An exploration of the first flush diverter by Jesse Savou, ARCSA A.P., Founder of BlueBarrel Rainwater Catchment Systems
To First Flush or not to First Flush: It's a topic of much discussion in the rainwater harvesting world. A first flush diverter (also known as a roof washer) is a simple contraption that diverts the first flow of water away from a rainwater catchment system. The first pass of water in any storm essentially washes your roof of all the sediments that have collected since the last rain. The idea is that diverting the first flush can help ensure cleaner water in your rain tanks or barrels.
Sounds like a good idea, right?
It turns out many rainwater harvesting professionals don't think so.
In this photo, the straight-pipe to the right of the tank is a first flush diverter. Water enters the straight-pipe first, and as sediments sink to the bottom, additional water spills over into the rain tank.
So why wouldn't we use a first flush diverter if it's so easy to do?
In fact, there are many reasons:
If your first flush is too big, you limit your ability to fill your collection tanks. Rainfall abstraction refers to the amount of water that is prevented from reaching your rain barrels or tanks. You can use a rainfall calculator to figure out how much water your rooftop generates, but you'll have to subtract the amount that a first flush diverts... every single time it rains.
If your first flush is too small, the unit will be overwhelmed and sediments will enter your primary storage anyway. In fact, if you have accumulated sediments in your first flush diverter from prior storms, you may even introduce extra particulates to your rain collection system. And this leads us into the maintenance issue...
The organic matter that accumulates on your roof between rains is actually good for your plants and soils. Why divert what amounts to a light application of fertilizer? (See #5.)
This photo shows a debris excluder ("leaf eater"): the white screened box that prevents debris from entering the rain barrel system. These rain barrels are used for garden irrigation, so there is no need for an additional first flush. The clear plastic inlet hose can be manually detached if the user wants to divert the first rain of the season.
Decide for yourself, but all in all, at BlueBarrel we find that first flush diversions are just that: a diversion. With the benefits so questionable, why not focus on what really matters: collecting the abundant fresh water source that falls on your roof. You have no time to lose!