National water agency PUB is inviting companies to submit proposals for the construction of the second Changi Newater plant.

To be completed by 2016, the Newater plant will add another 50 million gallons of Newater a day to Singapore's water supply, said the agency in a statement on Friday.

Its main process facilities will be built mostly on the rooftop of the Changi water reclamation plant, saving both land space and cost. The successful bidder will enter into a 25-year agreement to supply Newater to PUB from 2016.

Newater currently meets 30 per cent of Singapore's water demand, and this is set to increase up to 55 per cent in the long term. There are currently four Newater plants - in Bedok, Kranji, Ulu Pandan and Changi.