What Is Rain Water Harvesting?
D Hillel defined Water Harvesting as, “The collection of rainstorm-generated runoff from a catchment (the area from which the water flows into bodies) to provide water either for immediate utilization as irrigation or for subsequent utilization by storing in above-ground ponds or aquifers.” In layman’s terms, water harvesting is the direct collection of rainwater.
There are mainly two methods of rainwater harvesting Surface runoff harvesting and Rooftop rainwater harvesting methods.
Although three-fourths of the earth is water, a minute amount is available for human consumption or irrigation. Moreover, due to increasing global temperature and population, there is a lack of potable water sources (e.g., rivers, lakes, or aquifers) in semi-arid and arid areas.
The global problem is rapid industrialization and disposal of chemical waste into water-bodies leads to pollution of freshwater sources. On the other side, rainwater is the only supply of fresh water left. To deal with the water crisis, water harvesting is adopted.
“The earth, the land, and the water are not an inheritance from our forefathers but on loan from our children. So, we have to hand it over to them at least as it was handed over to us.”–[Mahatma Gandhi]
Purpose of Rainwater Harvesting
- To serve as domestic use with proper treatment (drinking, watering garden).
- To serve as landscape irrigation without filtration, especially for dryland farming.
- To increase groundwater recharge which further increases soil fertility.
- To reduce stormwater discharges, urban floods, and overloading of sewage treatment plants; Saves the fresh surface water free of fertilizers, pesticides, metals, and other sediments.
- To reduce seawater ingress in coastal areas.
Besides the methodology of rainwater harvesting surface runoff, it reduces soil erosion. It also reduces water consumption by approximately 40%.
68% of cultivated area in Indian agriculture belongs to dryland, which contributes about 44% of total food production. –[DHAN Foundation, 2006]
Rainwater Harvesting Methods
There are mainly two methods of rainwater harvesting,
- Surface runoff harvesting
- Rooftop rainwater harvesting
1. Surface Runoff Harvesting
This system is most suitable in an urban area where the rainwater flowing along the ground during the rains will be collected to designated water storage. The flow of small tributaries of rivers or reservoirs is redirected to store the surface runoff.
The surface runoff is stored in ponds, tanks, and reservoirs built for this purpose. During the storage of rainwater, efficient and effective water conservation methods (reducing evaporation) are incorporated. Several measures are taken to sustain healthy and hygienic water.
2. Rooftop Rainwater Harvesting
Roof Top Rainwater Harvesting Method can be adopted at individual homes or schools where the rainwater is captured from the roof catchments of domestic houses or commercial buildings, diverts, and stored in tanks.
Harvested rainwater can either be stored in a tank or diverted to an artificial recharge system to meet daily needs such as flushing toilets, washing machines, washing vehicles, gardening, showers, sinks, and baths.
This system can be installed in the following places
- Residential Homes or Villas
- Schools, Colleges, and other Educational Institutions
- Apartments/Flats, Multi-Storey Buildings
- Government Buildings
- Industries, Factories, Mills
- IT Parks, Hotels, Restaurants, Resorts
- Swimming Pools, Stadiums
The rooftop rainwater harvesting system consists of sub-components such as catchments, transportation, first flush, and filter.
1. Catchments: The catchment of the rainwater harvesting system is the surface area that receives direct rainfall and contributes rainwater to the harvesting system. It can be a terrace (flat RCC/stone roof or sloping roof), courtyard, or paved or unpaved open ground.
2. Transportation: Rainwater from the rooftop is carried to the harvesting system from gutters through water pipes or drains. The mouth of each drain should be surmounted by wire mesh to restrict floating material. Water pipes must be UV resistant to the desired capacity.
3. First Flush: Water received in the first shower is flushed off by a device – first flush. This is essential to avoid contamination in storage from vast contaminants of the atmosphere and catchment roof.
Hence, it helps in cleaning slits and other materials deposited on a roof during dry seasons. In addition, provisions of the first rain separator should be made at the outlet of each drainpipe. After flushing, rainwater is passed through filters.
4. Filter: There is skepticism about Roof Top Rainwater Harvesting due to the perception that rainwater may contaminate groundwater if a proper filter mechanism is not adopted or if underground sewer drains are punctured.
Filters are used for removing turbidity, color, and microorganisms from water. Gravel, sand, and mesh filter are designed to place on top of the storage tank. Its function is to remove silt, dust, leaves, and other organic matter from entering the storage tank.
Filter media should be cleaned daily after rainfall. Clogged filters are suitable for this purpose. The sand or gravel media should be taken out to be washed before it replaces in the filter.
Different Types of Filter Used In Rainwater Harvesting
In practice, there are different types of filters whose basic function is to purify water,
- Sand Gravel Filter
- Charcoal Filter
- PVC-Pipe Filter
- Sponge Filter
1. Sand Gravel Filter
In the solid granular media category, three materials have been used – sand, gravel, and pebbles. Media less than 2mm in diameter is sand and anything larger is referred to as gravel.
The rainwater flows relatively slowly through sand and is used for single-pass filters. However, gravel is used for recirculating filters which can accept larger amounts of runoff than single-pass sand filters. Each layer is separated by wire mesh.
2. Charcoal Filter
Charcoal Filter A charcoal filter is an extended form of sand gravel filter that serves the purpose of absorbing odor apart from unnecessary minerals.
It consists of 25cm of gravel layer, 25cm of sand layer, 10cm of charcoal layer, and 10cm of gravel layer, where each layer is separated.
3. PVC – Pipe Filter
From the name, it is clear that the filter consists of a PVC pipe of 1 to 1.2m in length, while the diameter of the pipe varies with the area of the roof.
The dimension of pipe per sq. ft of roof area is mentioned below
|Diameter of pipe||The surface area of the roof|
|6 inches||1500 sq. ft|
|8 inches||>1500 sq. ft|
The pipe has three compartments, each of which is filled with gravel and sand alternatively. The gravel and sand layer is separated by a layer of charcoal. The size of the filter has been reduced to the size of the inlets and outlets at the end. PVC – The pipe filter can be placed horizontally or vertically.
4. Sponge Filter
The sponge filter consists of a PVC drum having a layer of sponge in the middle of the drum, usually suitable for residential units.
Methods of Rooftop Rainwater Harvesting
Various methods of rooftop rainwater harvesting are described in this section.
1. Storage of Direct Use
In this method, the rainwater is collected from the roof of the building and diverted to a storage tank. The design of the storage tank is based on water requirement, rainfall, and catchment area.
It is mandatory to have a mesh filter at the mouth of each drainpipe, along with the first flush device followed by a filtration system which then leads to a storage tank.
Apart from this, an excess water overflow system is also attached to tanks. The excess water collected is diverted to the recharge system.
The main advantage of this system is to save energy incurred on transportation and distribution of water at the doorstep.
2. Recharging Groundwater Aquifers
The rainwater is forced to drain into the various kinds of structures that percolate the rainwater into the ground instead of flowing onto the ground.
The recharging methods being utilized are as follows
Recharging of Bore Wells
Rainwater is collected from the rooftop of the building, then diverted through drainpipes to a filtration tank (settlement), from where water is diverted to bore wells to recharge deep aquifers. Leftover bore wells can also be used for recharge.
The design of the filtration tank is based on an area of catchment, the intensity of rainfall, and the recharge rate. Entry of floating matter and silt may clog the recharge structure.
so the first one or two showers should be flushed out through a rain separator to avoid contamination.
Recharging of Dug Wells
Rainwater is diverted to dug wells. It is necessary to clean and desalt the dug well for a productive recharge rate. The filtration method utilized in bore well recharging could be used.
Recharge pits can be of any shape (rectangular, square, or circular), contracted with brick or stone masonry walls with weep holes at regular intervals, and of small size.
The top of the pit is covered with perforated covers whereas the bottom of the pit is filled with filter media. The recharge pit recharges the shallow aquifer and small houses.
Recharge trenches are suitable where the upper impervious layer of soil is shallow (such as small houses, playgrounds, parks, and roadside drains). A trench is excavated on the ground which is refilled with porous media like pebbles, boulders, or brickbats.
For productive percolation, bore wells can also be inserted inside the trench as recharge shafts. The expected amount of runoff decides the length of the trench (usually 0.5 to 1.0m wide and 1.0 to 1.5m deep).
Soakaways or Recharge Shafts
The place where the upper layer of soil is alluvial or less previous layer – soak away recharge is a suitable option. Usually, the design of the borehole is 30 cm in diameter with 10 to 15 m deep (Depth depends on the depth of the previous layer).
To prevent the collapse of the vertical sides, bores are lined with slotted/perforated PVC/MS pipe. A sump filled with filter media is constructed at the top of the soakaway to retain runoff.
Percolation tanks – artificially created surface water bodies, submerging a land area with adequate permeability to facilitate sufficient percolation to recharge the groundwater (suitable for big campuses where land is available and topography is suitable, such as gardens, open spaces, and roadside greenbelts of urban area).
The stored water can be directly used for gardening and purpose other than drinking.
Indian Irrigational Purpose Rainwater Harvesting
In a developing country like India where agriculture has a large contribution to GDP, usually, three types of rainwater harvesting techniques are practiced.
The following are techniques of rainwater harvesting,
Inter-row Water Harvesting
Inter-row water harvesting is also known as roaded catchments practice in arid areas where annual rainfall is less than 750mm(suitable for flat lands without the sacrifice of land). Crops are grown in narrow strips between wide intervals that are ridge and compacted to increase runoff to crop rows.
Triangular cross-sectional bunds (height of bunds ranges from 40 to 100 cm, built at an interval of 210m) or levees are constructed along the main slope of the land.
The runoff flowing down the slope is collected between the ridges and then directed to the reservoir at the end of a feed canal or to a crop cultivated between the ridges.
Inter-plot or Micro Plot Water Harvesting
Inter-plot water harvesting is the practice in an area where the rainfall is more than 750mm. water is harvested in the passages or furrows between the plots. Runoff from the sloping area directs the rainfall towards the crops on level land.
Water Harvesting In Farm Ponds and Reservoirs
In this technique, surface runoff is stored in farm ponds (small tanks) or reservoirs. These are meant for lifesaving irrigation, cattle, fish production, etc. The design of farm ponds is such that the depth is 3ft. while the width and length vary with the farmer’s land and water potential of the catchment in the field.
Dugout ponds, surface ponds, spring or creek-fed ponds, and off-stream storage ponds are the different types of farm ponds depending on the source of water and their location concerning the land surface. Waterborne asphalt emulsion is utilized to control seepage loss.
What is rainwater harvesting?
Rainwater harvesting is the process of collecting and storing rainwater for later use, rather than allowing it to run off and be wasted.
What are the benefits of rainwater harvesting?
There are many benefits to rainwater harvesting, including reduced demand for municipal water supplies, lower water bills, improved soil quality, and reduced erosion and runoff.
What are the different methods of rainwater harvesting?
The most common methods of rainwater harvesting include rooftop harvesting, surface runoff harvesting, and subsurface harvesting.
How does rooftop harvesting work?
Rooftop harvesting involves collecting rainwater from rooftops and storing it in tanks or other containers for later use. This method is typically used in urban areas where there is limited space for other types of harvesting systems.
What is surface runoff harvesting?
Surface runoff harvesting involves collecting rainwater from land surfaces such as roads, parking lots, and other impervious surfaces. The collected water is typically diverted to storage tanks or other containers for later use.
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