Water

Catch water where it falls: Urban rainwater harvesting

Every raindrop matters at this time when large parts of the world are staring at water scarcity. Here are the answers to some FAQs on urban water harvesting

 
By DTE Staff
Last Updated: Thursday 04 July 2019
Photo: Getty Images

In urban areas, rainwater can be collected from the roof, paved and unpaved areas of a house, a block of flats, a colony, a park, a playground, parking areas, schools, office complexes lakes and tanks.

There are two ways of using the harvested rainwater:

  • Storing in receptacles 
  • Recharging into the aquifer

There are instances where the citizens have come together to revive waterbodies, which act as groundwater recharge structures in urban centres.

How to calculate?

Thumb rule: 10 millimetre of rainfall over 100 square metres of roof area will fetch 1,000 litres (volume = rainfall x area).

Accounting for some loss due to evaporation or absorption by catchment surfaces, the actual volume can be ascertained by determining the run-off coefficient of the catchment — it indicates the proportion of rainwater that can be harvested from the total rainfall.

 Let’s take a roof, for example:

  • Area = length x breadth = 20 m x 10 m = 200 sqm
  • Run-off coefficient = 0.8
  • Annual rainfall = 500 mm 
  • Rainwater harvesting potential = 200 x 0.8 x 500 = 80,000 litres 
  • Water demand, family of four, consuming 540 l/day = 540 x 365 = 197,100 l/day 
  • Water demand, family of four, the 3 dry months = 540 x 90 = 48,600 l 
  • Water demand for toilet flushing and gardening (180 l/day) = 180 x 365 = 65,700 l/year

 Elements of rainwater harvesting

 A few basic elements are common to all rainwater harvesting (RWH) systems 

  1. The catchment area, where the rain falls
  2. The conveyance, or conduit, system that channels the flow of water in a given direction
  3. The first flush (a valve that ensures that run-off from the first spell of rain is flushed out and does not enter the system) and the filter system
  4. The storage area, consisting of tanks/receptacles
  5. The recharge area, where harvested rainwater is used to replenish groundwater

A simple RWH system. Illustration: CSE

The catchments

The catchment is a structure or land area that is used to collect rainwater and drain run-off. Clean catchments are necessary for maximising harvest. They can be either paved (roofs, courtyards, roads, etc) or unpaved (lawns, playgrounds, open spaces, etc). 

Types of catchments. Illustration: CSE

Charecteristics of catchments 

Collection efficiency: Not all the rain that falls on the catchments is available for rainwater harvesting. Some of the water that falls on catchments may be lost because of evaporation, seepage into the ground, absorption by roof materials, improper fittings, leakages from pipes and gutters and clogging in various parts of the system. These factors and the effective catchment area largely influence the collection efficiency. 

Run-off coefficient: This is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface. More water runs off smooth and impervious surfaces such as roofs or paved areas than soils and unpaved areas. Different catchment materials absorb water to differing extents.

Conveyance systems

Conveyance, or conduit systems, direct water flow from the catchment area to the storage area. A carefully designed and constructed conveyance system can divert more than 90 per cent of all the water that falls on to the roof.

A roof outlet. Illustration: CSE

Harvesting rain from a sloping roof. Illustration: CSE

Filter systems

Four types of filtration processes can be used in an RWH system:

  • Separation or screening: This is the first level that filters out gross pollutants such as leaves, droppings and other materials
  • First flush: At the second level, the first spell of rain containing dissolved impurities — the first flush — is allowed to flow away
  • Filtration: Filters remove dissolved organic and inorganic particles in the rainwater, before it is collected
  • Settlement tanks: Settling tanks remove silt and other coarse materials 

Harvesting for storage

Storage tanks for harvested rainwater can be built both underground and overground. In the cities of Gujarat and Rajasthan, where rooftop harvesting was practised traditionally, rainwater from the roof was collected in underground tanks in the courtyard or within the buildings. Called tankas, these tanks would supply drinking water throughout the year. Some are still in use today.

The number of tanks will depend on site conditions like:

  • Volume of rainwater that can be harvested
  • Water demand
  • Space available
  • Layout of the building
  • Size of the storage tank
  • Budget

 Siting of tanks

 

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