D. Brett Martinson and Terry Thomas
12th International Rainwater Catchment Systems Conference, New Delhi, India, 2005
Inlet filters are a common method for enhancing water quality in rainwater harvesting systems. They range from cheap cloth or gravel filters to complex and expensive multi-stage systems. Field experience has shown, however that filters often suffer from a lack of maintenance so self-cleaning is an advantage. Filters can clean themselves by dividing the water stream into two components; the first and largest is the clean water passed to the tank, the second much smaller component can be used to carry away suspended particulates.
This paper reports the results of a series of laboratory tests on the abilities of very simple filters to remove particulates from roof run-off water. The low-cost (<$5) filters tested were of stretched cloth: two cloth types were used. Self-cleaning and plain (debris-retaining) filter designs were compared. Each filter was tested with a standard (published) contaminant load based on sand and Polyethylene sheet “leaves” under a variety of representative flowrates.
Filter performance indicators are
- hydraulic efficiency – the fraction of water transmitted
- cleaning effectiveness – the fraction of particulates removed from the flow.
The initial tests showed that self-cleaning by using a sloping surface works satisfactorily and that simple cloth filters have a comparable performance to sophisticated filters found in German rainwater harvesting systems. However the German test configuration used was found to poorly represent roof run-off water in tropical countries and will be changed for continuing tests to be performed on refinements to the crude filters reported here.