In ultra-filtration or micro-filtration plants the commonly used method for fouling characterisation is based on an analysis of easily measurable process parameters such as membrane permeability. The results presented in this paper are included in a research project aiming to optimize membrane plant operation for drinking water production with a backwash procedure using air and water flow in dead-end ultra-filtration using inside-out hollow-fibres. Experiments on both semi-industrial and laboratory scale pilot plants were first performed with a clay suspension for different filtration conditions and backwash procedures. A comparison of backwash efficiency assessed by initial permeability recovery and by mass balances on particles was performed. From these experiments, it can be pointed out that: - Permeability at the beginning of the filtration cycle is constant whatever the backwash procedures (with or without air). However this cannot be considered as a criterion of good backwash efficiency, because of the following results. - Mass balances on backwash waters show that, despite total permeability recovery, some particles remain on the membrane surface, creating a so-called "remaining particulate fouling". In addition, the quantity of particles remaining on the membrane surface is influenced by backwash parameters and procedure. Air injection during backwash enhances particle removal. So this study demonstrates that there is remaining particulate fouling after backwash in spite of total permeability recovery. This remaining particulate fouling depends on both filtration and backwash conditions and might contribute to the long-term fouling. Indeed, experiments with natural surface water showed that the long-term decrease in initial membrane permeability (i.e. permeability at the beginning of each filtration cycle) was mainly due to the particles remaining at the end of a classical backwash and only slightly due to irreversible fouling by natural organic matter adsorption. In contrast, when the backwash procedure was hydraulically enhanced by air injection, the remaining particulate fouling was significantly lowered and the long-term decrease in membrane permeability was mainly due to irreversible fouling phenomena, which leads to an increase in process runtime. (C) 2010 Elsevier By. All rights reserved.