First flush diverters for rainwater tanks
The first rain after a dry spell carries the most dirt. A diverter keeps it out of your tank.
A first flush diverter sends the first part of each rainfall to waste instead of into your tank. It does not make tank water safe to drink. It simply stops the worst of the roof dirt from settling in the bottom of the tank.
What is in the first flush
Between storms, your roof collects dust, leaves, bird droppings and the odd dead insect. The first rain washes all of that into the gutters. If it flows straight into the tank, it adds sediment and organic matter that feed algae and bacteria.
Leaf screens catch the big pieces. They do nothing about the fine dirt that rides in with the water. That gap is what a diverter is for.
How a diverter works
Most are a vertical pipe with a floating ball or a similar valve. Dirty water fills the pipe first. When the pipe is full, the ball seals the top and the cleaner water carries on to the tank.
The pipe then drains slowly through a small outlet, ready for the next rain. Some designs sit on a downpipe. Others are built into a rain head or sit below the gutter line.
| Part of the system | What it catches | What it misses |
|---|---|---|
| Gutter guard or leaf screen | Leaves and larger debris | Fine dust and droppings |
| First flush diverter | The dirtiest water at the start of rain | Dissolved contaminants and anything after the pipe fills |
| Inlet strainer on the tank | Debris and insects before they enter | Dirt already in the water |
Are they worth it?
For most tanks that collect from a roof, yes. A diverter is cheap compared with the cost of cleaning sludge out of a tank. It is most useful if your roof is shaded by trees, or if you get long dry spells between storms.
It is less useful if you only use the tank for the garden and do not mind some sediment. Even then, less dirt in the tank means fewer blocked pumps and filters.
Diverters need looking after
A diverter that never gets cleaned fills with muck and stops working. If the drain outlet blocks, the pipe stays full and the dirty water gets through or the pipe overflows. Check it every few months and after big storms.
Maintenance matters for the rest of the system too. See why a tank overflows for what happens when inlets and outlets are not kept clear.
A simple diverter care routine
A diverter that is never cleaned fills with muck and stops working. A short routine keeps it doing its job.
| When | What to check | Why |
|---|---|---|
| Every few months | Look at the chamber and clear anything that has collected | Muck stops the ball sealing |
| After a big storm | Check the chamber is empty and the ball has dropped | The pipe has to be ready for the next rain |
| After a long dry spell | Look at the drain outlet before the first rain | Dust and leaves build up between storms |
| If the tank shows sediment | Check the outlet is not blocked | A blocked outlet keeps the pipe full, so dirty water gets through |
| If the pipe overflows | Clear the outlet and look at the downpipe | The chamber cannot drain between rains |
Only check what you can reach from the ground. Fitting or moving a diverter is licensed work. Our guide on why a water tank overflows covers what happens when inlets and outlets are not kept clear.
Does your tank need a first flush diverter?
For most tanks that collect from a roof, yes. Use this table to see where your own setup falls.
| Your situation | Worth fitting? | Reason |
|---|---|---|
| Roof shaded by trees | Yes | Leaves, droppings and dust collect more often |
| Long dry spells between storms | Yes | The first rain after a dry spell carries the most dirt |
| Pump or filter keeps blocking | Yes | Less dirt in the tank means fewer blocked pumps and filters |
| Garden-only tank and sediment does not bother you | Less useful | Even so, cleaner water is easier on the tank |
| You plan to drink the water | Helps, but does not solve it | It reduces dirt but does not treat water |
A diverter stops the worst of the roof dirt only. If you plan to drink tank water, have it tested by a NATA accredited laboratory. Read whether tank water is safe to drink before you decide.
Frequently asked questions
What does a first flush diverter do?
It diverts the first part of each rainfall, the dirtiest water off the roof, away from the tank so less sediment and organic matter gets in.
Does a first flush diverter make rainwater safe to drink?
No. It reduces dirt, but it does not treat water. If you plan to drink tank water, have it tested by a NATA accredited laboratory.
How often should a first flush diverter be cleaned?
Check it every few months and after big storms, and clear the drain outlet and the chamber so it empties between rains.
Do I need a licensed plumber to fit a first flush diverter?
Fitting one into roof drainage or connecting it to plumbing is usually licensed work. Check the rules with your state or territory regulator.
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)