Cleaning rainwater tanks now that it is finally raining has brought about an increase of the issue of tannin in drinking water. We have been receiving calls and a lot go something like this; “Our water has gone brown and it smells” It looks a bit like the water in…
This is called tannin. Tannin, also called tannic acid, any of a group of pale-yellow to light-brown amorphous substances in the form of powder, flakes, or a spongy mass, widely distributed in plants and used chiefly in tanning leather, dyeing fabric, making ink, and in various medical applications. These solutions are acid and have an astringent taste. Tannin is responsible for the astringency, colour, and some of the flavour in tea. Tannins occur normally in the roots, wood, bark, leaves, and fruit of many plants, particularly in the bark of oak species and in sumac and myrobalan. They also occur in galls, pathological growths resulting from insect attacks. (Encyclopaedia Britanica)
Because we haven’t had rain for so long the leaves, sticks, gum nuts etc. that have falling on your roofs have decayed. So when it rains the water picks up the tannin and washes it into your rain water tanks.
Is Tannin Safe to Drink?
Tannin in drinking water is generally safe. But the sight and smell will probably put you off. However, tannin will permanently stain your porcelain fittings like basins and toilets. Your whites will go brown if you wash your clothes in it.
How to Keep Tannin Out of Your Rainwater Tanks
- Regularly clean gutters.
- Have a good, well maintained first flush system.
How to Remove Tannin From Water
This is a bit complicated. Tannin can be removed from water with a carbon filter. But not the normal coal or coconut carbon sold by most retail shops. The best type is a wood based carbon. This makes some sense, that’s where the tannin is from. It’s only sold loose by the bag.
In addition there’s the time involved to remove it. It’s called contact time. The more tannin in the water the more time it needs in contact with the carbon to drag the tannin out. Around 5 minutes is a good starting point. If you have a Big Blue 20″x 4.5″ filter, and you turn on a single tap it will flow at around 7 litres a minute. The Big Blue filter holds 2.8L and 3kg of carbon, so the water will have a contact time of 30 seconds at best.
What’s required is a media tank. There’s one pictured below. We’re setting one of these up as part of our tank cleaning equipment and will offer them for sale with an install option in the local area. It’ll hold 30kg of carbon. and provide a contact time of around 5 minutes.
There’s a picture below taken by our wholesaler showing tannin in drinking water on the left, and after tannin removal. The system we’re implementing is the same as the one setup to do this.
I wouldn’t recommend buying one of these to remove tannin from rainwater unless absolutely necessary. It’s essentially a maintenance and water harvesting issue. However, if you have a source of water that you’d like to use but it contains tannin, like a natural spring, we can setup a system so you can take full advantage of this water source.
Another Major Issue
Before I finish there’s one more item I’d like to mention that’s really important. We’ve now seen two tanks where the water has gone a milky white. This is the paint oxide from the white tin roofs which has leached out of the paint and washed straight into the tank. I assume other colours do it, it’s just white is so much more obvious.
Do not drink this water. The only option here is to dump it and wait for rain or buy a load of water. A good first flush device would have stopped this.