Some impurities such as silica can only be removed by a strongly basic resin. For example, dissolved silica is a major component of most water supplies. Normally it exists as a neutral polymer, and it becomes negatively charged only at high pH levels. This means that it can only be removed from water in the highly alkaline environment of a strong base resin in the hydroxyl form. The exchange process is often made more efficient by introducing the regenerant at the bottom of the resin column and passing it upwards through the bed (counter current regeneration). This ensures that the resin at the bottom becomes more highly regenerated than that above it. Treated water leaving the column flowing downwards then comes in contact with this resin last and undergoes the highest possible degree of exchange.

2.3 Advantages and Disadvantages in the use of Ion-Exchange Resins The 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. When these limitations itemized appear to represent a formidable list and the impression can be given that ion exchange methods might have too many short comings to useful in practice. However, this is not the case as the advantages