Biofilm-Induced Critical Flux in Dead-End Ultrafiltration Processes: Phenomenon, Mechanism, and Economic and Environmental Benefits

Abstract

The concept of critical flux, introduced by R.W. Field, defines the flux below which the filtration resistance remains constant over time. Notably, this concept, originally for cross-flow filtration, faces challenges in dead-end filtration (the dominant mode used in drinking water ultrafiltration (UF)). Herein, leveraged by regulated membrane biofilms, we proposed a novel biofilm-induced critical flux specific to dead-end filtration. Below this critical flux, the membrane biofilm could act like a cross-flow to maintain mass balances by the biodegradation of foulants, thereby preventing a continuous increase in filtration resistance. Additionally, we demonstrated an optimized strategy to improve the critical flux─backwashing without air scouring, which doubled the critical flux from 6 to 12 L·m^-2·h^-1. A life cycle analysis revealed that operating at the biofilm-induced critical flux can reduce energy consumption and minimize membrane cleaning, thereby effectively lowering the overall operating costs (52%) and carbon emissions (61%) compared to conventional UF. Sensitivity analysis also indicated that extending membrane life and reducing membrane costs were crucial for lowering overall operating costs, while minimizing fossil energy usage was decisive for reducing carbon emissions. Overall, our study demonstrates that operating at a biofilm-induced critical flux offers a low-maintenance, cost-effective, and environmentally sustainable strategy for drinking water UF.