Potential hazards and treatment

To minimise detrimental impacts on rainwater quality, potential water pollutants should be intercepted prior to draining into the tank. Discoloured rainwater can be of concern to some for washing clothes and toilet flushing. Odours from poor quality rainwater due to breakdown of leaves and other organic matter in the tank can also be undesirable.

Below are a few options.

Screening  Screening devices positioned between the roof surfaces and the rainwater tank assist to separate this debris from the rainwater.

Combinations of coarse and fine mesh screens can be provided to remove different size debris and prevent tank access by mosquitoes, frogs, rodents and other animals.

Screens are typically provided at breaks in downpipes.

Coarse and fine mesh screens are usually provided in the roof of the rainwater tank to filter rainwater at inlets.

Finer screens are provided over outlets from the tank to prevent mosquito access.

Diagram showing screen at break in downpipe

Diagram showing coarse and fine mesh screens in the roof of the rainwater tank to filter rainwater at inlets.

First Flush Diverters  A first flush diverter should be provided prior to a rainwater tank inlet and after a screening device.


 First flush devices should be sized to divert the first flush of rainwater from the roof which typically contains the highest concentration of pollutants.

Diagram of a diverter

Passive Treatment

Passive water treatment occurs within the tank due to settling of sediments and formation of biofilms on the internal walls of the tank. To prevent excessive build-up of sediment, dewatering of the rainwater tank should occur every few years depending on location. A valve located at the base of the tank assists with completing this efficiently.

Sun-shades should be positioned under screens located in the roof of the tank to minimise light penetration into the rainwater tank to restrict the growth of algae and other unwanted biological activity in the tank that can lower the rainwater quality. Rainwater tanks should be positioned in the shade where possible. Sun-shades should be used to protect tanks from direct sunlight as direct sun can diminish the tank itself and light penetration into the tank can cause algae and other bacterial growth in the water stored in the tank.


 In most circumstances, screens and first flush diverters will be sufficient for providing rainwater of an appropriate quality for all non-potable rainwater uses. In circumstances where finer pollutants or discolouration of water is of concern, filters may be required. Although, filters can regularly become blocked by fine sediments requiring increased maintenance and energy input to pump water. If intending to connect rainwater tanks to all household uses, refer to the NSW Health guidelines for further information on filters and other advanced water quality treatment considerations (http://www.health.nsw.gov.au/environment/water - NSW Health website).