“First flush” system keeps sediment out of rain barrels
The “first flush” drains the dirtiest rainwater (from the first few minutes of any storm) off the roof and into the ground, bypassing the cistern or rain barrel. There are many ways to make a first flush system–one of the simplest systems is pictured here.
A section of 3″ diameter pipe comes straight down from the gutter downspout and ends in a cap. The cap must leak! You can screw the cap on loosely, or drill a hole in the cap and shove a drip irrigation “spaghetti tube” into the hole. When it rains, this pipe fills up. Water begins to trickle out the bottom, but at a much slower rate than rain is flowing in. Once the pipe is full, the (now cleaner) water spills through a branch (to the left in the photo below) and into the first barrel.
Photos and captions by Christina Bertea of Greywater Action, http://www.greywateraction.org.
First flush system for linked rain barrels (400-gallon capacity). Because the barrels are connected with HDPE tubing near the bottom, they all fill at the same time.
Here you see the gutter emptying into the 3″ pipe, which is strapped to the fascia board.
The first dirty water goes straight down until it fills that whole pipe, then as cleaner water comes down from the gutter it flows into the side branch to the tanks.
We used a hole saw to cut a hole in the plastic top of the tank. The pipe sticks down in about an inch, where it touches the mosquito screen. This setup will block most light from hitting the water–important for limiting algae growth.
The bottom of the first flush pipe is supported by bricks and the drain tube extends to wherethere are plants. When crud builds up inside, the entire plug at the bottom can be unscrewedand the pipe cleaned out.
Rainwater entering the top of the barrels. The overflow pipe exits on the side. Had I extended the 90 out farther from the side of the tank, it would have raised the water level in all of the tanks, since the slope of the tank wall made the outlet pipe angle upward. Remember that the overflow MUST be lower than the entrance point–or the tank will overflow!
The “full” water level in all tanks will be the same since “water seeks it’s own level”….
The 90 degree fitting has female threads on the tank side which screwed onto a 2″ male adapter sticking through the wall of the tank from the inside. The rubber washer is on the inside is compressed against the wall of the tank by tightening the 90 degree fitting onto the male threads.
The overflow drops to just below the level of the doors on the side of the garage and slopes 1/4″ per foot all the way to the rain garden. Note the plumber’s tape strapping it to the trim at either side of the doors.
The 2″ overflow pipe should be painted (lavender?) for protection from UV light.