The city of Mumbai is currently under intense water stress, with the water demand exceeding the water supply almost twofold. Gated communities comprising a significant portion of the demand, should look to alternate sources to ensure a steady supply for their resident and a means of precaution in a time of constant heatwaves and contaminated water sources.
The Brihanmumbai Municipality Corporation has therefore taken steps towards a category – B action plan of augmenting the water supply by looking into alternate sources of water.
Why RWH is required in Mumbai housing societies?
To address the current and future needs of your housing society. Due to excessive water extraction, there has been a significant drop in the freshwater level. When this happens, the natural barrier that prevents seawater from seeping inland is disrupted. As the freshwater level falls below that of the seawater, seawater begins to infiltrate our land. This not only affects our water supply but also poses a risk to the structural integrity of our buildings, as the seawater can come into contact with and corrode pile foundations.
By implementing rainwater harvesting, your society can help maintain the freshwater barrier, prevent seawater ingress, and ensure a sustainable and safe water supply for all residents.
Hence Mumbai Municipal Corporation made Rain Water Harvesting (RWH)
mandatory to the properties having plot area of more than 1000 sq. mts. after 1st Oct. 2002. Recycling has been made compulsory for buildings having centralized (water-cooled) A.C. plants. M.C.G.M. had granted residential water connections at 90 Ipcd i.e. no municipal water for flushing requirement.
Rainwater Harvesting Methods
The following techniques are used for Urban rainwater harvesting:
- Storage in artificial above or underground tanks.
- Recharging aquifer directly through existing dug-up wells & bore wells.
- Recharging aquifer by percolation/soakage into the ground.
- Pumping (putting under pressure) rainwater into the soil to prevent seawater ingress.
But before adopting any of these methodologies teaching oneself holistic
approach i.e. water reuse, recycling, and water conservation is a must.
Rubbing alcohol
It is necessary to ensure that the groundwater is not contaminated by mixing of
sewage/sullage or other impurities e.g. chemical/biological etc. So there should not
be any leakages in the premises, where the RWH system is to be installed. Societies should maintain their premises clean. Terraces should be cleaned before monsoon & be kept locked during monsoon. Pets should not be permitted there. Similarly, the street flood water in monsoon is contaminated. So it also should not be mixed with the RWH system. A mistake made by one person directly charging the aquifer can affect a number of persons at a time & there is no easy solution to clean the contaminated groundwater.
Paving
In housing societies, paved open spaces are often preferred for low maintenance and car parking, but they prevent water percolation. To balance paving needs and water absorption, avoid unnecessary paving. Instead, use porous pavements made with a higher percentage of coarse aggregate. These can include a geotextile layer at the bottom to direct water to specific areas. Additionally, pre-cast tiles with center holes set on sand bedding will facilitate better water percolation.
Forest
Understanding the relationship between soil, water, and green spaces is crucial for housing societies. Planting diverse tree varieties at different heights increases water retention by creating natural obstructions with their leaves and branches, slowing down rainfall. Fallen branches on the ground also help slow water flow. Humus from decomposed leaves releases water gradually and prevents soil erosion. As a result, areas with more trees have better water retention. To conserve soil and water, housing societies should maintain green cover and plant as many indigenous tree varieties as possible, which also supports the local ecosystem.
What are other regulations regarding rainwater harvesting?
The BMC’s health department looks into the maintenance of rainwater harvesting systems, particularly the 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 RWH. The Maharashtra State Water Policy, 2019 mandates the conservation of rainwater and makes rainwater harvesting mandatory in urban areas.
The Development Control and Promotion Regulations (DCPR-2034) contain detailed provisions for rainwater 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 as follows:
- The following systems may be adopted for harvesting the rainwater drawn from the terrace and the paved surface :
i. Open well of a minimum of 1.00 mt. dia and 6 mt. in depth into which rainwater may be channeled and allowed after filtration for removing silt and floating material. The well shall be provided with ventilating covers. The water from the open well may be used for nonpotable domestic purposes such as washing, flushing, and for watering the garden etc.
ii. Raid Water Harvesting for recharge of groundwater may be done through a bore well around which a pit of one-metre width may be excavated up to a depth of at least 3.00 mt. and refilled with stone aggregate and sand. The filtered rainwater may be channeled to the refilled pit for recharging the borewell.
iii. An impervious surface / underground storage tank of required capacity may be constructed in the setback or other open space and the rainwater may be channeled to the storage tank. The storage tank shall always be provided with ventilating covers and shall have draw-off taps suitably placed so that the rainwater may be drawn off for domestic, washing gardening, and other purposes. The storage tanks shall be provided with an overflow.
iv. The surplus rainwater after storage may be recharged into the ground through percolation pits or trenches or a combination of pits and trenches. Depending on the geomorphological and topographical condition, the pits may be of the size of 1.20 mt. width X 1.20 mt. length X 2.00 mt. to 2.50 mt. depth. The trenches can be 0.60 mt width X 2.00 to 6.00 mt. length X 1.50 to 2.00 mt. depth. Terrace water shall be channeled to pits or trenches. Such pits or trenches shall be backfilled with filter media comprising the following materials:
a) 40 mm stone aggregate as the bottom layer up to 50% of the depth
b) 20 mm stone aggregate as the lower middle layer up to 20% of the depth
c) Coarse sand as the upper middle layer up to 20% of the depth
d) A thin layer of fine sand as the top layer
e) The top 10% of the pits/trenches will be empty and a splash is to be provided in this portion in such a way that rooftop waterfalls on the splash pad
f) Brick masonry wall is to be constructed on the exposed surface of pits/trenches and the cement mortar plastered. The depth of the wall below ground shall be such that the wall prevents loose soil from entering into pits/trenches. The projection of the wall above ground shall at least be 15 cm.
g) Perforated concrete slabs shall be provided on the pits/trenches.
v) If the open space surrounding the building is not paved, the top layer up to a sufficient depth shall be removed and refilled with course sand to allow percolation of rainwater into the ground. - The terrace shall be connected to the open well/borewell/storage tank / recharge it/trench using HDPE / PVC pipes through filter media. A valve system shall be provided to enable the first washings from the roof or terrace catchment, as they would contain undesirable dirt. The mouths of all pipes and openings shall be covered with mosquito (insect) proof wire net. For the efficient discharge of rainwater, there shall be at least two rainwater pipes of 100 mm diameter. For a roof area of 100 sq.mt.
- Rainwater harvesting structures shall be sited so as not to endanger the stability of buildings or earthwork. The structures shall be designed such that no dampness is caused in any part of the walls or foundation of the building or those of an adjacent building.
- The water so collected/recharged shall as far as possible be used for non-drinking and non-cooking purposes. Provided that when the rainwater in exceptional circumstances will be utilized for drinking and/or cooking purposes, it shall be ensured that proper filter arrangement and a separate outlet for by-passing the first rainwater has been provided. Provided further that it will be ensured that for such use, proper disinfectants and water purification arrangements have been made.
- The following systems may be adopted for harvesting the rainwater drawn from the terrace and the paved surface :
