Effects and Mechanisms of Foam Suppression in Food Waste Anaerobic Digester Using Defoaming Agents

The defoaming effects of organic silicone defoamer S-559 (OSD), polyether defoamer S-717 (PD), polyether-polysiloxane defoamer S-722 (PPD), high-carbon alcohol defoamer S-714 (HCD), corn oil (CO), and biochar derived from biogas residue (BBR) were investigated based on the foaming potential test, biotoxicity test, and long-term semi-continuous test. Foaming potential experiments showed that the OSD, PD, PPD, and CO had the best initial and continuous defoaming efficiency. However, PD was biotoxic. The cumulative methane production of food waste from the batch anaerobic digesters decreased by 18.4% and 22.6% after adding 0.1% (v/v) and 0.5% (v/v) PD, respectively. After the OSD was used several times in semi-continuous experiments, the foam reduction efficiency of the OSD increased to more than 98% and the foam recurrence efficiency of the reactor decreased to <21%. In contrast, the foam reduction efficiency of the CO decreased to<18% after long-term use, and the foam recurrence efficiency increased to more than 120%. The reason for the difference in defoaming performance is that the OSD was composed of solid hydrophobic particles with sharp edges, and the liquid phase had good spread ability and low surface tension. Thus, OSD relied on the bridging-stretching mechanism, the bridging-dewetting mechanism, and the spreading effect for defoaming. CO is a liquid defoamer with low surface tension, which may only rely on the bridging-stretching mechanism for defoaming. These results provide a reference for selecting a defoamer for a food waste anaerobic digestion system, and provide insight for the development of new defoamers.