Filters for roof top rainwater harvesting
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Many products are available. The plumber too can make an economical filter
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One of the most frequently asked questions when rooftop rainwater is to be harvested is about filters. Where can I buy one? How much does it cost? How many filters do I need? Would they be easy to clean? How often should I clean them? Should they be on the ground or below it? These are some of the more common questions.
As we go about the business of harvesting rain from roofs, we must understand the limited role of the filter. All it does is to keep organic material such as leaves and some amount of silt out from the stored rainwater. Nothing more and nothing less.
By keeping the organic material out the quality of the stored rainwater remains better for a longer time. In the case of a recharge system like a well, the filter prevents the well from choking up.
Let us begin with the first rain separator or the bye-pass arrangement. The vertical down pipe coming from the roof is closed at the bottom with a threaded end cap costing Rs. 40.
This enables the roof to be cleaned and the wash water allowed to flow out whenever we do not want to collect the rain. Just open the threaded end cap and the wash water goes out. Close it and the rainwater rises and goes out of the down pipe into the filter at a T inserted for the purpose. The T costs Rs. 120. This leads the water into the filter from the top of the drum.
Filter
A 90 litre plastic drum costing around Rs. 400 is needed. One male and female threaded adapter is also required costing around Rs. 120. You can get it from the neighbourhood hardware/plastic ware shop.
This is then assembled with an opening at the bottom cut into the drum. Fill the drum with gravel up to 9 inches and about 3 to 3 ½ inches of coarse sand on top of the gravel.
Better still, just fill 12 inches of washed wood charcoal instead of the gravel and sand.
Tie three layers of netlon mesh to the top of the drum. The rainwater filter is ready.
Place the filter on the ground just where the down pipe comes down. The filter takes about two hours to assemble. A plumber should charge around Rs. 500 as labour for two filters.
This filter will handle rainwater from a 100 square metre roof with an intensity of rain of 60 mm/hour.
For one house one filter is enough. Since however there are likely to be two down pipes two filters may sometimes be necessary.
Visit www.rainwaterclub.org / www.arghyam.org
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