Explainer: How to start rainwater harvesting and why Mumbaikars should do it

If rainwater harvesting is implemented on a large scale in Mumbai, it will help in water conservation and reduce floods too.

Mumbai may have witnessed heavy rains but one must not forget that the city is still reeling under a 10% water cut since June 5, 2024. On the one hand water levels in the reservoirs are depleting, and on the other hand rainwater continues to flow out. In such a scenario, rainwater harvesting can be the game changer for Mumbai. If done right, it ensures water supply for individuals, fights larger water crisis and might help reduce floods too.   

Urban local bodies (ULB) like the Brihanmumbai Municipal Corporation (BMC) have been encouraging housing societies to set up rainwater harvesting to store rainwater for non-potable use and to recharge groundwater levels. In this explainer, we look at various aspects of rainwater harvesting including different methods, procedures, laws and incentives.

What is rainwater harvesting?  

Rainwater harvesting (RWH) in a housing society involves collection and storage rainwater falling on the roofs, terraces, open paved or unpaved surfaces within building sites and directing it to large underground artificial tanks or natural reservoirs (aquifers) for storage and reuse. 

Collected rainwater can also be used for groundwater recharging. It is the cheapest, simplest, indigenous technology practised in India for centuries to help self-sufficiency of water.

It also improves groundwater table, reduces soil erosion and improves the quality of groundwater by reducing its salinity. It is less expensive, easy to maintain and prevents sea water ingression.

What is the purpose of rainwater harvesting?

  • Water conservation – The Mumbai Climate Action Plan, 2022 (MCAP), which has a holistic water resilience approach, proposes 50% of the city’s water demand be met through localised water conservation and efficient use initiatives by the year 2030. It recommends large-scale implementation of RWH systems in Mumbai by developing a comprehensive policy for all typologies of buildings based on a detailed water audit report. It also recommends monitoring to ensure long-term utility and sustainability of these systems.
  • To reduce load on storm water drain network – RWH ensures that maximum rainwater is discharged directly into the ground in areas with insufficient natural ground to absorb rain water. The Eco-Housing Assessment Criteria recommends RWH for minimising load on storm water drains and sewage treatment plants.
  • RWH as a solution to reduce urban flooding – The MCAP discusses rainwater harvesting to help reduce flood risks, along with preserving water resources. “To address these flooding and surface run-off issues, the city needs to adopt a watershed approach, focusing on specific aspects such as rainwater harvesting and decentralised treatment of wastewater,” states the MCAP report. Rainwater harvesting will ensure water and sanitation equity and make Mumbai a water resilient city.
A simple rainwater harvesting system on the terrace connecting the staircase roof to a Rain Barrel.
A simple rainwater harvesting system on the terrace connecting the staircase roof to a Rain Barrel . Pic: via X (Twitter) account of Vishwanath S @zenrainman
  • To prevent sea water ingression in borewells – It is observed that some of the wells, especially borewells, which have sweet water initially turn saline, as excessive drawing leads to drop in fresh water. The fresh water barrier that restricts sea water from coming into the land is disturbed. Once fresh water level goes below the sea water level, sea water rushes into the land. The sea water now comes in contact with the pile foundations which can endanger structures in future. By harvesting rain water we can maintain this barrier and prevent sea water ingression.

Read more: In the news for heavy rains every year, why is Mumbai still water-deficient?

What are different ways in which rainwater is harvested? 

 Rainwater harvesting can be implemented by following four methods.

  • Storage of water from roofs in artificial tanks above or underground tanks – This involves collection of water from roofs, terraces through pipes into storage tanks. The BMC had implemented this concept at its Santacruz Transport garage, where it has set up a rooftop collection point on the terrace. 
  • Recharging aquifers directly through existing dug up wells and borewells – This involves direct recharge of subsoil strata through borewells. This saves costs of setting up the gutter network to divert water into the sea. This system was set up at the CTI and RC Center, Borivali and that has ensured that the borewell has not dried up. 
A resident shows the rainwater harvesting system in their housing society
Pawan Kumar Batra shows the rainwater harvesting system in their housing society. Pic: Pawan Kumar Batra
  • Recharging aquifer by percolation/soakage into the ground – This involves recharge of the subsoil strata through percolation. This system was followed by the BMC at its Sane Guruji Udyan, at Siddhi Vinayak garden, which requires 30,000 litres of water daily to water the garden. It is used to recharge one ring well and two recharge wells. The municipal water supply has been disconnected since November 2004 and saves about one crore litres of water per year, the BMC claims. 
  • Pumping rainwater into the soil to prevent sea water ingress – In this method, the subsoil soil strata is directly recharged through dug up wells. It has been done at the Bhagwati Hospital and is used by the fire brigade as a water filling station. 

What is BMC policy on rainwater harvesting?

In 2002, the BMC became the first municipal corporation to make rainwater harvesting mandatory for new housing societies as part of its building design failing which they were not eligible to have Occupation Certification (OC). Rainwater harvesting was also mandatory for old buildings that were altered or if additional Floor Space Index (FSI) was utilised. 

In 2002 all developments, whether residential or commercial, over 1000 sq metres had to undertake RWH. After a few changes to the policy, from May 8, 2019, as per DP-2034, rainwater harvesting is now mandatory for a developed area of 500 sq metres and more. 

The civic body also grants residential water connections at 90 litres per capita daily (Ipcd) instead of 135 lpcd, which means it does not provide municipal water for sanitation requirements. The housing societies need to supplement the gap for flushing water either by rainwater harvesting or by recycling

Does BMC give incentives for rainwater harvesting?  

New and upcoming housing societies have to set up rainwater harvesting system to get Occupation Certification. Old societies that implement rainwater harvesting were to get a 5% rebate in property taxes to make it more attractive for them along with rebates offered for undertaking segregation of garbage. 

However, this policy remains more on paper than in practice. Earlier, the rebate was announced in 2010 but in 2015 the then municipal commissioner rescinded it citing lack of clarity from the state government. The property tax rebate of 5% failed to take off due to lack of clear directives, he had stated. 

In 2019, the standing committee did sanction a 5% rebate on property tax for those utilising harvested rain water or grey water but nothing came out of it and it failed to be implemented.  

What are other regulations regarding rainwater harvesting? 

The BMC’s health department looks into the maintenance of rainwater harvesting systems, particularly prevention of mosquito infestation and cleanliness. 

The legal perspectives have been developed from various guidelines issued by various departments and Acts. The Model Building Bye Laws (MBBL), 2016 framed by the Union Ministry for Housing have included guidelines for setting up on RWH. These by-laws were adopted by 15 States and union territories including Maharashtra. 

The Maharashtra State Water Policy, 2019 mandates the conservation of rain water and makes rainwater harvesting mandatory in urban areas.

The Development Control and Promotion Regulations (DCPR-2034) contains detailed provisions forrainwater harvesting for recharging the groundwater. The DCPR also contains provisions to ensure that rainwater naturally percolates into the ground in open spaces. The Maharashtra Groundwater (Development and Management) Act, 2009 also looks at the management of groundwater. 

How can Mumbaikars set up rainwater harvesting?

Since Mumbai generally receives rainfall by large intensity showers, it may not be always possible to ensure absorption of water at the rate of rainfall. A large quantity of runoff water could be wasted. Therefore it is better to collect rooftop runoff in artificial tanks, which would suffice flushing demand for a few days.

The BMC has stipulated the details of construction and maintenance which are available here 

Overflowing water from these tanks along with surface run off could be utilised for recharging of ground water with proper precautions. 

Read more: How to start harvesting rainwater at your home in Chennai

Precautions while setting rainwater harvesting in a society 

The most important precaution, while setting up a rainwater harvesting system, is to ensure that it does not endanger the stability of building or earthwork. The design should ensure that the none of the walls or foundation of the building or those of an adjacent building get any dampness.

Though the rainwater thus collected is meant for non-potable use, it could be used for drinking or cooking purposes only in exceptional circumstances after ensuring that it is properly filtered.  Overall, the soociety has to ensure that a proper disinfectant and water purification arrangement is in place. 

Prevention from contamination:  It is necessary to ensure that the groundwater is not contaminated by mixing of sewage/sullage or any other impurities eg. chemical/biological waste.  Care should be taken to avoid any leakages in the premises where the RWH system is installed. 

Societies should maintain their premises clean and terraces should be cleaned  and locked prior to monsoon. Pets should not be permitted there. Similarly dirty street flood water should not be allowed to mix with the rainwater harvesting system. 

How should societies maintain the rainwater harvesting system? 

A housing society is to ensure that the rainwater harvesting structure is maintained well for storage of water for non potable purposes or recharge of groundwater at all times. The insecticide branch of the public health department of the BMC mandates that societies clean and maintain it to ensure that the facility does not become a breeding ground for mosquitoes

Failure to maintain the rainwater harvesting system, could lead to fines of not exceeding Rs.1000/- per annum for every 100 sq mtr of built up area.

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