elimination of organic matter

The various ion exchange methods presented previously assume that the raw water contains only dissolved minerals.

In reality, natural water always contains a certain amount of organic matters ( OM ), which act differently with the resins, depending on their affinity to the resin’s active sites. Some pass easily through the resin bed, others are reversibly attached to the resin and then removed during regeneration, and others are irreversibly retained, tending to poison the resins.

In practice, this last disadvantage affects mainly high alkaline anion exchangers, since cation exchangers are virtually insensitive and the weak alkaline anion exchangers tend to retain these products more or less reversibly. Most frequently, the systems described above yield satisfactory results in industrial applications if the resins are properly selected. In some cases however, and particularly when the raw water contains humic compounds, it has been observed that the conventional resin combinations do not produce the expected results.

With this type of raw water, the treatment requires either using a comprehensive pre-treatment designed to remove the humic matters (see the potable water treatment sequence, section surface water treatment systems), or at the beginning of the train using high porosity anion exchangers with a high adsorption capacity in order to protect properly the resins specifically used for demineralisation. These anionic resins known as “Scavenger” resins directly adsorb the organic matter, which may be eluted by treatment with sodium chloride followed by caustic soda or better still, with a mixture of salt and sodium hydroxide known as “alkaline brine”.