The catchment system is the roof area that collects rainwater and the network of gutters and downpipes that deliver the water into your storage tanks.  

After a dry spell, the roof area can be heavily contaminated with leaves, dust, pollen, insects, and animal waste. The first rainfall after a dry spell will wash these contaminants off the roof, into the gutters and into your water tank.  

Luckily, there are some steps you can take to prevent this contaminated water entering your tank, and these will make a significant difference to the quality of water in your tank. 

First flush diverters

The first option is to install a device called a first flush diverter. This diverts and separates contaminated water that washes from your roof with the first few millimetres of rain. This contaminated water is either diverted into the stormwater system or a secondary storage container.

Removing the first few millimetres of contaminated rainwater is a commonly used method for ensuring high water quality in rainwater harvesting systems. Once the contaminated water has been collected, the clean, uncontaminated water is allowed to flow in the water tank.

While there is almost universal acceptance that a first flush diverter is beneficial, there is no agreement on just how much water should be diverted, but research has suggested the following rule-of thumb of 0.40 litres for every square metre of roof area in low-pollution environments and 1.8 litres/m2 for high pollution environments.