Sulfur-containing microbial protein production from sulfide-rich biogas in hollow fiber membrane bioreactors

Anaerobic digestion of organic waste can unlock the upcycling of waste resources through the valorization of biogas into value-added products. This study proposed the conversion of sulfide-rich biogas into microbial protein (MP) through a microbial fermentation process carried out in hollow fiber membrane bioreactors by means of a consortium of methane- and sulfur-oxidizing bacteria, characterized by the dominance of Methylocystis spp. and Chryseobacterium spp. The efficiency of the gas-to-liquid mass transfer and the effects of biofilm formation on gas diffusion, as well as the effects of gas loading rate (10 L/d and 15 L/d) and sulfide content of the biogas (1500, 3000 and 4000 ppm of H2S) on the fermentation performance were evaluated. Biomass concentrations up to 768.5 (± 17.2) mg of volatile suspended solids (VSS) per liter were achieved when treating biogas containing 1500 ppm of H2S. Increasing sulfide concentrations (up to 4000 ppm of H2S) had negligible effects on the biomass productivity and yield. The biomass had a high protein (up to 64.7 %) and amino acids (up to 510.3 mg/g VSS) content, with beneficial effects of H2S on the content of sulfur-containing amino acids. Switching from 1500 ppm to 4000 ppm of H2S increased the content of methionine and cysteine by 57 % and 39 %, respectively.