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Polishing up the green belt

Environment Bioremediation

Saudi capital Riyadh is setting out to save its principal environmental and recreational resource, Wadi Hanifah, from obliteration. Andrew Mylius finds out how.

Riyadh, I think, is the fastest growing city in the world at present, ' says Alan Travers, group manager for river and coastal engineering at consulting engineer Buro Happold.

The Saudi capital, which was little more than a village a century ago, is now a sprawling metropolis the size of Greater London and home to 4.4M people. Its population is expected to have rocketed to 10.1M by 2021.

As the city has expanded, its principal environmental asset, a 120km long wadi that cuts a swath of green diagonally across Riyadh, has come under immense pressure. It has become an unauthorised dumping ground for thousands of tonnes of construction, municipal and industrial waste. Sewage spewed out by the city is discharged, often with little or no treatment, directly into the river channel that runs the wadi's length.

'Wadi Hanifah is Riyadh's main environmental resource and its only real recreational green space, ' says Travers. As the city grows in the next two decades its value will become much more significant. Yet without urgent action, the wadi's environmental and recreational role could suffer irreversible damage.

A wadi is a water course in which surface water flows only during periods of rainfall. However, there is normally a subter3per day of this base flow from Wadi Hanifah to slake its thirst.

But in the last 20 years flow regimes in the wadi have changed.

Riyadh started using desalinated sea water in the mid 1980s and now consumes 750,000m 3oftreated water a day. Most of this, in addition to the 400,000 drawn from Wadi Hanifah, eventually finds its way back as sewage or via leaks in the city's water supply system. So much water is now being returned to the wadi that there is a consistent yearround surface flow.

This is potentially very positive, says Travers. Trees, shrubs and aquatic plants have colonised the banks of the water course and farmers have tapped the stream to irrigate crops. The green of leaves and the shade cast by plants provide a wonderful respite from the surrounding arid, sun-baked desert. But with heaps of rubbish littering the sides of the Wadi Hanifah gorge, flotsam and jetsam floating on the polluted water, and mosquito-infested pools left stagnating after each flood, it is squandered potential.

Client Arriyadh Development Authority wants to turn Wadi Hanifah into a giant park at the heart of Riyadh, in which its citizens will walk and picnic on the banks of a clean, clear running river stocked with fish. But it has other good reasons for cleaning the wadi up. Water consumption is expected to rise to between 2.8Mm 3and 3Mm 3per day in the next couple of decades. This will inevitably require more desalination. But if the city can recycle 1Mm 3per day from surface flows in the wadi, huge cost savings can be achieved.

Tenders have just been returned for what will be a Riyals 580M ($155M) environmental improvement project, designed by Buro Happold with Australian firm Moriyama & Teshima Architects, and spanning the best part of the next decade. Arriyadh Development Authority hopes the contractor will start work in the next month, with removal of hundreds of thousands of tonnes of construction and demolition waste to disused stone quarries nearby. Tens of thousands of tonnes of municipal and industrial waste will be carted off to licensed landfill sites. In all, some 1.5Mm 3of material will have to be carted away, Travers reports.

Removal of the rubbish will help improve flood flows, which at present back up behind obstructions, causing fairly considerable damage to farmland and buildings. The aim is to accommodate all floods up to 1:20 year events within the main river and designed flood channels. To make the wadi more attractive aesthetically, work will also involve cleaning loose rock from the 30m-40m cliff faces that bound the wadi along much of its length, and regrading the wadi floor to prevent future ponding.

While the scale of this work is huge, it has not called for particularly unusual engineering design solutions, Travers concedes. Improving water quality is a different matter, though.

There is some chemical pollution from factories, but the most significant challenge to water quality is from sewage outfalls and tanker operators who often discharge the contents of septic tanks directly into the water course. A major sewage treatment works at Man Fouha will be upgraded to provide tertiary treatment, reducing the biological oxygen demand (BOD) level of its discharges to below 20mg/litre. Its capacity will be increased from 400,000m 3/day to 1Mm 3/day.

This alone will not be enough to make the river pleasant to sit by or to abstract from. So the water will be subjected to a process of bioremediation, in which pollutants will be trapped and broken down naturally.

'The wadi is actually cleaning itself through bioremediation, ' Travers says. 'The approach we've taken is to provide the right environment to maximise the process.' To start with, the channel is being lined with rip rap type rock that will provide a large surface area on which bacteria and algae can develop, which will feed on waterborne coliforms and pathogens. Small weirs at 500m intervals will oxygenate the water.

Meanwhile, a series of 135 bioremediation tanks will be provided to intensify the process. These will be arranged in groups on four levels, over a 1km stretch of the wadi. Water flow will be split into the first group of identical tanks, where it will go through a three stage process.

First, artificial weed colonised by bacteria will sweep suspended particles from the water and perform an initial attack on micro-organisms. The water will then filter through reed beds, where bacteria in the soil and plant roots will remove micro-organisms, break down hydrocarbons, and trap metals and inorganic compounds.

Finally it will flow between algae-covered boulders for further polishing. Fish feeding on micro-organisms will assist the cleansing process. The same process will be repeated on each of the four levels.

A key aim is to make the cells an attractive landscape feature, as well as a highly functional treatment facility, Travers notes.

A pilot project of 22 remediation cells will be built in the first year of the wadi clean up and run for 12 months to test performance.

When water quality is up to a standard where it can be recycled, a new 10m high, 1.5km long earth embankment dam will be built to impound between 5Mm 3and 20Mm 3of water, giving Riyadh a modest buffer against fluctuations in demand. Away from the main channel, meanwhile, on the slopes of Wadi Hanifah and in the many sub-wadis that feed into it, hundreds of small, crescent-shaped microdams will be erected to trap rainfall. The hope is that, in time, enough moisture can be locked into the ground to green the degraded desert landscape.

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