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Engineering LibreTexts

2.4: First-Flush

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  • The first-flush diverts the first and dirtiest portion of rainwater away from the storage and remaining system components. Pollution collects on the roof between rains and is subsequently washed during the beginning of the next rain. Typical pollutants include bird droppings, ash from fires and fireplaces, and accumulated air pollution from car exhaust and industrial emissions. Long dry seasons, heavily polluted environment, and porous collection areas will raise the amount of pollution present on the roof before the rain. The first-flush prevents much of this pollution from entering the system.

    Many systems are designed without a first-flush. In fact, without water quality testing, it can take a while to notice the impacts of not including a first-flush. While more studies need to be done, most studies show that removing the first part of the rain removes a significant portion of the pollutants.

    There are two primary types of first-flush: the tipping method and the floating ball method (Figure 2-18). Both techniques make the rainwater harvesting system healthier and longer-lasting without significantly increasing maintenance requirements. Both techniques include a method to evacuate the water and reset the system between rains.

    The floating ball method (Figure 2-19) uses fewer moving parts and is more durable than the tipping method. In a floating ball first-flush, the first portion of the rain fills the first-flush volume until the floating ball floats to the top and closes off the first-flush, allowing the remaining rain to run to the storage. A small borehole near the bottom of the first-flush allows for the first-flush to slowly evacuate after a rain. A removable cap allows for cleaning clogs and any debris that has found its way into the first-flush (Figure 2-20 through Figure 2-23).

    That said, even with the floating ball technique, there are some important caveats and difficulties. A main source of issues is the small borehole:

    • It easily clogs. Make sure to make the hole high enough so that any collected solids at the bottom of the first-flush do not block the hole. Using a removable cover for cleaning the first-flush is encouraged. In addition, attaching a tool to clean out the borehole encourages proper maintenance.
    • The small hole may eject a high-velocity stream of water, depending on the height of the water above it. Make sure to direct the stream in a way that is useful and does not encourage erosion.
    • The hole must be made small enough so that water does not evacuate too quickly. Current research suggests a trickle to divert less water from rain events close together in time.
    • Make sure to test the system by filling the first-flush with water.







    The water from the first-flush diversion can still be used for end uses that do not need to be clean, such as cleaning equipment or watering ornamental plants.

    The partially cocked valve solution in Figure 2-23 is slightly more expensive than a small borehole, due to the cost of the valve as opposed to just drilling a hole. This extra cost can be offset by not needing a removable bottom cap. The borehole method does introduce one interesting social issue—people will often close the valve all the way if they see it dripping. The valve needs to continue to drip so that the first-flush will be fully evacuated and ready for the dirty water from the next rain. Therefore, appropriate signage or a method to prevent fully closing the valve is needed.