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

Highlights: • Ca²⁺ addition effectively promoted macro-floc sludge formation. • Parts of sludge even granulated as large as approximately 900 μm with 410 mg/L Ca²⁺. • Ca²⁺ led to microbial community change but had no effects on COD removal (over 90%). 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 biofouling. 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. Particle size analyses revealed that MFS formation was overall enhanced by the Ca²⁺ addition and granular sludge with diameters of up to 900 μm 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.