3. System Components and

Design Considerations

2

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

3

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)

4

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)

5

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

6

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

7

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

8

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

9

10

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

11

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

12

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

13

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

14

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

15

16

17

18

RWH Brick Jars - Uganda

Source: Rees and Whitehead (2000), DTU, University of Warwick, UK

19

Rainwater Harvesting - Kenya

Source: John Gould (Waterlines, January 2000)

20

Ferro-cement jar for rainwater collection - Uganda

Source: DTU, University of Warwick (September 2000)

21

Underground lime and bricks cistern

22

Rainwater Harvesting – Sri Lanka

23

24

25http://www.greenhouse.gov.au/yourhome/technical/pdf/fs22.pdf

26

                                             

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

27http://www.arcsa-usa.org/

28

29

Source: http://www.greenhouse.gov.au

Rainwater Tanks

30

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.

31

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

32

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.

33

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

34

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

35

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

)

36

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

37

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

38

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

39

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)

40

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

41

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

42

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)

43

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)

44

Gutter configurations

Gutters - Shapes and Configurations

45

Gutters - Shapes and Configurations

46

Gutters and Hangers

47

Shade cloth guttering

Source: Peter Morgan (1998)http://aquamor.tripod.com/RAINWATER.htm

48

                                       

http://www.eng.warwick.ac.uk/DTU/pubs/wp/wp55/8gutter.html

Plastic sheet guttering

49

50

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)

51

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)

52

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

53

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

54

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.

55

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.

56

First flush system (3)

Developed by Khon Kaen University, Thailand

57

First flush system (4)

58

First flush system (5)

59

First flush system (6)

60

Device for separating rainwater from roof-accumulated impurities

61

Roof catchment system with filter and storage tank

62

Storage tank & first flush - Malaysia