Budget First Flush DIY System
The “First Flush” rainwater filter system diverts any small or dissolved pollutants from your roof and gutters into a separate pipe chamber, so it does not enter the tank. It also stops sediment from building up in the bottom of your tank.
When it first starts to rain, water slowly builds up in the roof gutter system before it exits through the downpipe. The first flush of water from the roof can contain amounts of bacteria from decomposed insects, lizards, bird and animal droppings and concentrated tannic acid. It may also contain sediments, water borne heavy metals and chemical residues, all of which are undesirable elements to have in a water storage system.
Gutter mesh and Leaf catchers (80mm or 100m) will help with leaves and larger debris but cannot stop small bits like dust and bacteria. That is why it is recommended that you install a First Flush Diverter. We all know that prevention is better than cure and so it makes sense to stop the pollutants from entering your tank.
How it works:
The first flush device is installed along with the gutter downpipe that will feed into your water tank. As the water level in the first flush diverter chamber (110mm pipe) rises, the ball floats and rises. Once the chamber is full, the ball rests on a seat inside the chamber preventing any further water entering the diverter. The subsequent flow of clean water is then automatically directed along with the pipe system through the T piece into the rainwater tank.
It is best to have one first flush diverter on each downpipe however a single unit can be placed close to the tank and perform the first flush for several downpipes. This will really depend on how big your roof catchment is and how clean you need the water to be.
The amount of water you need to first flush will depend on a few things like:
- The size of your roof- The recommended minimum first flush is 4L per 20m of the roof catchment area.
- The number of leaves and pollution on the roof (larger polluted areas need to be flushed more)
- The intended use of the water (drinking water will need a bigger first flush, as well as additional filtration systems).
(You should always check the quality of the water in your tanks before drinking it).
The amount of water flushed is determined by the length of the 110mm PVC (flush) pipe attached to the first flush kit. The average is 1m of 110mm PVC pipe will flush 50sqm of the roof catchment area. The 110mm pipe is not included in the DIY kit but is readily available at any good hardware store. (It is sometimes only sold in 4 or 6-meter lengths although some hardware stores or plumbing suppliers will cut a piece for you). As an example, 850 mm of 110mm pipe will give you an 8-liter system.
The first flush system is a simple and effective device. After the rain, just drain the system through the valve, ready for the next rain. The bottom of the unit (end cap) can be screwed off to fully clean the unit occasionally. During periods of frequent rain, it may not be necessary to drain the first flush device every time it starts raining as the roof and gutters may already be clean.
