Effect of loading rate and pH on glycerol fermentation and microbial population in an upflow anaerobic filter reactor

A reactor with silicone tubes as support medium was used for glycerol fermentation. The experimental set-up consisted of three phases. In P1, the applied glycerol loading rate (gly-LR) was in the range of 6-10 g.L^-1.d^-1 at an influent pH of 7.9 ± 0.4. In P2, gly-LR was kept constant (18.0 ± 1.8 g.L^-1.d^-1) with different doses of NaHCO₃. Finally in P3, two different gly-LR (9 and 18 g.L^-1.d^-1) were evaluated, dosing 1 g-NaHCO₃ per g-COD of glycerol. Glycerol consumption was close 90%. The main end-product was 1,3-propanediol (1,3-PDO) (0.40 mol.mol-gly^-1), but ethanol was also generated, particularly at pH above 8 and low gly-LR (0.20 mol.mol-gly^-1). After 1-year operation with glycerol as the only carbon source, a drastic shift in the bacterial community was observed. The 1,3-PDO producers Lacrimispora and Clostridium became dominant, although non-glycerol-degrading fermentative genera, e.g., Actinomyces and Eubacterium, thrived at the expense of cellular breakdown products.