First-Flush Diverter

by | April 1st, 2013 | Green Materials & Products, Open Source Designs, Water & Sewage | 3 Comments

How to make a first-flush diverter

In May 2012 an Arkitrek Camp team made a first-flush diverter for a rainwater harvesting system on their design for the Mantanani Bunkhouse. Here’s how it works.

This building has a roof area of approximately 45m² which means that 1mm of rain will yield approximately 45 litres of water for collection. The first flush diverter with a volume of 35 litres is designed to divert the first 1mm of each rainfall. There’s no science or research behind this figure, it just feels ‘about right’. The roof is traditional nipah palm thatch with a strip of galvanised corrugated steel at the eaves to collect water neatly into a gutter.

The following drawing is available in PDF format

Creative Commons Licence
Arkitrek First Flush Diverter by Arkitrek is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

Based on a work at

We have revised the above drawing to fix a problem we had with both keeping the end cap on and making it easily removable for cleaning. This design uses a screw-on end cap, which can be supplied by pcv pipe manufacturers.

If you’ve made a first-flush diverter yourself, please share your experience by leaving a comment below or contact us.

The photo below shows a slightly different design, which uses a vertical galvanised mesh screen rather than a floating ball to keep dirt out the storage tanks. It’s at the Bornean Sun Bear Conservation Centre.

3 Comments for First-Flush Diverter


great info on water trap to water tank flush system, i am going to make one for my house, i started a small company to help ppl go green.

Munaver jaman

Dear Sir/Madam

If you have small holes at the bottom of first flush diverter, when the water gets out through the small hole, the ball which at the top of diverter lowers simultaneously. Then again the gutter water fills the diverter again.

Give comment for the above…


    Munaver jaman, there are supposed to be small holes to drain the diverter after the rain stops. The trick is to get the hole size right so that during rain the diverter does not drain more quickly than it is being filled up.