roof top rain water harvesting - definition, components ,type of filters, runoff coefficient, advantages

1. Natural Resource Management
Roof top water harvesting
Revadh Balraj
2017-20-005

2. Rooftop Rain Water Harvesting is the
technique through which rain water is captured
from the roof catchments and stored in
reservoirs.
Harvested rain water can be stored in sub-
surface ground water reservoir by adopting
artificial recharge techniques to meet the
household needs through storage in tanks.
Roof Top Rainwater Harvesting

3. Roof Top Rainwater Harvesting Unit

4. Components of Rainwater Harvesting
System
• Cachements
• Coarse mesh
• Gutters
• Conduits
• First flush
• Filters
• Storage tanks and
• Recharge structures

5. 1. Catchments
The surface which directly receives the rainfall
and provides water to the system is called
catchment area. It can be a paved area like a
terrace or courtyard of a building, or an
unpaved area like a lawn or open ground. A
roof made of reinforced cement concrete
(RCC), galvanized iron or corrugated sheets
can also be used for water harvesting.

6. 2. Coarse Mesh
It prevents the passage of debris, provided in
the roof.
3. Gutters
Channels which surrounds edge of a sloping
roof to collect and transport rainwater to the
storage tank. Gutters can be semi – circular or
rectangular and mostly made locally from
plain galvanized iron sheet. Gutters need to be
supported so they do not sag or fall off when
loaded with water.

7. 4. Conduits
Conduits are pipelines or drains that carry
rainwater from the catchment or roof top area
to the harvesting system. Commonly available
conduits are made up of material like polyvinyl
chloride (PVC) or galvanized iron (GI).
5. First-flushing
A first flush device is a valve which ensures
flushing out of first spell of rain away from the
storage tank that carries a relatively larger
amount of pollutants from the air and
catchment surface.

8. 6. Filters
The filter is used to remove suspended
pollutants from rainwater collected from roof
top water. There are different types of filters in
practice, but the basic function is to purify
water. Different types of filters are described in
the following section:
a. Sand Gravel Filter
These are commonly used filters, constructed
by brick masonry and filleted by pebbles,
gravel, and sand. Each layer should be
separated by wire mesh.

10. b. Charcoal Filter
Charcoal filters can be made in-situ or in a
drum. Pebbles, gravel, sand, and charcoal as
shown in the figure should fill the drum or
chamber. Each layer should be separated by
wire mesh. The thin layer of charcoal is used
to absorb odor if any.

11. c. PVC –Pipe filter
This filter can be made by PVC pipe of 1 to
1.20 m length; Diameter of pipe depends on
the area of roof. Six inches dia. pipe is enough
for a 1500 Sq. Ft. roof and 8 inches dia. pipe
should be used for roofs more than 1500 Sq.
Ft. Pipe is divided into three compartments by
wire mesh

12. Each component should be filled with gravel
and sand alternatively as shown in the figure.
A layer of charcoal could also be inserted
between two layers.
Both ends of the filter should have a reduction
of the required size to connect the inlet and
outlet. This filter could be placed horizontally
or vertically in the system. A schematic pipe
filter is shown in Fig 3.

13. d.Sponge Filter
It is a simple filter made from PVC drum
having a layer of sponge in the middle of
drum. It is the easiest and cheapest form filter,
suitable for residential units. A typical figure of
sponge filter is shown in Figure

14. 7. Storage facility
There are various options available for the construction
of these tanks with respect to the shape, size, material
of construction and the position of tank and they are: -
Shape : Cylindrical, square and rectangular.
Material of construction: Reinforced cement concrete
(RCC), masonry, Ferrocement etc.
Position of tank: Depending on land space availability
these tanks could be constructed above ground, partly
underground or fully underground. Some maintenance
measures like disinfection and cleaning are required to
ensure the quality of water stored in the container.
If harvested water is decided to recharge the
underground aquifer/reservoir, then some of the
structures mentioned below are used.

15. 8. Recharge structures
Rainwater Harvested can also be used for
charging the groundwater aquifers through
suitable structures like dugwells, borewells,
recharge trenches and recharge pits.
Various recharge structures are possible –
some which promote the percolation of water
through soil strata at shallower depth (e.g.,
recharge trenches, permeable pavements)
whereas others conduct water to greater
depths from where it joins the groundwater
(e.g. recharge wells).

16. At many locations, existing structures like
wells, pits and tanks can be modified as
recharge structures, eliminating the need to
construct any fresh structures.
Some of the few commonly used recharging
methods are recharging of dug wells and
abandoned tube wells, Settlement tank,
Recharging of service tube wells, Recharge
pits, Soakaways /Percolation pit , Recharge
troughs, Recharge trenches, Modified
injection well.

17. How Much You Can Collect
Collection Efficiency
How efficiently the rainfall can be collected depends
on several considerations. Collection efficiencies of
80% are often used depending on the specific design.
Rainfall Reliability.
The first step is to determine how much water would
be generated from your roof area. Average monsoon
rainfall is used for this purpose.
Formula:
Total quantity of water to be collected (cu.m.) = Roof Top
Area (Sq.m.) x Average Monsoon Rainfall (m) x 0.8

18. Runoff coefficient
When calculating the amount of runoff that
can be from a roof, it is common to include a
runoff coefficient. The Table lists the runoff
coefficients for common roof materials.

19. This coefficient accounts for the fact that some
roof surfaces are more efficient than others at
collecting rainwater. For example, a pitched
metal roof is typically the most efficient type
of roof for collecting water, delivering 95% of
the water that falls on it (except for some
heavy snowfalls). Conversely, a flat tar-and-
gravel roof is typically the least efficient roof
type, delivering 80-85% of the water that falls
on it

20. Need for Rooftop Rain Water
Harvesting
• To meet the ever increasing demand for water
• To reduce the runoff which chokes storm drains
• To avoid flooding of roads
• To augment the ground water storage and control
decline of water levels
• To reduce ground water pollution
• To improve the quality of ground water
• To reduce the soil erosion
• To supplement domestic water requirement during
summer, drought etc.

21. Advantages of Rain Water Harvesting
• Provides self-sufficiency to your water supply
• Reduces the cost for pumping of ground water
• Provides high quality water, soft and low in minerals
• Improves the quality of ground water through dilution
when recharged to ground water
• Reduces soil erosion in urban areas
• The rooftop rain water harvesting is less expensive
• Rainwater harvesting systems are simple which can be
adopted by individuals

22. • Rooftop rain water harvesting systems are easy to
construct, operate and maintain
• In hilly terrains, rain water harvesting is preferred
• In saline or coastal areas, rain water provides good
quality water and when recharged to ground water, it
reduces salinity and also helps in maintaining balance
between the fresh-saline water interface
• In Islands, due to limited extent of fresh water aquifers,
rain water harvesting is the most preferred source of
water for domestic use
• In desert, where rain fall is low, rain water harvesting
has been providing relief to people

23. Thank You