Syngas-driven sewage sludge conversion to microbial protein through H₂S- and CO-tolerant hydrogen-oxidizing bacteria

Treating excess municipal sewage sludge (MSS) by means of thermochemical processes could enable its conversion into high-value microbial protein (MP) through syngas. Nevertheless, the variable composition and content of inhibitory compounds of the latter hinders the application potential of such a biorefinery scheme. Through a series of short- (48 to 96 h) and long-term (30 days) batch aerobic bioconversion tests, the present study aimed at investigating the potential of a mixed culture of hydrogen-oxidizing bacteria (HOB) to produce MP from a simulated syngas mixture characterized by variable H₂ and CO₂ concentrations, and different levels of CO and H₂S as potential inhibitors of the HOB-driven process. Syngas was converted into MP with a protein content as high as 74 %, reaching biomass yields of 0.25 g VSS/g H₂-COD, close to the maximum reported HOB yield of 0.28 g VSS/g H₂-COD,  and volumetric productivities of 16 mg VSS/L/h. The potential of the process to provide between 50 and 100 % of the total nitrogen requirement of HOB solely by means of the gaseous ammonia nitrogen recovered through syngas was also preliminarily calculated. The presence of H₂S and CO concentrations up to 0.4 % and up to 40 %, respectively, and a wide range of H₂/CO₂ ratios (2 - 10) had no negative influence on the main process performances. The role played by H₂S- and CO-tolerant HOB species was fundamental to guarantee a high tolerance to microbial inhibitors, and demonstrated the high potential of mixed cultures for resource recovery and valorisation.