Evolution of the fatty fraction during co-composting of olive oil industry wastes with animal manure: Maturity assessment of the end product

Olive mill wastewater sludge, resulting from the natural evaporation of olive oil processing effluent, was co-composted with poultry manure and changes in the lipid fraction investigated. Composting was achieved after approximately 9 months, leading to a compost with high stability and maturity (C/N ratio: 11.9; cation exchange capacity (CEC): 85.9 meq 100 g−1 organic matter, CEC/total organic carbon: 4.2 meq g−1; humic acids carbon/fulvic acids carbon: 2.2) useable directly in agriculture and having the same fertilizing capacity as farmyard manure.Composting led to a reduction in the lipid fraction by at least 95%. Unsaturated fatty acids, particularly polyunsaturated acids, were the most degraded (reduction of 55%) leading to an increase in saturated fatty acids. This change was confirmed by the relative increase in the peroxide index from 5 to 32.5 meq O2 kg−1 fats, and a decrease in the C18:2/C16:0 ratio from 0.9 to 0.3. In addition, this study demonstrated that 1.2% of the humic acids component of the compost comprised fatty acids.