Opportunities for expanding the use of wastewaters for irrigation of olives

Highlights

• Wastewaters present challenges for sustained long-term use in olive production.

• Organics and plant nutrients in the effluents are typically beneficial but depend on dosajes.

• Soil conditions can be affected if salts and trace elements are not monitored.

• Untreated wastewaters from the many industries have limited long-term potential for reuse.

• Health risk of table olives and olive oil is very low.

Abstract

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.

Introduction

Olive trees are a cultural and historical icon in the Mediterranean landscape. The cultivation of olive trees dates back thousands of years to the early Bronze Age and olives are frequently mentioned in Greek mythology (Connor, 2005; Liphschitz et al., 1991; Loumou and Giourga, 2013). Today, orchards in Spain, Italy, Greece, and Tunisia produce the vast majority of the world's olive oil supply. With the increased popularity in the use of olive oil for cooking and consumption, the global demand for this healthy oil continues to grow. And with this increase in popularity, olive oil production has, in recent times, expanded far beyond the Mediterranean region to California (UC Olive Center, 2018) and many countries in the southern hemisphere including Australia, Argentina, Peru, and Chile (Torres et al., 2017).

While olive orchards have thrived for centuries on rain alone (Connor and Fereres, 2005), olive oil production has become more dependent on irrigation, which has shown to improve oil production while sustaining quality (Dag et al., 2014; Orgaz and Fereres, 2004). However, in the Mediterranean region, like other semi-arid climates, water is scarce and climate change is expected to exacerbate water scarcity by adding uncertainty to precipitation patterns, producing more frequent and severe droughts and reducing the overall rainfall in the region (IPPC, 2013). With the uncertainty and likely future reduction of precipitation in the Mediterranean area, supplemental water supplies become not only attractive but a necessary option for the region to sustain its historical importance in olive oil production.

The likely source of the needed supplemental water for olive irrigation is recycled wastewater, much of which currently goes unused. Groundwater in the Mediterranean region is already over depleted, so a sustained increase in groundwater extraction from wells is unlikely, particularly in coastal areas experiencing salinization from seawater intrusion (Pedrero et al., 2018). There are many opportunities to utilize wastewaters from various sources such as treated wastewater from municipalities and wastewaters from agricultural production (e.g. olive mills, canneries, vineyards, food processing plants, drainage waters, etc.). Wastewaters, however, are typically poorer in quality than the water used to produce the waste. This poor-quality water can present challenges for sustained long-term use in olive production. Particularly problematic are industrial wastewaters from textile processing and oil production that have elevated heavy metal concentrations and/or organic fractions that pose health risks.

This review will address the potential opportunities and limitations of sustaining olive oil production in the Mediterranean region using wastewater of various sources. The review is divided into sections with specific categories identifying sources and their qualities that can either limit or enhance utilization of the water.