For permanent water supply
The common queries about rainwater harvesting are more to do with its logic, especially the way common people perceive it. Why store water when it is raining outside? What if we have to order tanker water in summer despite doing rainwater harvesting? These are not aimed at dismissing the idea of using rainwater, but trying to hint at the possible limitations of water harvesting. Of course the answers to such queries are simple – much depends upon the way we use the stored water.
Using the rainwater during the monsoon reduces our dependency on the public water, treated and moved from far-off places, besides saving on our bills. As the rainy season ends, we should not use our stock of water, but save it till the time public supply starts to dwindle. By pumping up limited quantity every time the overhead tank goes dry, we can manage this stock for many weeks.
The Rajasthan way
There is a better alternative for those who can afford upfront investment and seek long-term water supply. Rajasthan is known for a concept called ‘tanka’, a large-capacity tank fully underground that collects rainwater and stores it. Applying this idea for today, there can be such tankas upwards of 20,000 litres capacity dug within the site. The main criteria are to ensure total darkness in this chamber, which enables water to stay fresh for long, and ensure it gets full by the time rains stop so that we get maximum storage.
Tankas are better fitted with mono-block pump with only a suction pipe getting into the tank, though submersible ones can also be used. However pure the rainwater is, there will be certain dissolved particles and traces of dust from the terrace that may escape the first flush that will settle down at the bottom.
Since there is no outlet at the bottom to clean, manual cleaning is necessary when the tanka has no water. To get in for cleaning, keep a small, i.e. less than 3’x 3’ trap door either in the side or on top of the tanka as per site condition.
In many places, especially house sites, the setback or garden spaces left around the house may not be enough for the tanka or we may not like to lose such areas for an RCC roof. In such cases, the tanka can be dug under the house and fitted within the house walls, as if it is a basement space. The walls need to be structurally designed as retaining walls, to take care of soil and water pressures. Not only we save space, but also get to save water.
Investing on a tanka is worthwhile across many years, and may not suit budget buildings. However, those planning for a rainwater sump can plan to extend the water storage capacity. With water today, the advice is simple – save more, store more and use less.