Rain water harvesting is so easy, why can't Mumbai do it?

Mumbai has a growing population of 18.41 million, according to provisional 2011 census data and it’s only increasing. With many parts of Maharashtra - Marathwada, Solapur, Ahmednagar, Sangli - suffering from the regions worst drought in 40 years this year, serious questions have been raised about water consumption in Mumbai.

While many Mumbaikars committed the symbolic gesture of not using water during Holi earlier this year, most would agree that there needs to be more concrete steps taken to ensure Mumbai, as a city, is at least partly self reliant on water. What the city therefore needs is an overall water management system that looks ahead.

According to data released by the Brihanmumbai Municipal Corporation, the city’s demand is 4200 million litres daily (MLD) and the BMC supplies 3400 MLD, a shortfall of 800 MLD.

Hydraulic expert Madhav Chitale in a 1999 report submitted to the government says the city’s demand for water is expected to rise to 5,400 mld in 2021 with the city’s population at 1.65 crore.

Legislation In 2002, the BMC put in place legislation mandating that all constructions on an area of 1000 sq mts have to set-up a water harvesting plant as a prerequisite for obtaining a completion certificate.

In 2007, the BMC amended it and made it mandatory for plots measuring 300 sq mts and above to show a rain water harvesting plant in its plans. It has also made recycling compulsory for buildings having centralised AC plants.

But despite putting in place such legislation more than a decade ago, the government still doesn’t have concrete data to show the mumber of buildings that have implemented rain water harvesting.

Water consumption Data shows that on an average, a household uses only 20 percent of its water supply for cooking and drinking. Sixty percent of it is used for flushing, cleaning and bathing. Importantly, a single flush still uses 10-12 litres of clean water. This amounts to over 60 percent of potable water going down the drain.

Potential to tap rain water? Mumbai has an average rainfall of 2,146.6 mm in the island city, and 2,457 mm in the suburbs. With an area of 437 sq km, data by the Centre for Science and Environment shows the city has the potential to harvest 2394.52 MLD of water.

The data further suggests that if one considers 70 percent of the city to be paved and 50 percent of it roofed, collecting 70 percent of the rain water that falls over it will resulting in the harvesting of 589.34 MLD of water, which can be reused.

Rain water harvesting system Cities like Delhi and Ahmedabad use recharging techniques, while cities such as Chennai and Bangalore tend to store their roof water in sumps and recharge soil with surface run-off water. Mumbai needs a combined method of storage and recharging due to the monsoon pattern the city recieves, says a BMC report.

The process of rain water harvesting is an easy and indigenous process of water conservation and can ensure self sufficiency at an individual and community level in the long run.

According to a BMC report, Mumbai, which has incessant rainfall followed by dry spells, needs a combined method of recharging aquafiers and water storage. This stored water can then be used for various purposes - gardening, car washing, flushing, floor swabbing, etc.

Residents of Chennai, considered a city that has almost successfully implemented rain water harvesting to tackle its shortfall, have gone a step further and have started potable consumption of harvested rain water. All three storied buildings in the city have to mandatorily have a rain water harvesting system. New water and sewer connections are provided only after the installation of rainwater harvesting systems.

While implementation of existing laws, especially environment related, have always been sluggish, initiatives at the individual and community level can pave the way for a self reliant and sustainable system.

If you are in Mumbai and you want to start a rain water harvesting system at an individual level or in your colony, you can contact the BMC here:

Rain Water Harvesting Cell, 3rd floor Municipal H.O. annex building.

Tel. 022 - 22620251 ext.2309. There are also NGOs which can help setup a RWH system. You could also refer to  this guide book made available by the Centre for Science and Environment and this  brochure by the BMC .

Also, below are plans/blueprints by the BMC that can help you decide which system suits you best.