What is rainwater harvesting?

Rainwater harvesting is a technique for collecting and storing rainwater that runs off natural or artificial catchment areas such as hill slopes, compounds, rocky surfaces, and rooftops. The process is done by utilizing artificially created systems. The stored rainwater can be filtered and used for various purposes.

Importance and purposes of water harvesting  

• Using it for domestic purposes (drinking, watering the garden) with proper filtration.
• To serve as unfiltered landscape irrigation, especially for dryland farming. 
• To promote groundwater recharge, which will further boost soil fertility. 
• Preventing metals, pesticides, fertilizers, and other sediments from entering clean, fresh surface water; reducing sewage treatment plant overloads, urban flooding, and stormwater discharges. 
• To reduce the seawater influx into coastal towns.
• Compared to alternative purification or pumping techniques, which are more expensive and guarantee high-quality water. 
• The need for groundwater is decreased. A rainwater collection system increases aquifer productivity, which raises groundwater levels.

Advantages of harvesting rainwater 

• This renewable water source is quite accessible.
• Urban flooding is reduced via rainwater harvesting.
• Soil erosion can be avoided by rainwater harvesting.
• Harvesting rainwater is a fairly economical technique to conserve water.
• This doesn’t require a lot of labor.

Disadvantages of harvesting rainwater 

• The rainwater harvested might not be safe to drink unless treated adequately
• Water harvesting cannot be done in prolonged dry areas
• Water could get polluted if storage is not adequately maintained and become a breeding ground for insects. 
• The initial setup may be high, even though it is not expensive
• Depending on the seasonal rainfall, harvesting systems have an impact on the yield  

Methods of rainwater harvesting 

Mostly, two methods are utilized for harvesting rainwater: 

Rooftop water harvesting 

Rooftop water harvesting techniques are mostly employed in residential or commercial structures, where rainwater is collected through rooftop catchments, diverted, and stored in tanks. 

Harvezted water can either be maintained in a tank or moved to an artificial recharge system to meet daily demands such as washing cars, flushing toilets, washing machines, gardening, showers, and baths. 

Surface runoff harvesting 

This method is best suited for urban areas where rainwater falls during downpours and is collected and stored in designated storage spaces. To facilitate the storage of surface runoff, the flow of small rivers or reservoirs is changed. 

The surface runoff is stored in ponds, tanks, and reservoirs specially built for this purpose. While lowering evaporation, effective and efficient water conservation is employed to store rainwater. Maintenance of clean and sanitary water is approached by taking several steps. 

Rooftop rainwater harvesting techniques 

Below are several examples of rooftop rainwater harvesting methods: 

Groundwater aquifer recharge

In order to ensure that precipitation percolates into the ground rather than draining away from the surface, several structures can be utilized to recharge groundwater aquifers. The most common recharge methods are as follows:

• Replenishment of bore wells
• Filling of excavated well 
• Refilling pits
• Trenches for recharge 
• Shafts of recharge or soakaways 
• Filtration tanks 

Direct use storage 

This method involves moving the collected rainwater from the rooftop to a storage tank. The catchment’s availability, rainfall, and water consumption are the key considerations while designing the storage tank. 

Drainpipes connecting to the tank must have a filtering system,a first flush device, and a mesh at the mouth. Each tank must have a method for excess water overflow. 

This is the most cost-effective method to collect rainwater. Harvested water can be used for extracurricular activities like laundering and washing. 

The main advantage of collecting and using rainwater during the rainy season is not only to conserve water from traditional sources but also to lower energy costs for water distribution and transportation. 

Filling up bore wells 

Collected rainwater on the building’s roof is moved to a settlement or filtration tank through drain pipes. After filtration, water is moved to a bore well to refill deep aquifers. Abandoned bore wells can also be recharged. 

The construction of a settlement tank with appropriate capacity is based on the catchment area, amount of rainfall, and recharge rate. Floating debris and silt must be kept away from the recharging structure since they might clog it up. 

The first one or two showers should be cleaned off with a rain separator to avoid contamination. 

Recharge pits

These are small pits with a weep hole that are spaced at regular intervals and enclosed by a brick or stone masonry wall. The top of the pit may be covered with perforated covers. The bottom of the pit should be filled with filter media. 

The capacity of the pit can be calculated using the catchment area, the amount of rainfall, and the rate of soil recharge. Depending on the depth of the preceding stratum, the pit’s dimensions typically range from 1 to 2 meters in width to 2 to 3 meters in depth.

In these holes, water can be restored for small homes and shallow aquifers.

Filling up of dig wells

Dug wells may serve as structures for recharge. Rainwater from the roof is diverted to drill wells after passing through the filter bed. Dig wells must be regularly cleaned and desalted to accelerate the process of recharging. One alternative is to use the filtration method suggested for bore well recharging.

Recharge or soakaway shafts

Soakaway or recharge shafts are provided in alluvial or less porous topsoil areas. These are the 30 cm-diameter bored holes, which, depending on the thickness of the previous layer, may reach depths of 10 to 15 m. A PVC/MS pipe with slots or perforations should line the bore to prevent the collapse of the vertical sidewalls.

The requisite size sump is constructed at the top of the soakaway to catch runoff before it filters through. Filter media should be in the sump.

Trenches for recharge 

A recharging trench is provided where the upper impermeable layer of soil is shallow. The recharge trench is dug and excavated, then filled with porous materials like pebbles, boulders, or brickbats. It is frequently used to collect surface runoff.

Bore wells can also be inserted inside the trench as recharge shafts to enhance percolation. The length of the trench is determined by the expected amount of runoff.

This method can be used to improve roadside drainage, playgrounds, parks, and small homes. The recharging trench might be anywhere between 0.50 and 1.0 meters wide and 1.5 meters deep.

Percolation in a tank

A portion of land with sufficient permeability is submerged in man-made surface water pools called percolation tanks, which allow for sufficient percolation to recharge the groundwater. They can be built on sizable campuses with available land and appropriate topography.

Surface runoff and roof runoff can both be directed into this tank. Water that has been stored in the tank percolates through the solids to increase the groundwater.

The water that has been saved can be used for gardening and other practical purposes. Percolation tanks need to be placed in all urban greenbelts, parks, and gardens.

Components of rooftop rainwater harvesting 

Catchments: A catchment is a surface region that receives direct rainfall and supplies rainwater to a system for collecting it. It is possible to build terraces with sloping or flat RCC/stone roofs, courtyards, and open land that has or has not been paved.

Transportation: Rooftop rainwater is transported to the rainwater collection system by water pipes or drains. The wire mesh should be placed over each drain’s mouth to keep the floating material in place. Water pipes need to be UV resistant and have the right capacity.

First Flush: The first shower’s water is removed using a system known as first flush. This is essential to prevent contamination in storage due to high amounts of contaminants from the atmosphere and catchment roofs. As a result, it helps to remove material that is thrown onto a roof during dry seasons and cracks. Construction for the first rain separator should also be done at the end of each drainpipe. After being cleansed, rainwater is sent through filters.

Filter: Some are sceptical of rooftop rainwater collection because they think that groundwater contamination could occur if the proper filtration system is not used or if underneath sewage drains are broken.

Rainwater harvesting in India

In India, the topic of water is a state subject. Nonetheless, through technical and financial support, the federal government supplements state initiatives to conserve and recharge water, including rainfall conservation and harvesting. The administration has also been putting various rainwater harvesting and conservation plans into action, coordinating them with state plans, and enlisting public support.

The Jal Shakti Abhiyan, one of the government’s major projects, would encourage water conservation and water resource management in 1,592 blocks across 256 water-stressed districts from July to November 2019. On March 29, 2022, the Jal Shakti Abhiyan: Catch The Rain – 2022 project will also begin in all districts of the nation in an effort to conserve water and collect rainwater. The Atal Mission for Rejuvenation and Urban Transformation (AMRUT) and the Atal Bhujal Yojana both emphasize rainwater collection, while the National Water Policy promotes water efficiency and rainwater collection.

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