2. 2
• Introduction
• Uses of Rain Water Harvesting
• Advantages of Rain Water Harvesting
• Limitations of Rain Water Harvesting
• Rain water System components and design
considerations.
• Feasibility of Rain Water Harvesting
• Quality of Rain Water
• Conclusion
TABLE CONTENTS
3. 3
Rainwater harvesting (RWH): technology used for collecting
and storing rainwater for human use from rooftops, land
surfaces or rock catchments.
One of the world’s most important ancient water supply
techniques (practiced for more than 4,000 years), is beginning
to enjoy a resurgence in popularity.
Rainwater is an important water source in many areas with
significant rainfall but lacking any kind of conventional,
centralised supply system.
Introduction
●●●
4. 4
Rainwater is also a good option in areas where good quality
fresh surface water or groundwater is lacking.
It could be used as a supplement to piped water supply e.g.
for toilet flushing, washing and garden spraying
RWH is a decentralised, environmentally sound solution, which
can avoid many environmental problems often caused in
centralised conventional large-scale water supply projects.
Introduction
8. 8
In view of increasing migration to urban area and the emergence of
mega-cities in the next millennium, it is imperative that water supply
systems should be evolved to cater for such a development.
In areas with relatively high rainfall spread throughout the year, where
other water resources are scarce, RWH is an important option, for
example parts of Sri Lanka, Philippines, Indonesia, Nepal and Uganda.
Installation RWH system is mandatory for the construction of buildings
in some towns in India and on the Virgin Islands, USA.
Many government agencies and municipalities worldwide provide
grants/subsidies and technical know-how to promote RWH system.
Uses of Rain Water Harvesting
●●●
9. 9
In case of roof catchment systems, there is sufficient
flexibility to utilize systems that will be adaptable to suit all
socio-economic levels of population including the urban poor.
Examples of typical options in urban area
- Rainwater use in households as a supplement
- Public institutions
- High rise building in high density urban areas
- Collection of rainwater in industrial areas
- Use of runoff in airports
- Collection of rainfall from public open spaces for
recharging
Uses of Rain Water Harvesting
10. 10
RWH systems provide water at or near the point where water is
needed or used.
Rainwater is relatively clean and the quality is usually
acceptable for many purposes with little or even no
treatment.
System is independent and therefore suitable for scattered
settlements.
Local materials and craftsmanship can be used in construction
of rainwater system.
Ease in maintenance by the owner/user
Provides a water supply buffer for use in times of emergency or
breakdown of the public water supply systems
Advantages of RWH
●●●
11. 11
Flood control - by greatly reducing urban runoff;
Stormwater drainage - by reducing the size and scale of
infrastructure requirements;
Firefighting and disaster relief - by providing independent
household reservoirs;
Water conservation - as less water is required from other
sources;
Reduced groundwater exploitation and subsidence - as less
groundwater is required;
Financial savings – where rainwater can be used in place of
water purchased from water vendors.
Advantages of RWH
12. 12
The initial cost (mainly of storage tank) may prevent a
family from installing a RWH system.
The water availability is limited by the rainfall intensity
and available roof area.
Mineral-free rainwater has a flat taste, which may not be
liked by many.
The poorer segment of the population may not have a
roof suitable for rainwater harvesting.
Domestic RWH will always remain a supplement and not a
complete replacement for city-level piped supply or supply
from more ‘reliable’ sources.
Limitations of RWH
13. 13
Catchment Area/Roof
- the surface upon which the rain falls
Gutters and Downpipes
- the transport channels from catchment surface to storage
Leaf Screens and Roofwashers
- the systems that remove contaminants and debris
Cisterns or Storage Tanks
- where collected rainwater is stored
Conveying
- the delivery system for the treated rainwater, either by
gravity or pump
Water Treatment
- filters and equipment, and additives to settle, filter, and
disinfect
RWH System Components
●●●
14. 14
Rainfall quantity (mm/year)
Rainfall pattern
Collection surface area (m2)
Runoff coefficient of collection (-)
Storage capacity (m3)
Daily consumption rate (litres/capita /day)
Number of users
Cost
Alternative water sources
RWH System Components
15. 15
The technical feasibility of roof RWH as a primary
source of water is determined by the potential of a
rainwater to meet the demand more effectively than
other alternatives.
Often the attraction of RWH may be as a
supplementary water source to reduce the pressure
on a finite primary source or as a backup during the
time of drought or breakdown.
The total amount of water that is received in the
form of rainfall over an area is called the rainwater
endowment of that area.
The collection efficiency accounts for the fact that
all the rainwater falling over an area cannot be
effectively harvested.
Feasibility of Rainwater Harvesting
●●●
16. 16
The size of supply of rainwater depends on the
amount of rainfall (R), the area of the catchment (A)
and its runoff coefficient (C).
An estimate of mean annual runoff from a given
catchment can be obtained using the equation:
S = R * A * C
Where S = Rainwater supply per annum
R = mean annual rainfall
A = Area of the catchment
C = Runoff coefficient
The actual amount of rainwater supplied will ultimately
depend on the volume of the storage tank or reservoir.
Feasibility of Rainwater Harvesting
17. 17
The quality of rainwater is relatively good but it is
not free from all impurities.
Analysis of stored rainwater has shown some
bacteriological contamination.
The rainwater is essentially lacking in minerals,
the presence of which is considered essential in
appropriate proportions.
Cleanliness of roof and storage tank is critical in
maintaining good quality of rainwater.
The storage tank requires cleaning and
disinfection when the tank is empty or at least
once in a year.
Quality of Rainwater
18. 18
The quality of rainwater is relatively good but it is
not free from all impurities.
Analysis of stored rainwater has shown some
bacteriological contamination.
The rainwater is essentially lacking in minerals,
the presence of which is considered essential in
appropriate proportions.
Cleanliness of roof and storage tank is critical in
maintaining good quality of rainwater.
The storage tank requires cleaning and
disinfection when the tank is empty or at least
once in a year.
Conclusion
