What is a First Flush Diverter?

First Flush Diverter - What does it do?

The First Flush Diverter captures and diverts the first few litres of rainwater that wash off from your roof and gutters, preventing it from entering your water tank. Those first few litres of water are often contaminated, by dust (especially if you live in a windy area), airborne pollutants (if you live near an airport or factories), animal fecal matter (mostly from bird droppings) and leaf debris. Click here to see Recommended First Flush Diverters.

First Flush Diverter, 80mm

How does the system work?

The water container of a First Flush Rainwater Diverter is a vertical PVC pipe with a 80mm or 110mm diameter. Inside this pipe is a ball system - a flotation ball that floats on top of the water. When the water level rises through the vertical pipe and reaches the top, the ball floats at the surface of the water and closes the pipe hermetically, directing the water overflow into your rainwater tank. At the bottom of the Diverter, the end cap is fitted with a ball valve (or similar) to empty the contaminated water after the rain.

It is best to have one diverter on each downpipe. However, a single unit can be placed close to the rainwater tank and perform the first flush for several downpipes. This will really depend on how big your roof catchment is and how clean you need the water to be.

What are the benefits of a First Flush Diverter?

By filling up your tank with cleaner water, the First Flush system will help you:

  • limit the growth of algae, bloodworms, harmful bacteria and protozoa
  • limit the build-up of sludge, contaminants and sediments
  • extend the life of the water pump and filter cartridges connected to your tank

Where do you install a First Flush Diverter?

A First Flush system should be installed just before your rainwater tank, after a Leaf Catcher. The Leaf Catcher first diverts big debris and leaves, and the First Flush removes smaller debris, animal droppings, pollutants and dust, before they can enter the rainwater tank. 

First Flush systems work best when a leaf catcher is installed at the start of the downpipe. The leaf catcher box (available from us and sold separately) is fitted at the start of the downpipe either directly on the underside of the roof gutter at the facia board, or to the wall. 

First Flush DiverterFirst Flush Diverter installed below gutter

How big should a First Flush Diverter be?

The bigger the roof, the bigger the First Flush needs to be. To remove most dirt off a roof area of 50m², the device should ideally be able to collect about 25 litres of water. This represents a length of 2.5m for a 110mm diverter, or 5m for an 80mm diverter.

Recommended First Flush Diverters

80mm First Flush Fiverter
80mm First Flush Diverter
110mm First Flush Diverter
110mm First Flush Diverter

Is a First Flush Diverter recommended when harvesting rainwater?

At EcoDepot we always recommend using a First Flush. It extends the life of your pump and filters, and limits the growth of unhealthy bacteria in your rainwater tank, such as E. coli and Giardia.


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