Kinetics of fatty acid degradation by psychrophilically grown anaerobic granular sludge

The kinetic parameters of anaerobic granular sludge, grown at 10–12°C in an expanded granular sludge bed (EGSB) reactor and fed with volatile fatty acid (VFA) mixtures, were determined in batch assays. The homoacetogenic, hydrogenotrophic and acetoclastic specific activities of the sludge at 10°C, measured after 306 days exposure to psychrophilic conditions were 1.744, and 0.296 and 0.331 g COD g−1 volatile suspended solids (VSS) day−1, respectively. At 30°C these values were 18.024, 2.732 and 2.204 g COD g−1 VSS day−1, respectively. The temperature characteristics of the granular sludge and the homoacetogenic enrichment culture from cultivated sludge showed that the temperature optima were still in the mesophilic range (30–40°C) even after long term (306 days) operation at low temperature (10–12°C). In the overall conversion rate for acetate and propionate, evidence for the temperature compensation effect was found. In contrast, no temperature compensation effect was present for butyrate degradation. The results of EGSB batch reactor experiments revealed apparent Km of the acetate and propionate degraders in the ranges of 39–58 mg COD l−1 and 7–14 mg COD l−1, respectively. For butyrate degraders, higher Km values were found, i.e., 142–243 mg COD l−1. The observed low Km values could be attributed to prevailing mixing conditions in the EGSB reactor system.