manual to design rwh's system

Chapter 3

Components of Rainwater Harvesting System

3.1 Roof Catchment

In a rainwater harvesting system, the only area for house

owner to harvest rainwater is the roof. However, if the

users want to increase additional capacity, an open barn or

rain barn can be built beside the house roof. Harvested rain

water quality can varied according to different type of roof

catchment material, country’s climate, and surrounding

environment (Vasudevan, 2002).

3.1.1 Metal Roof

Smoother surface of roof can enhance the harvesting ability.

A common used roofing material for rainwater harvesting is

Galvalume, which consists of 55% aluminum and 45% zinc

alloy-coated sheet steel (Texas, 2005).

3.1.2 Clay or Concrete Roof

Clay and concrete tile are porous. These types of materials

are suitable for potable system as well as non-potable

system. However, it may cause 10% of runoff loss due to the

tiles texture. The solution is to coat it with sealant to

reduce loss. However, sealant may have chance of toxins

leaching even though it can prevent bacterial growth (Texas,

2005).

3.1.3 Roof Area Calculations

The size of roof area has a huge impact on the collection of

rainwater for a house or building. Before calculating the

roof area, it is important to determine which parts of roof

can be used for collecting rainwater. Figure 3.1.1 shows

three different types of roof slope along with their

formulae for roof area calculations (DID, 2012)

(a) Single Sloping Roof Freely Exposed to the Wind

Ac=Ah+Av

2

Eq. 3.1a

(b) Single Sloping Roof Partially Exposed to the Wind

Ac=Ah+12

(Av2−Av1) Eq.

3.1b

(c) Two Adjacent Sloping Roofs

Ac=Ah1+Ah2+12

(Av2−Av1)

Eq. 3.1c

Figure 3.1.1: Roof Catchment Areas (DID, 2012) 3.2 Gutter and Downpipe

Gutters are used to capture the rainwater running off from

the roof and downpipes are used to deliver the rainwater

into the rainwater storage tank. Inadequate number of

downpipes, excessive long roof length, steep roof slopes,

and less perform gutter maintenance, are among the reasons

of spillage or overrunning of rainwater. Therefore, it is

advisable to consult the gutter supplier for the best

installation.

In allocating potable use water system, gutter and

downpipes cannot use lead material. This is due to slightly

acidic quality of rain could dissolve lead and thus

contaminate the water supply. The most common materials of

gutters for both potable and non-potable systems are PVC,

vinyl, seamless aluminum and galvanized steel (Georgia,

2009). Figure 3.2.1 shows the typical half round gutters and

eaves gutters.

(a) Layout of half round (b) Eaves

Guttersgutters & downpipes

Figure 3.2.1: Examples of Roof Conveyance System

3.2.1 Roof Catchment Runoff Calculation

The general equation used to calculate the rainwater runoff

flow rate on the roof is as below:

Q (l/s) = catchment area (m2) x rainfall intensity (mm/hr) x impermeability factor ÷ 3600

(Eq. 3.2a)

However, for roof slope greater than 40°, the following

equation is adopted:

Q (l/s) = catchment area (m2) x rainfall intensity (mm/hr) x (1+ 0.462 tanϴ) x

impermeability factor ÷ 3600

(Eq. 3.2b)

Where ϴ is the roof pitch in degrees.

3.2.2 Calculations of Gutter and Downpipe Sizes

Equation 3.2c is used to calculate the size of the level

half-round gutter based on the calculated Q by Equation

3.2a or 3.2b:

Q = 2.67 x 10−5 x Ag1.25 l/s

(Eq. 3.2c)

Where Ag is cross sectional area of the half-round gutter in

mm2.

On the other hand, Equation 3.2d is used to calculate the

size of other shapes of level gutter based on the calculated

Q by Equation 3.2a or 3.2b:

Q = 9.67105 x √ Ao

3

W l/s

(Eq. 3.2d)

Where Ao is the cross sectional area of flow at gutter

outlet in mm2, and W is the width of water surface.

Note:

For 1:600 gradient of gutter, Q is increased by 40%; while

the frictional resistance of gutter can reduce Q by 10% and

each bending can reduce 25% of Q.

Tables 3.2.1a, 3.2.1b, 3.2.2a, 3.2.2b, 3.2.3a and

3.2.3b show the sizes of half round and rectangular gutters

with downpipes calculated based on Equations 3.2a - 3.2d for

various design rainfall intensities of 50-mm/h, 100-mm/h and

150-mm/h. The calculations were based on the assumptions of

(i) roof pitch is 30o; (ii) 1:600 gradient of gutter and Q

is increased by 40%; (iii) the frictional resistance of

gutter can reduce Q by 10%; and (iv) no bending gutter.

User can select the gutter and downpipe sizes from the

tables according to the roof area (m2), roof catchment

runoff rate (L/s), and the shapes of gutter and downpipe.

Table 3.2.1a: Half round gutters and downpipes for 50-mm/h

of design rainfall intensity

RoofArea(m2)

RoofRunoffRate(L/s)

Half Round Gutters (diameter/mm)

Circular Downpipe * (diameter/mm)

Endoutlet

CenterOutlet End outlet

CenterOutlet

Cal.Size

Ava.Size

Cal.Size

Ava.Size

Cal.Size

Ava.Size

Cal.

Size

Ava.

Size

50 0.66 85 17442.5 174 56.0 82

28.0 82

60 0.79 90 17445.0 174 59.5 82

29.5 82

70 0.92 95 17447.5 174 63.0 82

31.5 82

80 1.06 100 17450.0 174 66.0 82

33.0 82

100 1.32 110 17455.0 174 72.5 82

36.5 82

120 1.58 120 17460.0 174 79.0 82

39.5 82

150 1.98 130 17465.0 174 86.0 110

43.0 82

200 2.64 145 17472.5 174 95.5 110

48.0 82

*Downpipe size is 66% of gutter width

Table 3.2.1b: Rectangular gutters and downpipes for 50-mm/h


RoofArea(m2)

RoofRunoff

Rate(L/s)

Rectangular / EaveGutters(mm)

Rectangular Downpipe *(mm)

Cal. Size Ava. Size Cal. Size Ava. Size

width depth width depth width dept

h width depth

50 0.66 75 37.5 190 150 49.5 25.0 100 5060 0.79 80 40.0 190 150 53.0 26.0 100 5070 0.92 85 42.5 190 150 56.0 28.0 100 5080 1.06 90 45.0 190 150 59.5 30.0 100 50100 1.32 95 47.5 190 150 62.5 31.5 100 50120 1.58 105 52.5 190 150 69.5 35.0 100 50150 1.98 115 57.5 190 150 76.0 38.0 100 50200 2.64 125 62.5 190 150 82.5 41.0 100 50 *Downpipe size is 66% of gutter width

Note: Assumed the depth is half of the width of gutter



RoofArea(m2)

RoofRunoffRate(L/s)



Endoutlet


CenterOutlet

Cal Ava Cal Ava. Cal. Ava Cal Ava

.Size

.Size

.Size Size Size

.Size

.Size

.Size

50 1.32 110 17455.0 174 72.5 82

36.5 82

60 1.58 120 17460.0 174 79.0 82

39.5 82

70 1.85 125 17462.5 174 82.5 82

41.0 82

80 2.11 135 17467.5 174 89.0 110

44.5 82

100 2.64 145 17472.5 174 95.5 110

48.0 82

120 3.17 155 17477.5 174

102.5 110

51.0 82

150 3.96 170 17485.0 174

112.0 110

56.0 82

200 5.28 195 17497.5 174

128.5 160

64.5 82


Table 3.2.2b:Rectangular gutters and downpipes for 100-mm/h


RoofArea(m2)

RoofRunoff

Rate(L/s)





h width depth

50 1.32 95 47.5 190 150 62.5 32 100 5060 1.58 105 50.0 190 150 69.5 35 100 5070 1.85 110 105.0 190 150 72.5 36 100 5080 2.11 115 57.5 190 150 76.0 38 100 50

100 2.64 125 62.5 190 150 82.5 41 100 50120 3.17 135 67.5 190 150 89.0 45 100 50150 3.96 150 75.0 190 150 99.0 50 100 50200 5.28 165 82.5 190 150 109.0 55 120 80 *Downpipe size is 66% of gutter width




RoofArea(m2)

RoofRunoffRate(L/s)



Endoutlet


CenterOutlet

Cal.

Size

Ava.

Size

Cal.

Size

Ava.Size

Cal.Size

Ava.

Size

Cal.

Size

Ava.

Size

50 1.98 130 174 65 174 85.8 11042.9 82

60 2.38 140 174 70 174 92.4 11046.2 82

70 2.77 150 174 75 174 99 11049.5 82

80 3.17 160 174 80 174105.6 110

52.8 82

100 3.96 170 174 85 174112.2 110

56.1 82

120 4.75 185 17492.5 174

122.1 160

61.05 82

150 5.94 200 174 100 174 132 160 66 82

200 7.92 225 174112.5 174

148.5 160

74.25 82


Table 3.2.3b:Rectangular gutters and downpipes for 150-mm/h


RoofArea(m2)

RoofRunoff

Rate(L/s)





h width depth

50 1.98 115 57.5 190 150 75.9 38 100 5060 2.38 120 60 190 150 79.2 40 100 5070 2.77 130 65 190 150 85.8 43 100 5080 3.17 135 67.5 190 150 89.1 45 100 50100 3.96 150 75 190 150 99 50 100 50120 4.75 160 80 190 150 105.6 53 120 80150 5.94 175 87.5 190 150 115.5 58 120 80200 7.92 195 97.5 250 178 128.7 64 150 75 *Downpipe size is 66% of gutter width


Some local gutter manufacturers also produce gutters

and downpipes with different sizes from those stated in the

tables above, such as 4” x 4” and 3½” x 6” eave gutters.

Sample Calculation

Given:

For roof with 15-m of width and 4-m of length, the roof catchment area (ABCK) = 60-m2

Design rainfall = 100 mm/hRoof pitch ≈ 30°Roof permeability factor = 0.95

(i) Roof catchment runoff rate

Based on Equation 3.2a (less than 40 degree of roof pitch), the roof catchment runoff rate is:

Q = 60 m2 x 100 mm/hr x 1/3600 s/hr x 1/1000 m/mm x 1000/1 l/m x 0.95 = 1.58- l/s

Figure 3.2.2: Roof Catchment Area

(ii) Gutter size

Case I: Circular gutter (end outlet)

Cross sectional area of circular gutter, Ag , wascalculated using Equation 3.2c;

Used 1:600 gradient of gutter, where gutter flow rateincreased by 40%;

Fictional force reduced gutter flow rate by 10%Thus,

Q = 1.4 x 0.9 x 2.67 x 10−5 x Ag1.25 l/s

AndAg = 5472.07 mm2

Width of gutter,

W = (A√ g x 8 / π) = 118.04-mm (rounded to 120-mm)

For center outlet, adopt a smaller gutter size (half of

the size) as it only carries half the flow load.

Case II: Rectangular gutter (end outlet)

Cross sectional area of rectangular gutter, Ao was calculated using Equation 3.2d;

Used 1:600 gradient of gutter, where gutter flow rateincreased by 40%;

Fictional force reduced gutter flow rate by 10%

Thus,

Q = 1.4 x 0.9 x 9.67105

x √ Ao3

W l/s

And Ao=WD/2

Width of gutter (assumed that the depth is half of the width),

1.5833 = 1.4 x 0.9 x 9.67105

x √ W2D3

8 = 1.4 x 0.9 x

9.67105 x √ W5

64

W = 101.56-mm (rounded to 105-mm)

(iii) Downpipe size (for both circular and rectangular gutters)

Assumed to be 66% of gutter width, thus:

Case I:

Circular gutter should adopt 79-mm diameter of downpipefor end outlet design and 39.5-mm diameter of downpipe for center outlet design;

Case II: Rectangular gutter should adopt 69.5-mm width and 35-mmdepth of downpipe

3.2.2.1 MSMA’s Method for Eave Gutter Design

Table 3.2.4 shows the sizes of eave gutters read from Charts

3.2.1 and 3.2.2 from a design rainfall intensity of

100-mm/h. Charts 3.2.1 and 3.2.2 show the relationships

among roof catchment area, design rainfall intensity and

cross sectional area of eave gutter. Finally, Table 3.2.5

shows the respective sizes of downpipes.

Table 3.2.4: Sizes of gutters and downpipes (DID, 2012)

RoofArea(m2)

DesignRainfall

Intensity

(mm/h)

Cross Sectional Area of EaveGutters (mm2)

Slope 1:500and steeper

Slope flatterthan 1:500

50 100 5400 720060 100 6250 825070 100 7000 940080 100 7800 1040090 100 8500 11250100 100 9250 12400

Chart 3.2.1: Eave Gutter Design Chart for Slope 1:500 andsteeper (DID, 2012)

The chart assumes:

1) An effective width to depth is a ratio about 2:1:2) Gradient of 1:500 or steeper;3) Manning’s formula with ‘n’ = 0.016

4) The least favorable positioning of downpipe and bends within the gutter length;

5) Cross-section or half round, quad, ogee or square;6) The outlet to downpipe is located centrally in the sole

of the eaves gutter.

Chart 3.2.2: Eave Gutter Design Chart for Slope flatter than1:500 (DID, 2012)

The chart assumes:

1) An effective width to depth is a ratio about 2:1:2) Gradient of flow flatter than 1:500;3) Manning’s formula with ‘n’ = 0.0164) The least favorable positioning of downpipe and bends

within the gutter length;5) Cross-section or half round, quad, ogee or square;6) The outlet to downpipe is located centrally in the sole

of the eaves gutter.

Table 3.2.5: Required Minimal Nominal Size of Downpipe (DID,

2012)

Cross SectionalArea of EaveGutters (mm2)

Minimal Nominal Size of Downpipe (mm)Circular Rectangular

4000

7565 x 504200

4600 75 x 504800 855900 100 x 506400 906600 75 x 706700 1008200 100 x 759600 12512,800 100 x 10016,000 150 125 x 10018,400 150 x 10019,200

Not applicable20,000 125 x 12522,000 150 x 125

3.2.2.2 Method from the Handbook of Rainwater Harvesting

for Caribbean

Caribbean’s Rainwater Harvesting Handbook (UNEP, 2009) was

intended as a practical guideline to introduce and assist

the citizens in the Caribbean region to construct their

rainwater harvesting systems. The handbook provides several

technical information and guidelines that are useful for

tropical region like Malaysia. The rational method was used

to calculate the roof runoff.

Tables 3.2.6a and 3.2.6b show the recommended runoff

coefficients for various catchment types and the sizes of

gutters and downpipes, respectively. Type of gutters

recommended here is PVC gutters since they do not rust and

the rainwater quality can be maintained over a long period.

Gutters must slope towards the tank and the gradient must

not more than 1:100.

Table 3.2.6a: Runoff coefficients for various catchment

types (UNEP, 2009)

Type of Catchment Runoff coefficientsRoof catchmentsTilesCorrugated metal sheets

0.8 – 0.90.7 – 0.9

Ground surface coveringsConcreteBrick pavement

0.6 – 0.80.5 – 0.6

Untreated ground catchmentsSoil on slopes less than 10 percentRocky natural catchments

0.1 – 0.30.2 – 0.5

Table 3.2.6b: Gutters and Downpipes sizing for RWH systemsin tropical regions (SOPAC, 2004)

Roof area (m2)served by one

gutter

Gutter width(mm)

Minimum diameter ofdownpipe (mm)

17 60 4025 70 5034 80 5046 90 6366 100 63128 125 75208 150 90

3.3 Type and Configuration of Rainwater Harvesting Systems

There are three types of rainwater harvesting system for

supplying non-potable water for internal and external uses,

which are directly pumped, indirectly pump, and gravity fed

(Leggett et al. , 2001).

3.3.1 Indirectly Pumped System

Indirectly pumped system is a system that pumps up water

from storage tank to header tank using water pump. The

header tank is usually placed on the roof of building while

the storage tank is being undergrounded. When the storage

tank runs out of water, the primary water supply piping will

supply water into header tank. When storage tank is full

with water, an overflow pipe is necessary to prevent storage

tank from more than the normal water level in a tank. The

main advantage of indirectly pumped system is the supply of

water will not be cut-off if the water pump is on mechanical

or electrical failure. The water can still be supplied to

internal and external uses by the primary water supply

system. The main disadvantage is the water can be delivered

slowly due to low pressure. Thus, it leads to low water

pressure when using shower or slow refilling after flushing

the toilet. In addition, there may be insufficient space on

the roof to install the header tank.

Figure 3.3.1 : Indirectly Pumped System

3.3.2 Directly Pumped System

The main difference between indirectly and directly pumped

system is the water in directly pumped system is stored in

storage tank and then pumped directly to internal and

external uses in a building. There is no header compared to

indirectly pumped system and the main water supply is direct

from the storage tank. The main water supply will not fully

fill the tank but maintain the water on a minimum level for

short term demand. The main advantage of this system is the

water is provided on high pressure. The disadvantage is

there will be no water supply if the water pump is

experiencing faulty due to mechanical or electrical failure.

Figure 3.3.2 : Directly Pumped System

3.3.3 Gravity Fed System

The main difference of gravity fed system from directly and

indirectly pumped systems is the storage tank of this system

is located on top of the building. Rainwater which is

collected and harvested is directly stored in the storage

tank or also known as header tank. Water is delivered from

storage tank by means of gravity to appliances. The main

advantage of the system is water pump or electrical supply

to pump water is not required. Since no pump is required,

there is no risk of water pump failure and electrical supply

cut-off. However, the main disadvantage is the low water

pressure similar in indirectly pumped system. For example

slow refilling in toilet tank after flushing the toilet.

Figure 3.3.3 : Gravity Fed System

3.3.4 Examples of Rainwater Harvesting System

3.3.4.1 Residential House – Bungalow, Semi Detached and

Terrace

Figure 3.3.4: Schematic diagram for residential house

Basic components

Catchment surface, gutters, leaf guarder, downpipes, first

flush diverters, storage tanks (cisterns), overflow pipe

System Procedure:

1) Harvest rainwater from roof;

2) Filter roof dirt through leaf guarder and first flush

diverter;

3) Deliver rainwater using gutter and downpipe;

4) Storing clean rainwater in the storage tanks;

5) In case there is shortage of water or no rainfall, the

public water supply is topped up into the storage

tanks.

3.3.4.2 High Rise Building – Condominium, Commercial and Office Tower

Figure 3.3.5: Schematic diagram for high-rise building

Basic components

Catchment surface, gutters, leaf guarder, downpipes, first

flush diverters, storage tanks (cisterns), pump, elevated

rainwater header tank

System Procedure:

1) Harvest rainwater from roof;

2) Filter roof dirt through leaf guarder and first flush

diverter;

3) Deliver rainwater using gutter and downpipe;

4) Collecting the first flush rainwater before water

entering the storage tank.

5) Storing clean rainwater in the on the ground storage

tank;

6) The pump is used to lift the rainwater up to the

elevated rainwater header tank installed on the roof

top of the high rise building;

7) In case there is shortage of water or no rainfall, the

public water supply is topped up into the elevated

rainwater header tank.

3.4 Pumping System

There are two types of pumping systems, namely pressurized

water supply system and header pressure system. For

pressurized system, pressure tank is required to maintain

the pressure throughout the system. Pump is functioning when

the pressure is drop. For header pressure system, water is

lifted up by the pump from storage tank to elevated tank,

and the water supplies to devices by gravity force (UNEP,

2009).

3.4.1 Selection of Pump

To select an appropriate type of pump for a rainwater

harvesting system, the following five steps must be followed

(Alberta, 2010):

Step I: Select the appropriate pump

Submersible pump and jet pump are the most common types of

pump that used in the residential house. Submersible pump is

located inside the tank and can function properly even fully

submerged inside the water tank; while jet pump is located

outside the tank. Comparisons among submersible pump, jet

pump and centrifugal pump, and their advantages and

disadvantages, are shown in Table 3.4.1.

Step II: Select the configuration of pump (speed of flow

rate)

Two available pump controllers can be selected to configure

the rainwater supply system.

(a) Constant speed pump:

Following a large drop in the system pressure, a

constant speed pump will activate and pump water at a

fixed rate to replenish the volume of water stored in

elevated tank;

(b)Variable Speed Drive (VSD) / Variable Frequency Drive

(VFD) pump:

VSD/VFD pumps can increase or decrease the speed of the

pump impeller to provide more or less water as needed

by the pressure system.

Table 3.4.1: Advantages and disadvantages of different types

of pump (Alberta, 2010)

Type of Pump Usage Advantages Disadvantag

es

Submersible pump

Pumping process

More efficient

Pump must be physically

inside water

Can be used forwater supply, drainage, slurryand sewage pumping

have a longerlifespan thanjet pumps

Reduces the amount of equipment andspace needed outside of the rainwatertank

Low noise

extracted fromtank to perform inspection, repair and/or replacement

May be more difficult to detect pump dry running (or any malfunction) as operation of pump may not be audible

Pumps generally designed for vertical installation, but must be installed horizontally as vertical installation reduces usablecapacity of cistern (increases dead space volume)

Jet pump Use for well systems

Pump can be easily inspected, repaired and/or replaced

Generally less expensive than submersible pumps

More difficultto commission than submersible pumps, as theymust be ‘primed’

Pump must be located in a temperature controlled space (indoors, pump

house, etc.) Pump operation

may be noisyCentrifugal Pump

For domesticand light industrial applications

Quiet operation andcompact design

Easy installation

Not suitable for large building like shopping mall

Unable to provide constant pressure

Table 3.4.2: Advantages and disadvantages of constant speedand VSD/VFD pumps (Alberta, 2010)

PumpController

ConfigurationAdvantages Disadvantages

Constant speed pump

Generally less expensive than VSD/VFD pumps

Ideal for applications where minor variations in water pressure and flow rate are acceptable (i.e., refilling toilet tanks after flushingand operating a garden hose)

Pressure tanks can bequite large for applications requiring high flow rates

Flow rate and system pressure may spike when pump activates, and pressure may dropif water demands are too high

Variable Speed Drive (VSD) / Variable Frequency Drive (VFD) pump

Provide constant pressure to fixtures, regardlessof demand

Use very small pressure tanks, or micro-pressure tank inside the pump or control unit

Often have built in

Use of smaller pressure tanks requires a greater number of ‘pump starts’ potentially increasing pump wear

More expensive than constant speed pump systems

low/high voltage shutoff and dry run protection

Smaller space requirements in the building

Lower electricity consumption than comparable constant speed pumps.

Step III: Pump Flow Rate

The amount of flow that must be generated by the pump

depends on the types and number of fixtures connected to the

distribution system. This means that at here, we must

consider the peak hour flow rate for that particular

building before selecting the suitable pumping system.

Step IV: Pump Head

Determine pump head is an important step especially for high

rise building. The pump pressure not only takes account on

lifting up water supply but also along with friction loss,

and various type of minor loss.

Step V: Acceptability of the service

Last issue to consider is that whether usage of pump is

acceptable for that building. Some of the system may

interrupted by pump downtime. For small residential housing,

several times of service interruption is acceptable. For

multi-residential or commercial buildings, it is important

to avoid pumping service went down at peak hour. The most

common way is to install an elevated tank on top of the

building.

3.4.2 Alberta’s Method

Alberta (2010) provides simple way to calculate the required

pump capacity. For estimation of maximum peak demand flow

rate, Table 3.4.3 can be used. Table 3.4.4 shows the required

system pressure for different indoor and outdoor fixtures.

Table 3.4.3: Minimum recommended water flow rate for variousindoor & outdoor fixtures (Alberta, 2010)

IndoorFixtures

Minimum FlowRate

(Per Fixture)

OutdoorFixtures

Maximum FlowRate

(Per Fixture)

Shower or Bathtub

19 LPM[5 GPM]

Garden hose with 13mm [1/2in.] supply

11LPM[3GPM]

Lavatory 1 LPM[0.3 GPM]

Garden hose with 18mm [3/4in.] supply

19LPM[6GPM]

Toilet 2.7 LPM Irrigation Varies

[0.7 GPM] system(Consultsupplier/

contractor)

Kitchen Sink 1.6 LPM[0.4 GPM]

Washing Machine

19 LPM[5 GPM]

Dishwasher 7.6 LPM[2 GPM]

Table 3.4.4: Required minimum pressure for residential homefixture (Georgia, 2009)

Usage Pressure Pressure Flowft m psi kPa GPM LPM

Impact Sprinkler

93 28 40 275.8 4.5 17.0

Pressure washer 46 14 20 137.9 4.0 15.1Toilet 46 14 20 137.9 6.0 22.7Garden hose nozzle

81 25 35 241.32

5.0 18.9

Figure 3.4.1 shows the illustration of different kinds

of pump heads such as static lift, static height and

friction loss in a pumping system.

Figure 3.4.1: Illustration for components of pump head

The pump head can be calculated using following equations:

Pump Head (m, or ft) = Required System Pressure + Total Dynamic Head Eq. 3.4a

Where the required system pressure is the operating pressure

required for rainwater fixtures (275-415 kPa [~40 – 60 psi]

for typical residential applications). If the final

discharge of a pumping system is into a rainwater header

tank, then there will be no required system pressure or

equals to zero.

Total Dynamic Head = Static Lift + Static Height + Friction Loss Eq. 3.4b

In order to calculate the total dynamic head, the friction

head loss must first be calculated. Friction Loss formula is

shown as below:

Friction Loss = [(LP−SE + LF−SE) x F100−SE

100mpipe] + [(LP−SU + LF−SU) x

F100−SU

100mpipe]

Where,

Friction Loss = Combined Friction losses (m) for the

service piping (SE) and

supply piping (SU)

LP = Linear length of pipe (m)

LF = Equivalent length of pipe fittings (m)

F100 = Friction loss per 100m of pipe

There are two distinct sections of rainwater pressure

piping:

1) Rainwater serviced pipe: The section of pipe from

storage tank to a jet pump (or pressure tank/control

unit for submersible pumps)

2) Rainwater supplied pipe: The section of pipe from jet

pump (or pressure tank/control unit for submersible

pumps) to permitted fixtures

Table 3.4.5 shows the value of friction head losses (m)

based on the selected pipe diameters and pipe flow rates,

and Table 3.4.6 shows the equivalent length of pipe for

different fittings.

Table 3.4.5: Friction head losses for SCH40 PCV pipe atvarious flow rates (Alberta, 2010)

FlowRate,

Q(LPM)

F100 Friction Head (m / 100m pipe)Pipe Diameter

13mm[1/2in.]

18mm[3/4in.]

25mm[1 in.]

32mm[1 ¼in.]

38mm[1 ½in.]

50mm[2 in.]

8 4.8 1.2 0.38 0.119 25.8 6.3 1.9 0.5 0.230 63.7 15.2 4.6 1.2 0.6 0.238 97.5 26 6.9 1.8 0.8 0.357 49.7 14.6 3.8 1.7 0.576 86.9 25.1 6.4 2.9 0.9113 13.6 6.3 1.8

The above table assumed a SCH40 PVC pipe or similar material

such as PE-polyethylene or PP-polypropylene is utilized.

Table 3.4.6: Equivalent length of pipe for differentfittings (Alberta, 2010)

Fitting Equivalent Length of Pipe (m)Pipe Diameter

13mm[1/2in.]

18mm[3/4in.]

25mm[1

in.]

32mm[1 ¼in.]

38mm[1 ½in.]

50mm[2

in.]

75mm[3

in.]90° Elbow 0.5 0.6 0.8 1.1 1.3 1.7 2.445° Elbow 0.2 0.3 0.4 0.5 0.6 0.8 1.2Gate Valve(shut-offvalve)(Open)

0.1 0.2 0.2 0.2 0.3 0.4 0.5

Tee Flow –Run 0.3 0.6 0.6 0.9 0.9 1.2 1.8

Tee Flow –Branch 1.0 1.4 1.7 2.3 2.7 3.7 5.2

In LineCheck Valve(Spring) orFoot Valve

1.2 1.8 2.4 3.7 4.3 5.8 9.8

Sample Calculation

A pumping system is installed at the bottom of a 2-storey

house to elevate the water supply from ground level to the

elevated tank, as shown in Figure 3.4.2 below. It is assumed

that there are 3 bathrooms or WCs, and two valves and three

90° bending from the harvested rainwater tank to the

elevated rainwater header tank. Assumed that PVC pipe is

used.

Figure 3.4.2: Typical pumping system for a 2-storey house[Change static height to 8-m; draw and label the 2 valves;

label 2 more bendings]

From Table 3.4.3:

The total toilet flow rate = 3 x 2.7-LPM = 8-LPM

From Table 3.4.5:

For a flow rate of 8-LPM with rainwater serviced pipe and

rainwater supplied pipe sizes of 25mm [1 inch],

F100−SE = 0.38 m /100 m pipe

F100−SU = 0.38 m /100 m pipe

From Table 3.4.6:

For one (1) 90° bending in rainwater serviced pipe and two

(2) 90° bending in rainwater supplied pipe,

Serviced pipe with one 90° bending, LF−SE = 0.8 m

Supplied pipe with two 90° bending, LF−SU = 0.8 x 2 =1.6m

And,

Serviced pipe gate valve, LF−SE=¿ 0.2m

Supplied pipe gate valve, LF−SU=¿ 0.2m

Thus,

Friction Loss = [(LP−SE + LF−SE) x F100−SE

100mpipe] + [(LP−SU + LF−SU) x

F100−SU

100mpipe]

= [(2+(0.8+0.2)) x 0.38

100mpipe ] + [(9+(1.6+0.2)) x 0.38

100mpipe ]

= 0.052-m

Then,

Total Dynamic Head = Static Lift + Static Height + Friction Loss

= 2 + 8 + 0.05244

= 10.052-m

3.4.3 Pump head

The required pump heads (in kPa and horsepower) for

different flow rates and pipe sizes calculated for a typical

rainwater harvesting system, as shown in Figure 3.4.2, are

shown in Tables 3.4.7, 3.4.8 and 3.4.9.

Table 3.4.7: The required pump heads for 3/4-inch pipe size

MinFlow

Rate,L/m

StaticLift,m

(A)

Static

Height, m

(B)

Friction

Loss,m

(C)

TotalDynami

cHead,m

(A+B+C)

*Cal.PumpHead,

kPa(D)

#Req.PumpHead,kPa

(D) /0.7

*Cal.Pumphorsepowe

r(E)

# Req.Pumphorsepower(E) /0.7

8 2 8 0.158 10.158 100 1420.014 0.0098

19 2 8 0.832 10.832 106 1520.037 0.0259

30 2 8 2.006 12.006 118 1680.066 0.0462

38 2 8 4.264 14.264 140 2000.102 0.0714

57 2 8 6.560 16.560 162 2320.181 0.1267

76 2 811.471 21.471 211 301

0.324 0.2268

* Direct discharge to rainwater header tank# Assumed 70% of pump efficiency

Table 3.4.8: The required pump heads for 1-inch pipe size

MinFlowRate,L/m

StaticLift,m

(A)

Static

Height,m

(B)

Friction

Loss,m

(C)

TotalDynami

cHead,m

(A+B+C)

*Cal.PumpHead,

kPa(D)

#Req.PumpHead,kPa

(D) /0.7

*Cal.Pumphorsepower(E)


8 2 8 0.052 10.052 99 141 0.01 0.00719 2 8 0.262 10.262 101 144 0.03 0.02130 2 8 0.635 10.635 104 149 0.04 0.02838 2 8 0.952 10.952 107 153 0.06 0.04257 2 8 2.015 12.015 118 168 0.09 0.06376 2 8 3.464 13.464 132 189 0.14 0.098


Table 3.4.9: The required pump heads for 1 1/4 -inch pipesize

MinFlowRate,L/m

StaticLift, m

(A)

Static

Height, m

(B)

Friction

Loss,m

(C)

TotalDynami

cHead,m

(A+B+C)

*Cal.PumpHead,

kPa(D)

#Req.PumpHead,kPa

(D) /0.7

*Cal.Pumphorsepower(E)


8 2 8 0.015 10.015 98 140 0.01 0.00719 2 8 0.074 10.074 99 141 0.03 0.02130 2 8 0.176 10.176 100 143 0.05 0.03538 2 8 0.380 10.380 102 145 0.07 0.04957 2 8 0.559 10.559 104 148 0.11 0.07776 2 8 0.941 10.941 107 153 0.15 0.105113 2 8 2.000 12.000 118 168 0.25 0.175


3.4.4 Loading Unit Method

Flow rate can also be estimated using the loading unit

method. Table 3.4.10 shows the loading unit rating for

different types of appliances. After calculating the total

loading unit for a rainwater supply system, the rainwater

flow rate can be read from Chart 3.4.1.

Table 3.4.10: Loading Unit Rating for Various Applications(DID, 2012)

Type of Appliance Loading Unit RatingDwelling and Flats W.C. Flushing Cistern Wash Basin Bath Sink

21.5103-5

Offices W.C Flushing Cistern Wash Basin (Distributed Use) Wash Basin (Concentrated Use)

21.53

School and Industrial Buildings W.C Flushing Cistern Wash Basin Shower (with Nozzle)

233

Public Bath 22

Chart 3.4.1: Design Flow Rate (L/s) versus Loading Units(DID, 2012)

Sample Calculation

For a 10-storey tower, it is assumed that every floor

consists of 10 units and each of the unit has 2 bathrooms or

WCs. The design flow rate is equal to:

From Table 3.4.10, the loading unit for W.C Flushing Cistern

is 2 units.

Total loading units = Loading Unit for 1 unit x No. of units per floor x Total

floor

= (2 x 2) x 10 x 10

= 400-units

From Chart 3.4.1:

Q = 3.51-l/s = 0.00351- m3/s

3.5 Top-up System

There is always a time when there is insufficient of

rainwater to meet the demand. In this situation, it is

necessary to have another alternative water supply for the

water supply system. Top-up device can be used to solve this

problem. When the water level inside the rainwater tank is

getting lower, the top up system will start filling up the

rainwater tank by transferring water from the public water

supply.

Rainwater must not flow into the public water supply

system. Water from the public water supply can flow into the

rainwater tank subjected to it being equipped with a one-way

non return valve, or the overflow pipe in the rainwater tank

is located at least 225-mm lower from the inlet pipe to the

rainwater tank (Selangor, 2012).

3.5.1 Types of Top-up System

There are various types of top-up system available for the

rainwater supply system. It is advisable to select the top-

up system wisely to avoid overflow, storage run down or

extra expense for unnecessary system. Basically, there are

two types of top-up systems namely automatic top-up system

and manual top-up system. Table 3.5.1 shows the advantages

and disadvantages of automatic top-up system and manual top-

up system.

Table 3.5.1: Advantages and disadvantages of top-up systems

(Canada, 2012)

Make-upwater method Advantages Disadvantages

Manual top-up

Simplest method to design and install due to reduced control equipment requirements

May result in serviceinterruptions (for example, no water forflushing toilets) if tank not topped up

Lowest cost alternative

prior to going dry Requires homeowner tomonitor volume of stored rainwater in tank and top up pre-emptively if low

Automatictop-up

Reduces the number of service interruptions by automatically filling tank before it runs dry

Make-up system operates without theneed for monitoring or intervention by the homeowner

Improper design or installation of control equipment maycause insufficient orexcessive top-up volumes to be dispensed by the make-up system

Service interruption during power failure

3.5.2 Automatic Top-up System (with electronic device)

There are two types of automatic top-up system, with

electronic device and without electronic device. The

planning stages for the layout of the automatic top-up

system (with electronic device) are:

i. As shown in Figure 3.5.1, a top-up system consists of

the following components:

(a) Water level sensors located in the rainwater

storage tank;

(b) A solenoid valve located in the rainwater storage

tank;

(c) An air gap;

(d) Top-up drainage conveying make-up water to the

rainwater storage tank;

(e) Electrical conduits containing wiring from water

level sensors and pumps.

Figure 3.5.1: Schematic diagram of top-up system forrainwater supply system (Alberta, 2010)

ii. Determine the location of solenoid valve and air gap;

iii. Plan route of top-up drainage;

iv. Plan route of electrical solenoid valve and power

supply to the tank;

v. Contact municipality and other service providers to

ensure that the planning layout do not conflict with

the current building systems like sewerage, piping,

electricity, building structures, etc.

3.5.2.1 Control Equipment

The control equipment is shown in Table 3.5.2 below:

Table 3.5.2: Control equipment (Canada, 2012)

Controlequipment

Description Devices/optionsavailable

Water level sensor

A device inside the tank is used to sense water level

Can control (turn onor off) warning lights, solenoid valves and/or pumps,based on water level

Float switch Ultrasonic level

sensor Liquid levels

switch (Float switch is typicallyused for residential applications).

Shut-off valve

A device that is manually opened (or closed) to permit (or prevent) the flow of water

Integrated into the RWH pressure system to manage water flowand isolate components of the makeup system (for example, solenoid valves and backflow preventers)

Types: ball valves,gate valves

Shut-off valves selected must be approved for handlingwater under pressure.

Solenoid A valve that Come in a variety of

valve (automated shut-off valve)

activates (opens or closes) automatically when turned on

Connected to water level sensor to activate make-up water system

configurations

The solenoid valves selected must be approved for handlingwater under pressure

3.5.2.2 Installation Procedures

The installation procedures are as follow:

Step 1: Set suitable type of water level sensors for RWH

system

Float switches:

Float switches is installed in the water tank so that it

can be used to sense the water level, then controlling

the pump and top-up water. Generally, normally close

(N/C) float switch is used for top-up systems, and

normally open (N/O) float switch is used for pumping

system.

*(N/O) float switch: Supply power (turn on) when switch is “down”

*(N/C) float switch: Supply power (turn off) when

switch is in “up”

Other water level sensors should be selected properly and

installation procedure should be handled carefully with

applicable codes & regulations.

Step 2: Installation of solenoid valves and shut-off valves

(a) Select suitable valves type

(b) Valve size must be the same size as piping system.

(c) Top-up systems use an (N/C) solenoid valve.

(d) Solenoid valve should be installed on the top of the

air gap and soft close/slow close solenoid is

recommended.

(e) Solenoid valves must be wired into a power supply in

conjunction with water level sensor.

(f) Must be installed by a licensed plumber or

technician.

(g) If soft/slow close valve is not used, a water hammer

arrester shall be installed on public water supply

piping upstream.

(h) All procedure should handle with care with applicable

codes & regulations.

Step 3: Installation of Air Gap

Air gap is required to prevent backflow. It needs to be

noted that rainwater cannot be mixed with the public water

supply)

(a) The air gap height must be at least 25-mm, 1-inch or

twice the diameter of water pipe.

(b) To prevent splash and water damage:

i. install flow restrictor at upstream of valve

ii. install aerator at place public water supply

terminate

iii. extend length of pipe and cut an angle no less

than 45° at end pipe

(c) All procedure should handle with care with applicable

codes & regulations

3.5.3 Automatic Top-up System (without electronic device)

Top-up valve, as shown in Figure 3.5.2, can effectively

maintain supply when demand exceeds the rainwater supply and

it does not require electricity supply or complex float

switch devices.

3.5.3.1 Operating Principle

Under normal conditions, rainwater will fill the tank. If

the rainwater level drops below a pre-set level, the top-up

system valve will open to maintain the water level using the

mains water supply.

3.5.3.2 Installation Procedures

i. Valve must be installed horizontally;

ii. Do NOT install on an angle;

iii. Do NOT restrict inlet water flow;

iv. NOT to be modified;

v. NOT to be used in dual purpose tanks used for

stormwater detention.

Figure 3.5.2: Schematic Diagram of Top-up Valve

3.6 Leaf Guarder

Leaf guarder, which is also known as leaf screen or gutter

guarder, fit along the length of the gutter. It is one of

the filter components at the pre-treatment stage of the

rainwater harvesting system. It is usually a ¼-inch mesh

screens in wire frames. They do come in aluminium, plastics

and vinyl for user’s requirement.

Purpose of installing leaf guarder is to separate the

leaves and other debris those are washed down from the roof

catchment surface. The leaf guarder is said to be the first

stage filtration that screen out the large particles such as

leaves, bloom and twigs in the collected rainwater. Through

removing the large particles, the subsequent components and

devices in the rainwater harvesting system are said to be

protected as accumulation of the large particles in the

system may deteriorate the quality of the rainwater. Simple

maintenance is required to clean the leaf guarder regularly.

Decomposition of the leaves and other debris may expand the

bacteria activities and cause harmful consequences to the

rainwater collected. Maintenance may be operated weekly or

monthly depend on the debris accumulation speed.

In Malaysia, according to KPKT (2009), an ordinary net

mesh or stainless mesh can be used at roof drain and gutter.

It is suggested by the manual that installation of the leaf

guarder shall adopt a net or screen mesh of 2 to 10-mm is

satisfactory, as shown in Figure 3.6.1.

Figure 3.6.1: Net of gutter (KPKT, 2009)

3.6.1 Types of Leaf Guarder

There are numerous types of leaf guarders available in the

local and overseas market. It is noted that installation of

gutter leaf guarder depends on the size of the gutter.

Figure 3.6.2 shows some examples of leaf guarder available

in local and overseas.

Some of the available materials include powder coated

steel, Stainless steel, black rubberized vinyl, industrial

strength nylon, High Density Industrial Strength

Polyethylene (HDPE) and aluminum.

Figure 3.6.2: Types of leaf guarder

3.6.2 Advantages and Disadvantages

There are pros and cons for installing leaf guarders to the

rain gutters. Installation of leaf guarder ensures that the

collected rainwater will be free from large particles that

are undesirable. As mentioned previously in the

introduction, leaf guarder s do eliminates the risk of

clogging of large particles for the subsequent components of

the rainwater harvesting system. Therefore, safeguard the

quality of the rainwater from debris at the first stage of

the filtration.

However, installing a leaf guarder requires

periodically checking and maintenance. As large particles

and debris accumulated to a certain amount that may clog the

leaf guarder if no maintenance. There are numerous types of

leaf guarders available in the market currently, as shown in

Figure 3.6.2, which can overcome these disadvantages.

3.7 First Flush System

Rainwater is one of the purest forms of water and is

initially clean to be used. When it rains, rainwater wash

down from roof where contamination occurs. The rainwater may

collect certain amount of undesirable matters from the roof

such as fecal matter, dead animal bodies, chemical residues,

sediments, bacteria and etc. This rainwater is also known as

first flush water. Therefore, a first flush diverter is

necessary to carry out this first stage filtration. When the

first flush water is removed, bacterial activities from

fecal bacteria and other water borne bacteria would be

greatly reduced. Therefore rainwater harvested in the system

is generally cleaner and safer to be used.

3.7.1 Typical Design of First Flush Diverter

Figure 3.7.1 illustrates the typical type of T-junction type

first flush diverter for normal residential buildings. The

device is usually placed at in between the installation of

gutter system and storage tank. When rainwater is flushed

down from the roof, gutters and downpipes will divert the

contaminated first flush water into the first flush diverter

as shown in the illustration. Once the contaminated first

flush water has filled up the device, the following

rainwater will flow into the storage tank through pipe

system. Typically a floating ball valve or sealing ball is

installed in the device to prevent the contaminated first

flush water from washing back out and flow into the storage

tank. A small opening valve is provided at the end of the

device to ensure that the device can slowly drain the water

out and reset to accommodate the first flush of next

rainfall. Installation of a screw cap before the opening of

the device allows periodically cleaning of debris.

Figure 3.7.1: Typical First Flush Diverter (Rain

Harvest, 2013)

3.7.2 Types of First Flush Diverters

There are various types of first flush diverters with

different installation methods and locations available in

the market. They are available in diverse material and sizes

depend on the requirement of the users and the connections

to the whole rainwater harvesting system. Basically, common

examples of the device are downpipe first flush diverters,

post/wall stand water diverters, commercial diverters and

in-ground diverters. Figure 3.7.2 shows a brief introduction

of the different types of first flush diverters and their

mechanisms (Rain Harvest, 2013).

Types of first flush MechanismDownpipe first flush

diverters

Post/wall stand water

diverters

In ground diverter

Figure 3.7.2: Types of first flush diverters and the

mechanisms (Rain Harvest, 2013)

3.7.3 NAHRIM’s First Flush Diverter

Fitting an appropriately sized First Flush Water Diverter is

critical to achieve good quality water. Water Diverters

improve water quality, reduce tank maintenance and protect

pumps by preventing the first flush of water, which may

contain contaminants from the roof, from entering the tank.

When it rains, instead of flowing to the rainwater storage

tank directly, the first flush of water from the roof that

may contain amounts of bacteria from decomposed insects,

bird and animal droppings and concentrated tannic acid, is

diverted into the chamber of the first flush water diverter.

Figure 3.7.3a shows the illustration of NAHRIM’s First Flush

Diverter and Figure 3.7.3b shows the detailed components of

the diverter.

Figure 3.7.3a: Illustration of NAHRIM’s First Flush Diverter

This water diverter utilises a dependable ball and seat

system, which is a simple automatic system. As the water

level rises in the chamber, the ball floats. Once the

chamber is full, the ball rests on a seat inside the chamber

preventing any further water entering the chamber. The

subsequent of fresh water is then channeled into the

rainwater storage tank through an insect screen. A slow

release valve ensures the chamber empties itself after rain

and resets automatically.

Figure 3.7.3b: The detailed components of the diverter

3.7.4 Volume of First Flush Diverter

Users could also design the volumes of their first flush

diverters. A minimum design criterion is that the first

flush device should divert the first 0.5-mm (or, 1.0-mm) of

the rainfall (first flush depth). To calculate the volume of

rainwater needed to be diverted, multiplies the length and

width of the roof by the first flush depth.

Required volume of diverted water (m3) = roof length (m) * roof width (m) * first flush

depth (m)

Eq. 3.7a

For example, a house with 10-m long by 5-m wide would

need to divert at least 0.025-m3 (or, 0.05-m3 if 1.0-mm of

first flush depth is used) of first flush. This is the

amount of first flush that a simple-pipe first flush device

would have to divert. By dividing the required volume of

first flush with the cross sectional area of the pipe (πr2),

the necessary pipe length for the simple-pipe first flush

device can be calculated from the following equation:

Pipe length (m) = Required volume of diverted water (m3) / πr2

Eq. 3.7b

A first flush downpipe of 200 mm diameter (100 mm

radius) would need to be at least 0.8-m (1.6-m if 1.0-mm of

first flush depth is used) long.

3.7.5 The Malaysian’s Condition

3.7.5.1 KPKT

According to KPKT (2009), the dimension of the first flush

device to be adopted is with minimum diameter of 100-mm and

the layout of design is shown in Figure 3.7.4 below. Figures

3.7.5a and 3.7.5b show the types of first flush diverter

suggested by KPKT.

Figure 3.7.4: Device to separate first flush rainwater

(KPKT, 2009)

Figure 3.7.5a: Standpipe First Flush Diverter (KPKT,

2009)

Figure 3.7.5b: Standpipe with Ball Valve (KPKT, 2009)

3.7.5.2 DID

The Department of Irrigation and Drainage Malaysia (DID,

2011) gives the requirements for designing a first flush

system in Tables 3.7.1 and 3.7.2.

Table 3.7.1: Guidelines for residential first flush

quantities (DID, 2011)

Rooftops of 100m2 or smaller 25-50 litersRooftops of 100m2 or larger 50 liters per 100m2

Table 3.7.2: Guidelines for surface catchments or for very

large rooftops (DID, 2011)

Rooftops or surface catchments of

4,356m2 or larger2,500 liters

3.7.5.3 SIRIM Berhad

In SIRIM (2013), it is stated that the first flush system

installed in the buildings shall be able to cater a volume

equivalent to 0.5mm of rainfall before the consequent

rainwater entering the storage tank. Table 3.7.3 shows the

first flush requirement according to the roof areas of

buildings.

Table 3.7.3: First flush requirement according to roof area

(SIRIM, 2013)

Roof area (m2) First flush volume(m3)

<100100 to 4356

>4356

0.025 to 0.050.05 to 2.5

2.5

NOTE. Adopt first flush of 5m3 if surface contains excessive soil, dust or debris.

3.7.5.4 NAHRIM

NAHRIM (2013) stated that the first 1-mm of rainwater from

the rooftop is normally contaminated with undesired

particles. All the rainwater harvesting systems installed by

NAHRIM followed this design criterion to ensure good quality

of rainwater.

3.7.6 Advantages

It is noted that the first flush diverter is operating

automatically, which is simple and user friendly. Most of

the first flush diverter is simple and easy to be installed.

It does not require manual intervention and is almost

maintenance free.

The device could safeguard the quality of the rainwater

through prevention of undesirable matters and pollutants

from entering the storage tank at the first stage of

rainwater harvesting. Therefore, the rainwater collected

could be directly used for non-potable purpose without

further complicated treatments. Besides, without

accumulation of the undesirable contaminants allows

protection to the subsequent system. Also, no power is

required to operate a first flush diverter. The technology

is a low-tech and low-cost demand to improve the rainwater

quality in ease.

manual to design rwh's system

Documents