A typical first flush device uses a water storage chamber capped with a floating ball.  Contaminated rain water is initially diverted into the first flush chamber. As it fills, the floating ball seals the top of the chamber and water is allowed to flow into the tank.

In practice standard first flush devices have a lot problems:

• The amount of water they can flush is limited by the size of the chamber, typically a few hundred litres at most.
• The drip drain frequently clogs, allow contaminated water into storage
• High maintenance, the chamber needs to be regularly cleared of debris

There are a few variations on the typical first flush device that attempt to address some of these limitations.

Underground first flush pipes allow for larger chambers (and hence more water to be flushed) but they are still prone to the clogging and general maintenance issues affecting standard first flush devices.  They also require significant ground works to install.

Flow rate based devices use plastic balls suspend on elastic strings to divert water based on the volume of water flowing in the pipe.  They are an improvement over standard first flush devices because they can divert more water without requiring a large chamber.  However the amount of water they flush can vary significantly from storm to storm, and is difficult to accurately predict or adjust.  Over time the elastic stretches and they close too early, allowing contaminated water to enter storage.  Flow rate devices also require regular maintenance to clear debris from the valve chamber.

How much water should I flush?
Most manufacturers state something in the range of 0.5 - 1mm.  On a 200m² roof that's around 100 to 200 litres, which is about the upper limit of most devices.  On a larger roof say 450m², flushing 1mm would require a chamber able to store 450 litres.

Is 0.5 - 1mm enough?
No, it's not even close.  Probably the best scientific study available is Quantifying the first-flush phenomenon (Coventry, United Kingdom, DTU/University of Warwick).  To reach the WHO high quality water standard, you need to flush between 3.5 and 8.5mm, depending upon how long since it last rained.