Impact of Al-based coagulants on the formation of aerobic granules: Comparison between poly aluminum chloride (PAC) and aluminum sulfate (AS)

Abstract

As widely used Al-based coagulants, poly aluminum chloride (PAC) and aluminum sulfate (AS) were adopted in a short term at the start-up stage (from 10th to 16th) to enhance the formation of aerobic granules, and their effects on aerobic granulation were elucidated. The results suggested that both PAC and AS facilitated the granulation by improving the physicochemical properties of sludge. The reactor performance in pollutant removal was also enhanced. Specifically, in terms of extracellular polymeric substances (EPS), PAC dosing mainly stimulated the production of loosely bound EPS (LB-EPS), whereas more tightly bound EPS (TB-EPS) were secreted with the presence of AS. Based on the elemental analysis, polymeric Al hydrolyzed from PAC mainly worked on the exterior of microbial aggregates, and thus the attached aluminum in granules was gradually eliminated by ion exchange and hydraulic shear force. In contrast, the aluminum species in AS hydrolyzed into monomeric and oligomeric Al, and thus could diffuse into the interior of microbial aggregates and eventually created an “Al-core” in the granules. Overall, the present study describes the AGS formation with Al-based coagulants and the mechanisms of PAC- and AS-enhanced aerobic granulation.