Insights into the effect of extracellular polymeric substances on anaerobic digestion foaming: From perspectives of composition, hydrophobicity, and functional groups

[Display omitted] •Hydrophobic EPSs were the major contributors to foam formation.•Hydrophilic EPSs contributed to foam formation by increasing sludge viscosity.•Hydrophobic proteins increased foam stability by reducing surface tension.•Humic substances enhanced foam stability by increasing the viscosity of liquid film.•Humic substances and hydrophobic proteins may serve as key substances in defoaming. Foaming is a common issue limiting the efficient and stable operation of food waste anaerobic digesters, and extracellular polymeric substances (EPSs) have been identified as key contributors to the foaming process. To further elucidate the influence mechanism of EPSs on foaming, batch tests were performed under various conditions, including different total solid contents, substrate-to-inoculum ratios, stirring frequencies, and substrate compositions, to induce the production of EPSs with different characteristics. The contributions of EPS composition and hydrophilic and hydrophobic characteristics to the foam formation and foam stabilization processes were analyzed. Hydrophobic EPSs were the main contributor to foam formation (with a correlation coefficient [r] of 0.56 and p