Long-term effect of pH (4, 10, and uncontrolled) on short-chain fatty acid (SCFA) accumulation, microbial community and sludge reduction were investigated in waste activated sludge (WAS) fermentors for over 90 days. The average SCFAs accumulation was 1721.4 (at pH 10), 114.2 (at pH 4), and 58.1 (at uncontrolled pH) mg chemical oxygen demand (COD)/L. About 31.65 mg COD/L was produced at pH 10, accounting for 20% of the influent COD. Illumina MiSeq sequencing revealed that Alcaligenes (hydrolic bacteria) and Erysipelothrix (acidogenic bacteria) were enriched at pH 10, while less acidogenic bacteria existed at pH 4 than pH 10, and no acidogenic bacteria were detected at the uncontrolled pH. The ratios of archaea to bacteria were 1:41, 1:16, and 1:9 at the pH of 10, 4, and uncontrolled. This study elucidated the effects of pH on WAS fermentation, and established the correlation of microbial structure with SCFAs accumulations and sludge reduction. (C) 2015 Elsevier Ltd. All rights reserved.