Fish, insects, indigenous plants and microorganisms combine to clean up contaminated water in the Saudi capital. It is all part of an award-winning, 10-year programme to improve the environment - and its costs are a third of conventional methods. Matthew Teller reports
A 10-year project to rehabilitate a wadi that runs through Riyadh has already won the Aga Khan Award for Architecture, and continues to push scientific and social boundaries.
Rising in the central Arabian highlands of Al Hissiyah, Wadi Hanifah runs south-east for around 120 kilometres, traversing the Saudi capital before losing itself in the Al Sahbaa sands, on the fringes of the Empty Quarter.
Fed by more than 40 tributaries, it drains a huge part of the eastern Najd, with a catchment area of more than 4,500 square kilometres.
Centuries before the rise of Islam, the Banu Hanifah - their name derives from the Arabic word for "pure" or "upright" - were farming the valley, trading from a village which became known as Al Riyadh ("The Gardens").
As a small town, Riyadh grew sustainably. But rapid, unregulated growth in the 1970s quickly overwhelmed urban ecosystems.
Dumping of domestic and commercial waste partially blocked the wadi, which was further damaged by quarrying, mining and soil extraction. Seasonal floods swept pollutants into residential neighbourhoods leaving pools of stagnant water, jeopardising public health.
Saleh Al Fayzi saw the wadi's shabbiness before the Arriyadh Development Authority's (ADA) current rehabilitation project.
"I started working on Wadi Hanifah about 20 years ago," he says. "[At that time] it had a very bad reputation. It was the city's backyard dump."
In 2001, he began directing redevelopment. Under a master plan drawn up with ADA, the Canadian firm Moriyama & Teshima Planners and the UK-based engineers Buro Happold cleared debris from the wadi, graded, landscaped, introduced flood-profiling measures, replanted native flora and devised world-leading techniques to treat the city's polluted wastewater.
At Al Elb, 35km north of Riyadh, backside parks now feature carefully designed individual family picnic pods, backed by tall palms. Each comprises a horseshoe of pale, roughly finished limestone slabs, offering privacy as well as open views.
More slabs, laid horizontally, create steps down to the wadi bed, where extended families lounge under the acacias.
Towards the city, beneath an overpass carrying Riyadh's Northern Ring Road, birds chirp amid desert gardens on the wadi floor.
Stone-bedded plots fill out the curves of a walking trail, newly planted with indigenous species - tamarisk, the yellow flowers of "needle bush" (Acacia farnesiana), mature Acacia tortilis and Acacia gerrardii, fluffy fountain grass (Pennisetum setaceum) and banks of silvery saltbush (Atriplex halitus) and boxthorn (Lycium shawii).
Melon-sized chunks of local limestone form a trapezoidal dry flow channel: both the shape and the material minimise erosion damage. The wadi road, formerly used as a crosstown shortcut, stands quiet, re-engineered with speed bumps, roundabouts and a 40 kph limit to nudge traffic away.
Beside the low-income neighbourhood of Al Uraijah, a box culvert brings surface run-off into Wadi Hanifah from around the city. From this point on, the wadi holds a continuous flow - though, initially, the untreated water is not safe.
Downstream at Al Utaiqah, the project's core idea is revealed. Overlooked by ceaseless traffic on the motorway interchange where King Fahd Road meets the Southern Ring Road, a new bioremediation plant is producing water clean enough for irrigation and recreation.
Bioremediation, which means applying natural processes to repair environmental damage, refers here to the creation of a linked series of wetland habitats.
Three large ponds - their herringbone design clearly discernible on satellite mapping websites - host 134 bioremediation cells. Untreated run-off enters a main head pool over a weir, which adds dissolved oxygen, killing coliform bacteria.
Suspended solids begin to settle while an aeration system (the only mechanical element of the process) adds more oxygen. Water crosses weirs into each biocell, where microorganisms feed on organic matter and are themselves consumed by tilapia fish.
The water then passes over a low weir into a mini-wetland zone within each cell, about 50 centimetres deep, enclosing three small islands planted with trees and grasses to provide a riparian habitat alongside the aquatic environment.
Contaminants in the water continue to be assimilated by microorganisms and tilapia, with dragonflies and other insects, frogs, snails, aquatic molluscs such as clams, and birds also entering the food web.
After more oxygenation, water collects in a final head pool for further aeration. Organic sediment decomposes (helped by more tilapia) and inorganic sediment settles, for flushing and dredging.
Functioning with a precisely engineered gradient of just 30 centimetres over a 900-metre route, the system treats 600,000 cubic metres of urban wastewater every day, producing clear, odourless water that is safe for human contact.
None of the techniques is new, but nowhere else have they been brought together on such a large scale, or at such reduced cost: bioremediation represents roughly one-third the capital outlay of traditional treatments.
It is a startling process to observe, where dirty water enters and clean water departs, without human intervention.
Farms are the chief beneficiaries; plans are afoot to irrigate a sustainable nursery growing replacement flora for the wadi.
Fish caught in a 15km stretch downstream - to the point where effluent from a traditional wastewater treatment plant enters - are edible. Indeed, restoration has spurred the growth in fishing as a leisure activity.
"Before, I was afraid to come here - the wadi was too oppressive," said Fathi Noor Hassan, an Egyptian who was fishing with his children one afternoon. "Now I feel like I'm by the Nile."
Project officials are keen to stress civic pride, environmental rehabilitation expunging the shame of neglect.
"Wadi Hanifah has become a place to breathe," says Saud Al Ajmi, an ADA engineer.
Yet it is striking that the scheme has so far absorbed around a billion dollars, even while up to a third of Riyadh households remain unconnected to mainline sewerage.
It also seems no accident that money has been lavished on a place that lies at the heart of Saudi identity: Wadi Hanifah is both the origin and power base of the Al Saud.
More than one observer has mentioned "greenwash", the idea that creating an environmental good-news story might divert attention from the kingdom's inequalities.
Riyadh families benefit regardless, packing recreation zones up and down the wadi every weekend. The presence of flowing water also seems to be easing social restrictions.
At a downstream lake one Friday, Saudis and expatriates were mixing freely. Within some family groups the women were unveiled.
"This project is changing opinions on social gatherings," one local man said. "You simply can't police a wadi that is 80km long."
Mr Al Fayzi concurs. "Wadi Hanifah is different from other city parks," the project director said. "It is open. We cannot control it."
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