Anionic exchange is an alternative to traditional drinking water purification processes when the main problem is a high concentration of natural organic matter (NOM) in the raw water. A few treatment plants employing anionic exchange have recently been installed in Norway. Some of these have after some time experienced increasing colour of the processed water. In the present study we show that the resin selectively adsorbs fulvic acid from the water while colloidal aggregates of humic substances and iron, which also contributes to the water colour, are nearly unaffected by the treatment. The treatment efficiency is successively reduced as the resin becomes fouled with iron from the raw water. Resin cleaning with hydrochloric acid had little effect, while treatment with a sodium dithionite solution resulted in full recovery of the treatment efficiency.

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