Enhancing volatile fatty acid production using arrested anaerobic digestion of food waste and high salinity glycerol sludge

This study quantified volatile fatty acid (VFA) production using semi-continuous, arrested anaerobic digestion (aAD) of food waste (FW) and high salinity glycerol sludge (HSGS) derived from plant-based biodiesel processing. FW and HSGS were tested at five ratios (100:0, 75:25, 50:50, 25:75, 0:100). The alkaline pH (9 - 10) conditions and high organic load (6.3 - 17.8 g VS/L-day) with HSGS inclusion resulted in higher VFA production, despite the high salinity (214 g/L NaCl) of HSGS. The VFAs produced from mono-aAD of HSGS (36.04 g/L VFAs) were significantly higher than aAD of FW (9.29 g/L), with the highest VFA concentration observed on Day 51. The FW-only treatment produced the most VFAs per unit of VS loaded during steady state (0.65 - 1.24 g VFA/g VS), yet the wide range of salinity concentrations (53.5 - 214 g/L NaCl) with FW and HSGS co-aAD did not significantly influence VFA production efficiency (p > 0.05). The inclusion of HSGS increased VFA production and mitigated acidic inhibition during aAD of FW. No potassium hydroxide (KOH) additions were needed to buffer acidic conditions during aAD when HSGS inclusion was greater than 50%, and KOH additions with 75:25 ratio of FW:HSGS was 95% less than FW-only. HSGS inclusion drove a shift from a community dominated by Olsenella and Methanobrevibacter to communities with higher diversity and a higher proportion of acetic acid producing fermenters. This work showed that HSGS inclusion increases VFA production when used singularly or with FW, with high VFA concentrations available for processing into bioplastics, bioenergy, or pharmaceuticals.