Identification of microorganisms in the granules generated during methane fermentation of the syrup wastewater produced while canning fruit

•Syrup wastewater was treated using UASB methane fermentation.•Methane generation abruptly diminished when OLR increased up to 30.3 kg COD m−3 d−1.•Methanosaeta spp., were dominant when the methane gas evolved vigorously.•The genus Geobacter was observed with the deterioration of methane fermentation.

The wastewater produced in the process of canning fruit contains a syrup that consists mainly of sucrose. This syrup wastewater was treated by methane fermentation in an upflow anaerobic sludge blanket reactor. The organic loading rate of syrup wastewater was increased gradually as fermentation progressed. The higher the organic loading rate, the more methane gas evolved until the organic loading rate reached 30.3 kg COD m−3 d−1, at which point methane generation abruptly diminished because the loading rate was too high to stably operate the reactor. The changes in the microbial community, that of both bacteria and archaea in the granules, were analyzed simultaneously using PCR-DGGE during the fermentation process. Methanosaeta spp., which are methanogenic archaea that produce extracellular polymers indispensable for the formation of granules, were dominant when the methane gas vigorously evolved, and the iron-reducing bacterium belonging to genus Geobacter, which outcompetes methanogens, grew proportionally with the deterioration of methane fermentation.