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

Abstract

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 < 0.01), while hydrophilic EPSs increased the contribution of hydrophobic EPS by increasing sludge viscosity (r = 0.65, p < 0.01). For foam stabilization, hydrophobic proteins (r = 0.61, p < 0.01) and humic substances (HU) (r = 0.77, p < 0.01) were the main contributors. The former increased foam stability by reducing surface tension (r = 0.63, p < 0.01), while the latter contained functional groups such as amino, hydroxyl, and ether that might enhance the viscosity of the apparent molecular layer of the liquid film, thereby inhibiting foam breakup. Considering that the hydrophobicity of proteins was significantly correlated with the ratio of α-helix to (β-sheet + random coil) in their secondary structure (r = 0.80, p < 0.01), and that HU contributed to foam stabilization owing to the hydration of functional groups, disrupting the secondary structure of proteins and the hydration of HU might contribute to defoaming.