Abu Dhabi's new STEP network to improve sewage and wastewater treatment

Dubbed a mega-structure and a super tunnel, 2017 will see one of the world’s most impressive construction projects, the Strategic Tunnel Enhancement Programme (STEP), finally come on-line, right beneath the streets of downtown Abu Dhabi. The programme has won plaudits and awards from around the globe and was even the subject of a National Geographic documentary. One of the world’s longest gravity-driven tunnels, STEP will transform the way in which the emirate deals with its sewerage and wastewater issues. The project also highlights the seriousness and commitment of Abu Dhabi’s leadership to becoming a global leader in wastewater management, with recycling now a key part of strategies for addressing the future needs of this water-poor part of the world.

Beneath City Streets

Pre-qualification for STEP began back in 2008, with construction itself beginning the following year. The US-based consulting, engineering, construction and operations firm CH2M was appointed programme manager, with some $1.9bn committed to its realisation by the Abu Dhabi government, via the Abu Dhabi Sewerage Services Company (ADSSC). The ADSSC is the service provider for all sewerage services in the emirate, and the operator of the current sewerage network and several treatment plants.

The basic scheme of STEP involved the construction of a deep sewer tunnel, 41 km long, fed by 43 km of supply tunnels. There is also an underground pumping station in the system, located at Al Wathba, a district to the south-east of Abu Dhabi City. Al Wathba is also the system’s deepest point, with the tunnel reaching some 85 metres underground there and the pumping station at 105 metres. The main pipe begins at a depth of just 27 metres, however, beneath Abu Dhabi’s islands. It then passes under Maqta Creek to the mainland at a slope that allows the system to work solely on gravity, removing the need for other pumping stations and saving energy and money. Indeed, the government will be able to do away with some 35 existing pumping stations through the gravity method. The connecting tunnels also operate using solely gravity, intercepting flows from existing sewers upstream. These flows are then channelled into the main tunnel, with the final and only pumping station lifting the resulting sewage to the surface at the Al Wathba Independent Sewerage Treatment Plants. The main tunnel is designed to last for 100 years, Alan Thomson, managing director of ADSSC, told OBG, “The flow capability of STEP is three times current volumes, making it built for the future. But STEP could reach its full hydraulic capacity as early as 2040, depending on population growth.”

Contracts

The programme was divided into sections for contractors, with the first section, a 15-km tunnel known as T-02, awarded to Italy’s Impregilo Group in 2009. This section has a maximum 6.3-metre diameter, with six access shafts. Impregilo was also awarded the 10.5-km T-03 section the following year. The maximum diameter of this piece widened to 7 metres, and the tunnel deepened, with access shafts now descending 60-80 metres. Meanwhile, Samsung C&T Corporation was busy working on the T-01 tunnel. Two link sewer contracts then went to Zublin, which worked on the LS-01 and LS-02 stretches. The first of these represented some particularly complex engineering work, with the construction of some 247 shafts and diameters varying from 200 mm to 2.8 metres. The L-02 link server involved an additional 95 shafts.

Unibeton Ready Mix supplied concrete for the link tunnels, while Commodore Cement supplied the lining materials and over 40 km of jacking pipes. Al Wathba has eight pump drive trains supplied by GE, with each of these rated at some 6.38 MW. The design of the station itself went to Odebrecht, with Drake and Scull Water and Power undertaking the mechanical, electrical and instrumentation work as a sub-contractor. Mott MacDonald is the appointed engineer overseeing the contract. The result is a system with a bore width of 50 metres in the main tunnel, capable of handling peak pumping capacity of 1.7m cu metres per day. According to Thomson, of the six main contracts for STEP, five are completed, with the focus now on the final pumping station, which is in its last phase and should see commissioning in the near term.

Meeting The Challenge

The scale of STEP is indicative of the vital service it will provide for a rapidly expanding urban area. The Abu Dhabi Economic Vision 2030, the emirate’s economic blueprint, foresees that current infrastructure will need to work increasingly hard to cope with population and economic growth over the next two decades. Indeed, the population experienced an annual average growth rate of some 7.3% between 2005 and 2014, with the most recent available figure, for mid-2015, putting the total population at 2.78m, according to the Statistics Centre – Abu Dhabi (SCAD). Within this, 1.72m lived in the Abu Dhabi Region. Therefore, a major upgrade and expansion programme in transport, power and water is under way.

A vital element is the efficient treatment and recycling of wastewater. When the economic vision was drawn up in 2008, Abu Dhabi already had much to be proud of on this score. According to SCAD, some 86% of the population was already receiving wastewater network coverage, placing the emirate above regional peers such as Bahrain (70%) and Saudi Arabia (40%). The question for planners and policy makers since then has been more over what to do with the wastewater once it is collected. In 2014 Abu Dhabi generated some 284m cu metres of treated sewage, or 100% of its wastewater, according to the Environment Agency - Abu Dhabi, with some 60% of this reused. The remaining 40% was discharged into the sea. In 2014 the leadership set the goal of achieving 100% reuse by 2018. The main users of this are consumers in the agriculture and industrial sectors, along with forestry and landscaping. Wastewater is also sent by ADSSC to the emirate’s municipalities for use in irrigating parks and roadside green spaces. The STEP project will radically simplify the collection and treatment of wastewater in an expanding Abu Dhabi. Collecting a full urban area’s sewerage in one place, treatment can help reach the 100% re-use target. The future will also likely see the uses of wastewater expanded dramatically. The Regulation and Supervision Bureau (RSB), which looks after the emirate’s water and power sectors, is tasked with ensuring that the best use is made of resources. This will likely entail future recycling of wastewater for other non-potable purposes, such as district cooling.

At present, desalinated water, Abu Dhabi’s main source of potable water, is used for this increasingly popular method of air conditioning. This is both expensive and inefficient, given that the desalinated water is currently produced from thermal plants, burning natural gas, and is also of a higher quality than is necessary for running a block cooling system. The use of recycled wastewater will also have a major advantage for agriculture and forestry, as it will save the fast-diminishing reserve of groundwater in the emirate. This reserve has seen spiking levels of salinity in recent times, rendering some of it useless, local press reported in 2016. New strategies and much research is going into ways of using recycled wastewater for this purpose.

A New Network

Developing the distribution network for wastewater is another area being given much thought. Public-private partnerships are one way forward, with the UAE’s first sewage treatment plant of this kind opened in Ajman in 2009. With wastewater currently available for free from municipalities or ADSSC, however, finding an appropriate financial model for such services could be challenging. Yet in Ajman, a partnership between Besix, Veolia and Black & Veatch, overcame similar issues, and the current campaign by the authorities in Abu Dhabi emirate to increase public awareness of the value of water is likely to aid in greater acceptance of charges for wastewater. “We see great potential to channel more recycled water to the city and other areas, and distribution companies can play a role here,” Thomson told OBG. “Already we have been given the go ahead for two major projects beyond STEP, that will help utilise about 400,000 cu metres of water that is currently being processed but not re-used.”