From sewage sludge and agri-food waste to VFA: Individual acid production potential and up-scaling

Volatile fatty acid (VFA) production through anaerobic fermentation may constitute an innovative solution for organic waste management within the context of circular economy. In the present study, the evolution of individual VFA during laboratory-scale fermentation of sewage sludge (SS), winery wastewater (W_ww) and meat and bone meal (MBM) was assessed, focusing on the effect of pH (5.5 and 10) and temperature (35 and 55 °C). Up-scaling of the fermentation process was evaluated in batch operation. The latter showed that specific VFA could be produced, giving similar individual evolution to lab-scale testing. To be precise, acetic acid percentage ranged within 30-65% and increased up to 5900 mg O₂ L^-1 during SS fermentation at 55 °C and pH 9. In addition, 60% butyric acid was reached during W_ww acid fermentation at 55 °C, which corresponded to 6670 mg O₂ L^-1 concentration in the fermentation broth. Regarding valeric acid, over 20% proportion and 2700 mg O₂ L^-1 were reached in MBM acid fermentation at 35 °C. Finally, iso-valeric maximum level ranged within 15-17% in SS alkaline fermentation at 55 °C, which represented a concentration close to 2000 mg O₂ L^-1. Interestingly, co-fermentation of agri-food waste and SS at thermophilic temperature and alkaline pH, boosted the VFA concentration 1.7-2 fold, which suggests that anaerobic co-fermentation of substrates from different nature could give promising outcomes in full-scale operation.