at Clay Lane consists of ozonation, rapid gravity filtration (GAC media) and disinfection with sodium hypochlorite. Ozone and GAC are used primarily for pesticide removal. In 1997 a decision was taken to install additional treatment at one of the sources, Eastbury, which can supply up to 40 Ml/d to Clay Lane and which has on occasions been subject to surface water influence and poor quality following heavy rainfall. Given the potential risk from contamination by Cryptosporidium oocysts, a Fibrotex plant capable of achieving 99% (2 log) removal of oocysts was selected as the appropriate treatment. This paper reviews the results obtained during the performance testing, both on site during commissioning and subsequently during the Cryptosporidium oocyst challenge test. In addition, the operation of the plant over the last 18 months is reviewed.

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