Wastewater is a resource that is too valuable to throw away, especially in an increasingly water-scarce world.
Wastewater from large cities is often pumped directly into rivers or seas without treatment, leading to pollution and posing a threat to the health of ecosystems and people.
But under the right conditions, wastewater can be recycled for agriculture, irrigation or industry, all of which tend to use huge quantities of the stuff.
A new report by The Nature Conservancy, Beyond the Source: The environmental, economic and community benefits of source water protection, says that for one in six of the 4,000 cities analysed, the cost of implementing source water protection activities, such as forest protection, reforestation and the use of cover crops, could be recouped through savings in annual water treatment costs alone.
This year’s World Water Day, on 22 March, looks at the sustainable management of freshwater resources with a special focus on how to deal with wastewater in African cities.
Here are some great examples of what can be done.
Solar is used for pumping wastewater to the greenbelt © Targa Aide
Greening the desert in Morocco
In Ouarzazate city in central-southern Morocco, UN Environment is helping to showcase the benefits of using treated wastewater to support reforestation.
The project, which is funded by the Korean Forest Service and the Moroccan Government and supported by UN Environment and local partners, is using municipal wastewater to irrigate degraded land for tree planting and create a greenbelt around Ouarzazate.
This greenbelt will protect the city from strong winds and dust clouds, provide a place for local people to enjoy nature, create agricultural jobs and encourage public participation in the prevention of land degradation and biodiversity loss.
It will also improve local air quality, conserve biodiversity, provide fodder, and boost the livelihoods of urban and fringe communities.
"This project has created jobs for us and opportunities to use our knowledge and experience,” says Lhoussine Chetma, an inhabitant of Ouarzazate and employee of the project.
A side-benefit of the project is that less untreated wastewater is pumped into the Monsour Eddahbi reservoir, which feeds water to the nearby NOOR solar power station. One of the biggest in the world, this power station is set to provide electricity to 2 million households.
Drip irrigation in the greenbelt using treated wastewater © Targa Aide
Wastewater problems in Dar es Salaam
In the Tanzanian capital, Dar es Salaam, UN Environment, UN-Habitat, BORDA-Africa and other partners have recently started a project to show how decentralized wastewater projects can work in city areas with no sewerage facilities.
Less than 10 per cent of Dar es Salaam is connected to the “centralized” sewer network. As a result, some 90 per cent of the population use pit latrines or practice open defecation.
This threatens the marine environment around Dar es Salaam, which provides important benefits for local communities through small-scale, artisanal fisheries and tourism.
By reducing the discharge of domestic wastewater into marine ecosystems – including mangroves, sea-grass meadows, and coral reefs – everyone stands to benefit from this project.
How do decentralized treatment plants work?
Biogas Plants: Built into the ground using bricks, their function is to separate incoming wastewater in liquid and solid form and biologically digest organic solids, under anaerobic conditions. This system generates biogas useful for cooking, lighting and heating.
Anaerobic Baffled Reactors: These consist of a series of chambers in which the wastewater flows up-stream. Here, the suspended and dissolved solids in the pre-settled wastewater undergo anaerobic degradation. The activated sludge settles at the bottom of each chamber and the inflowing wastewater is forced through this sludge blanket where anaerobic bacteria make use of the pollutants for their metabolism. Progressive decomposition occurs in the successive chambers.
Planted Gravel Filters: These are a constructed wetland suitable for wasysical filtration and chemical absorption. The filters are made of planted filter bodies consisting of graded gravel.