Thermophilic anaerobic co-digestion of glycerol and cheese whey – Effect of increasing organic loading rate

Abstract

The organic loading rate (OLR) is one of the variables that directly affects anaerobic digestion. An increase in OLR can improve methane production, but it can also lead to process failures. Therefore, determining the proper OLR for the process is of great importance. This study analyzed the effect of increasing the OLR on cheese whey and glycerol co-digestion in a thermophilic anaerobic fluidized bed reactor (AFBR) by varying both the influent concentration and the hydraulic retention time (HRT). The increase in OLR from 5 to 20 gCOD.L−1.d−1 was initially performed by increasing the influent concentration from 5 to 7.5, 10, 15, and 20 gCOD.L−1, at HRT of 24 h. Organic matter removal (87.7%), methane yield (MY) (253.0 mL CH4.gCOD−1rem), and methane production rate (MPR) (3.2 L CH4. L−1.d−1) showed the best performance at 10 gCOD.L−1.d−1. Conversely, raising the OLR to 20 gCOD.L−1 enhanced the buildup of volatile fatty acids, resulting in a 38.5% decrease in MY. Based on these findings, the HRT was reduced from 24 h to 20 h and 16 h at influent concentration of 10 gCOD.L−1 resulting in OLR of 10, 12, and 15 gCOD.L−1.d−1, respectively. The HRT at 20 h (12 gCOD.L−1.d−1) provided the highest MY values (around 292.5 mL CH4.gCOD−1rem), while the maximum MPR was observed at 16 h (5.1 L CH4. L−1.d−1). The main metabolites observed in all conditions were acetic acid (31.1–66.8%) and propionic acid (10.9–61.5%). The OLR also affected the microbial community, with Proteobacteria accounting for 53.1% of the relative abundance at 5 gCOD.L−1.d−1 while Firmicutes accounted for 76.6% at 10 gCOD.L−1.d−1. At 10 gCOD.L−1.d−1, with maximum MY, the bacteria observed with the highest relative abundance were an unidentified genus of Lentimicrobiaceae (8.9%), “Blvii28 wastewater sludge group” (2.9%), and Tolumonas (1.4%). The archaea identified with the highest relative abundance at both OLRs (5 and 10 gCOD.L−1.d−1) was hydrogenotrophic Methanobacterium (27.3% and 2.6%, respectively).