In the present work, lactic acid (LA) production performance with granulation was investigated at various hydraulic retention times (HRTs), 8-0.5 h. Glucose was used as a feedstock, and anaerobic mixed cultures were inoculated in an up-flow anaerobic sludge blanket reactor. As HRT decreased, the average diameter and hydrophobicity of the granules increased from 0.31 to 3.4 mm and from 17.5% to 38.3%, respectively, suggesting the successful formation of granules. With decreasing HRT, LA productivity increased up to 16.7 g LA/L-fermenter/h at HRT 0.5 h. The existence of rod-shaped organisms with pores and internal channels at granule surface was observed by scanning electron microscope. Next generation sequencing revealed that Lactobacillus was the dominant microorganism, accounting for 96.7% of total sequences, comprising LA-producing granules. (C) 2014 Elsevier Ltd. All rights reserved.