High-rate in-vessel continuous composting of olive mill byproducts

The increasing production of agro-industrial organic residues in modern society is extremely concerning. One of the most polluting procedures in the agricultural industry is the production of olive oil. This process creates a large amount of waste with high organic load and phytotoxic components. In this study, composting of two-phase olive pomace (OP), olive leaves (OL) and dewatered anaerobic sludge (DAS) from an olive mill wastewater anaerobic digestion process was conducted in a pilot-scale in-vessel high-rate continuous composter. Five different feed scenarios were studied with different OP/OL ratio in the feed material, while the effect of the addition of pine tree bark pieces (PB) and DAS was examined. The OP:OL 95:5 % w/w ratio exhibited the best results in terms of product quality, while OL proved capable of acting as a bulking agent for the better aeration of the material. The final product in the optimum feed ratio was free of Salmonella spp., was stable in terms of static respiratory index (lower than 0.5 g O2 kg-1 VS h-1) but contained elevated E. coli levels (3.5 x 104 CFU g-1 with a limit of 1 x 103 CFU g-1), which was the only EU proposed compost quality criteria not met. The addition of a more easily degradable material in the feed mixture is expected to lead to elevated composting temperature and amend the presence of pathogens.

Automated summary

The increasing production of agro-industrial organic residues, particularly in the production of olive oil, poses a serious environmental concern. In this study, composting of olive pomace, olive leaves, and dewatered anaerobic sludge was conducted, and the best results were obtained with a 95:5% ratio of olive pomace to olive leaves. However, the final product did not meet the compost quality criteria in terms of E. coli levels, suggesting the need for further improvement.