The incident occurred on 20 July, when the valve on a chlorine tank at the works was left closed after a routine changeover. It remained shut for 4.5 hours, stopping normal disinfection. In the evening of the same day, the company issued a precautionary warning to customers to boil their water.
The DWI is critical of the company for failing to take operational samples of water leaving the works and at most of its service reservoirs. Monitoring after the event produced no firm evidence that the water supplied was unfit for human consumption, which is an offence. The chlorination failure was probably mitigated by the mixing of untreated with disinfected water at the works and in the distribution system.
Three Valleys Water was also censured for not taking remedial action within a reasonable time. The DWI says it contravened water quality regulations by failing to notify all local councils and the district health authority. At the time of the incident there were no powers to take enforcement action for such a breach. This has now been corrected by the Water Supply (Water Quality) (Amendment) Regulations 1991.
Monitoring and alarm systems, staff training and emergency procedures were also inadequate, the DWI found.