Tannins in well waters form from humic acid, which is one of the major components of humic substances (or Natural Organic Matter (NOM). These are the dark brown constituents of soil organic matter. They can also be found in peat, coal, many upland streams and ocean water. Tannins are astringent, bitter-tasting plant polyphenols that bind and precipitate proteins. The word tannin refers to the source of tannins used in tanning animal hides into leather; however, the term is widely applied to refer to brown color in water, caused by decaying organic substances..
Tannins can be removed by self-cleaning automatic tannin filters These filters remove tannin by ion-exchange, using anion exchange media. The units we use regenerate with rock salt (sodium chloride) in the same way water softeners function. Frequently we see shallow wells under the influence of surface run-off water, achieve high levels of tannins (turning the water brown) during heavy rainy periods. Tannin filters are often an excellent relatively low-cost technology to use for this type of problem, if the actual cause of the problem (such as sealing off the well from the surface) cannot be achieved.
Sometimes tannins are accompanied with iron or manganese. Water high in iron or manganese can sometimes be red, rust colored, brown, tan, black, or greenish in color. Chlorination or ozone, followed by a well designed iron filter can be very effective at removing tannin and these oxidized iron particles.
One method that we have successfully used to remove tannins in well water involves injecting a flocculant such as alum which allows the microscopic suspended particles that create the color in water, to lose their positive charge and “floc” together into larger clumps. This is easily done on small scale systems by using a metering pump and injecting 2 -5 ppm of “Cat-Floc” (one of many types of flocculant aids used for this purpose) into the water as it flows into a holding tank or storage tank. The water is allowed to settle, and is followed by filtration to remove any suspended floc.
A very effective method to remove tannin color is by using ultrafiltration (“UF”) membrane systems. UF systems can be used by homeowners, small communities and commercial sites to reduce turbidity and produce crystal clear water less than 0.1 NTUs. Care must be taken to properly pretreat the water if UF is used if iron, manganese or hardness minerals are present.
In some cases, by pretreating the water with chlorine or ozone, direct filtration can be used. Depending on the nature of the color, a backwashing sediment filter using a special type of zeolite filter media does a great job at clarifying water. Sediment down to the 5 to 10 micron range can be removed, and then backwashed out periodically by the automatic control valve.
There are many different designs and application criteria that must be considered, including water chemistry, flow rates, how the water will be disinfected, and what types of filtration to use. In some cases additional treatment such as reverse osmosis or other types of membrane filtration, such as nano-filtration are used in place of or in conjunction with polymer injection and settling, for color removal.