Guidance on use of rainwater tanks

First flush diverters

Page last updated: March 2011

There is some evidence that the first flush of water in a rain event washes the roof catchment and hence may contain higher than average amounts of accumulated dust, bird and animal droppings, leaves and other debris (Coombes et al. 1999, 2002b; Yaziz et al. 1989). Yaziz et al. (1989) showed that, for a small roof, water quality improved once the first 5 litres of water passed through the down-pipe from the roof guttering.

Although further investigations are needed to determine how effective first flush diverters are in reducing chemical and microbial contamination in all areas of Australia (for example, in temperate, subtropical and tropical zones), installation of simple devices designed to reduce collection of accumulated debris is still supported. For an average roof catchment it is suggested that the first 20-25 L could be diverted or discarded. First flush devices are commercially available.

First flush devices should be regarded as an additional barrier to reduce contamination and should not be used to replace normal maintenance activities designed to keep roof catchments reasonably clean.

First flush devices need to be empty when a rain shower begins. If water remains standing in the diverter from the last shower then the diverter cannot do its job. Many commercially available first flush diverters are designed to empty themselves. However, the devices employed to empty them are not always reliable. First flush diverters need to be inspected frequently and when necessary, emptied manually.

The inlet pipe to all rainwater tanks should be easily detachable so that, when necessary, the tank can be bypassed. Manual detachment could be used as an alternative to an engineered first flush device, although the level of control will not be as good.