Abstract：To investigate the driving factors of foaming in anaerobic digestion (AD) of food waste, overloading, ammonia inhibition, and long-chain fatty acid inhibition were introduced into three semi-continuous anaerobic digesters to induce foaming incidents, during which the responses of process parameters, the variations of digestate' rheological properties, surface properties, foaming potential and extracellular polymeric substances (EPS) concentration were explored. The results showed that with the disturbance enhanced, the three digesters all produced foam with a height of 150~300mm. The volatile fatty acids (VFAs), surface activity (SA), foaming potential, surface tension, and viscosity were 1948~5978mg/L, 8.28~30.15, 312.5~350mL, 37.89~41.04mN/m, and 120~183.6mPa×s, respectively, in the three digesters during the initial stage of foaming. These variations suggested that their foaming thresholds were different, and even the variation trends of some parameters were inconsistent with the development trend of the foaming. In contrast, the EPS-related parameters were significantly correlated with the heights of the foam observed in different reactors. The soluble EPS and its protein concentrations, the polysaccharide concentrations of tightly bound EPS, and the polysaccharide concentrations of total EPS were positively correlated with the foam heights in three reactors (P<0.01, R2:0.64~0.81). Also, tightly bound EPS and total EPS concentrations were significantly (P < 0.05) or highly significantly (P < 0.01) related to the foaming heights in three reactors (R2:0.61~0.81). The interaction analysis of those parameters showed that in the overloaded and long-chain fatty acid inhibition reactors, the increased EPS concentrations were attributed to the accumulated VFAs, while in the ammonia inhibition reactor, free ammonia was the main reason for the increase of EPS concentration. Though the internal causes of EPS accumulation in test reactors were different, the increased EPS always significantly increased the viscosity of the digestates, and even reduced their surface tension, which eventually hindered the elimination of biogas from the liquid phase and resulted in foaming incidents. Further research on the EPS elimination technology may be expected to solve the AD foaming in situ with a high efficiency.
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