Tannin Removal – Colour
If you have ever had a hot cup of Earl Grey tea or a cool iced tea, the distinct brownish colour of the product comes from tannin. Tannins are formed upon the decomposition of vegetation. These compounds are large molecular-weight organics that fall under two large primary categories: humic acids and fulvic acids. The structure of the tannin varies, depending on the plant life in a given area. Tannins can cause a yellow to brown cast in water and may also impart a taste and odour. They will generally be found in surface water supplies or shallow wells. Although these compounds are not a health risk, they are aesthetically displeasing.
Tannins are difficult to remove from water. One treatment method may be effective in one area, but may be totally ineffective 10 miles down the road. It is all dependent on the vegetation in a given area. Styrene-based macroporous anion resin has long been used to remove tannins from water. This type of resin has worked well in some areas but poorly in others. More recently, acrylic-based resins have emerged on the market and are producing better results when removing tannins from water. The acrylics are manufactured with a macroporous structure. The additional porosity enables the tannins to be regenerated from the resin more effectively. Evolution Water uses both resins for maximum effectiveness.
It is usually recommended that the water be treated by a softener prior to the tannin removal unit. The softener removes hardness and metals. It should be noted that tannins will pass through softening resin (cation resin) without any detrimental effects. Hardness levels greater than 10 grains passing into the tannin unit may precipitate calcium carbonate. This is a white precipitate that will restrict flow through the unit. Metals, like iron, can also foul the anion resin, reducing the anion’s ability to remove tannins.
The removal of tannins via anion resins occurs through both an ion exchange process and an absorption process. Tannins have a slight negative charge. Because of this, they are exchanged for chloride onto the anion resin. Organics will also be absorbed by the anion resin. Tannins will tend to migrate into the inner matrix of the anion resin. Once this occurs, it is very difficult to regenerate the tannins from the resin. Since an anion resin is being used to remove tannins, it will also remove other negatively-charged anions. Alkalinity will be removed during the initial part of the service cycle. With the reduction in alkalinity, a corresponding decrease in pH will occur.
Once the resin’s capacity for alkalinity has been reached, the pH will ultimately go back to its original level. Nitrates are also frequently removed by tannin removal resin.
Salt is used to regenerate the anion resin. Generally, the resin will be brined at 10 lbs per cubic foot. Most importantly, this regeneration should be performed every two to three days. Frequent regeneration will reduce the likelihood of organic fouling. Longer regeneration intervals will promote organic fouling, whether the anion is an acrylic- or styrene-based resin.
Frequently, tannin resin is mixed with softening resin. This reduces the amount of space required by the equipment and reduces the cost of the equipment. This type of design has performed satisfactorily when properly designed. If a unit is backwashed at a flowrate typical for cation resin, the anion resin can be backwashed out of the unit. An upper screen or basket can be used to eliminate the loss of tannin resin. Improper design can also lead to pressure drop and higher hardness leakage. This is due to calcium carbonate precipitation during the regeneration process.