After three consecutive dry winters from 2015 to 2017, “Day Zero” – when the taps would run dry – loomed large for  Cape Town and its surrounds in 2018.

Municipalities introduced a slew of water restrictions and, almost overnight, Capetonians became familiar with the idea of greywater. Many had previously let this untreated water from baths, showers, kitchen sinks, washing machines and the like run down the drain. Now they installed storage tanks or carried around buckets of it to flush their toilets or water their lawns. It was a boom time for domestic greywater technologies.

Thanks to residents’ sparing use of water, including the adoption of greywater use, Day Zero never arrived. But there are  lessons to be learnt from that experience, especially around the potential of greywater both in seasons of plenty and dry.

And the dry times are not over. Scientists have repeatedly cautioned that climate change means water shortages remain a real risk. Gqeberha is on the verge of its own Day Zero. The broader sub-Saharan region is also threatened with dwindling water supplies and access.

Researchers have long argued that greywater has the potential to contribute to South Africa’s food security if it is used to water domestic food gardens.

There’s a big problem, though: consumers fear that greywater isn’t safe for use in domestic gardens. Alongside colleagues, I have conducted several studies to understand this reluctance. In two of these studies, in Limpopo, we found that people believed greywater with household detergents shouldn’t be used in food gardens. We also examined the quality of greywater being used  and found that it is, by and large, safe for domestic irrigation.

Greywater is used all over the world. Studies have shown that using greywater to flush toilets or water domestic gardens can save up to 30% of potable, drinking-quality water. But if South Africans are to embrace this important water source, their concerns must be addressed.

We set out to understand what these objections are among people in Limpopo. The province was selected as the test area because it is predominantly arid and water-scarce. High temperatures, droughts and erratic rainfall contribute to crop failure and food insecurity. We also tested whether the greywater that’s available for use there can be safely used on domestic gardens. The answer is “yes, mostly”, with the caveat that any greywater containing harsh pollutant loads shouldn’t be continuously used for irrigation.

We asked people from two villages to share their perceptions about reusing greywater for home gardening. Respondents worried about using water that contained household detergents to water their gardens. They feared their plants would die and that the water would contaminate their food; some were worried the greywater may be poisonous or have unhealthy side effects. They were especially reluctant to use greywater when running water was readily available.

However, residents from both villages who did use greywater reported that their gardens produced more food than they had before this approach was used. Some also reported that the greywater appeared to repel some of the insects that would typically eat plants.

The second study focused on what is in greywater used in these villages and whether it’s safe for domestic irrigation. We concluded that levels of pH – the measure of acidity and alkalinity in a substance – and sodium (salt) in untreated greywater were within widely held acceptable ranges for what’s safe for human consumption.

There are caveats, of course. Some of the water we tested contained harsh industrial chemicals and was not suitable for irrigation. We propose that greywater is best used for home gardening when freshwater supplies fall short,rather than a wholesale alternative to freshwater, since overusing it may negatively affect soil quality.

• Dube is professor of geospatial sciences, University of the Western Cape. This article first appeared on The Conversation

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