3. system components and design considerations
DESCRIPTION
3. System Components and Design Considerations. RWH System Components. 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 - PowerPoint PPT PresentationTRANSCRIPT
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3. System Components and
Design Considerations
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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
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The material of the catchment surfaces must be non-toxic and not contain substances which impair water quality.
Roof surfaces should be smooth, hard and dense since they are easy to clean and are less likely to be damaged and shed materials into water
Precautions are required to prevent the entry of contaminants into the storage tanks.
- No overhanging tree should be left near the roof
- The nesting of the birds on the roof should be prevented
- A first flush bypass such as detachable downpipe should be installed
Design considerations for rooftop catchment systems (1)
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All gutter ends should be fitted with a wire mesh screen to keep out leaves, etc.
The storage tank should have a tight-fitting roof that excludes light, a manhole cover and a flushing pipe at the base of the tank.
The design of the tank should allow for thorough scrubbing of the inner walls and floor or tank bottom. A sloped bottom and a provision of a sump and a drain are useful for collection and discharge of settled grit and sediment.
Taps/faucets should be installed at 10 cm above the base of the tank as this allows any derbis entering the tank to settle on the bottom where it remains undisturbed, will not affect the quality of water.
Design considerations for rooftop catchment systems (2)
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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
Factors affecting RWH system design
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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
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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 rainfallA = Area of the catchmentC = Runoff coefficient
The actual amount of rainwater supplied will ultimately depend on the volume of the storage tank or reservoir.
Feasibility of Rainwater Harvesting
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The size of roof catchment is the projected area of the roof or the building’s footprint under the
roof.
To calculate the catchment area (A), multiply the length (L) and width (B) of the guttered area. It is not necessary to measure the sloping edge of the roof.
Note that it does not matter whether the roof is flat or peaked. It is the “footprint” of the roof drip line that matters.
Catchment Area Size
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Type Runoff coefficient
Notes
GI sheets > 0.9 Excellent quality water. Surface is smooth and high temperatures help to sterilise bacteria
Tile (glazed)
0.6 – 0.9 Good quality water from glazed tiles. Unglazed can harbour mould Contamination can exist in tile joins
Asbestos Sheets
0.8 – 0.9 New sheets give good quality water Slightly porous so reduced runoff coefficient and older roofs harbour moulds and even moss
Organic (Thatch)
0.2 Poor quality water (>200 FC/100ml) Little first flush effect; High turbidity due to dissolved organic material which does not settle
Characteristics of Roof Types
Source: http://www.eng.warwick.ac.uk/dtu/rwh/components2.html
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Example 1:
For a building with a flat roof of size 10 m x 12 m in a city with the average annual rainfall of 800 mm
Roof Area (A) = 10 x 12 = 120 m2
Average annual rainfall (R) = 800 mm = 0.80 m
Total annual volume of rainfall over the roof
= A * R = 120 m2 x 0.80 m = 96 m3 = 96,000 litres
If 70% of the total rainfall is effectively harvested,
Volume of water harvested = 96,000 x 0.7 = 67,200 litres
Average water availability = 67,200 / 365 ~ 184 litres/ day
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There are several options available for the storage of rainwater. A variety of materials and different
shapes of the vessels have been used.
In general, there can be two basic types of storage system:
- Underground tank or storage vessel
- Ground tank or storage vessel
The choice of the system will depend on several technical and economic considerations like, space availability, materials and skill available, costs of buying a new tank or construction on site, ground conditions, local traditions for water storage etc.
Storage System
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The storage tank is the most expensive part of any RWH system and the most appropriate capacity for any given locality is affected by its cost and amount of water it is able to supply.
In general, larger tanks are required in area with marked wet and dry seasons, while relatively small tanks may suffice in areas where rainfall is relatively evenly spread throughout the year.
Field experiences show that a universal ideal tank design does not exist. Local materials, skills and costs, personal preference and other external factors may favour one design over another.
Storage System
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A solid secure cover to keep out insects, dirt and sunshine
A coarse inlet filter to catch leaves etc.
A overflow pipe
A manhole, sump and drain for cleaning
An extraction system that does not contaminate the water e.g. tap/pump
A soakaway to prevent split water forming puddles near the tank.
Additionally features
- sediment trap or other foul flush mechanism
- device to inside water level in the tank
Requirements for Storage System
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RWH Brick Jars - Uganda
Source: Rees and Whitehead (2000), DTU, University of Warwick, UK
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Rainwater Harvesting - Kenya
Source: John Gould (Waterlines, January 2000)
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Ferro-cement jar for rainwater collection - Uganda
Source: DTU, University of Warwick (September 2000)
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Underground lime and bricks cistern
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Rainwater Harvesting – Sri Lanka
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25http://www.greenhouse.gov.au/yourhome/technical/pdf/fs22.pdf
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A wooden water tank in Hawaii, USA
Source: Rainwater Harvesting And Utilisation. An Environmentally Sound Approach for Sustainable Urban Water Management: An Introductory Guide for Decision-Makers. ITEC, UNEP, Japan
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27http://www.arcsa-usa.org/
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Source: http://www.greenhouse.gov.au
Rainwater Tanks
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Storage capacity
When using rainwater, it is important to recognize that the rainfall is not constant through out the
year; therefore, planning the storage system with an adequate capacity is required for constant use of rainwater, even during the dry period.
Knowledge of the rainfall quantity and seasonality, the area of the catchment surface and volume of the storage tank, and quantity and period of use required for water supply purposes is critical.
There are two commonly used method to estimate storage requirements.
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Storage capacity
Method 1 – Storage required for dry period
A rough estimate of the maximum storage requirement can be made based on the (i) per capita consumption (ii) no of users and (iii) length
of the longest dry period
For a household with a 5 people, assuming water use of 20 lpcd and if longest dry period is 30
days and rainwater is the only water source, storage required = 5 x 20 x 30 = 3000 litres
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Storage capacity
Method 1 – Storage required for dry period
This simple method assumes sufficient rainfall and catchment area which is adequate, and is
therefore only applicable in areas where this is the situation.
It is a method for acquiring rough estimates of tank size.
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Storage capacity
Method 2 – Based on rainfall and water demand pattern
A better estimate of storage requirement can be made using the mass curve technique based on rainfall and water demand pattern.
Cumulative rainfall runoff and cumulative water demand in year is calculated and plotted on the same curve.
The sum of the maximum differences, on the either side, between the rainfall curve and water demand curve gives the size of the storage required
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Storage capacity
Example 2:
Calculate the size of the storage tank required for a school with 65 students and 5 staff, assuming average water consumption of 5 litres/day.
Roof area = 200 m2.
Assume runoff coefficient of 0.9.
The rainfall pattern in the area is given in the table below
Average daily demand = 70 x 5 = 350 litres
Yearly demand = 350 * 365 = 127750 litres = 127.75 m3 Average monthly demand = 127.75/12 ~ 10.65 m3
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Storage capacity calculations
(a) Rainfall pattern - 1
Month Rainfall
mmJan 120Feb 90Mar 70Apr 120May 40June 50July 45Aug 15SepOct 45Nov 70Dec 45
0
50
100
150
J F M A M J J A S O N D
Month
Ra
infa
ll (
mm
)
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Calculation of required storage capacity (1)
Required storage capacity = 29.4 m3 say 30 m3
Month Rainfall Rainfall Water Cum. Rainfall Cum. Water Differenceharvested Demand harvested CH Demand CD CH - CD
mm m3 m3 m3 m3 m3
J 120 21.6 10.65 21.6 10.65 10.95F 90 16.2 10.65 37.8 21.3 16.5M 70 12.6 10.65 50.4 31.95 18.45A 120 21.6 10.65 72 42.6 29.4M 40 7.2 10.65 79.2 53.25 25.95J 50 9 10.65 88.2 63.9 24.3J 45 8.1 10.65 96.3 74.55 21.75A 15 2.7 10.65 99 85.2 13.8S 0 10.65 99 95.85 3.15O 45 8.1 10.65 107.1 106.5 0.6N 70 12.6 10.65 119.7 117.15 2.55D 45 8.1 10.65 127.8 127.8 0
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Mass curve for calculation of required storage capacity
0
20
40
60
80
100
120
140
J F M A M J J A S O N D
Month
Wat
er (
m3)
Cum. Harvested Cum. Demand
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Mass curve for calculation of required storage capacity
0
20
40
60
80
100
120
140
J F M A M J J A S O N D
Month
Cu
mu
lati
ve (
m3 )
Harvested Water demand
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Storage capacity calculations
(b) Rainfall pattern - 2
Month Rainfall
mmJan 120Feb 100Mar 100Apr 115MayJuneJulyAugSepOct 55Nov 100Dec 120
0
20
40
60
80
100
120
140
J F M A M J J A S O N D
Months
Rai
nfal
l (m
m)
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Calculation of required storage capacity (2)
Required storage capacity = 35.7 + 18.3 = 54 m3
Month Rainfall Rainfall Water Cum. Rainfall Cum. Water Differenceharvested Demand harvested CH Demand CD CH - CD
mm m3 m3 m3 m3 m3
J 120 21.6 10.65 21.6 10.65 10.95F 100 18 10.65 39.6 21.3 18.3M 100 18 10.65 57.6 31.95 25.65A 115 20.7 10.65 78.3 42.6 35.7M 0 10.65 78.3 53.25 25.05J 0 10.65 78.3 63.9 14.4J 0 10.65 78.3 74.55 3.75A 0 10.65 78.3 85.2 -6.9S 0 0 10.65 78.3 95.85 -17.55O 55 9.9 10.65 88.2 106.5 -18.3N 100 18 10.65 106.2 117.15 -10.95D 120 21.6 10.65 127.8 127.8 0
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Gutters are channels all around the edge of a sloping roof to collect and transport rainwater to the storage tank.
A carefully designed and constructed gutter system is essential for any roof catchment system to operate effectively.
When the gutters are too small considerable quantities of runoff may be lost due to overflow during storms.
The size of the gutter should be according to the flow during the highest intensity rain. It is advisable to
make them 10 to 15 per cent oversize.
Gutters
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A general rule of thumb is that 1 cm2 of guttering is required for every m2 of roof area.
Gutters can be semi-circular or rectangular and could be made using a variety of materials:
- Locally available material such as plain galvanised iron sheet (20 to 22 gauge), folded to required shapes.
- Semi-circular gutters of PVC material can be readily prepared by cutting those pipes into two equal semi-circular channels.
- Bamboo or betel trunks cut vertically in half.
- Wood or plastic
Gutters (2)
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Gutters need to be supported so they do not sag or fall off when loaded with water.
The way in which gutters are fixed depends on the construction of the house;
- it is possible to fix iron or timber brackets into the walls, but for houses having wider eaves, some method of attachment to the rafters is necessary.
A properly fitted and maintained gutter-downpipe system is capable of diverting more than 80% of all runoff into the storage tank, the remainder being lost through evaporation, leakage, rain splash and overflow.
Gutters (3)
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Gutter configurations
Gutters - Shapes and Configurations
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Gutters - Shapes and Configurations
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Gutters and Hangers
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Shade cloth guttering
Source: Peter Morgan (1998)http://aquamor.tripod.com/RAINWATER.htm
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http://www.eng.warwick.ac.uk/DTU/pubs/wp/wp55/8gutter.html
Plastic sheet guttering
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Gutter sizingRecommended gutter widths for use in humid tropics
Gutter width (mm) Roof area (m2) served by 1 gutter
55 13
60 17
65 21
70 25
75 29
80 34
85 40
90 46
95 54
100 66
Source: (Still and Thomas, 2002)
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Gutter sizingOptimum roof area drainable by square gutters
(considering only conveyance)Square gutters
Slope (%)
0.5 1 2 4
Gutter width
Optimum roof area served by gutter (m2)
33 mm 10 14 20 28
50 mm 29 42 60 85
75 mm 88 125 177 250
100 mm 190 269 380 538
Source: (Still and Thomas, 2002)
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Guttering for a 60 m2 roof
Source: http://www.eng.warwick.ac.uk/DTU/rwh
Square
0.5% slope
Square
1% slope
Half round
1.0% slope
45o Triangle 1.0% slope
Material use
(mm)
214 189 150 175
Gutter width at top (mm)
71 63 96 124
Cross sectional area (cm2)
47 39 36 38
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Source: www.sopac.org
Guide to sizing of gutters and downpipes for rainwater harvesting systems in tropical regions
Roof area (m2) served by one gutter
Gutter width (mm)
Minimum diameter
of downpipe (mm)
17 60 40
25 70 50
34 80 50
46 90 63
66 100 63
128 125 75
208 150 90
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First flush system (1)
Debris, dirt, dust and droppings will collect on the roof of a building or other collection area.
When the first rains arrive, this unwanted matter will be washed into the tank. This will cause
contamination of the water and the quality will be deteriorated.
Many RWH systems therefore incorporate a system for diverting this ‘first flush’ or ‘foul flush” water so that it does not enter the storage tank.
Several first flush system are in use. The simplest one is a manually operated arrangement whereby the inlet pipe is moved away from the tank inlet and then replaced again once the initial first flush has been diverted.
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First flush system (2)
For an average roof catchment it is suggested that the first 20–25 L could be diverted or discarded.
First flush devices should be regarded as an additional barrier to reduce contamination and should not be used to replace normal maintenance activities designed to keep roof catchments reasonably clean.
The inlet pipe to all rainwater tanks should be easily detachable so that, when necessary, the tank can be bypassed. Manual detachment could be used as an alternative to an engineered first flush device,
although the level of control will not be as good.
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First flush system (3)
Developed by Khon Kaen University, Thailand
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First flush system (4)
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First flush system (5)
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First flush system (6)
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Device for separating rainwater from roof-accumulated impurities
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Roof catchment system with filter and storage tank
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Storage tank & first flush - Malaysia