Fouling is still the single most important problem that hinders the more widespread use of membrane separation processes. Membrane fouling manifests itself by the decline in permeate flux. The flux decline results from a number of complex kinetic processes that lead to gradual deposition of solids on the membrane surface and/or gradual blockage of the membrane pores. Four types of foulants may be distinguished: chemical foulants, which cause scaling, physical foulants, which are related to deposition of particles, biological foulants, which can form bio fouling and organic foulants, which can interact with the membrane.

The present paper is focussed on the scaling problem which is widely encountered in membrane water treatment processes. The deleterious effects of scale deposits are not only in permeate flow rate decline but also in flow maldistribution, augmented frictional pressure drop, increased salt passage and irreversible membrane damage. Clearly, an essential requirement for a viable water purification operation is pretreatment of the feed in order to prevent (or more realistically, minimize) fouling and scaling.

One of the main difficulties in planning an RO or NF system is the assessment of the pretreatment level required for effective fouling and scaling control subject to economic constraints. The difficulties stem from the wide varieties of raw waters, the limitations of available techniques for characterization of scaling and fouling propensity, and the efforts needed to tailor the pretreatment system to the specific anticipated fouling and scaling process. This paper briefly reviews current understanding of basic phenomena involved in scale formation, outlines current scale control techniques and indicates some promising research directions.