The effect of four alkali reagents (NaOH, KOH, Ca(OH)(2), mixed alkali) on waste activated sludge (WAS) hydrolytic acidification and microbial community was studied in semi-continuous fermentation systems at low temperature (15 degrees C) over long term operational time (65 day). The results showed that protein and polysaccharide of NaOH (124.26, 11.92) was similar to that of KOH (109.53, 11.30), both were higher than Ca(OH)(2) (70.66, 3.74) and mixed alkali (90.66, 8.71). The short chain fatty acids (SCFAs) of NaOH (231.62) was higher than KOH (220.62 mg chemical oxygen demand (COD)/g VSS). Although Ca(OH)(2) system had strong acidification capacity, the shortage of SCFAs occurred due to the low activity of hydrolase. Illumina MiSeq sequencing revealed that Tissierella and Erysipelothrix were enriched in the NaOH and Ca(OH)(2) systems, where Peptostreptococcaceae incertae_sedis was enriched in the NaOH and KOH systems, less Anaerolinea was involved in Ca(OH)(2) condition. (C) 2015 Published by Elsevier Ltd.