Effects of Earthworms on Olive Mill Wastewater Percolate and Soil Enrichment

Purpose: Olive mill wastewater (OMWW) is produced in large quantities in several oil-producing countries and is of particular concern due to its harmful long-term effects on soil fertility. Although OMWW is phytotoxic, it contains numerous nutrients suitable for reuse, particularly in agriculture. We hypothesized that the activity of earthworms, digesting OMWW and excreting the products into the soil, reduces the load of toxic organics and harmful effects of OMWW by the "vermicomposting" process, and increases soil fertility and nutrient availability. Methods: OMWW was diluted (at concentrations of 0%, 12.5%, and 25% of the original OMWW concentration) and added to a clay-loamy soil cultivated with durum wheat (the eastern Algerian variety) containing 20 adult individuals of the Eisenia fetida earthworm species. Based on the infiltration-percolation technique and the “vermicomposting” bioremediation, we analyzed the OMWW utilization by the earthworms and nutrient release. Results: The concentration of 25% OMWW with earthworms had the highest contents of nitrogen, phosphorus, and exchangeable cations (Ca2+, K+, and Na+). The biological oxygen demand increased 11-fold compared to the control soil (without earthworms), while the electrical conductivity increased from 1.3 ± 0.2 mS cm−1 to 2.7 ± 0.9 mS cm−1 in the presence of earthworms. Earthworms reduced organic matter content in soil with OMWW percolates by 25% as compared to the control soil (without earthworms). Conclusion: The presence of earthworms raises the availability of nutrients in soil and percolates and increases sodium and phosphorus contents compared to earthworm-absent controls. The concentration of 25% of OMWW in the presence of earthworms increased the nutrient availability in the soil. © 2025 Elsevier B.V., All rights reserved.