On August 6 last year, United Utilities (UU) began advising customers in Preston, South Ribble, Chorley and the Fylde coast to boil their water for all drinking and eating purposes after traces of cryptosporidium were detected during routine tests at Franklaw water treatment works.
UU flushed through 2,500 miles of pipes and storage reservoirs to kill off the Cryptosporidium parasite, and deployed portable ultraviolet ray equipment.
The incident, which lasted 30 days, cost the company about £25 million in compensation pay-outs to more than 300,000 customers.
The company said the DWI will make its findings public once the investigation is complete.