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Rainwater Harvesting (Practical Action Technical Brief)
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==== First-flush systems ==== Debris, dirt, dust and droppings will collect on the roof of a building or other collection area. When the first rains arrive, this unwanted matter will be washed into the tank. This will cause contamination of the water and the quality will be reduced. Many RWH systems therefore incorporate a system for diverting this ‘first flush’ water so that it does not enter the storage tank. [[File:pract.act.rain.water.fig.7a.jpg|thumb|left|Figure 7a: Tipping gutter mechanism]] The simpler ideas are based on a manually operated arrangement whereby the inlet pipe is moved away from the tank inlet, and then replaced again once the initial first flush has been diverted. This method has obvious drawbacks because there has to be a person present who will remember to move the pipe. Other systems use tipping gutters to achieve the same purpose. The most common system (as shown in Figure 7a) uses a bucket which accepts the first flush and the weight of this water off-balances a tipping gutter which then diverts the water back into the tank. The bucket then empties slowly through a small-bore pipe and automatically resets. The process will repeat itself from time to time if the rain continues to fall, which can be a problem where water is really at a premium. In this case a tap can be fitted to the bucket and will be operated manually. The quantity of water that is flushed is dependent on the force required to lift the guttering. This can be adjusted to suit the needs of the user. [[File:pract.act.rain.water.fig.7b.jpg|thumb|left|Figure 7b: Floating ball first flush system]] Another system relies on a floating ball that forms a seal once sufficient water has been diverted (see Figure 7b). The seal is made as the ball rises into the apex of an inverted cone. The ball seals the top of the ‘waste’ water chamber and the diverted water is slowly released, as with the bucket system above, through a small bore pipe. Again, the alternative is to use a tap. In some systems (notably one factory manufactured system from Australia) the top receiving chamber is designed such that a vortex is formed and any particles in the water are drawn down into the base of the vortex while only clean water passes into the storage tank. The ‘waste’ water can be used for irrigating garden plants or other suitable application. The debris has to be removed from the lower chamber occasionally. Although the more sophisticated methods provide a much more elegant means of rejecting the first flush water, practitioners often recommend that very simple, easily maintained systems be used, as these are more likely to be repaired if failure occurs.
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