This is a question from our FAQ Water Tanks Page where we needed to supply a much more detailed explanation.

A first flush filter is usually fitted to every down pipe that is to feed water to the rainwater tank. The diverter prevents the initial 10-25 litres of water collected from the roof, from entering the inlet of the rainwater tank.

The reason for this is that this water is usually dirty,  contaminated with leaves, debri and sometimes mosquito eggs from the gutter or roof that is been harvested.

A standard first flush filter consists of a PVC tee and a ball float and unscrewable cap. A PVC drop pipe is connected to the bottom part of the tee. This drop pipe should be made as long as is possible so as to collect as much water as possible. The bottom of the drop pipe has a cap on it which holds a ball float.

As the pipe is filled with the diverted water the ball floats up the pipe until it reaches the top of the tee and makes a seal. Once the bottom part of the tee is sealed with the ball, the water is then diverted to the tank.

How The Diverter Works

A small rubber orifice is made in the cap at the bottom of the drop pipe to slowly release the dirty water so the filter is ready for use the next time it rains. Every few weeks, depending on rainfall, the cap should be unscrewed and the small strainer inside cleaned out and replaced.

You can view a video on the function of the filter by clicking on the video below.


If you have anymore questions about the first flush diverter please use our comment form below and we will respond in a timely manner.