Is physicochemical evaluation enough to characterize olive mill waste compost as soil amendment? The case of genotoxicity and cytotoxicity evaluation

Composting is considered a viable solution for olive mill waste. Among others, composting duration, bulking agents and their ratios are crucial factors that determine the quality of the final product. To examine optimal ratios and bulking agent types (i.e., olive leaves, rice husk, sawdust, wood shavings and chromium treated reeds), six parallel compost experiments were conducted with olive pomace. Experimental results indicate that all final composts presented similar physicochemical characteristics, while their values met EU legislation limits for soil amendments. Furthermore, the germination index (GI) in all final composts was above 100%, indicating the absence of any phytotoxic effect. Genotoxic and cytotoxic evaluation of the composts revealed that all initial mixtures presented cytotoxic effects. Genotoxic and cytotoxic effects were more intense during the thermophilic phase (all compost mixtures presented both genotoxic and cytotoxic effects). The majority of mature composts (102 days) did not have any genotoxic and cytotoxic effect. However, genotoxic and cytotoxic effects could appear if compost is not mature or specific materials (i.e., olive pomace and rice husk) have been used in excess. This work points out the significance of genotoxic and cytotoxic evaluation in olive mill waste compost, thus, indicating that similar evaluations should be performed for other compost materials deriving from agro-industrial and municipal wastes. Therefore, the revision of EU regulations is proposed to include genotoxic and cytotoxic evaluation of composts that enter the human food chain.