To First Flush, or not to First Flush

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.
As a case in point, all 5 panelists at the American Rainwater Catchment Systems Association's national conference in 2013 agreed that they prefer not to use first flushes. In a separate session of the same conference, keynote speaker Barnabas Kane of TBK Design also highlighted his distaste for first flush diverters, noting that over the years he's been hired to remove more of them than to put them in.
First Flush Diverter
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:


1. First flush diverters need to be sized correctly for optimal performance, and this is difficult if not impossible to do.
There are many variables that go into determining the optimal size for a first flush diverter. These include rainfall intensity and duration; length of time between rains; roof size, slope, and material; gutter size; wind direction and speed; and air quality. Since most of these factors can vary tremendously even in a single location, you may find that the "optimal size" for your first flush diversion is different for every storm.  And yes, a mis-sized first flush is a bad thing:

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...

healthy plants and soils
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.)
2. A first flush diverter is the only part of a rainwater system that requires significant maintenance, and if neglected, it can worsen the problem it aims to solve.
Luckily for most of us, non-potable rainwater catchment systems are amazingly low-maintenance. However, first flush diverters must be cleaned out regularly in order to serve their purpose, and in fact should be emptied prior to every storm event to prevent mixing diverted ("dirty") water with fresher flow. Professionals have witnessed that even the most well-meaning user will neglect this maintenance and reap the consequences.
First flush diverters can be designed with a continuous drain to eliminate the need to empty between rains, but with so much "bleeding," this can be a liability, especially in climates with light rains and/or long dry seasons. It is like having a constant leak that draws water away from your rain tanks or barrels.
3. First flush diverters create a weak point in the conveyance system.
A first flush diverter is usually made of exposed pipe material. This makes it more vulnerable to physical impacts and freeze-cracking than other parts of the rainwater catchment system. Since a first flush device is "upstream" of the storage tanks by design, a damaged first flush will divert too much water—and potentially all water—from the storage vessels. This will result in slow fill rates, or even empty tanks.
4. You have to screen the water anyway. Does a first flush diverter provide additional benefit?
Many states have adopted code to establish simple standards for building safe and effective rain catchment systems. In the state of California, for example, one key requirement is that systems must be equipped with a "debris excluder" (e.g. a leaf eater), and in fact all openings must be protected by 16th" mesh, including the inlet.  This mesh keeps particulates from clogging the system, and also prevents the entrance of insects and other small creatures. If the code requires screening as the preferred method for rough-filtration at the intake, what we're left to determine is whether there's significant additional benefit to adding a first flush diversion.
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.
5. It turns out plants actually like the organic matter that the first flush of rain delivers.
If you are harvesting water for irrigation use (as most of our customers do), the plants actually benefit from the organic accumulation that the first flush delivers. Why go to the effort to divert what amounts to a nice fertilizer mix? In the case of non-potable rain catchment systems that are used primarily for garden irrigation, a first flush may be attempting to solve a problem that's not really a problem.
6. It may be easier to divert the first flush manually.
You may find that only the very first rain of the season is dirty enough to justify diverting. Rather than losing all that water in subsequent rains, take the control into your own hands and remove your downspout diverter while the first storm washes your roof. This may be one of the world's greatest ironies, but if an automatic first flush requires manual emptying after every storm, the manual first flush method described here requires less manual input than the "automatic."

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!