Rainwater Harvesting

About 1,050 mm/year or approximately 88 mm/month of rain falls on the entire surface of our planet.

Rainwater Harvesting | Greensutra | India

1.What is Rainwater Harvesting?

Rainwater Harvesting is the activity involving collection of rain water for purposes of direct use, indirect use, storage, recharging of ground water levels, etc.

2. What are the advantages and disadvantages of Rainwater Harvesting?

Rainwater Harvesting is one of the simplest and oldest methods of self-supply water for domestic purposes. Like everything else, harvesting rainwater comes with its share of advantages and disadvantages.


  1. Flexible System : One of the beauty of these systems is their flexibility. It can be as simple as placing a tank/barrel under the rain gutter for purposes like watering the plants, cleaning, etc or it can be complex engineered, multi-filter, multi-tank system for residential and commercial supply.
  2. Easy Maintenance : Maintenance for these systems require little time and energy.
  3. Multi Purpose : Rainwater can be  used for various things from flushing toilets, to washing clothes, cars, etc.
  4. Reduced Floods & Soil Erosion : Collecting rainwater, prevents it from reaching the ground. It is possible to prevent flooding if carried out on a large enough scale. In many low lying areas, it helps reduce soil erosion by preventing surface run-offs.
  5. Reduced Water Bills : Collected rainwater can substitute for several domestic purposes by families and businesses. This may lead to a significant reduction in utility bills.
  6. Helps Preventing Water Contamination / Pollution : Non harvested rainwater flows freely and rejoins a larger water body. While freely flowing, it carries with it multiple impurities and toxins which then contaminate the larger water body. Such instances can be easily prevented by Rainwater Harvesting.


  1. Initial Setup Cost : Determining the best system configuration and getting it set up is not always budget friendly.
  2. Unpredictable Nature of Rainfall : Prediction of rainfall is not an exact science yet. This can limit the supply of rainwater.
  3. Storage Limitations : Rainwater collection and storage systems are fairly limited and restrictive.

3.What are the different components of a Rainwater Harvesting System ?

There are several types of rainwater harvesting systems that range in complexity and capacity depending on the requirements. Despite the varied systems available, the essential components of rainwater harvesting systems remain the same.

The components of Rainwater Harvesting Systems are listed as follows:

  • Catchment Area – Refers to the targeted surface area that will be used for harvesting the rainwater
  • Coarse Mesh – Refers to the mesh installed at the inlet point to avoid the passage of debris, dried leaves etc.
  • Gutters (optional) – Refers to the channels created along the perimeter of the catchment area to transport rainwater
  • Conduits or Piping Systems – Refers to the piping system installed for transporting the water from the catchment surface to the storage devices
  • First Flushing Devices – Refers to the non-return valve system that can be engaged to flush out the runoff from the first rainfall ensuring that it does not enter system
  • Filter Devices – Refers to the filter devices installed to remove suspected pollutants entering the storage devices
  • Storage Devices – Refers to the sump or storage devices where the harvested rainwater can be stored for later use.

4. What type of filters are required for Rainwater Harvesting?

Before storing harvested rainwater, it needs to be filtered to ensure that it is kept in the best possible condition, to avoid:

  • Degradation of Biological Material
  • Development of Odours

Rainwater harvesting systems may have different types of filters.

  • Mechanical Pre-tank Filters: Stopping contaminants from entering the rainwater harvesting storage tank is the most most effective way of keeping rainwater clean. To achieve this, most systems use a pre-tank or in-tank filter. The degree of filtration can be controlled by the fineness of the mesh which is measured in microns. The lower the micron rating, the finer the degree of filtration.
  • Microscopic Filtration: Use of particulate filters offers a much finer degree of filtration.  Particulate filters are generally in the form of a sealed housing containing a cartridge or bag which traps very fine particles to provide an even cleaner standard of water which require periodic changing. For these filters to operate efficiently, they require pressure. Due to this, such filters are only found on externally pumped systems.
  • Disinfection: While mechanical filters are effective at removing particles from water, they cannot remove bacteria. This is not an issue if the use of water is going to be for purposes of flushing, washing and gardening. Additional step of disinfection becomes mandatory, if the rainwater is also destined for personal use; drinking, showering, etc.
  • Carbon Filters: Carbon Filters are used in cases where rainwater is to be used for purposes such as drinking. Passing water through carbon helps improve the taste and odour significantly and also discolouration to a degree. Carbon is also effective at removing chlorine and other volatile organic substances (VOCs).

Here are some examples of these filter systems:

  • Sand Gravel Filter: These filters are made using bricks which are then filled with three layers of pebbles, gravel, and sand each that are separated using a wire mesh. These are commonly used filters, constructed by brick masonry and filleted by pebbles, gravel, and sand as shown in the figure. Each layer should be separated by wire mesh.
  • Charcoal filters: These filters can be made on site using a drum with three layers of gravel, sand and finally charcoal that are separated using a wire mesh. The charcoal in this filter also helps in absorbing any foul odor if it is present.
  • PVC- Pipe filter: These filters that can be placed both horizontally or vertically are made using PVC pipes of a diameter of 6-8 inches and length of 1-1.2 meter. The pipe is then divided into three layers which are separated by a wire mesh. The first layer has gravel, the second layer has charcoal and the third layer has sand. The ends of the filters should have a reduced size that matches the size of the inlet and outlet.
  • Sponge Filter: This filter is made using a PVC drum with a layer of sponge in the middle of the drum. This filter is quite suitable for home use. It is also the cheapest kind of filter.

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