Vermicomposting of two-phase olive mill waste with Eisenia fetida to reduce the environmental impact of olive oil production and produce a high-quality soil amendment

Olive oil production is integral to Mediterranean culture and economy. Yet, the disposal of olive mill waste (OMW) poses major environmental risks due to its phenolic content, low biodegradability, and phytotoxicity. Vermicomposting using Eisenia fetida offers a sustainable method to convert two-phase OMW into a nutrient-rich soil amendment, while reducing its toxicity. This study evaluated the feasibility of vermicomposting OMW mixed with dairy cow manure to produce a high-quality soil amendment without impairing earthworm performance.

A completely randomized design with four treatments and four replicates was used, testing OMW-to-manure ratios of 0:100 (control), 33:67, 50:50, and 67:33 as a feedstock in 20x20x9cm plastic containers. Each container received ten mature E. fetida earthworms and was incubated for one month at 21°C and 75% moisture. Parameters monitored included electrical conductivity (EC) and pH, while phytotoxicity was assessed by germination of lettuce seeds exposed to extracts of the resulting vermicompost.

Vermicompost pH was 7.7 across treatments, while EC increased with increasing OMW concentrations, ranging from 685 μS/cm (control) to 1062 μS/cm (OMW-67%). Earthworm biomass increased in all OMW treatments relative to the control. However, cocoon production declined with higher OMW content, suggesting temporary reproductive inhibition. During a subsequent recovery phase in which earthworms were transferred to dairy manure, biomass remained similar across treatments, and cocoon production increased above control levels, demonstrating a temporary rather than permanent reduction in reproductive activity. Lettuce seed germination exceeded 90% in all treatments (91-93%), indicating low residual phytotoxicity.

These findings indicate that OMW can be effectively converted into a high-quality soil amendment through vermicomposting when mixed with cow manure, while maintaining viable, reproductively active earthworm populations. This process offers a practical and environmentally sound approach to managing a major agro-industrial byproduct in Mediterranean regions, supporting circular economy practices and sustainable agricultural development.