For the sake of the soil in which your plants must grow and thrive, think twice before deciding to re-use laundry greywater in your garden. If you really want to use it as part of your greywater system, at least then chose liquid washing detergent over washing powder, as it still degrades the soil, but not as much. That is the message from soil scientists at Stellenbosch University, who published the first ever comprehensive research on the topic in the Journal of Hydrology.
The study was led by Dr Ailsa Hardie of the Department of Soil Science in the Faculty of AgriSciences at Stellenbosch University. It is based on the MSc work of one of the co-authors, Ms Ncumisa Madubela, who in January 2020 received an award for the best junior research paper presented at the annual congress of the Soil Science Society of South Africa. Ms Madubela graduated in March 2020. Dr Cathy Clarke and Mr Vink Lategan, also of the SU Department of Soil Science, contributed to the study.
Laundry greywater is one of the largest and easiest sources of greywater to reuse, as it simply requires extending the washing machine drainage hose, whereas direct access to drains is required for re-using other sources such as bathing water. It however has its drawbacks.
“When Day Zero loomed for Capetonians, many admirably responded by re-using greywater to irrigate their gardens. While it serves to keep some of plants alive, the re-used laundry water isn't benefitting the soil in which they are planted," says Dr Hardie.
“The same chemicals in washing detergent that strip dirt and grime from clothing also strip beneficial humus from the soil. Humus is the best part of the organic matter in soil, and contributes to its health, fertility and water holding capacity," she explains.
This “stripping effect" causes clay particles in the soil to disperse and blocks soil pores. This in turn causes a crust to form on the surface that seals the soil.
“It becomes difficult for water to soak into soils and increases run-off when it rains or when you water your garden. In essence you are then actually wasting water, because in time very little water gets to penetrate into the soil to reach plant roots," says Dr Hardie.
The blocking of soil pores also decreases the water holding capacity of soils. In times of drought this makes soils even drier. Soils become increasingly saline (brackish) and alkaline.
“Plants cannot thrive in such soils, or take up water and certain essential nutrients," explains Dr Hardie. “The degradation sets in from the very first addition of laundry greywater to your soil. In time, it becomes very difficult to fix."
During the course of the study, it was found that powdered laundry detergent greywater is far more harmful to soils than liquid laundry detergent greywater. This is because of the difference in their main chemical ingredients. Washing powder is very alkaline (pH 9-10) and contains a lot of sodium carbonate, while water is the main ingredient of liquid detergents. Both types contain a host of ionic and non-ionic surfactants, ion sequestering agents, bleaching agents and enzymes that help to remove soil and grime from fabrics.
“It makes sense that by their very nature detergents would also therefore degrade and strip soil when used for irrigation," explains Dr Hardie.
“Powder detergent greywater should not be used to irrigate soils due to its aggressive soil degrading qualities. Liquid detergent-based greywater should be used cautiously," says Dr Hardie.
The study was conducted using soil samples from Cape Town and Stellenbosch, and showed that certain soils are more susceptible to the negative effects of washing powder-based laundry greywater than others.
“We found that sandy soils are the most susceptible to the stripping effect that washing powder greywater has on soil humus," says Dr Hardie, who adds that around 58% of soils around greater Cape Town are sandy.
Granite and shale derived soils with a lower iron content were most susceptible to pore sealing, which influences ability of soil to take up and store water.
The researchers found that red or yellow soils rich in iron withstood the degrading effect of laundry greywater the best but were still prone to becoming polluted by sodium and saline. Plants struggle to flourish in such soils, and it would exacerbate drought stress in the dry months, says Dr Hardie.
Based on these results it is recommended that powdered laundry detergent greywaters are not applied to soils. Liquid laundry detergents are substantially less damaging to soils, but the short-term gains in terms of irrigation need to be weighed up against the accumulated negative effects of repeated greywater application on soil quality and longterm drought resilience.
From unpublished research that they have conducted, Dr Hardie says it is far better for the soil (and therefore also for plants) to reuse shower, bath or kitchen sink water, because these sources tend to contain less aggressive soil degrading chemicals.
Funding permitting, Dr Hardie and her co-investigators would like to extend the current research to find affordable ways on how to fix soils that have already been degraded by laundry greywater.
- First comprehensive study on effect that laundry greywater has on soils.
- Soil degradation sets right from the start.
- Soil pores are blocked. In due course water cannot soak into the soil. Very little reaches plant roots. Run-off increased.
- Liquid washing detergent is better to use than washing powder if you really want to use laundry greywater to water plants.