Bio-based production of medium-chain carboxylic acids from food waste and sludge without chemical addition: The pivotal role of mix ratio and pretreatment

Medium-chain carboxylates such as caproic acid, have a plethora of applications, ranging from food additives to bioplastics, and can be produced via microbial chain elongation (CE), a process that can be more sustainable than conventional production routes. During CE short fatty acids accept electrons from donors as lactate or ethanol elongating into medium-chain carboxylates with higher economic value and easier recoverability. Nowadays, waste activated sludge (WAS) and food waste (FW) are the most abundant waste steams generated in European cities impacting dramatically on environment, society, and economic sectors. A novel sustainable biotechnology is here proposed to produce marketable caproic acid from mixed WAS and FW with in-situ self-formed lactate without buffering agents addition. Different mix ratios between WAS and FW, and thermal pretreatment strategies were compared by fermentation batch tests. The production of lactate as electron donor (ED), together with acetate and butyrate, and hydrogen assured high production of caproate only in presence of thermal pretreated WAS, able to reduce the potential risk of lactate accumulation and drop in pH. The findings of this study demonstrate that the initial soluble carboydrate/protein ratio significantly affected the pH of the fermentation broth in the first hours of fermentation, and that only using FW and pretreated WAS mixed using a ratio of 60/40 in terms of volatile solids, CE pathway and lactate utilization as ED were promoted obtaining the highest percentage of caproate (about 21% of the total produced carboxylic acids; 2 gCOD/L).