This article was originally published by Earth Day Network – India in its eBook ‘Pathways To Green Cities: Innovative Ideas From Urban India’
Rainwater harvesting, a buzz word today, has been prevalent in India since ancient times. The tanka, a subterranean water tank, formed the basis of a traditional rainwater harvesting system. During the monsoon season, rainwater was collected on terraces and channelled into a copper pipe that carried it down to an open chamber or tanka below the central courtyard. The water from the first few showers was allowed to flow out, keeping in mind that it might contain some impurities and also, as a way to clean the pipes.
This outlet was then plugged and water from the next showers directed straight to the tanka. Astrology was used to predict the right time to do this—based on predictions of times when bacteria and microbes grew the least.
The catchment during the monsoon provided household-adequate water to see them through till the next monsoon rains came to refill the tanka and so, water was not a problem even in Ahmedabad, a major city that is located in a semi-arid zone. Most of the tankas below the houses in the walled city (or historic area) of Ahmedabad are lying sealed today. Their closure was ordered years ago by the then British rulers as they were unsure about monitoring the quality of water stored in tankas. Also, they feared the tanks might be used as hideouts by freedom fighters.
With growing water problems, the Ahmedabad Municipal Corporation (AMC) considered reviving this novel water storage system, one that uses ancient wisdom about hydrology, and rainfall patterns and where water quality surprisingly conforms to international standards. The first task was to identify, open up, and clean a few of the tankas. Before beginning the cleaning work, the AMC circulated a form for citizens to fill out. This helped garner their interest as the AMC was keen that there was public participation in the exercise. On the basis of responses, 10 tankas were selected.
Keeping in mind that the tankas had been lying closed for so many years, an assessment of the risk factor involved was needed. A team of four skilled personnel was formed and equipped to carry out the task. Exhaust fans were used to remove any foul gases and to enhance the ventilation inside the tanka while cleaning work went on. After opening the tankas, eight of them were found to contain water up to depths of 8–9 ft. A submersible pump was pressed into service for removal of the stagnant water. All the silt deposited in the bottom was removed and the internal surfaces cleaned. AMC also began research and documentation. One of the most interesting aspects of the research was laboratory testing of the water that was lying locked in these tankas for decades.
To everyone’s surprise, the water was found to be drinkable. The factors which contribute to achieving these exceptional results are: rainwater is directly collected, minimising chances of pollution, the lack of light in the perpetually dark interior of tankas hinders the growth of chlorophyll, containing algae and other microbes and bacterial growth is restricted by the presence of lime which is used in the construction of the tanka. After the monsoons, the addition of lime changes the pH factor of the water, making it more alkaline (many bacteria cannot endure alkaline conditions).
The entire operation generated immense interest in the community. Many ventured down the less than 2 ft wide shaft into the dark tanka. The youth, in particular, were keen to learn about the construction and working of the tanka system. They had never seen an operational tanka. The owners of homes got completely involved with the process. In one of the homes, the lid was missing. The owner made special efforts to find the traditional copper lid and was delighted that he didn’t have to use a ‘modern’ concrete one.
Reviving the tankas, triggered an effort to preserve other traditions as well. Many owners of houses where the tankas were revived are now looking to conserve other traditions as well. They welcome people to come and visit. Measure drawings were made of two houses in Dhobi-ni-Pole. The plans and water holding capacity of the tankas of the other eight houses were documented.
So that the traditional system of tankas has wide acceptance, data based on scientific tests and theoretical explanation of the processes are being disseminated widely. AMC plans to further the positive atmosphere for rainwater harvesting. There are about 10,000 dormant tankas in the walled city of Ahmedabad. Their revival is high on the AMC agenda. Going by a conservative estimate of 25,000 litres of water per tanka, there is potential to harness 25,00,00,000 litres per year in the walled city. And the walled city is just a fraction of Ahmedabad! AMC also plans to make it mandatory for any new construction to include a tanka.
From the health point of view as well, hygienic water, free of chemical pollutants and undesirable salts, will be a boon. This versatile rainwater harvesting system that provides adequate and safe water and reduces flooding on streets can easily be adapted by other cities as well. A little more research into related issues such as how the use of lime controls the development of microbes, the effect of darkness in controlling bacteria and devising better methods of filtration can make the tanka even more efficient.
This project was coordinated by Debashish Nayak as an advisor to the Heritage Cell, Ahmedabad Municipal Corporation.
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