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Rainwater First Flush Diverters: How Do They Work?

First flush diverters help make rainwater potable. But how do they work?

You've decided to install a rain catchment system. Depending on what you plan on using the water you water_samples_60catch for, you may need to consider installing a first flush diverter. First flush diverters are designed to discard the first 5 or 10 gallons per 1,000 sq ft of roof during a rainstorm.

In areas where there may be roof contaminates—or if you intend to use the water for potable purposes—discarding this initial wash of water ensures the safety of the collected water. Water catchment systems used for gardening, toilets or washing machines do not require first flush diverters, but according to the University of Hawaii, it is still important to make sure that leaves and debris are not building up in your water tank or pipes—these could damage your plumbing.

An MIT study completed in 2006 found that failure to discard the first flush of water contaminates the entire rainwater tank. However, this contamination does not occur if rains happen within three days of each other, because the roof is still clean.

There are a number of commercial first flush systems available that fit in line and are easily installed. It is also possible to make your own first flush system when designing your system. There are two common first flush diverters in the US. The first is called a constant-volume first-flush container. In this system an in-line vessel collects all of the water until it is full. This vessel is appropriately sized for the roof and any rainwater that occurs after the vessel is full is diverted to the main storage tank.  

The other common first flush diverter is a valve that is sensitive to the rate of rainwater flow. It relies on a  = ball to determine when it is safe to send water to the main storage tank. Because these diverters are more sensitive and thus better designed to prevent contamination, valve type diverters are considered a better choice for a rain catchment system. First flush diverters can be installed anywhere in the rain catchment system—either near the gutter or near the tank. Example

Regardless of the type of first flush system used, it is important to inspect it regularly and ensure it's working properly. Some systems will have a filter to remove particulates—which must be cleaned regularly.

Additionally, you must inspect the roof, gutters and piping to ensure they are free of debris or buildup may occur that will cause the first flush system to fail.