5 Steps to Building a Rainwater Harvesting System: A Solution to Mumbai’s Water Crisis

Around 52% of the world’s population – projected at 9.7 billion people – will live in water-stressed regions by 2050

The water on our Earth today is the same water that’s been here for nearly 5 billion years. Only a tiny bit of it has escaped out into space.

Freshwater that is available on the surface is relatively scarce. Streams, Rivers, and lakes are the most visible part of the water cycle which supply fresh water which is vital for life, supporting ecosystems and human civilizations.

While nearly 70% of the world is covered by water, 97.5%  of the water is salt-water and only 2.5% is freshwater. The rest is saline and ocean-based. Off the 2.5% freshwater, just 1% is easily accessible, as much of it is trapped in glaciers and snowfields.

According to a NASA-led study, many of the world’s freshwater sources are being drained faster than they are being replenished.

Jay Famiglietti, the senior water scientist at NASA, says that “the water table is dropping all over the world. There’s not an infinite supply of water.”

While our planet as a whole may never run out of the water, it’s important to remember that clean freshwater is not always available where and when humans need it.


Water scarcity arises with low availability of freshwater which is further escalated by existing water resources being polluted by human activities. The solution to this problem is rainwater harvesting.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-scarcity

More than half of the world population is expected to live in water-stressed condition by 2050, and rainwater harvesting could help them (Water Aid)

It is the best sustainable solution as it does not have any energy requirements for functioning, it does not deplete any natural resources, on the contrary, it conserves and protects the natural habitat with surplus availability of water for usage.

What is rainwater harvesting?

The saline water from sea evaporates due to the heat from the sun, forms clouds and falls as rain as freshwater. A significant part of this rainwater runs off into oceans and drains. If we can harness this effectively it can be an important source which can be used for various applications. 

Rainwater harvesting is the process of storing rainwater for reusing rather than to allow it to run off.

The main ecological advantage by using rainwater harvesting system is that less water would have to be drawn from the environment.


  • Watering the garden
  • Filling up the pool
  • Washing cars
  • For Drinking, cooking etc.
  • Recharging groundwater aquifers
    • Groundwater aquifers can be recharged by various kinds of structures to ensure the percolation of rainwater in the ground instead of draining away from the surface. Commonly used recharging methods are:-
      • Recharging of bore wells


Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-catchment

A typical Rainwater Harvesting system showing parts and areas for collection and storage (Center for Science and Environment)

STEP 1: Determine Your Rainwater Harvesting Potential

The rainwater harvesting potential is the amount of rainfall which could be collected in the given area. It is calculated as the surface area you have available to collect the rainwater multiplied by your annual precipitation.

Area available to collect x Annual rainfall

The surface that receives rainfall directly is called the catchment of the rainwater harvesting system. It may be a terrace, courtyard, or paved or unpaved open ground.  Therefore the catchment is the area, which contributes rainwater to the harvesting system.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-freshwater-catchment area (1)

Image illustrating the various types of the catchment area for collecting rainwater (ResearchGate.net, Andrade, Rolland)

For example: Let’s say the catchment area is 100 m2 and the area receives an average 1000 mm of rainfall per year then, 100 m3 will be the harvesting potential 

The extent of the collection could also differ depending on:

  • The type of surface you are collecting from as the collection surfaces have different levels of run-off.
  • The rainfall distribution, pattern or event size

STEP 2: Determining the Layout

The layout and the location of the tank, pipelines are determined in such a way as to maximize the use of collection surfaces.

There are several options but the major ones are:

a) Spread out tank option

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-spread out catchment

b) Cluster tank option

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-clustered catchment

STEP 3: Setting up the Storage

The rainwater collected from the roof of the building is diverted to a storage tank. The storage tank has to be designed according to water requirements, rainfall, and catchment availability.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-filter

A filtration unit to remove debris and silt from the harvested water (deepleeninsta.com)

Each drain pipe and collecting system should have:

  • A mesh filter at the mouth and first flush (also known as roof washer) device which will divert the first flow of water away from the tank.
  • A filtration system before connecting to the storage tank.
  • An air gap to prevent stormwater backflow.
  • Each tank should have an excess water overflow system.
  • Excess water could be diverted to the recharge system.

Water from storage tanks can be used for secondary purposes such as washing and gardening

The main advantage of collecting and using rainwater during the rainy season is not only to save water from conventional sources but also to save energy incurred on transportation and distribution of water at the doorstep.

This also conserves groundwater, if it is being extracted to meet the demand of the freshwater.

  • Determining the amount of storage is dependent on the site and the purpose for which water would be used.
  • To get a good idea of how much tank volume would be needed one need to have access to some good rainfall data. If there is no access to good data then will have to fill in the blanks with estimates.

STEP 4: Decide on the Features in the System

1. First Flush Diverters

  • The first flush is a device used to flush off the water received in the first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof.
  • It will also help in cleaning of silt and other material deposited on the roof during dry seasons. Provisions of first rain separators should be made at the outlet of each drainpipe.
Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-flush unit

Diagram explaining the working of First Flush Diverters (ecosustainablehouse.com.au)

2. Filter

  • Filters are used for the treatment of water to effectively remove turbidity, colour, and microorganisms. After the first flushing of rainfall, water should pass through filters. 
  • Gravel, sand, and ‘netlon’ mesh filters are designed and placed on top of the storage tank
  • It prevents silt, dust, leaves, and other organic matter from entering the storage tank.

The filter media should be cleaned daily after every rainfall event. Clogged filters prevent rainwater from easily entering the storage tank and the filter may overflow.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-filter unit

A typical filter arrangement in a Rainwater harvesting system (theconstructor.org)

3. Limit Sources of Contamination

  • Check your roof surface materials is suitable for collecting appropriate quality water and trim overhanging vegetation
  • Install gutter mesh to prevent leaves and debris from blocking gutters and pipes

4. Additional features which could be added:

  • Add a UV filter in case it is to be used for drinking water
  • Gravity feed to tanks 
  • Pumped distribution 
  • Each tank can individually be emptied for cleaning 
  • Recharging of groundwater aquifers
  • Automated irrigation system

STEP 5: Installing the Pipes and Tank

For installing pipes:

  1. Seek the necessary permissions ( if required ) from the authorities to lay the pipes 
  2. Dig Trenches, if laid underground.
  3. It is preferable to have the pipe as a single fall to avoid sediment collecting in places that cannot be reached easily for cleaning.
  4. Lay the pipes in trenches or otherwise and connect using connectors.

For the system to work the collector site must be at a higher level than the inlet to the tank and hence the sump.

It is important to get a sufficient head between the collector site and tank to deal with quick and large rain events.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-tank

Installing the pipe system for the rainwater collection (instructables.com, Bechtel8)

For installing Tanks, Sump:

  1. Make a tank stand suitable to the property
  2. Put the tanks on the stand and ensure that they don’t blow away when empty 
  3. Connect them up so they can behave as one volume. Recommend putting a valve for each tank mostly for maintenance
  4. Install first flush diverters to keep the most contaminated rainwater out of your rainwater tank.
  5. Connect some sort of overflow arrangement. If the tanks are in a sump it would require overflow arrangement and lots of drainages. When the overflow is undersized, pumps will flood.
  6. Make the final connections between the PVC collector pipes and the tanks.
  7. Install a tank gauge to regularly monitor your water level and usage
Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-tanks

Building a storage tank for rainwater collection (instructables.com, Bechtel8)

For installing pumps and managing standing water:

  1. Use a pump to distribute the water or pump it to an overhead tank or for watering the garden etc.
  2. The pump should be right-sized or it can be marginally oversized.
  3. Adequate pressure must be maintained by using pressure couplings.
  4. Put some water in the collectors and check the water flows into the tanks and the functioning of the entire system.
  5. For standing water, use a wet-dry valve or first flush in-ground diverter to drain your pipes in between rainfall events
  6. Install a solar shield to block sunlight and prevent algae growth


Rainwater harvesting proved to be a solution for the water crisis in many parts of India, but the Municipal Corporation of Greater Mumbai is not pushing it in the city. Even after making it mandatory in 2002 and suggesting it in Development Plan 2034. 

While many Mumbaikars have taken symbolic gestures of saving water – like not using water during the festival of Holi – more concrete steps need to be taken to ensure Mumbai, as a city, is at least partly self-reliant on water.

Mumbai-India-Environmental-NGO-Earth5r-UN-Rain water-harvest-system-mumbai flood

A people walking through a flooded street in Mumbai. Mumbai receives heavy rains every year which gives it a potential to excel at rainwater harvesting (The Indian Express)

Mumbai has a population of 18.41 million and it’s growing. With many parts of Maharashtra – Marathwada, Solapur, Ahmednagar, Sangli – suffering from droughts regularly, serious questions need to be raised about water consumption in Mumbai.

According to data released by the Brihanmumbai Municipal Corporation (BMC), the city’s demand is 4200 million litres daily (MLD) and the BMC supplies 3400 MLD, that is a 20% shortfall of 800 MLD.

Could Rainwater Harvesting Be Implemented By The Authorities?

In 2002, the BMC put in place legislation mandating that all constructions on an area of 1000 m2 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 m2 and above to show a rainwater 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 number of buildings that have implemented rainwater harvesting.

Where Is The Water Being Used?

Data shows that on average, a household uses only 20% of its water supply for cooking and drinking.

60% 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% of potable water going down the drain.

Does Mumbai have the potential to tap rainwater?

Mumbai city has the potential to harvest 2394.52 MLD ( Million litres per day ) of water, which 950 times as big as an Olympic Swimming Pool during the monsoon period

The data further suggests that if one considers 70% of the city to be paved and 50% of it roofed, collecting 70% of the rainwater that falls over it will result in the harvesting of 589.34 MLD of water ( almost 15% of Mumbai’s water requirement ) which can be reused.

A significant portion of the water shortfall can be made up.


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 the soil with surface run-off water.

Mumbai needs a combined method of storage and recharging due to the monsoon pattern the city receives.

According to a BMC report, Mumbai, which has incessant rainfall followed by dry spells, needs a combined method of recharging aquifers and water storage. 

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

30 Indian cities will face ‘water risk’ by 2050

While the 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

Several societies in Mumbai and Thane have taken a  pro-active approach and voluntarily implemented RWH system which have helped them to reduce the water problem in their societies and also reduce water bills.

The process of rainwater 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.


Rainwater Harvesting and Distribution: 9 Steps

Apply the 8 Rain Harvesting Steps

Methods of Rainwater Harvesting [PDF]: Components, Transport, and Storage 

Rainwater Harvesting – Solution to water crisis – Technology and Systems

Rainwater harvesting is so easy, why can’t Mumbai do it? – Mumbai News


Earth5R is an environmental organization from India with its head office at Mumbai. It works with the NGO sector, Companies and helps them conduct environmental corporate social responsibility (CSR) programs across India. Earth5R specializes in circular economy based projects. Earth5R also offers short term and long term environmental courses.

Earth5R’s Global Sustainability Hub is a cross-sector and cross-country collaboration in pursuit of UN Sustainable Development Goals. It is an excellent opportunity for governments and the private sector to engage with communities, use Sustainability-based models to drive economic changes, and create social and environmental impact.

Reported by Sukumar Krishnamurthy, Edited by Riya Dani

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