The Zoroastrians brought the consciousness of water related rituals and the concept of harvesting water from ancient Iran. The tanka is a unique water harvesting system, providing pure drinking water at Parsi homes in Bharuch, South Gujarat. Following traditional methods of collecting rainwater, this system has a series of filters, which purify the water.
The tanka is an underground tank, accommodated inside the house, preferably under the kitchen or dining room, made of chiselled blocks of stone, in lime mortar. It is unlined but made waterproof by an indigenous herbal mix which renders the inner surface waterproof, seals minor cracks and prevents bacteriological growth. The tanka is large enough to store sufficient drinking water for a family for six to eight months, its average capacity being around 25,000 litres. With sizes reaching nearly 20 feet by 60 feet and a height of 12 feet, arches and vaults are needed to support the earthwork and the superstructure on top of the tanka.
The tanka is filled from rainwater collected through roof runoff. This simple system of collection, via a 3" to 4" pipe, depends on successive sumps whose water is collected and overflows on its way to the tanka. Settled impurities are flushed out through an overflow pipe.
During the first days of the rainy season, water is made to run down the overflow pipe, ensuring a maximum cleaning of all surfaces. When the owner is certain of the cleanliness, done by constant visual testing and actual tasting of water, the overflow is plugged and the tanka inlet opened. This starts the flow of water into the tanka.
The tanka has a hatch cover which is kept closed except for the time when water is pulled out as from a well, by a bucket on a pulley. The water retention capacity of these tanks is seen in the form of a particular 'danger level' indicated inside by the depiction of a sculptured 'fish' along the inlet neck. Filling the tank above this mark is considered dangerous as the hydraulic pressure inside could exceed the retaining capacity of the tank wall.
The tanka is filled gradually up to the 'fish' mark, and the stored water is used long after the rains have stopped. The opening of the tanka is placed so that direct sunlight does not enter, and there is no contamination. This makes the tanka a most precious source of drinking water, especially in the hot summer months. Most owners clean the tanka only once in 5 to 10 years. The water in Bharuch has been tested and found to be of potable quality.
The tankas, built without cement or concrete, have withstood devastating earthquakes in Gujarat for the past 200 years. Even when houses have fallen, tankas, full of enormous volumes of water, have remained unaffected.
This article was first published in UNESCO Power of Creativity Magazine; Vol. 2, August, 2008
Share on Facebook