Use of earthworms (Eisenia fetida) to reduce phytotoxicity and promote humification of pre‐composted olive oil mill wastewater

BACKGROUND: Olive mill wastewaters (OMWW) contain a high recalcitrant organic load and an associated toxicity that make their treatment necessary before environmental application. The aim of this study was to evaluate the feasibility of promoting the valorization and reducing the phytotoxicity of OMWW through a pre‐composting process together with straw‐chip bulking materials followed by the application of earthworms (Eisenia fetida) in the presence of oat seedlings (Avena sativa L.) seedlings. RESULTS: After 3 months, the pre‐composted material showed properties similar to a partially digested compost with some significant amelioration of chemical–physical and biochemical properties. The application of earthworms permitted a significant decrease in chemical (total organic carbon, water‐extractable organic carbon, total nitrogen) and biological parameters (dehydrogenase enzyme activity), and an increase in humic substances and available nitrogen forms. In the presence of plants a higher C/N ratio and a lower content of nitrates were observed. In addition, the reduction in phenolic compounds observed in treatments with earthworms caused a decrease in phytotoxicity by about 50% with respect to the pre‐composted material, which results in an increase in germination index. CONCLUSION: The utilization of earthworms, in particular in the presence of plants, may be an ecologically sound and economically feasible technology to obtain a non‐toxic, high‐value product useful for agricultural purposes. Copyright © 2010 Society of Chemical Industry