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Reusing grey water is a relatively easy task compared to reclaiming black water
The 10th in our series ‘Waterwise’ brings to you ideas on how one can effectively deal with waste water. The stream of letters pouring in and the overwhelming response that the column has generated from city and overseas are by
itself explanations on cultivating saving habits. And rainy reason is the best time to look at the drops and plan your ways of accumulation.
Once water is used up at a household level it is converted to what is commonly called as waste water. To help us understand this waste water stream better, it is classified into grey water and black water. Grey water is water that comes out of the bathroom and the kitchen. Black water is what comes out from the toilet. Black water is highly pathogenic while grey water is relatively benign.
Reusing grey water is a relatively easy task as compared to reclaiming black water. In Chennai it is actually mandatory to reuse grey water for all buildings though it may not be enforced as strictly as rainwater harvesting is.
Where does this grey water originate? From the bath we have, from the clothes we wash, from the utensils we clean and sundry other non-consumptive uses that we do with water. If we use 135 litres of water every day it is likely that about 65 litres will be grey water that can be reused.
How do we reuse this grey water?
A simple step is to make a planted filter. This is a long narrow channel in the ground about 2 feet wide, about a foot-and-a-half deep and about 10 to 12 feet long. This channel is filled with 40 mm size gravel to about 9 inches depth and sand about 3 inches deep and then soil about 6 inches deep.
Lead the water from the bathroom and the kitchen to this planted filter through a pipe. The pipe should be at least 9 inches below the surface.
Let the water flow through the 40 mm size gravel and occupy all the length of the channel. The rule of thumb is 10 square feet of planted filter for every 100 litres of grey water generated.
Plants such as banana, reeds, turmeric can be planted on the top of the planted filter.
The roots of these plants will go in through the soil and the sand into the gravel and into the grey water and use up all the water for their growth.
They also make use of the soaps and detergents too. You can expect a luxuriant foliage since this planted filter will be permanently watered. Harvest and replant the planted filter regularly.
If there is a large amount of grey water it is possible to create a small trough at one end of the planted filter from where the grey water will emerge. This trough could be about 3 feet x 3 feet x 2 feet deep.
The water collected in this trough can be used for non-potable purpose such as washing the driveway or for watering plants.
Some precautions to take with a planted filter is to site it where there is enough sunlight throughout the day. Plants grow well in sunlight. If possibly the place should also be well ventilated.
The grey water should never be in contact with the air and should always be fed sufficiently below the top layer. The movement of the grey water should be free through the large voids of the 40 mm gravel.
If necessary a small solids trap and grease trap can be designed before the planted filter to avoid the choking of the gravel in the planted filter. Using ph balanced soaps and mild detergents helps the plants do better.
Reusing grey water provides additional water for non-potable use, reduces the burden on the city sewage lines and provides a nice garden. It is a sound ecological move.
If you are building a house or have space for a grey water reuse system, using a planted filter do consider the move.
I grow paddy on my terrace using grey water only but that is another story.
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