Interaction of bisbiguanide chlorhexidine with sludge organics and microbes regulated sludge anaerobic digestion

Emerging non-antibiotic bisbiguanide antimicrobial chlorhexidine (CHX) is increasingly discharged into sewage sludge, potentially impacting biological treatment processes by interacting with sludge's biotic and abiotic components. This work systematically revealed the interactive processes between CHX with sludge organics and microbes, and disclosed the impact mechanisms on sludge anaerobic digestion (AD). CHX drove extracellular polymeric substances secretion and organic release by preferentially interacting with protein-related groups and changing protein secondary structures. Subsequently, low CHX concentrations of 0.5-5 mg/g total solids enriched metabolic microorganisms and genes involved in glycolysis, amino acid degradation, pyruvate oxidation, fatty acids generation, and acetotrophic methanogenesis by stimulating acidifying bacteria activity and enhancing microbial adaptability, resulting in 8-38 % increase in methane production. Besides, the interaction of CHX with microorganisms was slower than that with abiotic components, leading to shortened AD lag time. By comparison, the toxic effects of high-dose CHX destroyed AD-related microbes and induced metabolic shifts toward survival mechanisms, decreasing 40 % of methane production. The inhibitory effect could be alleviated by biochar, possibly through buffering contact. This study highlights the intricate interactions of non-antibiotic antimicrobials with sludge components, providing insights for mitigating their impacts on AD performance.