Abu Dhabi Sewerage Services Com-pany (ADSSC) is inviting selected contractors to prequalify for the AED2bn ($545m) deep-tunnel sewer as part of its plans to overhaul the sewerage network in the emirate.

The tunnel will run for 42 kilometres from the existing pump-ing station on Abu Dhabi island to the Mafraq wastewater plant, before heading towards the Al-Wathba wastewater treatment plant.

The tunnel is being tendered as three design-and-build contracts. One will be for a four-metre diameter 16.2km-long tunnel from Abu Dhabi island onto the mainland. Another will be for a 15.6km tunnel with a diameter of 5 metres from the Musafah area to Mafraq.

The third involves the construction of a 10km tunnel with a diameter of 5.5 metres, between Mafraq and Al-Wathba.

Each contract requires the design and construction of the main tunnel, access shafts and short connecting tunnels. It also covers any additional work, such as data acquisition, liaising with other agencies, seeking statutory approvals and relocating services and equipment.

The tunnel is a major component of Abu Dhabi’s AED4bn Strategic Tunnel Enhancement Programme (Step), formerly the Strategic Investment Programme.

The project also includes developing one or two pumping stations along deep-tunnel sewer routes, a series of smaller sewers connecting existing sewerage systems to the deep tunnel sewer, and the removal of existing pumping stations.

The gravity tunnel and the sewers feeding into it will enable ADSSC to eventually decommission several existing pumping stations, thereby reducing its operating and maintenance costs.

US-based CH2M Hill is the programme manager for Step and the entire programme is expected to take six years to complete.

Sewerage is a growing concern in the capital as the development of major real estate projects will dramatically increase the population on Abu Dhabi island and the surrounding areas.

The Urban Planning Council expects the population to exceed 3 million by 2030, as residents move into island projects such as Reem, Saadiyat, Yas and other developments on Abu Dhabi island itself.

Several other sewerage schemes are under way in the emirate. Two consortiums have been awarded 25-year concessions to build, own, operate and transfer sewage treatment plants at Al-Wathba, serving Abu Dhabi, and at Al-Saad, serving Al-Ain.

One contract was awarded to a consortium of the UK’s Biwater Group, Kuwait’s Mohamed Abdulmohsin Kharafi & Sons and the local Al-Qudra Holding.

The other was awarded to the French/Belgian joint venture of Veolia and Besix.

Both groups will build two plants, one at each location. The plants for Abu Dhabi will have capacity of 300,000 cubic metres a day (cm/d), and the plants for Al-Ain will be able to treat 65,000 cm/d (MEED 13:8:08).