New sewage plants to play double role

Facilities will tackle growing volume of waste due to Abu Dhabi's population increase as well as reducing the need for desalinated water.

Construction of a new waste water treatment plant in Abu Dhabi.

ABU DHABI // The first two of four new waste water treatment plants for Abu Dhabi are on track to be completed next year, officials say. The plants are expected to not only reduce the load on the emirate's sole existing facility but also on desalination plants by providing treated water suitable for agriculture.

Work on the two plants, in Al Wathba, 40 kilometres south-east of the capital, and Al Hamah, 40 kilometres from Al Ain, is 41 per cent complete, officials from the company behind their development, Al Wathba Veolia Besix Waste Water, say. The plants are being built at a cost of US$400 million (Dh1.5 billion) and will have a combined capacity of 430,000 cubic metres of sewage per day, enough to supply 1.5 million people.

The Al Wathba plant is scheduled for completion in August next year and the Al Hama plant a few months later in December. The emirate's rising population has placed a heavy load on the capital's only sewage treatment plant, in Mafraq. It receives more than 450,000 cubic metres of waste water daily, which is almost twice its design capacity. The four new sewage treatment plants, which will be able to process more than 800,000 cubic metres of waste daily, will also serve another important objective: the recycling of waste water.

High-quality desalinated water, which requires large amounts of energy and is expensive to produce, has been used indiscriminately, but officials are now advocating that certain needs, such as irrigation, be met with lower quality and cheaper treated waste water. At present some 40 per cent of the outflow from the Mafraq Waste Water Treatment plant is released into the sea due to lack of the infrastructure needed to treat and send the water back to the city for reuse.

"The outflow is a valuable product and we know there is a need higher than what is being produced," said Marc Richli, Wathba Veolia Besix Waste Water's executive managing director. The special-purpose company was established between the Abu Dhabi Water and Electricity Authority (ADWEA), the water solutions provider Veolia Water and the construction company Besix. Another special-purpose company that includes ADWEA, Emirates Utilities Company Holding and the Biwater Group is building another two sewage treatment plants, at Al Wathba and Al Saad, that will have a combined processing capacity of 380,000 cubic metres per day.

Although they are being built and operated by different companies, the two Al Wathba facilities are adjacent on a one square km plot. The water they produce will be pumped into a giant common reservoir before being sent back to Abu Dhabi. The outflow will be carried to the capital via a network that is being upgraded and should be finished by the end of this year. It is due to the current lack of infrastructure that treated waste water is not being reused, said Philippe Paulissen, technical director at Al Wathba Veolia Besix.

"We will produce water suitable for irrigation," he said. "The idea is to have no discharge at sea." The Al Wathba facility will have a capacity of 300,000 cubic metres of sewage per day. Water will take 13 hours to travel through the system. The journey will start at a metering room being built in a 20-metre-deep excavation. "This is the deepest point," said Mr Richli, during a recent tour of the building site. "This chamber will fit the meter to measure the incoming flow."

After the meter room is the pumping station, where six pumps will draw the sewage from underground to the plant level. The waste will first go through a pre-treatment stage where grease, sand and other substances are removed. Water is then extracted from the sewage and sent to two tanks, where bacteria begin the cleaning process by consuming the carbon and nitrogen dissolved in it. "This is the heart of the plant," Mr Richli said.

The water is then further treated in another series of tanks that are more than 10m high, called clarifiers, where the last remaining impurities settle to the bottom. The water at the top is piped off, filtered and treated with chlorine until it is ready for use. The sludge left behind is treated further to extract more water before being sent to two tall round towers called digesters. These contain special bacteria that thrive in the absence of oxygen and eat away at the sludge, reducing its volume by half.

The end product will be poured on to 14 large drying beds where the sun will evaporate more than 80 per cent of its moisture. It is estimated that the plant will produce 135 tonnes of sludge per day, which will be collected by the Abu Dhabi Sewage Services Company and could be used for compost. [e-mail]