Technical Yearbook 2024

MARCH

Treated municipal wastewater for irrigation (Part 1): Irrigation application and water quality

By Carolyn Howell, Karla Hoogendijk, Philip Myburgh, Vink Lategan & Eduard Hoffman

Abstract Low annual rainfall, limited supply of fresh water that can be stored on farms and water restrictions imposed by authorities during drought highlighted the necessity for alternative water sources for vineyard irrigation for the South African wine industry. Therefore, the impact of irrigation with treated municipal wastewater (TMW) on soil and grapevines was assessed under field conditions in vineyards in the Coastal region of South Africa. Grapevines were uninterruptedly irrigated using TMW from the City of Cape Town over 11 years. Grapevines were either rain-fed, irrigated with TMW via a single dripper line, or received twice the volume via double dripper lines. As expected, the quality of the TMW used for the irrigation of the vineyards was below the minimum criteria stipulated by the General Authorisations to irrigate with wastewater in terms of pH, electrical conductivity (ECw), chemical oxygen demand (COD), faecal coliforms and sodium adsorption ratio (SAR). The mean sodium (Na+) concentration in the TMW exceeded the critical value of 100 mg/L for irrigating grapevines in South Africa. However, chloride (Cl-) levels were well below the threshold value of 700 mg/L, at which toxicity in grapevines could occur. The low N content in the TMW could not supply the annual N requirement of the grapevines. The annual amount of P applied via the single dripper lines was slightly below grapevine requirements, whereas double the TMW irrigation applied excessive amounts of P. Amounts of K+ applied via TMW irrigation were more than annual grapevine requirements for the double dripper line treatment, which could affect wine quality negatively. The amount of Ca2+ and Mg2+ applied via the TMW also exceeded annual grapevine requirements. Regular analyses of TMW are essential when using it as an alternative water source for vineyard irrigation. This will ensure that the legislated limits given in the General Authorisations are adhered to and that the chemical load conforms to recommended thresholds and norms. This study’s objectives were to assess the quality of treated municipal wastewater (TMW) used for irrigation of commercial vineyards and to quantify the amount of plant nutrients applied via TMW irrigation. In this regard, the study formed part of a long-term project to assess the sustainability of using TMW for vineyard irrigation in the Coastal region of the Western Cape.

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TECHNICAL YEARBOOK 2024