Olive trees are iconic to the Mediterranean landscape and in recent times, have expanded to other regions across the globe that share similar climatic conditions. Olive oil production benefits from irrigation, but with a changing climate and uncertainty in precipitation patterns, wastewaters will likely play a larger role supplementing irrigation water requirements. However, due to their relatively poor quality, wastewaters present challenges for sustained long-term use in olive production. Wastewaters include all effluents from municipalities, agricultural drainage, animal production facilities, agricultural processing and industrial processes. This review focuses on potential opportunities and limitations of sustaining olive oil production in the Mediterranean region using wastewater of various sources. The primary challenges for using such wastewaters include concerns related to salinity, sodicity, metals and trace elements, nutrients, organics, and pathogens. Organics and plant nutrients in the effluents are typically beneficial but depend on dosages.
Many studies have shown that saline wastewaters have been successfully used to irrigate olives in Greece, Israel, Italy, Jordan and Tunisia. Still, olive varieties and rootstocks have different tolerances to salinity and could respond differently and oil quality may improve or be compromised. Salts and trace elements need to be monitored in plants and soil to make sure accumulation does not continue from year to year and that soil physical conditions are not affected. Some food industries generate effluents with suitable characteristics for irrigation but one must balance the benefits (e.g. addition of nutrients), detriments (e.g. addition of salts or other limiting chemicals) and costs when determining the feasibility and practicality of reuse. Long-term accumulation of trace elements and metals will likely limit the feasibility of using industrial-originating effluents without treatment processes that would remove the toxic constituents prior to reuse. Therefore, untreated wastewaters from the many industries have limited long-term potential for reuse at this time. Application of olive mill wastewater may be agronomically and economically beneficial, particularly as a local disposal solution, but there are concerns associated with high-concentrations of polyphenols that may be phytotoxic and toxic to soil microbial populations.
With regards to human safety, risk of contamination of table olives and olive oil is very low because irrigation methods deliver water below the canopy, fruits are not picked from the ground, processing itself eliminates pathogens and the irrigation season typically ends days or weeks before the harvest (depending on the climate condition). Finally, considering physiological, nutritional and intrinsic characteristics of this species, it is clear that olive trees are appropriate candidates for the reuse of recycled water as an irrigation source.