Inhibition of metformin on sewage sludge anaerobic digestion: Focusing on changes in microbial metabolites and community/functional genes

Metformin (MEF) is commonly used in the treatment of diabetes and is often detected in sewage sludge, but its effect on anaerobic digestion (AD) for methane (CH₄) production has rarely been investigated. This study revealed the influence of MEF on AD of sewage sludge. MEF significantly interfered with AD process, causing CH₄ yield reduction of 9.67 % and 30.18 % exposure on 6 mM and 9 mM of MEF, compared to the control groups without MEF, respectively. Liquid phase analyses revealed that MEF led to the accumulation of butyric and acetic acids and the increase in protein and polysaccharide contents at the initial stage of AD, showing the sludge dissolution. In addition, the high-throughput sequencing identified Firmicutes as the main contributor to the accumulation of volatile fatty acids (VFAs), with its relative abundance rising from 35.45 % to 47.48 %. Conversely, the relative abundance of Methanosaeta decreased from 56.21 % to 36.0 %, causing low CH₄ yield. Furthermore, an analysis of the changes in enzyme abundance involved in metabolic processes indicated that MEF could disrupt the metabolic pathway, thereby inhibiting CH₄ yield. Surprisingly, MEF could not be decomposed by anaerobes while it could be adsorbed by the sludge in AD. Thus, low cost and efficient pretreatment techniques for the PPCPs containing sewage waste are recommended to obtain suitable substrates for more CH₄.