A circular economy approach using a dual waste system of coal fly ash and food waste to optimise biodegradability of lignocellulosic agricultural residues for enhanced methane production in anaerobic digestion

Lignocellulosic agricultural residues (LARs) represent a largely untapped resource for renewable energy production, particularly through anaerobic digestion (AD), but their recalcitrance limits biodegradability and methane (CH4) yield. Lignin surrounds cellulose and hemicellulose, acting as a barrier to microbial hydrolytic enzymes and limiting microbial access to these compounds as carbon sources. This study investigated a novel integrated approach utilising several waste materials to maximise the AD of LARs, using barley straw (BS), coal fly ash (CFA) for alkali pretreatment and food waste (FW) for acid neutralisation. AD experiments in the solid-state system were conducted with two variations of alkalinity (CFA) and acidity (FW) to optimise process efficiency. Alkali pretreatment using calcium oxide (CaO); as a principal alkalinity source of CFA, equivalent to ~5,000 mg calcium carbonate (CaCO3) L 1 was found to be optimal for pretreatment, leading to a significant change in fibre composition (e.g., 28% lignin reduction) and a 29% CH4 yield increase, compared to water-treated BS Control. Increasing alkalinity to ~11,000 mg CaCO3 L 1, however, did not result in significant differences in both fibre composition and CH4 yield. Interestingly, CFA pretreatment (~11,000 mg CaCO3 L 1) showed a 39% CH4 yield increase compared to the equivalent alkalinity CaO pretreatment. This suggests that unique properties in CFA, particularly conductivity, stimulated microbial interaction mechanisms presumably associated with direct interspecies electron transfer (DIET) and a more efficient indirect interspecies electron transfer (IIET). High alkali-pretreated BS was effectively neutralised by 20% volatile solids (VS) fermented FW, maintaining ideal pH for efficient CH4 conversion. The study concludes that integrating CFA and FW in AD of LARs reduces reliance on synthetic chemicals and potentially lowers pretreatment costs. Moreover, this approach supports circular economy principles by valorising organic and industrial waste streams, offering a sustainable solution for waste management and renewable energy production.