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 13 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.