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Rainwater Harvesting (Practical Action Technical Brief)
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==== Filtration systems and settling tanks ==== [[File:pract.act.rain.water.fig.8.jpg|thumb|left|Figure 8: the WISY filter (downpipe and high-capacity below ground versions)]] Again, there are a wide variety of systems available for treating water before, during, and after storage. The level of sophistication also varies from extremely high-tech to very rudimentary. A German company, WISY, have developed an ingenious filter which fits into a vertical downpipe and acts as both filter and first-flush system. The filter (Figure 8) cleverly takes in water through a very fine (~0.20mm) mesh while allowing silt and debris to continue down the pipe. The efficiency of the filter is over 90%. This filter is commonly used in European systems. The simple trash rack has been used in some systems but this type of filter has a number of associated problems: firstly it only removes large debris; and secondly the rack can become clogged easily and requires regular cleaning. The sand-charcoal-stone filter is often used for filtering rainwater entering a tank. This type of filter is only suitable, however, where the inflow is slow to moderate, and will soon overflow if the inflow exceeds the rate at which the water can percolate through the sand. Settling tanks and partitions can be used to remove silt and other suspended solids from the water. These are usually effective, but add significant additional cost if elaborate techniques are used. Many systems found in the field rely simply on a piece of cloth or fine mosquito mesh to act as the filter (and to prevent mosquitoes entering the tank). Post storage filtration include such systems as the upflow [[sand filter]] or the twin compartment candle filters commonly found in developing countries. Many other systems exist and can be found in the appropriate water literature.
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