First Flush System 90mm

Protect your rainwater quality by diverting the most contaminated water that washes off your roof with the first few millimetres of rainfall with a First Flush Downpipe Diverter.

Calculate your diversion needs

Generally speaking, the more water you divert, the better the quality of the rainwater that ends up in your tank. However, diverting more rainwater than necessary can lead to volume shortages, so it’s important to calculate your diversion needs, and divert water accordingly.  

The following formulas offer general guidelines for calculating how much water you should divert: 

• Areas with low pollution (open fields, no trees, no bird droppings or animal matter, clean environment) = divert 0.5 litres of water per square metre of roof 
• Areas with higher pollution (leaves and debris, bird droppings, animal matter/carcasses, pollution) = divert at 2 litres of water per square metre of roof.  

This calculation will allow you to determine what length and size pipe or pipes you need for your diversion chambers.

1 metre of 90mm pipe = 5.7 litres

1 metre of 100mm pipe = 8.8 litres

By diverting and discarding the most contaminated water from your roof, the First Flush Downpipe helps keep hazardous bacteria and fine particles out of your tank. This improves the quality of the rainwater you collect and is a key step in effective Rain Harvesting practices.

Supplied in a kit with T junction, ball and drip-drainer included, this first flush diverter can be installed on walls, posts, stands or even under your eaves. Simply add a customised length of pipe to divert your desired volume of dirty, contaminated water from each downpipe.

Available accessories to suit the First Flush System

Leaf Eater

Leaf Beater