The effect of quorum sensing on anaerobic granular sludge in different pH conditions

The effects of several signal molecules on anaerobic granular sludge at different pHs were investigated. The results showed that pH affected the synthesis and maintaining of the signal molecules. In an alkaline medium (pH 9), sludge bacteria secreted less acyl-homoserine lactone (AHLs), which had an opposite variation with the synthesis of extracellular polymeric substances (EPS), and its effect did not present. Additionally, the synthesis of EPS (mainly loosely bound layer) was majorly promoted by auto-inducers-2 (AI-2), but a lower negative role of diffusible signal factor (DSF). In neutral and weak alkaline media, the combining regulation of increasing AI-2 content and decreasing DSF content enhanced relative hydrophobicity (RH) and granular strength in granular sludge, which made granules to have a greater diameter and a more glossy granular shape. Whereas, in acid medium, the decreasing AI-2 content and increasing DSF content made a decrease in RH and granular strength, resulting in the degeneration of anaerobic granular sludge.