Estimating Actual Operational Capacity in Full-Scale Membrane Bioreactors Using a Hydrodynamic Index and Fouling Resistance

Abstract The widespread adoption of membrane bioreactor (MBR) technology is constrained by high operational expenditures for fouling control. Although research has advanced intelligent control algorithms, practical operation often relies on fixed set points and rule-based logic that do not reveal how much usable capacity remains under changing fouling conditions. This study evaluates an integrated dual-metric framework across three full-scale facilities. The hydrodynamic index (H value) expresses the system-scale operating margin associated with air input, permeate demand, apparent viscosity, solids concentration, and hydraulic resistance, whereas resistance components diagnose the underlying fouling state. Actual capacity (JA) is defined as the flux-equivalent boundary corresponding to a fixed, site-calibrated target H value (HTarget), and the capacity-utilization ratio (CUR = J/JA) compares current demand with this boundary. Across the three facilities, high-fouling-rate events increased from 2.7–4.6% below the capacity boundary to 35.4–56.3% above it. The framework identified flux-driven sensitivity in Plant A, low-flux resistance-state progression in Plant B, and high-stress instability in Plant C. Together, this framework diagnoses whether usable capacity is constrained by hydrodynamic stress, accumulated resistance, or both, and provides a practical basis for selecting appropriate adjustments to flux, aeration, and cleaning to maintain stable membrane performance.