Innovative use of ash additives in anaerobic co-digestion: Optimizing methane production from agro-industrial wastes within a circular economy framework

This study investigates the impact of adding residual ashes in anaerobic co-digestion of olive mill wastewater (OMWW), poultry manure (PM) and cheese whey to boost methane production. It evaluates the effects of olive pomace bottom ash, slaughterhouse waste incineration bottom ash and biochar on methane yield. These specific biomass ashes are utilized for the first time in anaerobic digestion. Experiments were conducted in two phases: (1) anaerobic digestion of agro-industrial waste and (2) effect of ash and biochar on anaerobic digestion. Kinetic models including Logistic function (LF), Modified Gompertz (MG) and Reaction curve-type were used to analyse production dynamics. The results revealed that the optimal substrate ratio was the one with highest content of OMWW (60%) that produced 447.5 mlCH₄ gVS^-1. Second phase showed the highest methane yield of 688.5 mlCH₄ gVS^-1 from slaughterhouse ash at 0.75g gVS^-1, followed by 612.5 mlCH₄ gVS^-1 with biochar at 0.50 g gVS^-1. In contrast, olive pomace ash generally yielded lower methane production compared to control, likely due to high chloride content and potential polycyclic aromatic hydrocarbons. MG and LF explain better the experimental data. The findings underscore the potential of anaerobic co-digestion to valorize agricultural and agro-industrial waste, advancing sustainable energy production and waste management strategies in alignment with circular economy principles.