When rain hasn't fallen for a few weeks, your roof becomes a collection area for bird droppings, dust, leaves, atmospheric pollutants, and dead insects. When the rain finally arrives, the very first few millimeters wash all of this debris directly into your gutters. If you allow this initial "dirty" water to enter your rainwater tank, it provides a feast of nutrients for algae and drastically lowers your water quality. This is where the First Flush Diverter becomes your system's unsung hero and first line of defense.
How First Flush Diverters Work
The concept is brilliantly simple and relies entirely on gravity and basic mechanics; no electricity is required. A first flush diverter is essentially a chamber (usually a length of PVC pipe) installed between the downspout and the tank inlet. Inside this chamber is a buoyant ball.
As the initial, contaminated rain flows down from the roof, it falls into the diverter chamber rather than flowing across to the tank. As the chamber fills with this dirty water, the buoyant ball rises. Once the chamber is completely full, the ball seats itself against the top opening, creating a waterproof seal. At this point, the diverter is essentially full, and the subsequent, much cleaner rainwater can now flow straight over the ball and into your storage tank.
How Much Water Should You Divert?
The amount of water you need to flush depends primarily on the size of your roof and your local environment. A heavily wooded area with lots of birds requires more flushing than a clean metal roof in a suburban setting.
A general rule of thumb is to divert between 0.01 to 0.02 gallons per square foot of roof area. If you have a 1,000-square-foot roof, you should aim to divert the first 10 to 20 gallons of rainfall. The length and diameter of the PVC pipe used for your diverter chamber determine its holding capacity. For instance, four feet of 4-inch PVC pipe holds roughly 2.6 gallons.
Required Maintenance
A first flush diverter is only effective if it's empty before the next rainstorm. At the bottom of the diverter chamber is a small, adjustable drip valve. This valve slowly leaks the dirty water out onto the ground or into a drain over the course of a few days. Once the chamber drains, the buoyant ball drops back down, resetting the system for the next rain event.
Maintenance is minimal but crucial. The drip valve can easily become clogged with the very sludge the diverter is designed to catch. Every few months, especially during heavy pollen or leaf-fall seasons, unscrew the bottom cap of the diverter to clean out any built-up sediment and ensure the drip valve is flowing freely.
Conclusion
A downspout screen keeps out large leaves, but only a first flush diverter can stop dissolved pollutants and fine sludge. By sacrificing a few gallons of dirty water at the start of a storm, you ensure thousands of gallons of crystal clear water in your tank.