Extent of inhibition and utilization of volatile fatty acids as carbon sources for activated sludge microbial consortia dedicated for biodiesel production

The extent of inhibition and utilization of volatile fatty acids (VFAs) acetic acid, propionic acid, and butyric acid fed to activated sludge was studied using inhibition test and microbial oil accumulation. The effects of total VFAs loading and mass fraction of acetic acid, propionic acid and butyric acid in the VFAs mix were found significant factors on the inhibition on activated sludge. Almost complete inhibition occurred at total VFAs loading 10 g/L and 20 g/L. These trends were confirmed using microbial oil accumulation experiments at the regions of maximum and minimum inhibition. The fatty acid methyl esters (FAMEs) profiles from the microbial oil extracts were suitable for biodiesel production. The individual effects of pH and acid concentration were also tested and found as significant contributing effects into the inhibition. To demonstrate the improvement in microbial oil accumulation of activated sludge a fed-batch feeding of acetic acid via acetic acid-sodium acetate buffer was done in 5-L bioreactor. The acetic acid fed was consumed, activated sludge biomass growth was continuous, and microbial oil increased. These findings indicated that VFAs loading and composition must be accounted in design of an activated sludge microbial system dedicated for biodiesel production operating on VFAs platform.