Effects of combined ultrasonic and grinding pre-treatments on anaerobic digestion of vinegar residue: organic solubilization, hydrolysis, and CH4 production

Abstract

ABSTRACT The high lignocellulose content of vinegar residues (VR) limits their biochemical methane potential (BMP) in anaerobic digestion (AD). However, unlike reported high cellulosic materials such as straw and grass, single pre-treatment with ultrasonication or grinding only slightly improved VR AD, due to the high protein and carbohydrate contents of VR. This study used statistical analysis to show that the methane yield, protein and polysaccharide release, and hydrolysis performance during VR AD were significantly enhanced with a combined grinding-ultrasound pre-treatment. Specifically, at 60 min of ultrasonic, the group with the combined pre-treatment (60 min + RS) showed the highest VR BMP (∼307.1 mLCH4/gVS), 68.7% greater than that in the control group. This group also exhibited optimal conditions for dissolution of polysaccharide and protein, with accumulated amounts of ∼500 and 1600 mg/L, respectively. The highest volatile fatty acid (VFA) concentration in the 60 min + RS group was 61.5% higher than that in the control group. Both dissolution and hydrolysis experiments suggested that ultrasound accelerated protein release from VR, particularly after the particle size was reduced, and that the grinding pre-treatment had a positive effect on polysaccharide release.