Ethanol pre-fermentation of food waste in anaerobic digestion to enhance biogas production and digestate quality

Abstract

Ethanol pre-fermentation (EP) of substrates with yeast before anaerobic digestion (AD) mitigates organic overload during acidogenesis by favoring ethanol formation over volatile fatty acids. However, studies on EP-treated food waste (EP-FW) often overlook the influence of the substrate-to-inoculum ratio (SIR) on digester stability and biogas yield. While the biofertilizer role of yeast is recognized, the fertilizer potential of the resulting digestate remains underexplored. This study examined a two-stage system comprising EP followed by AD. During EP, yeast diverted approximately 110 mg carbon/g VS (22.6%) to ethanol and carbon dioxide, achieving volatile solids (VS) and chemical oxygen demand (COD) removal efficiencies of 29.7% and 10.2%, respectively, relative to the control. In the subsequent AD stage, eight SIR ratios (0.25–2) were tested. The treatment digesters received EP-FW, and the controls used non-fermented FW. The digester with an SIR ratio of 0.5 had the highest biogas and methane yield (517 ± 7 Nml/g VS and 310 ± 2 Nml/g VS) representing 25.3% and 25.8% increases over the control. This optimal ratio enhanced the removal efficiency of COD (9.7%) and VS (9.9%) by maintaining digester stability. 0.5EP-FW enhanced nutrient solubilization, increasing nitrogen, phosphorus, and potassium concentrations by 49%, 40%, and 146%, respectively, and produced a stable, agronomically safe, non-phytotoxic digestate (∼100% germination; GI ∼ 3.0–3.5), while the non-fermented group showed variable toxicity and reduced germination at higher loadings. Overall, EP with optimized SIR, enhanced methane recovery, stability, and agronomic digestate quality, adding scientific value to food-waste valorization in a circular bioenergy-biofertilizer system.