title: Microbial protein production from sulfide-rich biogas through methane- and sulfur-oxidizing consortia
Creators: name: Areniello, Marica
Publisher: name: University of Galway
Year: 2024
Subjects: subject: microbial protein
subject: single cell protein
subject: biogas
subject: hydrogen sulfide
subject: gas-based fermentation
subject: amino acids
subject: mixed culture
subject: methane oxidizing bacteria
subject: sulfur oxidizing bacteria
subject: Biological and Chemical Sciences
subject: Microbiology
subject: FOS: Biological sciences

description: Microbial protein (MP) represent a potentially powerful solution to help relieve the pressure on the agri-food sector by addressing the expanding protein gap at the global level. Particularly, MP production from waste and residues paves the way towards their valorization into high value products. Biogas from anaerobic digestion of organic waste has been widely investigated as a potential substrate to support gas-based fermentation towards MP production using methane oxidizing bacteria (MOB). The latter, nevertheless, may be potentially inhibited by the presence of H2S in biogas (0-10000 ppm), thereby posing the need for dedicated and expensive pretreatments. Therefore, the overarching aim of this study was to enable an improved H2S-rich biogas-to-MP conversion by combining MOB with sulfur oxidizing bacteria (SOB), capable of oxidizing sulfide and reducing its toxicity. A MOB-SOB mixed culture was selected and enriched from an inoculum grown on methane in the presence of sulfide and several operating conditions were investigated through batch tests (Chapter 3). The most optimal operating conditions for biomass growth and protein production identified were a CH4:O2 ratio of 2:3, a starting pH of 7.0 and an equivalent H2S concentration in the biogas of 1500 ppm. The enriched MOB-SOB culture, dominated by Methylocystis spp. (MOB) and Chryseobacterium spp. (SOB), could withstand H2S concentrations in the gas phase as high as 4000 ppm, with inhibition occurring at 6000 ppm, thereby suggesting its suitability for direct conversion of sulfide-rich biogas into MP. Thereafter, the extent of H2S inhibition on MP production from biogas was evaluated under different operating conditions and bioreactor configurations. The enriched MOB-SOB mixed culture was utilized for the continuous production of MP from H2S-rich biogas through bubble column reactors (Chapter 4) and hollow fiber membrane reactors (Chapters 5-6). The continuous operation in the bubble column reactors (Chapter 4) showed promising results in terms of biomass growth (up to 559.9 ± 13.3 mg volatile suspended solids (VSS) per liter in the presence of 1500 ppm of equivalent H2S), being it affected by higher H2S levels as the biomass concentration decreased up to 40% when fed a biogas with 4000 ppm of H2S. The utilization of hollow fiber membrane bioreactors (Chapter 5), aiming at more efficient supply of the gaseous substrates, promoted both higher biomass growth (720.3 ± 19.8 mg VSS/L in the presence of 1500 ppm of equivalent H2S) and improved tolerance to higher H2S levels, with biomass concentrations decreasing only by 11% under 4000 ppm of H2S with respect to the presence of 1500 ppm. This higher resistance to sulfide inhibition could be attributed to the beneficial effects of the presence of attached biomass on the membranes, rich in sulfur oxidizing species, which favored overall better process performance in this bioreactor configuration. Moreover, the potential of the sulfide in biogas to act as source of sulfur, as well as the recovery and the concomitant valorization of nitrogen from anaerobic digestate during MP production, were evaluated (Chapter 6). Particularly, the feasibility of direct nitrogen stripping from digestate performed using the recirculating biogas/oxygen-containing gas stream of the reactor was demonstrated, with encouraging stripping efficiencies when stripping was carried out at 45 °C. The biomass produced and harvested during continuous MP production from biogas showed an overall high protein content (up to more than 65%), and an amino acid profile rich in essential amino acids, comparable to those of conventional animal- or plant-based protein sources. Moreover, increasing sulfide concentrations in biogas promoted the accumulation (up to 2.5 times more than in the absence of sulfide) of nutritionally-relevant sulfur- containing amino acids, such as methionine and cysteine (up to 61.3 and 58.3 mg/g biomass, respectively).