First Flush

What is First Flush?

During the first moments of a rain storm pollen, mold spoors and other contaminants are washed off a rooftop and into its drainage system. If the downspout tube does not have some type of first flush device to remove this foul water it will be washed into a rainwater harvesting system. The water will then stagnate and make it even unfit for washing clothes. The combined pollutants can stagnate water quickly inside a rainwater tank so a first flush device is required.

Two of the more popular types of devices are first flush units and constant volume containers. Constant volume containers simply fill to capacity then spill over into the pipe leading to a rainwater tank. First flush valves are optimized to meet the rate of water flow using a filler ball device.

  • Constant Volume Container – If sized correctly it should remove 5 gallons of water per 500 sq. feet of catchment area. The container usually has a ball that rises and falls with the water level to block off the flow when the basin reaches its required capacity. A clever constant volume device will use something like a soaker hose at its base for slow water dispersal (which empties it when it stops raining) to not allow loss of cherished rain during a storm.
  • First Flush Valves – These valves are optimized to meet the rate of water flow. It uses a ball that allows a set amount of rainwater to fill its small core, and then when it reaches a required weight it presses a valve that blocks off the flow to the first flush device.
  • The problem with first flush valves is that they do not divert water to a rainwater tank unless there are ample amounts of rain coming off a rooftop. For smaller roof surfaces the constant volume container is a good choice. For places that have monsoon like rain showers a first flush valve will work fine.

    Both the constant volume and the first flush units work great for different weather. If they are correctly set to meet the flow of rainwater from a rooftop catchment area they are very useful in removing contaminants. Make sure if you are purchasing a first flush device that it has a removable screw cap so you can clean it out from time to time.


    Posted in Filters by Administrator on July 8, 2005.

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