Fouling is a broad, generic term used to identify a multitude of time-dependent phenomena, which, singly or in combination, impact membrane performance. Just as illness can take many forms -- from a relatively mild cold to life-threatening cancer-- fouling also can have a range of effects.
Some degree of fouling has already been accounted for in any properly designed system. From a practical standpoint, fouling is of interest only when it negatively affects a system's productivity.
Calculations such as the Langelier saturation index (LSI) or silt density index (SDI), found in ASTM standards D4189-94 and D3739-94, respectively, can provide insight into the fouling problems that are inherent in any membrane system. Ideally, these calculations are used in the original design of the system. However, operators may find it useful to work through the calculations to anticipate the potential effects of a system upset.