A plume of sewage stains the ocean off Green Point in this aerial image taken in 2015. The City of Cape Town discharges city wastewater to sea from three marine pipelines at Green Point, Camps Bay and Hout Bay, along with a fourth pipeline for the Chevron fuel refinery at Milnerton.
Image: Jean Tresfon /Facebook
Teboho Mofokeng
As the planet gets hotter and freshwater sources dry up, cities and towns will not be able to continue the global norm of using millions of litres of clean, drinkable water to flush toilets. South Africa’s Water Research Commission recently commissioned a study into using seawater to flush toilets in Cape Town. I have been researching water management and reuse in South Africa and co-authored the study.
The first problem is that the city uses clean, treated drinking water for toilet flushing. Generally, 20%-30% of the city’s drinkable water supply to households is used to flush toilets.
The water used for bathing, cooking, laundry, cleaning and flushing toilets goes down the drain. It is treated at wastewater plants to remove harmful germs before being pumped back into rivers.
The second problem is that in some Cape Town communities (as well as the coastal cities of Durban and Gqeberha in South Africa), wastewater is discharged into the sea with little treatment to remove harmful germs. Although the government permits a certain amount of discharge, this pollutes the ocean.
The third problem is that as demand for housing grows, so does the need for water and sanitation. When more and more wastewater is discharged into the sea, it is a lost opportunity to keep the water within the urban water management system, and use it again.
South Africa’s water supply network is ageing, has not been well maintained and is due for replacement or repairs. When new housing developments are connected to the water supply network, there is an opportunity to come up with alternatives to using drinking water to flush toilets. One option is seawater.
Another could be recycled water. Toilet water could be treated to remove harmful bacteria and reused for flushing. This would need a closed water system for flushing toilets. It would eliminate the current loss of drinking water flushed down toilets and then discharged into the sea.
Using fresh water to flush toilets is increasingly going to become an unaffordable luxury
Image: Supplied
Studies have shown that most households would prefer to flush with recycled water than clean, drinkable water. However, our study is the first to investigate how people feel about flushing with seawater.
We asked 239 people if they’d be willing to flush with seawater or recycled water or continue to flush with drinking water but pay more to do so. We also asked how they felt about toilet wastewater being discharged into the sea.
Our survey had mixed findings. Overall, 90% of people were willing to move away from using drinking water for toilet flushing, but only if the new water source did not stain the toilet bowl, was clear and had no smell.
We found that 58% of the people we surveyed preferred using seawater to recycled water for toilet flushing. However, there were differences in preference according to income, gender and household size. About 45% of the people we interviewed earned more than R12 800 per month. They preferred to use drinking water to flush toilets. But females with a higher education qualification were more likely to pay extra for both seawater and recycled water options. Homes where more than three people lived were more likely to pay for recycled water than seawater.
On average, 10% to 15% of Capetonians’ municipal bills goes to paying for water. We looked at the preferences of households that pay R350 to R900 per month for water and found they were willing to pay 5%-10% more to use clean and clear alternative water. They were also willing to use water with a slight odour if they got a discount of up to 60% on their monthly water bills.
Treating wastewater before discharging it into the sea would cost money. We found that people were not willing to foot this bill. They were in favour of the city treating wastewater only if it reduced their water bill by up to 7%.
Firstly, it would need a duplicate network to be set up. This would be one network of water pipes for the drinking water supply and another for the toilet flushing water supply. This would mean that both new and ageing water systems would be set up or replaced by systems with a duplicate pipe network.
Usually water supply networks use concrete, steel or plastic pipes. But because seawater corrodes, plastic pipes will be needed for the seawater flushing pipe network. Plastic manufacturing and the raw materials needed have a significant impact on the environment. On the other hand, steel or concrete pipes would need to be replaced more frequently than plastic pipes.
Secondly, storage facilities like reservoirs would need to be built to store the seawater and a new system set up to treat seawater before it is piped to homes.
Thirdly, the chemicals and electricity required to clean the water, the supply of water to consumers and eventually the disposal of the wastewater would add to the costs. Since South Africa’s energy is still electricity made by burning coal, using up more energy in a duplicate water system would increase global warming significantly. So, a renewable energy system would need to be set up.
Despite these drawbacks, a duplicate system has worked well in Hong Kong. Using less freshwater also benefits the environment more.
Climate change will result in a drier south-western Africa. Unpredictable rainfall and frequent droughts will mean less freshwater available for people to drink. Therefore, it is crucial that coastal cities like Cape Town, whose populations are growing, start investing now in water supplies from unconventional sources.
People have to become more aware that not all household activities need to use drinking water. Increasing public awareness and education about the benefits of using alternative water can help people to accept that they won’t be able to flush their toilets with clean, drinkable water for much longer. | The Conversation
Teboho Mofokeng is a lecturer in Civil Engineering, University of Cape Town