Syngas biomethanation by co-digestion with brewery spent yeast in a lab-scale reactor

Abstract

The achievement of the current decarbonization goals requires optimizing biomass conversion processes to reduce processing costs and unlock the valorization of a wide array of feedstocks. Gasification enables exploiting recalcitrant biomasses to produce syngas which can be upgraded to biomethane. In this context, the possible integration of syngas biomethanation and waste treatment into a single unit is particularly relevant. This study investigates the methanation performance of the co-digestion of syngas (45% H2, 20% CO, 25% CO2, 10% N2) and brewery spent yeast (BSY) in a lab-scale continuously stirred tank reactor (CSTR) operated for 85 days, with the aim of evaluating the microbial culture capacity to simultaneously convert syngas and BSY and the stability of the process over different stages. Adaptation of mixed microbial cultures was performed both in batch bottles and by continuous feeding of syngas in a CSTR reaching after 44 days a methane productivity of 150 mL·L−1·d−1 and a 27% degradation of the initial COD. The adapted culture in CSTR was then subjected to a continuous syngas load of 1.13 L·L−1·d−1 and 0.55 gVS·L−1·d−1 of BSY, reaching methane productivity of up to 233 mLCH4·L−1·d−1. The H2 and CO conversion efficiencies were on average over 80% towards the end of the adaptation stage and during the 21 days of the continuous liquid feeding stage. While sulfur limitation inhibited methanogenic activity after 21 days of co-feeding of syngas and BSY, satisfactory process stability was achieved and efficient syngas conversion by microbial cultures could be demonstrated in concomitance with a considerable organic load reduction during the previous stages. The study demonstrates that BSY is a valuable substrate for syngas co-digestion, with a high potential for process scale-up under optimized operating conditions. Future work should optimize the methane productivity especially by focusing on secondary nutrients management.