About 1,050 mm/year or approximately 88 mm/month of rain falls on the entire surface of our planet.
Rainwater Harvesting is the activity involving collection of rain water for purposes of direct use, indirect use, storage, recharging of ground water levels, etc.
Depending on the target catchment areas, there are many possible configurations and degrees of complexity to a rainwater harvesting system. Based on how the rainwater is pumped, it can be divided into the following categories:
- Direct Pumped
- Indirect Pumped
- Indirect Gravity
- Direct Gravity
Common Types of Rainwater Harvesting Systems
- Water Butt – This is one the most basic form of rainwater harvesting where the water collects in the container from the drain pipes and/or natural rainfall. It is usually used for gardening purposes.
- Submersible – This type of rainwater harvesting systems is most commonly used for domestic purposes. The pump is located within the underground tank and harvested water is simply pumped directly to the WCs or other appliances.
- Suction – This type of rainwater harvesting systems is located within a control unit within the establishment(e.g. utility room). The unit also deals with the backup from mains water supply, so there is no need to send mains water down to the tank.
- Indirect Gravity – This type of rainwater harvesting system first pumps the harvested rainwater to a high level tank (header tank) and then allowed to supply the outlets by gravity alone. With this arrangement, the pump only has to work when the header tank needs filling.
- Indirect Pumped – This type of rainwater harvesting system first pumps the harvested rainwater to a tank. This tank can be at any level in the building, as it does not rely on gravity to supply the outlets. Instead a booster pump set is used to provide a pressurized supply. This system employs the benefit of not having to feed mains back-up water to the underground tank, whilst also offering great flexibility as the booster pumps can be tailored to suit the flow and pressure requirements of the building.
- Gravity Only – In some situations It may be possible to have a system that functions purely through gravity. This type of rainwater harvesting system is only ever possible where the storage tank can be located below the level of the gutters, yet higher that the outlets that it will supply. Only the power of gravity is needed to feed collected and filtered water to various parts of the home for use, so it is an ultra-energy efficient option.
Rainwater Harvesting Potential
Rainwater harvesting potential of a particular catchment area is the amount of rainwater that can be effectively harvested from the said catchment area.
To calculate the rainwater harvesting potential, the following data is required:
- Surface area of catchment in sq. ft.
- Runoff Co-efficient of catchment surface.
- Amount of water received in rainfall in mm.
(Amount of rainfall can be found out from local authorities website or any other local news sources)
Formula to calculate rainwater harvesting potential:
Rainwater Harvesting Potential = Surface Area of catchment (sq. ft.) x Runoff Co-Efficient of Catchment Surface x Amount of Rainfall (mm)
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.
Types 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.
Characteristics of a Good Rainwater Harvesting System
All good rainwater harvesting systems have the following characteristics that if adopted ensure maximum efficiency:
- Completeness: To ensure that runoff from maximum collectible areas can be harvested.
- Apportioning of water: To ensure that the system is not overloaded which may lead to wastage of rain water by means of overflowing or other losses.
- Proper design: To ensure consideration of the total volume of water that will be harvested and released to flow through the soil in the area or sewerage after use should be considered.
- Maintainability: Design should incorporate features allowing for periodic maintenance, cleaning and scaling of the structure.
Advantages and Disadvantages
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.
- 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.
- Easy Maintenance : Maintenance for these systems require little time and energy.
- Multi Purpose : Rainwater can be used for various things from flushing toilets, to washing clothes, cars, etc.
- 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.
- 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.
- 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.
- Initial Setup Cost : Determining the best system configuration and getting it set up is not always budget friendly.
- Unpredictable Nature of Rainfall : Prediction of rainfall is not an exact science yet. This can limit the supply of rainwater.
- Storage Limitations : Rainwater collection and storage systems are fairly limited and restrictive.