Filtration systems and settling tanks
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 shown on the right cleverly takes in water through a very fine (0.17mm) 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 problems attached: firstly it only removes large debris; and secondly the rack can become clogged easily and requires regular cleaning.
An upflow sandfilter is sometimes used for filtering rainwater entering a tank. This type of filter is only suitable, however, where the inflow is slow, and will soon overflow if the inflow exceeds the rate at which the water can percolate through the sand:
Basic Filter from India
A basic filter developed for rural roof-top rainwater harvesting projects.
The filter is a funnel with a net (plastic) at the bottom, 75 mm of sand on top of the net and one more net at the top.
The test being carried out ensures a good flow as well as good filtration.
Settling tanks and partitions can be used to remove silt and other suspended solids from the water. These are effective where used but add significant additional cost if elaborate techniques are used.