Muscat Wastewater Project, Oman

Scheduled for completion by 2017 – and including the construction of what is expected to be the largest membrane bioreactor (MBR) plant in the world – the Muscat Wastewater project is an ambitious programme which will ultimately serve 90% of the city’s inhabitants.

The scheme has two main elements – the provision of the 53,000m³/day treatment works at Al-Ansab in the Bausher region and the extension and further development of facilities in the area of Al Seeb – which will be delivered in three phases. Establishing an integrated approach to wastewater management for the area, the project is intended to meet the needs of the growing population to the year 2025.

In addition to building the new WWTP, the work involves extending the capacity of the Darsait WWTP by 7,500m³/day, as well as constructing a 25km trunk line and a drainage basin near Muscat. It also calls for the provision of vacuum sewage collection systems and the design of an associated network to cover an area of more than 500km²/ in extent.

The project cost is $1bn.

BACKGROUND

Muscat, capital of the Sultanate of Oman, is the seat of the country’s government, home to most of the private sector companies and is an increasingly popular tourist venue. With an ever-increasing population – around 630,000 according to the 2003 census and projected to exceed 1 million by 2025 – the government set up the Oman Wastewater Services Company (OWSC) to address the urgent need for a wastewater management system.

A state company, established in December 2002 to work on a commercial basis before being eventually privatised in 2010, OWSC drew up a master plan for Muscat. This aims to serve 80% of the population in the six wilayats (districts) of Greater Muscat, Muttrah, Bausher, Seeb, Al Amerat and Quriyat by the end of 2014 and a further 10%, three years later.

The current scheme represents the implementation of this strategy. It will ultimately cover an area of 3,670km², provide sewage services to 46,000 households and supply treated water for irrigation.

BAUSHER AND THE AL-ANSAB PLANT

Given the scope and scale of the work involved – it covers all of the Bausher area and calls for a new collection network running from Hamryah to Azaibah in addition to the major new treatment plant – construction has been awarded as two separate contracts. Both of these are being implemented in a series of phases, spread throughout the duration of the overall scheme. The design stage began in April 2003 and the last elements are scheduled for completion during the final phase of the project.

The new facility itself is being built adjacent to Al-Ansab’s existing WWTP. With an initial design capacity of 53,000m³/day, with scope to expand this to 80,000m³/day, once commissioned the plant will be the largest of its kind in the world – a series of submerged units housing a total of 124,000 flat-sheet membrane cartridges.

The MBR system selected has been developed to permit a high level of treatment and good energy efficiency, while demanding a very small footprint. The units are installed within the activated sludge tanks – the aeration required for treatment generating an upward cross-flow over the membranes and keeping their surfaces clean. Since this design effectively acts as a high-concentration treatment, no settling or sludge thickening tanks are required, allowing the whole system to be simple and compact – the flow typically only needing to be screened and de-gritted prior to entering the MBR.

The sheets are made from chlorinated polyethylene, ultrasonic-welded on both surfaces and with a nominal pore size of 0.4 microns, though in use, the biofilm layer which forms on their surface reduces the effective pore size to less than 0.01 – within the ultra filtration range.

In addition to carrying out onsite treatment with the installed MBR technology, the works will also act as the network's operational centre, making extensive use of SCADA and distributed control systems to monitor the remote stations and direct operations. To accommodate this, the construction programme involves the provision of offices, staff accommodation and buildings to house equipment.

The associated sewer contract will provide the region with a network around 300km in length, with some 18,000 connections. It also includes pumping stations at Azaibah and Al-Qurum, lift stations, rising mains to supply the new plant and an extensive irrigation network to allow reuse of the treated effluent, along with the installation of all electrical, control and automation systems.

AL SEEB

Work to upgrade the Darsait WWTP – a major programme of rehabilitation and replacement – was completed by the beginning of April 2005, expanding the plant’s daily capacity by 7,500m³ to 21,000m³/day. A year later, the rehabilitation of the ageing plants at Al-Khod, Shattie Al-Qurum and Ma’abailah – part of the project which began in 2004 – had also been concluded.

A further element of the project will extend wastewater provision to A’seeb. This involves the provision of both a new treatment plant and a complete new network of gravity sewers, pumping stations and rising mains to collect and convey the flow. Once the system is completed, households within the catchment area will be connected up and the existing septic tanks taken out of service. This part of the scheme is split into sub-stages – two for the plant and five for the collection system – and scheduled for delivery during phases II and III of the project.

An associated pipeline network will be installed here also, again to enable the treated water to be distributed locally for reuse in irrigation.

KEY PLAYERS

Oman Wastewater Services is responsible for overseeing the project, with Veolia as the operations/performance management contractor. The network design was done by Mott MacDonald. The design and supervision contractors for the Bausher collection network, Al-Ansab WWTP and Darsait WWTP upgrade were Khatib & Alami and Metcalf & Eddy. This role was played by Energoprojekt Entel for the rehabilitation work at the Al Khod, Al-Ansab, Shattie Al-Qurum & Ma’abailah WWTPs and by Parsons International for the A’seeb element of the scheme.

Galfar Engineering is the construction contractor for the Al-Ansab WWTP, Sino Hydro for the Bausher collection network and Bahwan for the Darsait WWTP upgrade. Berwin Leighton Paisner provided legal advice to Veolia regarding the performance management contract. Kubota membranes form the MBR system at the Al-Ansab plant.