Fouling is a general term that describes the interference of solid materials with uid ow . One of its most challenging aspects is the in situ formation of inorganic salts or colloids. Severe fouling at times forces system operators to discard critical equipment components. Even if mechanical or chemical cleaning are viable options, they require several work hours, total system shut downs, and high costs . Foulants could be organic or inorganic. Organic foulants are a result of poor system biocontrol and are beyond the scope of this chapter . Inorganic foulants include mineral salts of alkaline earth metals such as calcium, barium, and strontium. Amorphous and colloidal deposits are considered to be “special cases.” One such case is colloidal silica, which will be the subject of this chapter .