External and internal mass transfer kinetics of phosphate removal in a UASB reactor treating landfill leachate: A generalized Fulazzaky kinetic analysis

Abstract

Understanding phosphate (PO₄³⁻) transport during anaerobic treatment is essential for improving nutrient removal from municipal landfill leachate. This study tested the hypothesis that the Generalized Fulazzaky (GF) equations can quantitatively distinguish external, internal, and global mass-transfer processes and identify the dominant rate-controlling resistance governing PO₄³⁻ removal in anaerobic systems. A laboratory-scale up-flow anaerobic sludge blanket (UASB) reactor equipped with a gas-liquid-solid separator was operated under mesophilic conditions (38 °C) for 197 d to treat municipal landfill leachate. During the initial operating stage, PO₄³⁻ removal was limited because the anaerobic sludge (ANS) was still acclimating and granule development was incomplete. As the biomass matured and the influent PO₄³⁻ concentration increased from 5.02 to 10.93 mg L⁻¹, the removal efficiency improved from 25.25% to 57.79%. Linear GF analysis showed an excellent fit (R² = 0.9929), enabling determination of the β and B parameters for kinetic simulation. The global mass-transfer factor closely followed the internal mass-transfer factor, whereas the external mass-transfer factor remained consistently lower, indicating that external mass-transfer resistance governed overall PO₄³⁻ removal under the investigated conditions. Analysis of the C/N/P ratio further demonstrated the coupling between nutrient availability, ANS development, and phosphate transport. These findings provide new mechanistic insight into PO₄³⁻ biosorption in anaerobic systems and demonstrate that the GF framework is a useful mechanistic tool for interpreting nutrient mass-transfer behavior in laboratory-scale UASB reactors, providing a basis for future pilot-scale validation.