Why build a huge sump?

Architect Sathya Prakash Varanashi says that instead of a blind practice, the frequency of municipal water supply and family size should together decide the ideal volume of storage required, to avoid both dead investment and unhealthy water

For most families, a sump for 3,000 and overhead tank for 1,500 litres is more than sufficient for municipal supplies. Photo: K. Ramesh Babu

What has a water storage sump got to do with an eco-friendly approach? If you think they are unrelated, you are in for a surprise. It is that grey area most house owners do not think about and builders simply follow a routine without ever wondering about the right practice. The fear of water scarcity in cities like Bangalore adds fuel to the fire with people building larger and larger sumps, spending much money on them. The idea of sump impacts water consumption, construction costs, water stagnation, setback space utilisation and such other indicators of a green home.

We all know that stored water does not stay fresh for too long, a point to reckon with while planning capacity of idle storage in sumps and tanks together. As per survey data, urban dwellers use 150 to 200 litres per day per person, so a family of five needs around 750 to 1,000 litres of water every day. This figure covers all tasks including drinking, cooking, cleaning, bathing, vehicle washing, gardening and occasional extra use due to guests or events. There are many families living with less consumption by avoiding car wash, lawns, high flow fixtures, large built-up area, hardy plants or activities in flowing water. However it is safe to assume the average consumption for size calculations.

How it works

If the city water supply comes every alternative day and we have an overhead tank for 2,000 litres and a sump for 5,000 litres, in every refill less than 2,000 litres out of 7,000 litres gets replaced. Over time, there will be more of old stagnant water than fresh water in the tank. In other words, instead of a blind practice, the frequency of municipal water supply and family size should together decide the ideal volume of storage required, to avoid both dead investment and unhealthy water.

For most families, a sump for 3,000 and overhead tank for 1,500 litres is more than sufficient for municipal supplies. It also fits into the theory of sump being double the capacity of the overhead tank. Virtually, every family has the habit of daily pumping up as a morning routine; as such, the previous day’s consumption, which may never cross 1,500 litres, gets refilled in the tank. In the event of power cut or being unable to pump up on a given day, the tank storage would suffice for the second day also. The sump and tank together can hold up to 4,500 litres, adequate for a minimum of five days for a family of five. If water gets supplied every two or three days, at least half the stored water gets replaced during the supply time, reducing the risks of water stagnation.

The eco-conscious can further reduce this total storage capacity based on the lifestyle of the family, but should not be increased unless specific reasons demand so.