ELEMENT:
First Flush Device
AIM:
As part of the strategy to minimise the use of external mains water at
the Healthy Home a 22 000L rainwater tank was installed. First Flush devices
are used to protect water quality in the tank by intercepting the intial
amount of (contaminated) runoff from the roof (the first flush) and divert
it to waste, rather than the rainwater tank.
STRATEGY:
In between rainfall events dust, car exhaust, leaves and animal droppings
accumulate on the roof of the Healthy Home. It is generally accepted that
the initial amount of runoff from a roof, say the first 1mm, is more heavily
contaminated than the subsequent runoff volumes. A First Flush device
is designed to prevent these contaminants from entering the rainwater
tank and adversely affecting the stored rainwater.
First Flush devices store the first millimetre or so of rainfall, and
divert this water away from the rainwater tanks. The remaining clean rainwater
collected by the roof is then directed into the rainwater tank. As a rule
of thumb, 100L of first flush water is required for each 100m squared
of roof area. The Healthy Home has 124 square meters of roof area connected
to the 22 000L rainwater tank, and three first flush devices which each
capture 25 litres of first flush water. The 75 litres of First Flush storage
is equivalent to 0.6mm of rainfall runoff.
The First Flush devices at the Healthy Home operate by filling of a small
storage tank which causes a "captured" floating ball to rise
to the top of the tank. The captured ball seals the inlet of the storage
tank, allowing all subsequent roof runoff to be diverted into the rainwater
tank. The inlet located at the top of the first flush device is covered
by a domed shaped fine mesh screen to intercept larger pollutants such
as leaves and insects.
The First Flush storage tank has a trickle draining valve located at
the bottom that continuosly releases a small amount of water. During most
rainfall events, roof runoff rate exceeds the loss rate from the trickle
valve, resulting in the filling of the storage tank and diversion of clean
water to the rainwater tanks. After the rainfall event, the trickle valve
empties the storage tank resetting the system for the next rainfall event.
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