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Home > Accessories > First Flush Diverters

First Flush Diverters

Downpipe First Flush Water Diverters improve water quality and reduce tank maintenance by preventing the first flush of water, which may contain roof contaminants, from entering the tank. They help ensure cleaner water is available for use, which protects rainwater pumps and internal household appliances such as clothes washing machines, toilets, hot water systems, etc.

Downpipe First Flush Diverters, such as the 90mm, 100mm and 150mm kits, are installed at each downpipe that supplies water to the tank. They utilise a dependable ball and seat system - a simple automatic system that does not rely on mechanical parts or manual intervention. As the water level rises in the diverter chamber, the ball floats, and once the chamber is full, the ball rests on a seat inside the diverter chamber preventing any further water entering the diverter. The subsequent flow of water is then automatically directed along the pipe system to the tank.
Features and Benefits

Prevents sediment, bird droppings, spiders, insects, mosquito eggs and debris from entering the rainwater tank.


Improves water quality, protects pumps and internal appliances.
Ideal to use in conjunction with a rain head.
Easy installation, just add pipe and glue.
No mechanical parts.
Low maintenance requirements.

How do they work?

Fitting an appropriately sized First Flush Water Diverter is critical to achieve good quality water. Water Diverters improve water quality, reduce tank maintenance and protect pumps by preventing the first flush of water, which may contain contaminants from the roof, from entering the tank.

When it rains, water slowly builds up in the roof guttering system before it exits through the downpipe. The first flush of water from the roof can contain amounts of bacteria from decomposed insects, skinks, bird and animal droppings and concentrated tannic acid. It may also contain sediments, water borne heavy metals and chemical residues, all of which are undesirable elements to have in a water storage system.

Instead of flowing to the water tank, these pollutants are diverted with the initial flow of water into the chamber of the water diverter. The water diverters from Rain Harvesting utilise a dependable ball and seat system - a simple automatic system that does not rely on mechanical parts or manual intervention.

As the water level rises in the diverter chamber the ball floats, and once the chamber is full, the ball rests on a seat inside the diverter chamber preventing any further water entering the diverter. The subsequent flow of water is then automatically directed along the pipe system to the tank.

For a water diverter to work effectively, the contaminated water in the chamber must be sealed off. This prevents the water passing to the storage tank and ensure the water does not "syphon" off the contaminated water in the chamber. The simplest and surest way to do this is with a floating ball and seat.

A slow release valve ensures the chamber empties itself after rain and resets automatically. The diverted water need not be wasted water because the drain pipe from the diverter chamber can be fitted to a standard drip irrigation system.

How much water to divert?

It is important to prevent heavy sediments and other roof pollutants from entering the rainwater tank. The amount of water diverted should be a minimum of 20 litres per 100 square metres of roof area (or 0.2L per m2). In calculating the amount of water to divert, consideration can be given to (1) the surface area of the roof, and (2) the amount of pollutants on the roof and gutters.

The following factors can be used as a guide in determining the volume of water to be diverted.

As a rule of thumb, the more water that is diverted the better the quality of water in the tank.

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