Tanks can serve many a purpose in the urban context.
Known as the city of tanks for years, Bangalore has seen large-scale destruction of, and encroachment on, its surface waterbodies over the years. Public outcry, activist groups and the courts have ensured that some efforts are on to save what remains and conserve them.
Built primarily as irrigation structures, these tanks have outlived that purpose as urbanisation has changed land use and converted the ‘atchcuts’ or command areas, where crops used to be grown, into housing and other sites. These tanks were also not perennial; most of them would go dry during summer. What is therefore a new role for them in a modern city?
Tanks could serve many a purpose in the modern urban context. They could be a space for biodiversity acting as wetlands where many birds and aquatic life could flourish. They could be recreational spaces where activities such as walking, jogging, boating and even swimming could happen. They could be micro-environment moderating spaces cooling with evaporation and water vapour during the hot summer months. They could hold stormwater during heavy rain and act as a water buffer, moderating floods. They could be percolating structures recharging the groundwater.
The most and sustainable role seems to be that of recipients of treated sewage water. The city gets over 1,400 million litres per day from the Arkavathy and the Cauvery rivers. Waters which are not natural but brought from far to quench the thirst of the city. Of this over 1,120 million litres per day will flow as waste water or sewage. This water will need to be treated if we have to do justice to the ecosystem. The best way to do it is to locate sewage treatment plants attached to all the remaining tanks in the city.
In addition, an equivalent of 3,000 million litres per day is received as rain in the city. Over half of this can be picked up in tanks if they are designed properly to receive the waters from urban catchments.
Some relevant examples
There are many examples dotting the landscape. The tank at Lalbagh has been refurbished and receives water from the small sewage treatment plant set up upstream of it. Almost 1.5 million litres per day thus becomes perennial flow to the tank, keeping it full. This recharges the groundwater and makes it available as open well water and bore-well water to slake the thirst of Lalbagh.
The Nagavara tank receives treated wastewater from the Jakkur sewage treatment plant and is always full for recreational activities.
The Jakkur tank, with a water spread of 50 hectares, has a 10 million litres per day sewage treatment plant upstream. The treated water is let into an artificial wetland for further treatment before it reaches the main water body of the tank.
This then recharges the ground water around and fills up the wells.
The utilisation of tanks to receive treated waste water and then to pick it up as groundwater has tremendous potential for the city.