Motivation: Rainwater is usually very clean - drinkable, in fact. Unfortunately, we live in a very dirty world so when rain falls on something, it picks up dirt. I'm not goin gto use my rain barrels as a source of drinking water but I do want the water to be clean. So, to help clean the water headed to the barrels I put a first flush device just upstream from the barrels.

Background:

In between rains dirt accumulates on surfaces. This dirt is washed away when it rains but it is not washed away evenly; early runoff is much dirtier than later runoff. Knowing that the beginning of a rainfall is where most of the dirt is, I built a first flush device (FFD) to divert the first ~5 gallons of rainwater from the barrels. This diversion prevents a large amount of dirt from getting into the barrels.

There are many different ways of building FFDs and you can buy them too but buying one would be no fun. In designing/building my FFD I wanted low to no maintenance, few to no moving parts and an elegant design. We're always looking for the elegant solution...

Most FFDs I saw are very simple devices but many seem overly complex, require a lot of attention and/or have too many moving parts. The design I settled on is essentially a long tube with a small ball in it (I'll add a detailed diagram in the future). The tube is about 5 gallons in volume and placed between the collection surface (my roof) and the barrels. At the bottom of the tube I have a spigot which I keep cracked open slightly so it takes 5-10 hours for the tube to fully drain.

When it begins raining the first five gallons of water does not get to the barrels, it fills the tube. Once the tube is full the ball floating in it blocks off the top (think of a ping pong ball in a 2 liter bottle - when the bottle is almost full, the ball will block more liquid from entering the bottle). With the tube full and the top blocked, the rainwater then is free to fill the barrels with cleaner water. The spigot at the bottom slowly drains the tube but it doesn't take much rain to continually top off the tube. When the rain stops the tube slowly drains until empty and is ready for the next rainfall.

Coming in the future: diagrams, materials, building instructions and more photos.