Abstract
Foaming negatively affects anaerobic digestion of food waste (FW). To identify the causes of foaming, reactor performance and microbial community dynamics were investigated in three mesophilic digesters treating FW. The digesters were operated under different modes, and foaming was induced with several methods. Proliferation of specific bacteria and accumulation of surface active materials may be the main causes of foaming. Volatile fatty acids (VFAs) and total ammonia nitrogen (TAN) accumulated in these reactors before foaming, which may have contributed to foam formation by decreasing the surface tension of sludge and increasing foam stability. The relative abundance of acid-producing bacteria (Petrimonas, Fastidiosipila, etc.) and ammonia producers (Proteiniphilum, Gelria, Aminobacterium, etc.) significantly increased after foaming, which explained the rapid accumulation of VFAs and NH4+ after foaming. In addition, the proportions of microbial genera known to contribute to foam formation and stabilization significantly increased in foaming samples, including bacteria containing mycolic acid in cell walls (Actinomyces, Corynebacterium, etc.) and those capable of producing biosurfactants (Corynebacterium, Lactobacillus, 060F05-B-SD-P93, etc.). These findings improve the understanding of foaming mechanisms in FW digesters and provide a theoretical basis for further research on effective suppression and early warning of foaming.
Highlights
Anaerobic digestion is the most feasible method for treating food waste (FW) and recovering renewable energy due to its high organic and moisture contents and low calorific value compared with traditional treatments such as incineration or landfills[1]
Because microbial communities are sensitive to operational conditions, the microbial community structure in foaming digesters with different temperatures or substrates may vary from those found in the studies mentioned above[13,15]
This study provides a theoretical basis for timely foam suppression and effective early warning in anaerobic digesters used to treat FW
Summary
Firmicutes and Bacteroidetes were the most common major phyla in the three reactors before foaming, accounting for 56.2% to 89.0% of total sequences (Fig. 3c) Members of these two phyla are closely related to hydrolysis and acidification of organic substrates and they have been reported as predominant phyla in other mesophilic anaerobic digesters treating FW13,19,21. The relative abundance of acid-producing bacteria (Petrimonas, Fastidiosipila, etc.) and ammonia producers (Proteiniphilum, Gelria, Aminobacterium, etc.) increased after foaming, which may help to explaine the rapid accumulation of VFAs and NH4+ after foaming. To eliminate or reduce the contribution of accumulated NH4+ to foaming, ammonia stripping methods can be considered[48]
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