Colour in water is normally due to iron and manganese or organic tannins (often from peaty water). If the colour is due to tannins the colour can be reduced by specialised ion exchange resins
Where does colour come from? Colour in water (after it has been filtered) is typically caused by both iron and manganese compounds or by organic compounds. If the colour is due to iron or manganese there are specific treatments (see the iron and manganese data sheets). If the colour is due to organics then treatment is normally by ion exchange or activated carbon. Organic colour comes from decaying vegetation and may be completely soluble or particulate. Tannins (Humic and Fumic acids) are by far the most common class of compounds and give the water a yellow/brown tint. In highland areas where water runs off peat the water can be virtually brown.
Colour Removal The two most common forms of colour removal are ion exchange resins or by specialised activated carbon
by Ion Exchange When the water passes through anion resin beads the organic molecules are bound to the resin. When the resin has become saturated and can hold no more colour forming compounds they need to be removed. Resin selection is critical as many require caustic soda to remove the organics. However there are resins available which can be regenerated with brine on its own or in combination with caustic. Brine has the advantage of being readily available and safe. Typically every 25 litres of resin will take out 2000 ppm of organic colour before it needs regenerating. Flow rates are reasonably fast, typically 12 to 15 times the volume of resin.
by Activated Carbon AC absorbs the organic colour. These systems are particularly useful when the colour levels are very low or the amount of water required is very low or more usefully as a polisher as a final treatment. Once the carbon is exhausted it needs to be discarded and new carbon put in. Phosphate rich activated carbons are particularly good at holding organic colour as well as some metals. The time the water needs to be in contact with the carbon is very long so flow rates are correspondingly very slow. Trickle feed systems are by far the best option or using the system as a final polisher is a good option.
How Does Ion Exchange Colour Removal Work?
An automatic colour removal ion exchange system consists of a pressure vessel filled with resin. Located on the top of the pressure vessel is the control valve. The water is passed through the control valve and down through the vessel. As the water passes across the resin bed, the colour compounds attach to the resin. Periodically, depending on how much water is used, the resin needs to be refreshed. This is done by flushing a small amount of salt (stored in an external brine tank), though the resin vessel. Once this process has been completed the resin is refreshed and ready to begin again.