Inhibition by fatty acids during fermentation of pre-treated waste activated sludge

Fermentation of waste activated sludge produces volatile fatty acids (VFAs), which can be used as the carbon sources for numerous biological processes. However, product inhibition can limit extent of fermentation to VFAs. In this study, product inhibition during fermentation of waste activated sludge pre-treated by a thermal hydrolysis process (THP-WAS) was investigated. Product inhibition was confirmed as spiking reactors with high levels of a mix of VFAs prevented fermentation taking place. Various inhibition models were trialled and it was found that a threshold model (based on thermodynamics) provided the best fit between model and data. This is the first time that threshold type inhibition has been shown for a mixed substrate, mixed population system. Batch fermentations carried out with THP-WAS of different dilutions were used to evaluate the impact of different organic loadings. The threshold VFA concentration for the systems studied was determined to be 17 ± 1 gCODVFA L−1. Inhibition was shown to be due to the presence of a combination of VFAs containing 2–6 carbon atoms each. When evaluated individually, by spiking individual VFAs, all VFAs except for acetate had the same impact at this threshold; acetate being approximately 50% as inhibitory as the other organic acids (COD basis). Based on this, a weighted model could be proposed to better represent the data. Strategies to improve overall yield could be increased production of acetate, or dilution to below the inhibitory level.

► Product inhibition restricts fermentation of pre-treated waste activated sludge.
► A threshold model (based on thermodynamics) best describes fermentation of THP-WAS.
► Threshold VFA concentration for THP-WAS fermentation is about 17 gVFA(COD)/L.
► Product inhibition is due to a combination of VFAs (C2-C6).
► All VFAs except for acetate have the same impact; acetate is 50% as inhibitory.