Anaerobic digestion of grape pomace: Effect of the hydraulic retention time on process performance and fibers degradability

To optimize the anaerobic digestion of grape pomace under mesophilic conditions, continuous digesters were operated at different hydraulic retention times (HRT) (30, 20, 15 and 10 days) equivalent to organic loading rates (OLR) of 2.5, 3.7, 5.7 and 7.3 kg COD m^-3 d^-1, respectively. At HRTs of 30 and 20 days, steady state conditions were observed with methane yields of 0.984 ± 0.013 NL d^-1 and 1.362 ± 0.018 NL d^-1, respectively. The HRT of 15 days was found critical because of acids accumulation through the experiments. When the OLR of 5.7 kg COD m^-3 d^-1 was reached, methane production was found to be instable. Finally, at HRT of 10 days, a failure of the system was observed due to the washing of the methanogenic microorganisms. Regarding the degradability of the lignocellulosic fractions, the maximum reduction yields for hemicellulose and cellulose were noted for HRTs of 30 and 20 days, while lignin was not degraded throughout the different experiments. For an optimization of the process, HRT of 20 days can therefore be recommended for productive use in large-scale applications.