An Integrated Management Approach for the Potential Reuse of Olive Pickling Waters in Olive Tree Irrigation

Olive pickling industries use large amounts of water in their production. Depending on the type of product, water usage can range between 1 and 6 m³ per ton of olives, with composition varying significantly depending on the processing step. Generally, the main issues with these waters are their high organic matter and salt content. They are mostly stored in evaporation ponds for treatment. In parallel, olive tree production faces serious threats from water scarcity due to climate change, especially in Mediterranean areas. Therefore, alternative water sources are needed for olive tree irrigation, allowing for the reuse of resources consumed by the sector and contributing to a circular economy.

This work presents an integrated management approach where olive pickling waters are automatically analyzed and stored, provided the electrical conductivity (EC) is less than 8 dS m⁻¹. These waters are subsequently used for precision irrigation of olive trees. The system is based on open-source hardware and software associated with an IoT platform, with units located both in the industry and the olive orchard. The industry unit automatically measures, analyzes, and controls the EC of the water produced, selectively separating it for irrigation. Meanwhile, the field unit monitors soil status (soil moisture and EC) and potential evapotranspiration to determine irrigation requirements.

The system has been operational for two years in an olive pickling industry in southwestern Spain, where most of the fruits are processed as Spanish-style green olives. During the experiment, almost 40% of the controlled waters had an EC below 10 dS m⁻¹, accounting for about 15,000 m³ of water that would otherwise need to be evaporated from a 2.5 ha pond. Instead, this water was used to irrigate more than twice as much land. Olive trees were irrigated with water having an EC of approximately 7 dS m⁻¹ over two seasons and compared with control trees that received no irrigation.

In the first season, deficit irrigation was applied, while in the second season, irrigation was based on crop evapotranspiration plus a 20% leaching fraction. In both seasons, higher crop yields (though not statistically significant), fruit weight (significant, p<0.05), oil content (significant, p<0.05), and pulp-to-stone ratio (significant, p<0.05) were observed. Soil EC significantly increased in the irrigated trees, reaching up to 1 and 1.5 dS m^-1 (1:5 extraction ratio) in the top 0.6 m after the first and second irrigation seasons, respectively.

Salt buildup in the soil indicates that medium-to-long-term sustainability of this type of irrigation must be considered, especially if natural rainfall is insufficient for adequate leaching. Soil modeling can be useful for assessing risks and deciding whether irrigation can continue in subsequent seasons. Nonetheless, using parts of the olive pickling waters for irrigation can be seen as a more sustainable alternative to evaporation for treating such waste.