The gold standard for disinfection of water systems has for a long time been the use of free chlorine, usually provided by hypochlorite / DICD or similar product, at a concentration and duration prescribed by BS 6700 (the British Standard for Designing, Installing, Testing and Maintaining Water Systems). The standard details the use of 50 mg/l (ppm) for a contact time of 1 hour after which the level should be retested, with a minimum of 30 mg/l (ppm) indicating successful disinfection, and under 30 mg/l requiring a repeat dose. The majority of contract specifications for routine disinfection work / tank cleaning identify chlorine as the preferred disinfectant.