Juas Water Treatment Works is now reopen to supply drinking water to the northern parts of Guernsey.
The reopening of the site is all part of Guernsey Water's strategy to make savings of £10 million over the next 25 years.
It will now mean treatments works at Longue Hougue Reservoir will be replaced by Juas Water.
Guernsey Water's general manager has said the new water treatment strategy will significantly reduce costs.
On average, islanders consume 12.5 million litres of water a day and Juas has been designed to provide 10 million litres a day at maximum output.
Juas was first purchased in 1933 and was finally decommissioned in 2013.
Its redevelopment work has taken 20 months to complete with the site having been fully refurbished including the installation of a UV disinfection system.
Guernsey Water has said the treatment work at Longue Hougue will be decommissioned over the next 6 months but will still be used as a valuable water storage facility.