South Africa Wine Technical Yearbook 2025

JUNE

Winery wastewater irrigation (Part 5): Effect on soil K, Na and pH (KCl) By Reckson Mulidzi & Carolyn Howell

Abstract Due to environmental legislation, the South African wine industry needs solutions for wastewater treatment, or use thereof. The feasibility of using diluted winery wastewater (WWW) was assessed in a pot experiment under a rain shelter over four simulated irrigation seasons. Four soils varying in clay content were irrigated with WWW diluted to 3 000 mg/L chemical oxygen demand (COD), whereas the control received municipal water. The four different soils responded to irrigation with WWW containing relatively high levels of potassium (K + ) and sodium (Na + )

compared to the municipal water control. The rate of K + increase in the soil containing 20% clay was higher than in soils containing 13% clay or less. This suggested that heavy soils will aggravate the risk of high K + levels. The risk of Na + accumulation increased linearly with clay content. Irrigation with diluted WWW increased soil pH (KCl) substantially in all soils over four simulated seasons. The soil pH increase was attributed to the addition of organic/bicarbonate salts to the soil. It must be noted that the results represent a worst-case scenario, i.e. in the absence of rainfall or crops.

Introduction Increased wine production in South Africa is putting more pressure on natural resources. Due to the intensification of legislation pertaining to the environment, 1 the wine industry needs to find solutions for the treatment of winery wastewater (WWW) or use thereof. 2 This initiated the development of guidelines for the management of wastewater and solid waste at wineries. 3 A shortage of good quality water often leads to an increasing demand to irrigate with poor quality water, such as saline groundwater, drainage water and treated wastewater. 4 The effects of using industrial and municipal wastewater for agricultural irrigation and their impact on the soil are well-documented. 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 However, the impact of

soils where WWW was used for irrigation were attributed to structure degradation caused by high Na + concentrations added to the soil. 17 This was confirmed when irrigation with diluted WWW reduced the hydraulic conductivity of differently textured soils. 18 Using WWW for irrigation could also result in K + accumulation in the soil, resulting in the leaching of Ca 2+ and Mg 2+ and increasing the instability of the soil structure in the long run. 11 Since K + has affinity for clay minerals, high soil K + can cause clay swelling and dispersion where wastewater is used for irrigation. 12 Similar to Na + , K + in WWW can reduce soil hydraulic conductivity. 19 However, knowledge regarding negative effects of K + on soil structure stability is limited compared to Na + .

irrigation with treated wastewater on the environment has not been widely investigated. 15 Replacement of calcium (Ca 2+ ) and magnesium (Mg 2+ ) by potassium (K + ) and sodium (Na + ) during irrigation can potentially lead to the breakdown of the soil structure. Exchangeable soil Na + tends to increase where wastewaters containing high levels of Na + are used for irrigation. 16 Where wineries use Na + -based cleaning agents, e.g. sodium hydroxide (NaOH), accumulation of monovalent cations on the exchange sites has the potential to degrade soil structure through clay dispersion and flocculation. 11 Consequently, soil hydraulic conductivity can be reduced where WWW is used for irrigation. 13 Indications of poor soil aeration and water infiltration observed in various

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TECHNICAL YEARBOOK 2025