K.4.2 Sewage flow calculation

Sewers should be designed to avoid possible overflows by making provision for the following flow contributions:

  • Regular flow (domestic and/or commercial sewage return flow) plus leakage and base flow (from night flow, leaking cisterns, leaking taps, etc.)
  • Infiltration – groundwater seeping through joints/cracks in the pipelines and junctions
  • Stormwater ingress – during rainstorms, runoff ingress into the system via illegal connections and inundated junctions

The general procedure for calculating the design flow is illustrated in Figure K.15:

Figure K.15: General procedure for calculating design flow
 

K.39

PDDWF (excl. infiltration) (kL/d) = Regular flow (kL/d) + Leakage & base flow (kL/d)

ADDWF (excl. infiltration) (kL/d) =
(PDDWF (excl. infiltration) (kL/d)/Peak day factor
Average PDDWF (L/s) =
PDDWF ( kL/d) / (24 h × 60 min × 60 sec)

IPDWF (L/s) = Peak Factor × Average PDDWF (L/s)

IPWWF or Design Flow (L/s) =
IPDWF (L/s) ( kL/d) / (1 ‒ Required spare capacity)

where:

PDDWF = Peak Daily Dry Weather Flow (total wastewater flow representing the peak day in a week)
ADDWF = Average Daily Dry Weather Flow (total average wastewater flow)
IPDWF = Instantaneous Peak Dry Weather Flow
IPWWF = Instantaneous Peak Wet Weather Flow (design flow)
Figure K.16: Sewer outflow hydrograph
 

K.40

K.4.2.1 Regular flow

Three methods can be used to calculate the regular flow (including leakage and base flow):

  • Unit hydrograph method – if the Average Annual Daily Demand (AADD) is unknown, but land use and Peak Daily Dry Weather Flow (PDDWF) are known
  • AADD method – if the AADD, percentage AADD sewer contribution, and land use are known
  • Sewer flow and peak factor method - if the PDDWF and land use are known (traditional method)

These three methods are discussed in detail and examples are provided.

(i) Unit hydrograph method

This method uses contributor unit hydrographs for different land use categories to calculate the expected theoretical peak flows and sewage volumes. It can be used when the AADD is not known for the specific land use(s). The inflow at a time (t) is calculated using the following formula:

Where:

UHt = Unit hydrograph value for land use type at a specific time step (from Table K.5)
Peak = Hydrograph peak flow for land use type (from Table K.5) in L/min
Leak = Hydrograph leakage and base flow for land use type (from Table K.5) in L/min
UQ = Unit PDDWF for land use type (from Table K.4, second last column) in kL/d/unit
EE = Number of units or land parcels per land use type
HQt = Calculated unit flow at a specific time step for land use type in L/min/unit
TQt = Calculated flow at a specific time step for land use type in L/s

From the unit hydrograph method the following design values can be calculated:

  • IPDWF (excl. infiltration) (Instantaneous Peak Dry Weather Flow) calculated as the maximum value of TQt for each land use for the peak day as follows:

    IPDWF (excl. infiltration) (L/s) = max (TQ t1..24 )
     
  • PDDWF (excl. infiltration) (Peak Daily Dry Weather Flow) calculated as the average value of TQt for eachland use for the peak day, or the sum of the number of units multiplied with the unit PDDWF for each land use:

    PDDWF (excl. infiltration) (kL/d) = Number of units [EE] × Unit PDDWF [UQ] (kL/d/unit)
     

K.41

  • ADDWF (excl. infiltration) (Average Daily Dry Weather Flow) can be calculated as the PDDWF (Peak daily dry weather flow for the peak day in the week) divided by the Peak day factor:

    ADDWF (excl. infiltration) (kL/d) = PDDWF (excl. infiltration) (kL/d) ÷ Peak day factor
     

Table K.4 provides the AADD, PDDWF and recommended UH-type for various land use types and densities. The recommended hourly ordinates of each UH-type are tabulated in Table K.5 and illustrated in Figure K.16 for a 24 hour period.

Worked example S1 – Unit hydrograph method

This worked example describes the Unit Hydrograph Method in determining the flow for 50 medium-density residential (UH2) units and 30 business/commercial property (UH7) units. The input data is obtained from Table K.4 and Table K.5 and the calculated values are shown in Table K.6.

Calculate each hourly interval for the ‘medium-density residential’ land use type excluding infiltration:

Example: Calculation for hour 1 of 24-hour time step:

Unit flow* (HQt) for hour 1 = UHt x Peak + Leak
  = 0.15 x 0.64 L/min/unit + 0.19 L/min/unit
  = 0.286 L/min/unit
Calculated unit PDDWF* = Average of HQt for the 24h period L/min/unit x ( 24 h x 60 min ) ÷ 1 000 L
  = 0.542 (L/min/unit) x 24 h x 60 min ÷ 1000 L
  = 0.780 kL/d/unit
Flow* (TQt) for hour 1 = HQt L/min/unit x ( UQ kL/d/unit ÷ Calculated Unit PDDWF kL/d/unit ) x EE units
  = 0.286 L/min/unit x ( 0.60 kL/d/unit ÷ 0.78 kL/d/unit ) x 50 units ÷ 60 sec
  = 0.183 L/s

Note: * Flow excludes infiltration

Calculate design flow for ‘medium-density residential’ land use type excluding infiltration:

IPDWF* = Maximum of Flow [TQ] over the 24h period
  = 0.532 L/s
PDDWF* = Average of flow [TQ] over the 24h period x (24h x 60min ) ÷ 1 000 L, or
  = Number of units [EE] x Unit PDDWF [UQ]
  = 50 units x 0.60 kL/d/unit
  = 30.0 kL/d
ADDWF* = PDDWF ÷ Peak day factor
  = 30.0 kL/d ÷ 1.1 factor
  = 27.3 kL/d

Note: * Flow excludes infiltration

Similarly, the flows for the 30 business/commercial (UH7) units can be calculated with the calculated values shown in Table K.6.

K.42

Table K.4: Demands and hydrographs for different land use categories
Land use Density #1 units/ha Stand size #2 Unit of measeure Water demand (AADD) Sewer flow (excl. infiltration) (Unit PDDWF) #4
kL/ ha/ d kL/ unit/ d #3 % AADD kL/ unit/ d #3 Unit Hydrograph (UH)
Residential stands High density, small sized 20 to 12 400 to 670 kL/unit 11 0.60 to 0.80 80% to 70% 0.48 to 0.56 UH5
Medium density, medium sized 12 to 8 670 to 1 000 kL/unit 9 0.80 to 1.00 70% to 60% 0.56 to 0.60 UH3
Low density, large sized 8 to 5 1 000 to
1 600
kL/unit 8 1.00 to 1.30 60% to 55% 0.60 to 0.72 UH2
Very low density, extra-large sized 5 to 3 1 600 to
2 670
kL/unit 7 1.30 to 2.00 55% to 40% 0.72 to 0.80 UH1
Stands for low-income housing (waterborne sanitation) High density, small sized 30 to 20 270 to 400 kL/unit 9 0.30 to 0.40 95% to 90% 0.29 to 0.36 UH4
Medium density, medium sized 20 to 12 400 to 670 kL/unit 7 0.40 to 0.50 90% to 85% 0.36 to 0.43 UH4
Low density, extra-large sized 12 to 8 670 to 1000 kL/unit 6 0.50 to 0.60 85% to 80% 0.43 to 0.48 UH4
Stands for low-income housing (dry sanitation) High density, small sized 30 to 20 270 to 400 kL/unit 7 0.25 to 0.30 n.a. n.a. n.a.
Medium density, medium sized 20 to 12 400 to 670 kL/unit 6 0.30 to 0.35 n.a. n.a. n.a.
Low density, extra-large sized 12 to 8 670 to 1 000 kL/unit 4 0.35 to 0.40 n.a. n.a. n.a.
Group/ cluster housing High density 60 to 40 130 to 200 kL/unit 21 0.40 to 0.45 95% to 90% 0.38 to 0.41 UH5
Medium density 40 to 30 200 to 270 kL/unit 17 0.45 to 0.50 90% to 85% 0.41 to 0.43 UH5
Low density 30 to 20 270 to 400 kL/unit 14 0.50 to 0.60 85% to 80% 0.43 to 0.48 UH5

K.43

Table K.4: Demands and hydrographs for different land use categories
Land use Density #1 units/ha Stand size #2 Unit of measeure Water demand (AADD) Sewer flow (excl. infiltration) (Unit PDDWF) #4
kL/ ha/ d kL/ unit/ d #3 % AADD kL/ unit/ d #3 Unit Hydrograph (UH)
Flats Very high density 100 to 80 80 to 100 kL/unit 25 0.25 to 0.30 100% to 98% 0.25 to 0.29 UH6
High density 80 to 60 100 to 130 kL/unit 23 0.30 to 0.35 98% to 97% 0.29 to 0.34 UH6
Medium density 60 to 50 130 to 160 kL/unit 21 0.35 to 0.40 97% to 96% 0.34 to 0.38 UH6
Low density 50 to 40 160 to 200 kL/unit 19 0.40 to 0.45 96% to 95% 0.38 to 0.43 UH6
Agricultural holdings Including irrigation < 3 < 2670 kL/unit 12 4.00 40% 1.60 UH1
Domestic water only < 3 < 2670 kL/unit 6 2.00 80% 1.60 UH1
Golf estate - excl. golf course water requirements < 3 < 2670 kL/unit 9 3.00 40% 1.20 UH2
Retirement village 20 to 12 400 to 670 kL/unit 11 0.60 to 0.80 80% to 70% 0.48 to 0.56 UH5
Business/commercial FAR = 0.4 n.a. kL/100m² #2 21 0.65 80% 0.52 UH7
Industrial FAR = 0.4 n.a. kL/100m² #2 13 0.40 80% 0.32 UH10
Government institutions FAR = 0.4 n.a. kL/100m² #2 13 0.40 80% 0.32 UH9
Warehousing FAR = 0.4 n.a. kL/100m² #2 10 0.30 80% 0.24 UH11
Institutional FAR = 0.4 n.a. kL/100m² #2 20 0.60 80% 0.48 UH9
Municipal services FAR = 0.4 n.a. kL/100m² #2 20 0.60 80% 0.48 UH9
Educational FAR = 0.4 n.a. kL/100m² #2 20 0.60 65% 0.39 UH8
Cemeteries n.a. n.a. kL/ha 12 n.a. n.a. n.a. n.a.
Parks n.a. n.a. kL/ha 12 n.a. n.a. n.a. n.a.
Sports fields n.a. n.a. kL/ha 12 n.a. n.a. n.a. n.a.

Notes:
#1 - Assumed net area factor = 0.8 x gross area (20% allowance for roads, servitudes and open spaces)
#2 - Floor area
#3 - Unit type as defined in column ‘Unit of measure’
#4 - Regular flow + leakage and base flow
FAR (Floor Area Ratio) is the ratio of the floor area of a building to its site area. Also referred to as FSR (Floor Space Ratio).

K.44

Table K.5: Sewer unit hydrographs
Definition of unit hydrographs
  Unit Hydrograph (UH) Number
UH1 UH2 UH3 UH4 UH5 UH6 UH7 UH8 UH9 UH10 UH11 UH12 UH13 UH14 UH15
Land use(s) following typical UH pattern
Hr Dimensionless flow ordinates (relative to hydrograph peak)
1 0.15 0.17 0.15 0.09 0.15 0.15 0.08 0.08 0.08 0.09 0.08 0.17 0.00 0.09 0.09
2 0.08 0.10 0.09 0.05 0.09 0.09 0.07 0.07 0.07 0.07 0.07 0.10 0.00 0.07 0.07
3 0.06 0.05 0.07 0.06 0.07 0.07 0.06 0.06 0.06 0.06 0.06 0.05 0.00 0.06 0.06
4 0.05 0.05 0.05 0.19 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.00 0.05 0.05
5 0.05 0.05 0.15 0.49 0.15 0.15 0.06 0.06 0.06 0.06 0.06 0.05 0.00 0.08 0.08
6 0.11 0.44 0.77 0.80 0.77 0.77 0.08 0.08 0.08 0.10 0.08 0.44 0.00 0.25 0.25
7 0.67 1.00 1.00 0.83 1.00 1.00 0.15 0.15 0.15 0.47 0.15 1.00 0.00 0.69 0.69
8 1.00 0.91 0.98 0.93 0.98 0.98 0.34 0.34 0.34 0.68 0.34 0.91 0.00 0.95 0.95
9 0.87 0.94 0.73 1.00 0.73 0.73 0.83 0.83 0.83 0.84 0.83 0.94 0.00 1.00 1.00
10 0.85 0.84 0.66 0.96 0.66 0.66 0.94 0.94 0.94 0.93 0.94 0.84 0.00 0.89 0.89
11 0.82 0.74 0.72 0.89 0.72 0.72 1.00 1.00 1.00 0.94 1.00 0.74 0.00 0.87 0.87
12 0.71 0.59 0.65 0.75 0.65 0.65 0.98 0.98 0.98 0.89 0.98 0.59 0.00 0.83 0.83
13 0.56 0.48 0.61 0.71 0.61 0.61 0.94 0.94 0.94 0.75 0.94 0.48 0.00 0.60 0.60
14 0.50 0.41 0.57 0.74 0.57 0.57 0.89 0.89 0.89 0.81 0.89 0.41 0.00 0.59 0.59
15 0.46 0.40 0.63 0.76 0.63 0.63 0.88 0.88 0.88 0.95 0.88 0.40 0.00 0.53 0.53
16 0.44 0.38 0.66 0.73 0.66 0.66 0.92 0.92 0.92 1.00 0.92 0.38 0.00 0.53 0.53
17 0.41 0.39 0.70 0.70 0.70 0.70 0.84 0.84 0.84 0.89 0.84 0.39 0.00 0.47 0.47
18 0.38 0.48 0.68 0.66 0.68 0.68 0.35 0.35 0.35 0.66 0.35 0.48 0.00 0.37 0.37
19 0.45 0.53 0.72 0.59 0.72 0.72 0.22 0.22 0.22 0.35 0.22 0.53 0.00 0.28 0.28
20 0.49 0.52 0.70 0.51 0.70 0.70 0.15 0.15 0.15 0.22 0.15 0.52 0.00 0.24 0.24
21 0.45 0.51 0.68 0.38 0.68 0.68 0.12 0.12 0.12 0.17 0.12 0.51 0.00 0.20 0.20
22 0.50 0.49 0.57 0.26 0.57 0.57 0.11 0.11 0.11 0.14 0.11 0.49 0.00 0.16 0.16
23 0.40 0.42 0.44 0.18 0.44 0.44 0.10 0.10 0.10 0.12 0.10 0.42 0.00 0.15 0.15
24 0.29 0.28 0.22 0.13 0.22 0.22 0.09 0.09 0.09 0.10 0.09 0.28 0.00 0.13 0.13
  Unit hydrograph parameters (L/min)
Hydrograph Peak 1.69 1.04 0.64 0.21 0.37 0.24 2.46 4.97 1.93 2.19 1.75 0.59 0.00 0.55 0.50
% of AADD 40% 55% 60% 70% 80% 90% 80% 65% 80% 80% 60% 80% 0% 55% 60%
Leakage & base flow 0.26 0.21 0.19 0.15 0.14 0.21 1.05 2.12 0.83 1.04 0.75 0.15 0.00 0.23 0.21
  Flow hydrograph volumes (L/d)
Regular flow 1090 697 507 169 293 190 1513 3057 1187 1490 1076 395 0 333 302
Leakage & base flow 374 302 274 216 202 302 1512 3053 1195 1498 1080 216 0 331 302
TOTAL FLOW 1464 999 780 385 495 492 3025 6109 2382 2988 2156 611 0 664 605

K.45

Figure K.17: Hourly flow ordinates relative to Hydrograph Peak (residential land use)

K.46

Figure K.18: Hourly flow ordinates relative to Hydrograph Peak (other land uses)
 
Table K.6: Worked example S1 – Unit hydrograph method
Input Data and data from Table K.4 and Table K.5
Land use #1 Medium-density residential Business and commercial Total
Number of units [EE] #1 50 30  
Unit hydrograph type#2 UH3 UH7  
Peak flow (L/min) #3 0.64 2.46  
Leakage and base flow (L/min) #3 0.19 1.05  
Unit PDDWF [UQ] (kL/d/unit) #2 0.60 0.52  
Peak day factor [EE] #1     1.1

K.47

Table K.6: Worked example S1 – Unit hydrograph method
Calculations (Regular flow + leakage and base flow)
Hour [t] Unit hyd. values #3 [UH] Unit flow [HQ] (L/min/ unit) Flow [TQ] (L/s) Unit hyd. values #3 [UH] Unit flow [UQ] (L/min/unit) Flow [TQ] (L/s) Total flow (L/s)
1 0.15 0.286 0.183 0.08 1.247 0.107 0.290
2 0.09 0.248 0.159 0.07 1.222 0.105 0.264
3 0.07 0.235 0.150 0.06 1.198 0.103 0.253
4 0.05 0.222 0.142 0.05 1.173 0.101 0.243
5 0.15 0.286 0.183 0.06 1.198 0.103 0.286
6 0.77 0.683 0.437 0.08 1.247 0.107 0.545
7 1.00 0.830 0.532 0.15 1.419 0.122 0.654
8 0.98 0.817 0.524 0.34 1.886 0.162 0.686
9 0.73 0.657 0.421 0.83 3.092 0.266 0.687
10 0.66 0.612 0.392 0.94 3.362 0.289 0.681
11 0.72 0.651 0.417 1.00 3.510 0.302 0.719
12 0.65 0.606 0.388 0.98 3.461 0.297 0.686
13 0.61 0.580 0.372 0.94 3.362 0.289 0.661
14 0.57 0.555 0.355 0.89 3.239 0.278 0.634
15 0.63 0.593 0.380 0.88 3.215 0.276 0.656
16 0.66 0.612 0.392 0.92 3.313 0.285 0.677
17 0.70 0.638 0.409 0.84 3.116 0.268 0.677
18 0.68 0.625 0.401 0.35 1.911 0.164 0.565
19 0.72 0.651 0.417 0.22 1.591 0.137 0.554
20 0.70 0.638 0.409 0.15 1.419 0.122

0.531

21 0.68 0.625 0.401 0.12 1.345 0.116 0.516
22 0.57 0.555 0.355 0.11 1.321

0.114

0.469
23 0.44 0.472 0.302 0.10 1.296 0.111 0.414
24 0.22 0.331 0.212 0.09 1.271 0.109

0.321

Hydrograph PDDWF (kL/d/unit) 0.780     3.025    
IPDWF (excl. infiltration) (L/s     0.532   0.302 0.719
PDDWF (excl. infiltration) (kL/d)   30.0     15.6 45.6
ADDWF (excl. infiltration) (kL/d)   27.3       41.5

Notes:
#1 - Example input data
#2 - Obtain from Table K.4 : Demands and hydrographs for different land use categories
#3 - Obtain from Table K.5 : Sewer unit hydrographs

K.48

Figure K.19: Worked example S1: Outflow hydrograph
 

Note: A peak factor is already included in the hydrograph used to calculate the flows. Therefore a seperate peak factor does not need to be applied.

(ii) AADD Method

The AADD method, as opposed to the unit hydrograph method, uses the actual or theoretical average annual daily demand (AADD), calculated per land use, to determine the sewage flow in the pipe at any time (‘t’). This method can only be used if the AADD is known. This is a more accurate method as it gives a more realistic view of the flow pattern. The flow is calculated using the following formulae:

Where:

UHt = Unit hydrograph value for land use type at a specific time step (from Table K.5)
Peak = Hydrograph peak flow for land use type (from Table K.5) in L/min
Leak = Hydrograph leakage and base flow for land use type (from Table K.5) in L/min
UQ = Unit PDDWF for land use type (from Table K.4, second last column) in kL/d/unit
AADD = Unit AADD for land use type (from Table K.4) in kL/d/unit
Ratio = The Ratio % of AADD (from Table K.4)
EE = Number of units or land parcels per land use type
HQt = Calculated unit flow at a specific time step for land use type in L/min/unit
TQt = Calculated flow at a specific time step for land use type in L/s

K.49

Worked example S2 – AADD method

This worked example describes the AADD method to determine the flow for 50 medium-density residential (UH2) units and 30 business/commercial property (UH7) units. The first two equations are the same as used for the Unit Hydrograph method to calculate unit flow and unit PDDWF, but the equation for TQt is slightly modified to incorporate the AADD adjustment. The input data is as obtained from Table K.4 and Table K.5 and the calculated values are shown in Table K.7.

Calculate for each hourly interval for the ‘medium-density residential’ land use type excluding infiltration:

Example: Calculation for hour 1 of 24-hour time step:

Unit flow* (HQt) for hour 1 = UHt x Peak + Leak
  = 0.15 x 0.64 L/min/unit + 0.19 L/min/unit
  = 0.286 L/min/unit
Calculated unit PDDWF* = Average of HQt for the 24h period L/min/unit x ( 24 h x 60 min ) ÷ 1 000 L
  = 0.542 (L/min/unit) x 24 h x 60 min ÷ 1000 L
  = 0.780 kL/d/unit
Calculated PDDWF* (UQ) = AADD kL/d/unit x Ratio % ÷ 100
  = 1.00 kL/d/unit x 60% ÷ 100
  = 0.60 kL/d/unit
Flow* (TQt) for hour 1 = HQt L/min/unit x ( UQ kL/d/unit ÷ Calculated Unit PDDWF kL/d/unit ) x EE units
  = 0.286 L/min/unit x ( 0.60 kL/d/unit ÷ 0.78 kL/d/unit ) x 50 units ÷ 60 sec
  = 0.183 L/s

Note: * Flow excludes infiltration

Calculate design flow for ‘medium-density residential’ land use type excluding infiltration:

IPDWF* = Maximum of Flow [TQ] over the 24h period
  = 0.532 L/s
PDDWF* = Average of flow [TQ] over the 24h period x (24h x 60min ) ÷ 1 000 L, or
  = Number of units [EE] x Unit PDDWF [UQ]
  = 50 units x 0.60 kL/d/unit
  = 30.0 kL/d
ADDWF* = PDDWF ÷ Peak day factor
  = 30.0 kL/d ÷ 1.1 factor
  = 27.3 kL/d

Note: * Flow excludes infiltration

Similarly, the flows for the 30 business/commercial (UH7) units can be calculated with the values shown in Table K.6.

K.50

Table K.7: Worked example S2 – AADD method
Input Data and data from Table K.4 and Table K.5
Land use #1 Medium-density residential Business and commercial Total
Number of units [EE] #1 50 30  
Unit hydrograph type#2 UH3 UH7  
Hydrograph peak (L/min) #3 0.64 2.46  
Leakage and base flow (L/min) #3 0.19 1.05  
AADD (kL/d/unit) #2 1.00 0.65  
Ratio % of AADD #2 60% 80%  
Unit PDDWF [UQ] (kL/d/unit) 0.60 0.52  
Peak day factor #1     1.1
 
Table K.6: Worked example S1 – Unit hydrograph method
Calculations (Regular flow + leakage and base flow)
Hour [t] Unit hyd. values #3 [UH] Unit flow [HQ] (L/min/ unit) Flow [TQ] (L/s) Unit hyd. values #3 [UH] Unit flow [UQ] (L/min/unit) Flow [TQ] (L/s) Total flow (L/s)
1 0.15 0.286 0.183 0.08 1.247 0.107 0.290
2 0.09 0.248 0.159 0.07 1.222 0.105 0.264
3 0.07 0.235 0.150 0.06 1.198 0.103 0.253
4 0.05 0.222 0.142 0.05 1.173 0.101 0.243
5 0.15 0.286 0.183 0.06 1.198 0.103 0.286
6 0.77 0.683 0.437 0.08 1.247 0.107 0.545
7 1.00 0.830 0.532 0.15 1.419 0.122 0.654
8 0.98 0.817 0.524 0.34 1.886 0.162 0.686
9 0.73 0.657 0.421 0.83 3.092 0.266 0.687
10 0.66 0.612 0.392 0.94 3.362 0.289 0.681
11 0.72 0.651 0.417 1.00 3.510 0.302 0.719
12 0.65 0.606 0.388 0.98 3.461 0.297 0.686
13 0.61 0.580 0.372 0.94 3.362 0.289 0.661
14 0.57 0.555 0.355 0.89 3.239 0.278 0.634
15 0.63 0.593 0.380 0.88 3.215 0.276 0.656
16 0.66 0.612 0.392 0.92 3.313 0.285 0.677
17 0.70 0.638 0.409 0.84 3.116 0.268 0.677
18 0.68 0.625 0.401 0.35 1.911 0.164 0.565
19 0.72 0.651 0.417 0.22 1.591 0.137 0.554
20 0.70 0.638 0.409 0.15 1.419 0.122

0.531

21 0.68 0.625 0.401 0.12 1.345 0.116 0.516
22 0.57 0.555 0.355 0.11 1.321

0.114

0.469
23 0.44 0.472 0.302 0.10 1.296 0.111 0.414
24 0.22 0.331 0.212 0.09 1.271 0.109

0.321

Hydrograph PDDWF (kL/d/unit) 0.780     3.025    
IPDWF (excl. infiltration) (L/s     0.532   0.302 0.719
PDDWF (excl. infiltration) (kL/d)   30.0     15.6 45.6
ADDWF (excl. infiltration) (kL/d)   27.3       41.5

Notes:
#1 - Example input data
#2 - Obtain from Table K.4 : Demands and hydrographs for different land use categories
#3 - Obtain from Table K.5 : Sewer unit hydrographs

K.51

Figure K.20: Worked example S2: Outflow hydrograph

Note that a peak factor is already included in the hydrographs used to calculate the flows and therefore a separate peak factor does not need to be applied.

(iii) Sewer flow and peak factor method

This method uses the Peak Daily Dry Weather Flow (PDDWF) and applies a peak factor to determine the peak flow. Table K.4 should be used to determine the unit PDDWF. The total PDDWF is calculated as the sum of the unit PDDWF for the respective land uses and number of units/stands for the design area, as calculated below.

A peak factor should be used to determine the peak flow. Typical design peak factors are provided in Table K.8 for various land uses. Figure K.21 provides a graph to select the peak factor for the residential zone based on the anticipated population. Consult with the local authority to obtain specific peak factors, if available.

K.52

Table K.8: Peak factors
Land use Peak factor
Residential – see Figure K.16 1.8 to 2.5
Business/commercia 1.3 to 1.5
Industrial – light 2.5 to 4.0
Industrial – heavy 2.0 to 3.0
Clinics, restaurants, laundromats and hotels 1.8 to 2.5

Note:
This method does not use the hydrograph peak, and thus a peak factor needs to be applied.

The peak flow rate, IPDWF (excluding infiltration), should be calculated as follows for each land use category:

IPDWF (excl. infiltration) (L/s) = Number of units [EE] × PDDWF (kL/d/unit) × PF × 1 000 ÷ (24 h × 60 min × 60 sec)

PDDWF (excl. infiltration) (kL/d) = Number of units [EE] × Unit PDDWF [UQ] (kL/d/unit)

ADDWF (excl. infiltration) (kL/d) = PDDWF (kL/d) ÷ Peak day factor

Where:

PF = Peak factor (from Table K.8)
UQ = Unit PDDWF for land use type (from Table K.4, second last column) in kL/d/unit
EE = Number of units or land parcels per land use type

Use Figure K.21 for selecting a peak factor for residential areas where the anticipated population is known. The peak factor reduces due to attenuation of peak flows in gravity sewer systems as the contributor area and population increase. The maximum recommended peak factor for residential areas is 2.5. If actual local peak factors are available, these should be used instead.

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Figure K.21: Peak factors for residential land use

Worked example S3 – Sewer flow and peak factor method

The same land-use categories and units are used for this worked example as for the examples above.

Table K.9: Worked example S3 – Sewer flow and peak factor method
Input and data from Table K.4, Table K.8 and Figure K.16
Land use #1 Medium-density residential Business and commercial Total
Number of units [EE] #1 50 30  
Unit PDDWF [UQ] (kL/d/unit) #2 0.60 0.52  
Persons per unit #3 5 n/a  
Peak factor #3 2.5 1.50  
Peak day factor #1     1.1
Calculations
Total number of persons 250 n/a  
IPDWF (excl. infiltration) (L/s) 0.868 0.271 1.139
PDDWF (excl. infiltration) (kL/d) 30.0 15.6 45.6
ADDWF (excl. infiltration) (kL/d) 27.3 14.2 41.5

Notes :
#1 - Example input data

#2 - Obtain from Table K.4: PDDWF (excluding infiltration) for different land use categories

#3 - Obtain from Table K.8: Peak factors and Figure K.16: Peak factors for residential land use

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Comparing this to the flows as determined from the previous two methods, the design flow calculated from the peak factor method is much higher. This is because this method does not account for the difference in the timing of the peak, which does not occur at the same time for the Unit Hydrograph and AADD Method.

Calculate design flow for ‘medium-density residential’ land use type excluding infiltration:

Total number of persons = Number of units [EE] × persons per unit
  = 50 units × 5 persons per unit
  = 250 persons
Peak factor = From Figure K.16 (or Table K.8)
  = 2.5
IPDWF* = 50 units × 0.60 kL/d/unit × 2.5 Peak factor x 1 000 L ÷ (24h x 60min × 60sec)
  = 0.868 L/s
PDDWF* = Number of units [EE] × Unit PDDWF [UQ]
  = 50 units × 0.60 kL/d/unit
  = 30.0 kL/d
ADDWF* = PDDWF ÷ Peak day factor
  = 30.0 kL/d ÷ 1.1 factor

Where:

EE = Number of units or land parcels per land use category
UQ = Unit PDDWF for land use category (from Table K.4, second last column) in kL/d/unit

Note: * Flow excludes infiltration

K.4.2.2 Groundwater infiltration

Typically, 35% of the total base flow measured in sewers is due to groundwater infiltration through joints and cracks in the sewer pipe system. Assuming this to be the case, a groundwater infiltration rate of between 0.03 and 0.04 (L/min/m pipe/m Ø) should be allowed for (see Table K.10).

The infiltration is dependent on the length of the pipe and the outside diameter of the pipe. This is because the outside of the pipe is exposed to the ground. Where the total pipe length is unknown, use Table K.11 to estimate the sewer pipe lengths per stand (or unit) for different land use types. Multiply the unit pipe length with the proposed number of stands to obtain an indicative total pipe length (reticulation) for a proposed development. Separate allowance should be made for bulk or collector outfall sewers based on site-specific conditions. Due to the wide range of possible stand sizes for business/commercial units, the pipe lengths should be calculated using site-specific conditions.

Table K.10: Groundwater infiltration
Condition Infiltration (L/min/m pipe/m Ø)
Minimum groundwater infiltration 0.03
Maximum groundwater infiltration 0.04

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The Neighbourhood Planning and Design Guide
Creating Sustainable Human Settlements

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