Effects of single and combined olive mill wastewater and olive mill pomace on the growth, reproduction, and survival of two earthworm species (Aporrectodea trapezoides, Eisenia fetida)

The extraction processes of olive oil produce huge amounts of by-products, such as olive mill wastewater (OMWW) and olive mill pomace (OMP), these by-products have significant polluting properties related to their undesirable color and odor, high organic load, acidic pH, high phenolic compounds, high salinity, while their effect on soil organisms is unclear. The study into the effects of these by-products on soil bioindicators, such as earthworms should therefore be prioritized. Indeed, the purpose of this study was to evaluate the effect of environmentally realistic concentrations of olive mill wastewater and olive mill pomace on the growth, reproduction, and survival of the earthworms Aporrectodea trapezoides, and Eisenia fetida. Bioindicators were exposed under laboratory conditions to increasing doses (12.5%, 25%, 50%, 75%, and 100% w/w) of OMWW and OMP separately and in combination. The results showed a higher growth rate on Eisenia fetida when exposed to 12.5% OMP, with a neutral effect on reproduction and survival. Toxicological effects were found at higher doses, namely: there was no cocoon production, growth inhibition, and mortality; these effects were probably due to the higher level of phenolic compounds and higher salinity. In general OMP was found to have toxicity higher than OMWW, the combination of both by-products showed stronger effects on some earthworm endpoints. Comparative responses between the two species showed that Aporrectodea trapezoides proved to be more tolerant. Dilution and bioremediation of these by-products is recommended as a possible solution to reduce their toxicity.