Rainwater harvesting is the process of collecting and storing rainwater for future use. This article focuses on the details of rainwater harvesting including - what is rainwater harvesting is, its uses, methods, technologies, etc.

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What is Rainwater Harvesting?

A rainwater harvesting system (RWHS) is a process consisting of the collection and storage of precipitation or surface run-off for both potable and non-potable uses. With increased global freshwater demand, climate change, and environmental pollution, freshwater sources are depleting at an alarming rate. Rainwater harvesting is a sustainable method for freshwater sourcing in this era of the water crisis. It can be used for a variety of purposes, including irrigation, drinking water, and domestic use.

Rainwater Harvesting Areas

Countries like Uganda, Ethiopia, etc. which lack basic access to clean water, harvest rainwater to suffice their water needs to an extent. Not only underdeveloped countries, countries like New Zealand, Brazil, Thailand, etc. also implement rainwater collection to further boost their clean water availability. People living in hilly areas can collect rainwater in catchments to reduce surface runoff and consequent soil erosion. In arid lands, rainwater collected during rain can be used for dry-season necessities.

Technology Regarding Rainwater Harvesting

There are three fundamental components in a rainwater harvesting system.

Collection Area

The collection area is the surface area from where rainfall is collected. Usually, roofs or impervious surfaces of a structure are considered as collection areas. The slope of the surface, material, and area affects the collection quality and quantity of rainwater. Chemically inactive materials such as wood, plastic, aluminum, or fiberglass are used for making them. 

Conveyance System 

This is comprised of the gutters and pipes that connect the collection area to the storage tanks. Opening pipes can have wire mesh or screen to prevent material fragments and insects from entering the main flow. 

Storage Facilities 

This is where water is stored for future water needs. Storage tanks can be placed near or away from the collection area. Usually, these facilities are made of reinforced concrete, stainless steel, or fiberglass. 

Treatment of Harvested Rainwater

Rainwater comes in contact with various organic and inorganic materials during rainfall. Carbon-dioxide gas, dust particles, industrial micro-particles, bird feces, rotting leaves, etc. get mixed into the water and contaminate it. Hence, treatment is necessary according to the purpose of stored water.

First Flush Diversion

Usually, after the initial pour, the air and roof get cleared of the contaminants. So, if the first flush is diverted, rainwater can be easily kept clean.

Settlement

Rainwater, if kept still for a certain time being, heavy suspended particles settle at the bottom of the tank. Settlement helps in removing turbidity.

Filtration

Smaller particles can be cleaned by filters or membranes.

Coagulation and Flocculation

Alum coagulation and flocculation will settle small colloidal particles which can’t be filtered through filters. This also helps in removing the remaining turbidity or color of the water.

Disinfection

Lastly, water is disinfected using chlorination or other chemical methods to destroy harmful pathogens.

Factors Affecting the Quantity of Rainwater Harvested

There are a number of factors that can affect the quality of rainwater that is harvested. The followings are the major factors that affect the rainwater harvesting quality:

  • Rainfall intensity, duration, and frequency
  • Water quality
  • Method and technology behind the collection of rainwater
  • Water demand in that area
  • Location of catchment
  • Catchment area features and storage facilities
  • Run-off coefficient
  • Water treatment methods
  • Government regulations

Famous Rainwater Harvesting Project

The world’s largest rainwater harvesting project is the ‘Rainwater Harvesting in Rural Karnataka’ which is funded by the Rural Development and Panchayat Raj Department (RDPR) of the Government of Karnataka, India. 23,683 schools were selected after careful observation regarding water accessibility, fluoride contamination, and nearness to usable water sources. They used a simple rooftop harvesting system with PVC pipes for channeling to closed surface tanks.

Reference:

  1. Gayani K., Harshini M., J.D.E.M. G. (2013). “Rainwater Harvesting in Urban Buildings” Special Session on Green Building, 4th International Conference on Structural Engineering and Construction Management 2013, Kandy, Sri Lanka, 13th, 14th & 15th December 2013
  2. https://www.kscst.org.in/rwh_files/pdf/161.pdf
  3. https://www.britannica.com/technology/rainwater-harvesting-system
  4. https://www.watercache.com/education/rainwater-harvesting-101
  5. https://www.wateraid.org/bd/sites/g/files/jkxoof236/files/final-report-on-assessment-concept-design-and-layout-for-proposed-rainwater-harvesting-infrastructure-in-southwestern-bangladesh_0.pdf

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