WATR WISE
Store rain water in the sump
S. VISHWANATH
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As we have just seen the first few showers this season, it is the right time to look into the nitty-gritty of harvesting the precious drops in sumps.
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Rainwater harvesting is the process of collecting and storing rain for future productive use. It is an age old practice all over the world but especially prevalent in arid and semi-arid areas. In Rajasthan and Gujarat, rainwater stored in underground tanks called ‘tankas’ provided families with clean drinking and cooking water. Rituals have been built around rainwater harvesting, for example the practice of cleaning the terrace and locking the door to it just b
efore the monsoon, keeping a pot of limestone in underground tanks to keep the water from spoiling.
Is there a modern application of the ‘tanka’ in an urban context? The answer, surprisingly, is a most certain “yes.” Most houses and apartments in cities have a sump built to store water from the main line which comes intermittently. The same sump can be used to harvest rainwater too.
Have a clean roof
How is the harvesting to be done? First of all, we need a reasonably clean roof. It should not have massive amounts of leaf litter or be used to store junk. A clean catchment improves the quality of rainwater collected. If the roof is accessible, sweep it clean often.
Then we must have a good conveyance system for the rainwater. In the case of sloping roofs, a well-designed gutter system will pick up the rain and bring it to the filters. In the case of flat roofs, the down pipes will convey the water to the filter. The gutters and down pipes are usually of PVC for want of an environmentally safer material.
A first rain separator is then to be designed and put in place.
What this does is to provide us with the opportunity to allow rainwater to flow away into the ground when we want to clean the terrace or we do not want to harvest rainwater. It is a bye-pass arrangement.
A first rain separator is crucial to the collection of clean rainwater in sump tanks.
Filtration
Then a filter is designed to keep organic material such as leaves and twigs out of the stored rainwater. Filters are of many kinds. One gets filters as a product also but a simple way of making a filter is illustrated in the drawing.
The filter is essentially a drum or a ferro-cement box of about 90 litres capacity. The inlet rainwater comes in through the top from the down pipe. Inside the drum gravel or charcoal can be filled to varying depths but usually 9 to 12 inches of it is sufficient. A ‘netlon’-type mesh is then kept on the top preferably in three layers. To prevent the gravel or charcoal from slipping out a similar nylon mesh is tied to the outlet pipe from the inside of the filter.
A good filter will also have a simple bye-pass mechanism at the top. This will take the rainwater in case of a heavy downpour and allow it to go straight into the sump. A filter needs regular inspection and cleaning.
Once all these simple tricks are in place, which a good local plumber should be able to do, you are in the business of rainwater harvesting.
Sump size
How large should the sump be? A common question is asked regarding the size of the sump. Even a small storage system can generate large collection provided we use the rainwater every time it rains.
In Bangalore, for example, with it’s evenly spread rains, a 500-litre sump tank can generate 25,000 litres of rainwater in a year. An ideal size is 2,000 litres of sump storage for 100 square metre of roof area. This is as per the building bye-law for Bangalore. A total of 6,000 litres of storage for 100 square metre roof area should give you 70,000 litres of water in a well-distributed rainfall year.
The excess water from the sump during heavy rains should go out through an outlet pipe and fall into a recharge well or a borewell.
Precautions
The roof and the entire system needs to be regularly checked and kept clean. Post-storage, the water can be used for every purpose provided the drinking water is boiled or passes through a UV filter. Water used for bath can pass through a solar water heater and will be safe.
Simple common sense and eternal vigilance will ensure the availability of sustainable water for us.
(The author can be contacted at 23641690 or zenrainman@gmail.com)
www.rainwaterclub.org
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