PUB, Singapore's national water agency, appointed three contractors yesterday to design and build the first batch of deep tunnels and link sewers for the Deep Tunnel Sewerage System (DTSS) Phase 2.

Over the next seven years, the three contractors will design and construct about 30km of deep tunnels and link sewers, as well as their supporting structures.

Construction will begin at the end of this year. In total, the contracts are worth $1.51 billion.

The three contractors are the Singapore branch of Ed Zublin AG, a joint venture between Penta-Ocean Construction and Koh Brothers Building & Civil Engineering Contractor, and the Singapore branch of Leighton Contractors (Asia).

Their work will be part of a network of 40km of deep tunnels and 60km of link sewers for DTSS Phase 2.

The contracts for the rest of the network will be awarded from next year. By its expected completion in 2025, the DTSS will be connected to the Tuas Water Reclamation Plant. The DTSS will then transport used water from the whole of Singapore into three centralised water reclamation plants for treatment.

There, the water will either undergo more purification to make Newater, or it will be discharged into the sea.

Mr Yong Wei Hin, director of DTSS Phase 2 at PUB, said: "As the backbone of Newater production, the DTSS ensures the sustainability and resilience of the used-water network to facilitate large-scale water recycling in Singapore."

He added that this "contributes to the goal of increasing the overall water-recycling rate from 40 per cent to up to 55 per cent of total water demand in the long term"