How to clean a rainwater tank
Sludge builds up slowly and you rarely see it. The tap water tells you first.
A rainwater tank collects a layer of sludge on the floor over the years. And most tanks need cleaning every few years, sooner if the roof is shaded by trees. You cannot judge it from the outside, so watch the water and check the tank interior from the top.
Signs your tank needs cleaning
Cloudy water, a musty smell or a dark film in a glass are the usual clues. Blocked pump filters and sediment in the tap strainers point the same way. Mosquito larvae mean the inlet screen has failed, which is a separate fix.
If the tank has never been cleaned and is more than a few years old, assume there is sludge down there. Dirt that ran in through the inlet, leaf mulch and fine roof dust all settle to the floor.
What the job involves
There are two ways to do it. The first is to siphon or vacuum the sludge off the floor while the tank stays full. The second is to drain the tank, scrub the walls and floor, and rinse before refilling.
| Method | Good for | Drawback |
|---|---|---|
| Vacuum or siphon with the tank in use | Regular upkeep, light sludge | Stirs up fine sediment if done roughly |
| Drain, scrub and rinse | Heavy sludge, old tanks, a bad smell | You lose the stored water and the tank is empty for a while |
| Specialist tank cleaner | Large or buried tanks, confined spaces | Costs more than doing it yourself |
Getting inside is not a casual job
Entering a tank is a confined space risk. Fumes, low oxygen and a slippery floor are real dangers, and people have died in tanks. If the job needs someone to climb in, hire a tank cleaner who has the training and the gear. Do not send a family member down a ladder.
Work from the outside where you can. A vacuum head on a pole reaches most of a small tank through the manhole.
Fix the cause or the sludge comes back
Cleaning without fixing the inlet is a short-term fix. Check the leaf screens, the inlet strainer and the overflow screen. A first flush diverter stops much of the fine dirt before it gets in. Make sure the overflow is clear and screened as well.
Be honest about what cleaning achieves. It removes sludge. It does not disinfect the water, and it does not make a tank safe for drinking.
Hiring a tank cleaner: what to ask
If the job needs someone to climb in, use a professional. These questions help you pick a good one.
- Are they trained for confined spaces? Fumes, low oxygen and a slippery floor are real dangers.
- How will they clean it? Vacuuming with the tank in use and draining to scrub are different jobs.
- What happens to the stored water? Draining means you lose it.
- What will you get afterwards? Cleaning removes sludge. It does not disinfect or make water safe to drink.
- Will they check the screens? If the inlet is not fixed, the sludge comes back.
- Do they carry the right gear? Ask what safety equipment they use.
A cleaning day plan for light sludge
For a small tank with light sludge, you can often work from the outside. Never climb in.
- Check the weather. Pick a dry stretch so the tank is not filling as you work.
- Switch off the pump. Stirred-up sediment can clog it.
- Clear the screens and gutters. This stops fresh dirt entering while you clean.
- Reach through the manhole. A vacuum head on a pole reaches most of a small tank.
- Work slowly. Rough work stirs up fine sediment.
- Check the pump filter afterwards. Clean it if flow drops.
If the sludge is heavy, the smell is strong or the tank is buried, book a trained cleaner. See also the yearly maintenance checklist.
Frequently asked questions
How often should a rainwater tank be cleaned?
Most tanks need it every few years. Tanks under overhanging trees, or with a failed inlet screen, may need it sooner.
Can I clean a rainwater tank myself?
Light sludge can be siphoned from outside through the manhole. Never climb into a tank yourself, because it is a confined space with real risks. Use a trained tank cleaner for that.
Does cleaning make tank water safe to drink?
No. Cleaning removes sludge but does not disinfect. If you drink the water, have it tested by a NATA accredited laboratory.
Why does my tank water smell musty?
A musty smell usually means sludge or organic matter has built up on the floor. Check the inlet screens as well, or the problem returns.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- NHMRC. Australian Drinking Water Guidelines
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)