Turning the ultrafiltration bio-cake from risk carrier to functional barrier: Ca²⁺-enabled ecological backwash for membrane fouling mitigation and microbial safety improvement

Ultrafiltration is critical for water reclamation, yet membrane fouling, biostability, and biosafety remain coupled hurdles. This study identified the bio-cake as an ecological interface linking the three challenges and introduced a Ca²⁺-enabled backwash to reshape its function. The optimal Ca²⁺ level (0.05 mM) reduced pore fouling, promoted a thinner and more porous bio-cake formation, thereby improving hydraulic performance. Source tracking revealed that permeate microorganisms originated mainly from biofilm detachment on the membrane permeate-side and pipe surface, rather than incomplete membrane retention. The engineered bio-cake enhanced assimilable organic carbon (AOC) removal, with permeate AOC dropping by 42% on day 22 compared to the control group. This AOC reduction restricted the in-situ niche of the relevant microbial sources, leading to a lower initial total cell count (TCC) in permeate, and, critically, attenuated microbial regrowth during storage, thereby improving biostability. From a biosafety perspective, the bio-cake microorganisms accelerated pathogen inactivation through carbon competition, thereby preventing the pathogen leakage into the permeate. Mechanistically, the optimal Ca²⁺ level stimulated microbial activity and selected heterotrophic taxa with broad substrate utilization. Enrichment of carbohydrate-active enzymes and diverse metabolic pathways enhanced AOC biodegradation, while improved quorum sensing, cofactor supply, and stress resistance supported stable community function. This work recasts the bio-cake from a risk carrier to a functional barrier and provides an ecological engineering strategy that couples fouling control with simultaneous gains in reclaimed water biostability and biosafety.