Tannins are a natural organic material that can be the by-products of nature’s fermentation process, be created as water passes through peaty soil and decaying vegetation. This can cause water to have a faint yellow to tea-like colour, and can cause yellow staining on fabrics, fixtures, china and laundry. Tannins may give a tangy or tart aftertaste to water. They may also cause water to have a musty or earthy odour. Tannins – also known as fulvic or humic acid – are more common in surface water supplies and shallow wells than in bore hole water points. Water in marshy, low-lying, or coastal areas is also more susceptible to tannins.

Tannins are considered an aesthetic problem. While they may make water unappealing to drink and stain laundry, they present no health hazard. Tannins create a light yellow to dark brown discoloration in the water. A simple test for tannins involves filling a clear glass with water and letting it sit for a few days. If the colour settles to the bottom of the glass, the discoloration is most likely caused by iron and/or manganese and not tannins. If the intensity of the colour remains intact, it is most likely caused by tannins.

Common tannin treatment uses an anion exchange resin. The anion resin is sensitive to hardness, so most systems include a water softener as pre-treatment. The water softener extends the life of the anion resin and increases tannin absorption. Anion exchange resin systems should be regenerated occasionally with a baking soda and salt water solution to improve the effectiveness of the resin. When cleaning is needed, the water will have a “fishy” odour caused by the fouled anion resin. Anion exchange resin systems can also change the chloride, alkalinity and sulphate levels of the water, so you may wish to monitor these substances more closely once the system is installed.