Colloids in Water
Colloids in water are substances that are larger than ionic material but smaller than particulate matter. In general, colloids will exhibit a size of 0.01 to 0.1 µm or larger. On the basis of the ability of ultra-filtration membranes with stated molecular weight cut-offs to remove colloids, it would appear that colloids have a molecular weight in the range of 10,000 to 100,000 Da or higher. While colloids are commonly found in raw water supplies from a surface water source, they can also be present in raw water supplies from a ground water source.
An increasing number of treatment facilities inject chemicals to convert both soluble and particulate forms of iron to colloidal iron to minimize staining of domestic facilities using water. For water purification systems, colloids of silica, iron, and aluminium are important. For raw water supplies from a surface source, or a groundwater source under the influence of a surface source, colloids are generally complexed with organic material.
Colloids exhibit a slight negative charge. In high-purity water systems, this negative charge may be balanced with a hydronium ion. The high equivalent conductance of hydronium ions in a high-purity water system, for even trace amounts of colloidal material, will inhibit the production of water with a conductivity equivalent to that of theoretically ion-free water. The presence of colloids in pharmaceutical water purification systems and support systems is important. For example, the presence of colloidal silica in the feed water to a multiple-effect distillation unit may result in volatilization of the colloidal silica.
If the deposits are not removed during periodic maintenance, liquid carryover to the distilled product water may occur, potentially resulting in the presence of bacterial endotoxins in product water. The presence of colloids in feed water to RO unit can result in colloidal fouling of the membranes, requiring periodic chemical cleaning of the membranes.
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