Stability improvement of aerobic granular sludge (AGS) based on Gibbs free energy change (∆G) of sludge-water interface

Abstract

• Δ G sw a can normalize the stability of AGS . • Microbial and chemical properties were more important for Δ G sw a than physical ones. • Reasonable increase in OLR or HSF reduced Δ G sw a and enhanced stability of AGS. The stability of aerobic granular sludge (AGS) has been evaluated by lots of physicochemical and microbial characteristic indicators, but none of them could singly and comprehensively characterize the stability of AGS. Gibbs free energy change (∆G) could reflect the stable state of the whole system by the second law of thermodynamics, so ∆G of the sludge-water interface ( Δ G sw a ) was proposed to characterize the stability of AGS in this study. Organic loading rate (OLR) and hydraulic shear force (HSF) as the primary external energy inputs were selected at different levels in four reactors. Results showed that a reasonable increase in OLR or HSF could reduce Δ G sw a , and its influence on Δ G sw a had two paths: affecting the surface roughness of sludge directly (Path 1); on the other hand, EPS producers like denitrifiers were enriched, promoting the secretion of PN such as tryptophan and protein-like to affect Δ G sw a (Path 2). These indicators were correlated with Δ G sw a significantly, especially genus Paracoccus spp. (class Alphaproteobacteria) (-0.74, p <0.001) and Amide III (-0.81, p <0.001), indicating that Path 2 had a more significant influence on Δ G sw a . In addition, the absolute values of correlation between physical stability indicators and Δ G sw a were around 0.80 ( p <0.001). Therefore, Δ G sw a can be normalized to characterize the stability of AGS. This study establishes Δ G sw a as the normalized indicator for the stability of AGS, which provides specific theoretical guidance for operating parameter regulation in engineering application.