Effect of pH, substrate loading, oxygen, and methanogens inhibitors on volatile fatty acid (VFA) production from citrus waste by anaerobic digestion

Citrus waste from e.g., juice production is a potential substrate for anaerobic digestion (AD). However, due to the toxic citrus peel oil content, citrus waste has several challenges in biogas production. Hence, volatile fatty acids (VFAs) are very interesting intermediate products of AD. This paper was aimed to investigate VFA production from citrus wastes by boosting its production and inhibiting methane formation. Therefore, the effects of inoculum to substrate ratio (ISR), O₂ presence, pH, and inhibitor for methanogens, in VFA production from citrus waste through acidification process were studied. The addition of 2 g/L methanogens inhibitor and the presence of O₂ in the reactors were able to reduce methane production. The highest yield of VFA (0.793 g VFA/g VS_added) was achieved at controlled pH at 6 and low substrate loading (ISR 1:1). Acetic acid (32%), caproic acid (21%), and butyric acid (15%) dominate the VFA composition in this condition.