Kinetics and metagenomic analysis for assessment of biomethanation of gram crop waste in an anaerobic baffled reactor

The present study evaluated the potential of rarely-studied lignocellulosic agricultural residue- gram crop waste (GW) to serve as substrate for anaerobic digestion (AD). High amenability of GW to AD due to its high cellulose (39.37%) and hemicellulose (28.7%) content was clear from its biochemical methane potential (BMP) which showed the highest cumulative biogas and methane yield of 503 mL and 236 mL/g VS added respectively. Further confirmation by AD of GW in a four-chambered anaerobic baffled reactor (ABR) at a hydraulic retention time of 30 d resulted in cumulative methane yield of 6.89 L/g VS added during 120 days of reactor operation. Among the various kinetic models applied for validating the AD potential of GW, the first-order kinetic model displayed the best fit in batch studies, whereas the Logistic function model best fitted the biomethanation kinetics in the ABR. Correlation of the reactor performance with the activity associated microbial plasticity indicated the abundance of potent hydrolytic and fermentative genera Clostridium, Eubacterium, Bacteroides etc. in chamber C-I, acidogenic genera Atopobium, Olsenella, Prevotella, Bacillus, Propionibacterium, Acinetobacter in C-II, and acetogenic genera Acinetobacter, Sphingopyxis, Rhodopseudomonas in C-III. Among the members of phylum Euryarchaeota, the methanogenic genera Methanosarcina and Methanosaeta were dominant in C-IV in comparison to other chambers. Study provided deeper insights into the differential acclimatization of the microbial community in different chambers in presence of recalcitrant lignocellulosic substrate. This subsequently resulted in efficient biogas production by overcoming several short-comings associated with digestion of carbohydrate rich substrates in single stage digesters.