The mechanisms of membrane fouling in a pilot scale membrane enhanced biological phosphorus removal (MEBPR) system were investigated. It was found that the long-term fouling in the MEBPR process was hydraulically irreversible and was attributed to organic adsorption and deep pore clogging. The reversible fouling caused by sludge deposition and/or superficial pore blocking was reduced to the minimum due to the vigorous aeration and frequent backflushing applied. Soluble organic substances, such as soluble microbial products (SNIP), were thought to be responsible for the irreversible fouling. The mixed liquor with higher SMP concentrations exhibited a greater fouling propensity than the mixed liquor with lower SNIP concentrations. The significance of soluble organic substances to the irreversible fouling was confirmed through modeling of the evolution of trans-membrane pressure (TMP) in the MEBPR process. The modeling results indicated that the long-term irreversible fouling in the MEBPR process followed a simple exponential relationship with filtration time, volume of permeate filtered, or cumulative loading of soluble COD on the membranes. In particular, the degree of the irreversible fouling observed was most closely related to the cumulative amount of soluble organic material carried to the membranes by the flux of mixed liquor during the filtration process. Operating flux and/or process hydraulic retention time had little impact on the evolution of the organic loading-based irreversible fouling. (C) 2007 Elsevier B.V. All rights reserved.