Greywater (GW) reuse for irrigation is a common method of reducing domestic consumption of fresh water. Most of the scientific research and legislation efforts have focused on GW's health risks, while less attention has been given to its environmental outcomes. One of the environmental risks of GW irrigation is its possible effect on soil hydraulic properties. This research examined the ability of GW to induce soil hydrophobicity, as well as its degree and persistence. Fresh water (control) and three model GW solutions representing raw, treated and highly treated GW were used to wet fine-grained sand. Every treatment was subjected to five cycles of wetting, incubation (at 5 °C or 30 °C) and drying (60 °C). After each cycle, capillary rise was measured and the contact angle (CA) was calculated. Samples were also tested by the Wilhelmy plate method to retrieve advancing and receding CA and reservoir surface tension. Water repellence of the sand, as implied from the CA, increased with increasing GW concentration and was highest in the sand coated with the model raw GW and incubated at 5 °C. However, none of the treatments resulted in what is considered to be “water-repellent soil”. Furthermore, when raw GW-coated sand was immersed in water, its surface tension was significantly reduced relative to the other treatments, implying a release of surface-active compounds from the sand into the water. It was postulated that untreated GW may induce sub-critical water repellence in sand. However, this effect is sensitive to biodegradation and washing processes and is therefore temporary.
What are hydrophobic surfaces?