Ion-exchange resin type of pollution
cut lineThe advantages of ion exchange processes are the very low running costs. Very little energy is required, the regenerant chemicals are cheap and if well maintained resin beds can last for many years before replacement is needed. There are, however, a number of limitations which must be taken into account very carefully during the design stages, for example, the resin of pollution.
Ion-exchange resin type of pollution in water treatment.
Calcium sulphate fouling
Sulphuric acid is the cheapest cation resin regenerant for demineralisers and is used where possible. Some water supplies contain a high proportion of calcium and when this acid is used calcium sulphate precipitates can form during regeneration. This fouls the resin and blocks drain pipes with a build up of scale. Under such circumstances, hydrochloric acid must be substituted.
Iron fouling
Bores yielding anaerobic water from underground supplies nearly always contain soluble iron in the Fe2+ state. Small amounts are readily removed by sodium cycle softeners but care must be taken to prevent contact with air prior to treatment. Aeration allows oxidation of Fe2+ to Fe3+ and consequent precipitation of ferric hydroxide which clogs resin beads and prevents ion exchange. Iron fouling is the commonest cause of softener failure.
Adsorption of organic matter
One of the commonest problems results from the presence of organic matter in water supplies. Untreated water from lakes and rivers usually contains dissolved organic material derived from decaying vegetation which imparts a yellow or brown colour. These substances can become irreversibly adsorbed within the anion beads, reducing their exchange capacity and leading to a reduction in treated water quality. Removal of organics prior to demineralisation is usually achieved by flocculation with alum or ferric salts followed by filtration which removes the metal hydroxide floc and the coprecipitated organic compounds. This treatment also removes any fine silt which represents another source of resin fouling. Both organic and iron fouled units can be chemically cleaned on site but complete removal of impurities is rare and resin performance usually suffers after fouling.
Organic contamination from the resin
The resins themselves can be a source of non-ionized organic contamination. New commercial grade resin often contains organics remaining after manufacture, while very old resin will shed organic fragments as the polymer structure opens up very slowly (de-crosslinkage). Such contamination may be disregarded for many uses, but when removal is needed, the demineralised water can be passed through an ultra filtration membrane.
Bacterial contamination
Resin beds do not act as filters for the removal of bacteria or other micro-organisms. They very often tend to worsen such contamination as traces of organic matter, which invariably accumulate, constitute a nutrient source for continued growth. When sterile water is required it can be obtained by treating the demineralised water by non-chemical means such as heat, ultra violet irradiation or very fine filtration. Resins beds can be decontaminated with disinfectants such as formaldehyde, but heat or oxidising disinfectants such as chlorine must not be used as these damage resins.
Chlorine contamination
As stated above, chlorine damages resins. This means that even town supply water is an unsuitable demineraliser feed because of the trace of chlorine it contains. It is customary to treat such feeds by passing them through activated carbon which removes chlorine very efficiently.