Bioaugmentation-induced assimilatory sulfate reduction and chain elongation mitigate H₂S and CH₄ emissions in sewers

Controlling H₂S and CH₄ emissions is critical for ensuring the safety of sewer systems. Bioaugmentation offers a cost-effective and environmentally friendly alternative to chemical dosing, yet its performance and mechanisms in sewers remain unclear. This study investigated a bioaugmentation strategy by inoculating laboratory-scale sewer reactors with Bacillus subtilis and Saccharomyces cerevisiae. A 7-day inoculation (10⁷ CFU/mL) reduced H₂S and CH₄ emissions by 86.1% and 62.9%, respectively, with half-recovery times of 12 and 22 days. The strategy extended the duration of effective H₂S control by 20-300% compared with chemical dosing. Life-cycle assessment and cost analyses further indicated a 7.4% reduction in operating costs and a 60.6% reduction in greenhouse-gas emissions compared with a typical chemical dosing strategy. Bioaugmentation-induced assimilatory sulfate reduction was the primary pathway for H₂S mitigation. By diverting sulfate into the biosynthesis of sulfur-containing amino acids (an 859.3% increase), B. subtilis and S. cerevisiae reduced the electron acceptor available for sulfate-reducing bacteria (SRB), thereby reducing the relative abundance of SRB from 17.5% to 0.2% and suppressing H₂S emissions. Meanwhile, bioaugmentation-induced chain elongation was the dominant pathway for CH₄ mitigation. The inoculum selectively enriched lactic acid bacteria and Clostridium spp., which converted acetate to medium-chain fatty acids (increased by 150.5%) that are not utilized by methanogenic archaea (MA). This limited electron-donor availability reduced MA relative abundance (17.4% to 0.5%). These findings support the potential of bioaugmentation for sustained control of H₂S and CH₄ in sewers.