Assessing effects of Ca2+ addition on membrane bioreactor performance and macro-floc sludge characteristics

Calcium ions (Ca2+) can trigger coagulation-flocculation process to form macro-flocculated sludge (MFS). Thus, dosing Ca2+-containing reagents into membrane bioreactors (MBRs) is considered as a promising approach to mitigate membrane biolouling. However, a mechanistic understanding of Ca2+ addition to MBR performance remains elucidated, such as physicochemical characteristics of MFS and their functionality variations. Consequently, this study was sought to understand the interplays of Ca2+ addition and MBR performance with a focus on characterizing MFS in detail. Three parallel MBRs were amended with 82, 208 and 410 mg-Ca2+/L final concentrations. Partide size analyses revealed that MFS formation was overall enhanced by the Ca2+ addition and granular sludge with diameters of up to 900 fun was formed in the 410 mg-Ca2+/L scenario. We believed that cationic bridges facilitated by elevated Ca2+ concentrations in conjunction with coagulation-flocculation were primary mechanisms of the formation of large flocs. Moreover, significant portions of soluble proteins and polysaccharides were flocculated and precipitated by Ca2+, which demonstrated a negative correlation between extracellular polymeric substances (EPS) concentrations and the formation of MFS. Furthermore, the population abundancies of Thiotrichaceae, Sphingomonadales and Hyphomicrobiaceae decreased in the sludge with Ca2+ addition resulted in profound changes of the microbial communities in the MBRs. But MBR performance, such as chemical oxygen demand removal (over 90%), showed no variation during the MBR operation. On the contrary, total nitrogen removal was inhibited in the MBRs. It was because the enlarging MFS formed diffusion barriers to prevent organic component from entering into the sludge flocs to be consumed. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Calcium ions can trigger the coagulation-flocculation process to form macro-flocculated sludge, where an elevated Ca2+ concentration in MBRs can lead to the formation of granular sludge with diameters up to 900 μm. The addition of Ca2+ can flocculate and precipitate soluble proteins and polysaccharides, resulting in a negative correlation between extracellular polymeric substances concentrations and the formation of MFS. The microbial communities in the MBRs undergo significant changes with the addition of Ca2+, affecting total nitrogen removal due to the formation of diffusion barriers by the enlarging MFS.