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First flush filters and diverters, theory and application..

First flush filters and diverters, theory and application..

So you've started on the rainwater harvesting journey and are wondering what components you need to optimise your system.

The key elements are: filter, storage and distribution.

Capture is usually through a water butt connector or combined diverter-filter of which are there are many to choose. These diverter-filters allow all rainfall diverted to be filtered, the filtration level is determined by the mesh size.  

You may have heard the term "first flush" being used and are wondering what this means. First flush refers to the idea that the first rain that falls cleans the capture surface (roof) of debris and contaminants and so this initial rainwater carries the most debris, contaminants, bird droppings etc. This concept has been around for some time. Many reservoirs in the UK for example have a bypass channel at the head of the reservoir to divert this initial first flush of rainwater around the reservoir perimeter preventing this initial poor quality water entering the reservoir. Following the first flush the water quality improves.

Being able to divert the first flush would mean less contaminants in the main tank and fewer issues when this water is distributed, for example reduced blockages of valves and pumps etc.

First flush filters are unlike conventional filters in that they do not actually filter the rainwater as such. The first flush filters simply work by diverting a set amount of the initial rainfall to a drain and not to the water butt or storage tank. Once the initial rain has passed it and is running clear of contaminants it is diverted to conventional storage. The filter has a slow reset which means the next time it rains the filter is ready to divert. Clearly the filter reset time must be long enough so that if another rainfall event follows immediately after an initial rainfall event this water is stored and not bypassed.

Typically, first flush filters length of pipework -equivalent to the first flush volume contains a ball. During a rainfall event the first flush volume is filled initially, the ball rises until it seals off the first flush volume. At this point the rainwater should consist of T-fitting in the pipework. Assuming the T-runs horizontally as shown below, the vertical a be running clear and continues to run onwards to the rainwater tank.


Of course the first flush volume needs to be sized correctly so that clean water is minimised. There is plenty of research on the subject. A brief summary below: 

People for Rainwater Japan states that according to research done in March - April 1980 on rainwater collected from the roof of an office building in central Tokyo, about 0.5 - 1 cm of rainfall was needed to wash the surface of the roof. Other research, done in August - September 1986, also in central Tokyo, measuring the degree of change in water quality in rainwater collected from a roof after a period of about 11 days of no rain, showed that water quality is generally stable after 1.5 - 2 cm of rain.

The MSc thesis: Sizing the First Flush and its Effect on the Storage-Reliability-Yield Behavior of Rainwater Harvesting in Rwanda (K Doyle, 2006 Villanova University) suggests that with "1 mm diversion after 3 days... water quality of the stored water will greatly improve"

Quantifying the First-Flush Phenomenon: Effects of First-Flush on Water Yield and Quality provides a detailed methodology of determining the first flush volume by selcting the contaminant removal efficiency desired. "A more sensible diversion is over 2mm which both calculation and simulation yield a removal efficiency of about 90%"

So for example if you have 100m2 roof area and wish to divert the first 1mm (0.001m) this is equivalent to 100 litres (100m2*0.001m=0.1m3 or 100l). So a first flush filter can be fitted to any 100l tank to achieve the desired first flush diversion.  

Here at freeflush we have our own firstflush devices which can be fitted to any storage tank. Allowing the user to dictate the first flush volume. Get in touch to learn more.

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