Application of organic silicon quaternary ammonium salt (QSA) to reduce carbon footprint of sewers: Long-term inhibition on sulfidogenesis and methanogenesis.

Sulfidogenic and methanogenic processes are undesirable in sewer management, yet the derived problems regarding organic losses are often neglected. Traditional chemical dosing methods aimed at sulfide and methane control commonly involve similar mechanisms of oxidation and/or precipitation. Moreover, previous focuses were centered on elevating control efficacy rather than investigating interactions between dosed chemicals and biofilms. In this work, organic silicon quaternary ammonium salt (QSA) of 75 mg-N/L was firstly applied in laboratory pressurized sewer reactors. After three dosing events, it took 20 days for sulfidogenic activities to recover to 50 % without further elevations. Meantime, methanogenic activities were stable ca. 11 % without significant inclinations to recover. Notably, consumption rate of chemical oxygen demand (COD) was suppressed to 50 % at most, and no microbial resistance to QSA but better control efficacy was observed. Characterizations of physicochemistry, microbial community and metabolism were conducted to elucidate mechanisms. Results showed that QSA was attached on sewer biofilms via electrostatic attraction to exert enduring control efficacy. Biofilms tended to become more hydrophobic and compact after QSA exposure. Microbial analyses indicated that relative abundances of microbes regarding hydrolysis, acidogenesis and methanogenesis were sharply decreased together with down-regulation of pivotal enzymatic activities. Additionally, denitrification batch tests initially suggested that the biodegradability of effluent was significantly enhanced, which ensured the safety of QSA dosing into sewers. Overall, results of this work were expected to lay a theoretical foundation on employing QSA to wastewater management.