Over the years, the city of Bangalore has seen a rise in the number of contaminated water sources causing an increase in the cases of waterborne diseases. In the case of gated communities, which make up a significant portion of the city’s water demand. With multiple benefits such as minimal collection costs, elimination of distribution expenses, improved water quality where groundwater is poor, prevention of appliance scale build-up, reduced urban flooding and non-point source pollution, managed summer water demand peaks, and reduced water bills.
With this in mind, the Bangalore Water Supply and Sewerage Board ( BWSSB) has set in place guidelines for the set up of rainwater harvesting systems that your community can inculcate to ensure you live in a water-wise society.
Standard Measurements
Rainwater storage (surface tank or underground sump) & ground recharge (RCC precast ring well) of a minimum of 60 ltrs. Per Sq. Mtr of roof area and a minimum of 30 ltrs. Per Sq. mt. paved open space. Provision shall be made. The open well/recharge well of depth of 3 meters (minimum) & diameter of .9 mtr. (minimum) without filling in the well (like aggregates jelly, sand, etc.,) provision shall be made.
The outer periphery of RCC rings shall be covered with 40 mm and up six jelly to a width of 0.15 meters from the bottom of the well up to the ground level. This will ensure proper seating of RCC rings and easy infiltration of rainwater.
Basic Requirements
A rainwater harvesting system designed to catch and store rainwater for further use should have the following components:
- Catchment surface: The catchment surface encompasses the building’s roof and the surrounding open area for rainwater harvesting. Various materials can be used for constructing the roof, and the water quality collected depends on the type of roof material, climatic conditions, and the surrounding environment. In the case of sloping roofs, gutters and downspouts can attached to the edge of the roofs to direct rainwater towards storage tanks installed. In the case of a roof, the pipes are fixed on the wall around the building to drain out the rainwater collected on the rooftops.
- Gutters and downpipes:
Gutters are channels attached to the lower edges of a roof to collect and direct rainwater to a storage tank. Common materials for gutters and downpipes include half-round PVC, vinyl, seamless aluminum, and galvanized steel, available in semi-circular and rectangular shapes. Gutters should be sloped towards the downpipe, with the outer edge lower than the inner edge to ensure proper drainage away from the building wall. Downpipes, typically made of 75 mm to 110 mm (3 to 4 inch) PVC pipes, connect the gutters to the storage tank’s filter unit. In RCC buildings, drain pipes often serve as downpipes and must be interconnected to channel water to the storage tank. - Storage tanks: The Storage tank is used to store the water that is collected from the Rooftops and it is the most expensive component of the rainwater harvesting system.
For storing larger quantities of water, the system will usually require a bigger tank with sufficient strength and durability. Storage tank designs would depend on factors such as rainfall quantity, pattern, intensity, collection surface area, and capacity. - Filtration system:
- Sand bed filter: The process of filtration forms the most important stage in the purification of water. The process involves water passing through a thick sand layer, which removes suspended and colloidal impurities. The filter unit, a chamber filled with coarse sand, charcoal, pebbles, and gravel, eliminates debris and dirt from the water entering the tank
- Wall Mounted Filters: These filters are to be aligned in between the pipes coming from the rooftop and pipes moving towards the storage tank. Normally these filters are capable of filtering the rainwater coming from the roof area of 100 to 250 sq. mt. Hence, these filters can be used for buildings having roof surfaces of 100 to 250 sq. mt., and for roof areas of more than 250 sq. mt., multiple no. of filters may be used.
- First Rain Separator: For roof areas larger than 500 sq. mt. the method of first rain separator combined with sand bed filter for the filtration of rainwater has to be adopted. There are various designs available in the market promoted by different agencies. The First rain separator should be designed to separate the rainfall from the first 1 mm of rainfall. Hence if the rooftop is 500 sqm, 500 litres should be separated. This water will not be filtered or used for any domestic purposes.
- This first rain separator can be a drum/storage of equivalent capacity. There should be an arrangement to drain out the first rain separator. Since the volume of the first rain will be significant, a tap can be provided to the first rain separator and this drained-out water can be used for gardening.
- A bye-pass arrangement should be provided so that if required all the water can be let out and none of it is harvested. This can be provided along with the first rain separator.
- In Bangalore, given that the maximum intensity of rainfall is about 60 mm/hour, the filter should be sized for 1 mm of rainfall. Hence if the rooftop is 500 sqm, 500 litres should be the effective capacity of the filter. This filter should be filled with aggregate for not more than half its total volume. Recommended aggregates are 12 mm jelly, 20 mm jelly, 40 mm jelly, coarse sand, wood charcoal, and netlon mesh. The choice of aggregates will depend on the expected quality of the output water. An overflow pipe should be provided in the filter to handle rainfall that is of greater intensity than 60 mm per hour.
- Delivery system: Once collected and treated, the water so collected has to make its way to your household taps. The delivery system varies based on the building structure. Fixture and regular maintenance of these pipes become essential for a constant supply.
- Analysis and treatment :
To ensure water collected is safe for consumption, it is to be analysed for impurities and bacteria and treated to avoid any illnesses.
Maintenance of Catchment Area, Water Drains, and Recharge Structures
- Maintain the catchments neat and clean, do not use the rooftop/terrace of the building and open spaces around the buildings for dumping of unwanted items and scrap material.
- Do not let the washing machine water which has having heavy dose of detergents, enter into the water drains which are connected with recharge structures.
- Open water drains covered with perforated detachable RCC slabs are best as the maintenance of these drains is easy and pollution especially bacteriological pollution can be avoided. If the stormwater drainage is through a pipe system, provide manholes and chambers at regular intervals and also near the suspected silt and waste accumulation places within the channel.
- Protect the drainage system from tree leaves, polythene bags, plastic bottles, and pouches of eatables which are often seen dumped in stormwater drains.
- Put up the sign boards mentioning that the campus of building is equipped with a rainwater harvesting system which is being recharged to the groundwater system. Mention the ill effects and health impacts if the stormwater drains are not properly maintained. Educate the staff who are maintaining the stormwater drains to keep the drains neat and clean.
- Provide a wire mesh filter just before the inlet. Provide a silt check wall within the drain bed at a convenient place, to clean. If more silt is expected provide a check wall at regular intervals in the stormwater drains.
- The periodic removal of the material deposited on the surface by scraping the silt accumulated on top of the filter bed regularly. Precaution should be taken to avoid domestic wastewater entering into the recharge structures.
- Recharge tube wells shall be developed periodically by hand bailers to avoid clogging of the slots.
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 :
