Antifoam, dilution and trace element addition as foaming control strategies in mesophilic anaerobic digestion of sugar beet pulp

Abstract The effectiveness of foaming control strategies, including trace element addition, feedstock dilution and use of antifoams, was assessed for the anaerobic digestion of sugar beet pulp (SBP) at laboratory scale under mesophilic conditions. Semi-continuous digestion at an organic loading rate (OLR) of 2 g VS L−1 day−1 resulted in methane yields of 0.316 L CH4 g−1 of volatile solids (VS), with a VS destruction rate of ~91%. At OLR of 3 g VS L−1 day−1 or above, foaming occurred, accompanied by small increases in alkalinity ratio and volatile fatty acid (VFA) concentrations and a slight fall in specific biogas and methane production, indicating a minor reduction in process stability and performance. At OLR 5 g VS L−1 day−1 foaming was persistent and severe. In contrast, digesters fed at OLR 5 g VS L−1 day−1 and supplemented with selected trace elements showed stable performance with no foaming. Dilution has proved an effective control strategy for other feedstocks, but feeding with diluted SBP (1:1 w/w) led to a decline in gas production and operational stability, with no significant benefits in eliminating foaming. Seven antifoams were tested for foam reduction and effects on batch digestion at a range of concentrations. Addition of certain antifoams at dosages of 0.5 mL L−1 or more could inhibit gas production. A selected antifoam agent was then trialled in semi-continuous digestion at OLR 5 g VS L−1 day−1. Repeated high antifoam dosing of 1 mL L−1 digestate was required to reduce foaming, and eventually proved inhibitory. More reliable screening tests are needed for antifoam selection, as batch toxicity testing may not provide a satisfactory guide to performance in continuous operation.