Enzymatically hydrolyzed brewer's spent grain (BSG) was digested in two expanded granular sludge beds (EGSBs, named BSG1 and BSG2, respectively). Both reactors were operated with the same organic loading rate (OLR) from 1 to 10 kg COD m(-3) d(-1) during the first 45 days. Hereafter a rapid OLR increase was applied to BSG2 from 10 to 16 kg COD m(-3) d(-1) within three weeks, while the OLR of BSG1 was increased by less than 2 kg COD m(-3) d(-1) in the same period. Results showed that a 30% decrease in COD removal and 70% decrease in methane yield appeared in BSG2 after the rapid OLR increase, and volatile fatty acid (VFA) accumulated more than thirty times compared to BSG1. The biomass structure deteriorated and 15% of the biomass was lost from the BSG2 reactor. 454-PyroTag and qPCR analysis revealed a rapid growth of acidifiers (i.e., Bacteroides) and a unique microbial community in BSG2 following the rapid increase in OLR. (C) 2015 Elsevier Ltd. All rights reserved.