How to clean your water tanks

Water tank

Winter is the ideal time to clean your water tank.

Words: Nadene Hall

If your water tank collects the rain that falls on your roof, you will have contamination of some kind in an unprotected water tank. Dirt and dust, leaves, animal droppings and other substances will settle in the bottom of a water tank over time.

Research by the Auckland Public Health Protection Services showed over half the collections of drinking water from roofs involved in a pilot study contained contaminants from faecal matter such as salmonella, campylobacter, E coli and giardia.

You can install devices on spouting that diverts the ‘first flush’ (40 litres or so) of rain away from your tank, before allowing it to fill as normal, which washes away most debris and dust. There are also siphon systems that will suck debris up and out of the over-flow pipe, working for your all year round.

However, even if you have this kind of system, it’s a good idea to clean out your tank, ideally twice a year. Winter is an ideal time because you’re more likely to get it quickly refilled with rain.

You can get professional companies to do this for you, especially if you’re not keen on getting inside your tank. If you choose to do it yourself, make sure you have an assistant who can help you (who is outside the tank and has a spare ladder), and be very cautious of air flow and ventilation. Wear a respirator, gloves, something over your legs and arms, gumboots and eye protectors. Don’t stay inside a tank for long periods, and ideally stick your head out the trapdoor for fresh air every so often. Note, it’s very enclosed in a tank, and you need to have narrow hips to fit through the entry trapdoor.

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Turn off your pump, and get a bottle of household bleach (eg, unscented Janola), a stiff-bristled brush and a broom.

Empty the tank until there’s around 1000 litres or so in the bottom. Scrub the bottom and walls of your tank with Janola – use a 2:1 dilution of water to Janola – then remove the sludgey water. You may be able to use a broom to push the watery sludge out of the bottom through a scour valve (if your tank has one) or you will need a wet vacuum system or pump to suck it out. While it’s draining, use a hose to wash any debris off the sides of your tank.

Refill your tank. You can add Janola to the fresh water at a rate of 1 litre per 3300 litres of water if you want to flush your pipes as well. Bleach will naturally dissipate in water, and this dilution isn’t one you’re going to taste or that is harmful to humans, but it is enough to kill off bugs.
Alternatively, you can order a tanker of town-supply water which will already contain treated water.

– make sure the paint on your roof is lead-free. If you are repainting, make sure there is a sign on the label that says it is suitable for a water supply
– an uncoated metallic roof can react with rainwater, which tends to be slightly acidic, causing leaching so you will need have it painted
– underground pipes are difficult to clean so it can pay to keep your tank as close to the roof as possible. If you do use pipes underground, make sure it has a constant gradient so water can’t sit in one spot.
– Make sure there are no trees overhanging your roof
– Your downpipe should be easily detachable so you can divert water from your tank when you are cleaning or painting the roof
– If you live on a dirt road or in an area where sprays are often used, you might want to reconsider using the water collected off your roof for drinking.

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