Table K.11: Typical pipe length (reticulation) per stand/plot
Land use Stand Size #1 Pipe Length #2
m
Residential stands High density, small sized 400 to 670 10 to 13
Medium density, medium sized 670 to 1 000 13 to 16
Low density, large sized 1 000 to
1 600
16 to 20
Very low density, extra-large sized 1 600 to
2 670
20 to 26
Stands for low income housing (waterborne sanitation) High density, small sized 270 to 400 8 to 10
Medium density, medium sized 400 to 670 10 to 13
Low density, extra-large sized 670 to 1 000 13 to 16
Group/cluster housing High density 130 to 200 6 to 7
Medium density 200 to 270 7 to 8
Low density 270 to 400 8 to 10
Flats Very high density 80 to 100 4 to 5
High density 100 to 130 5 to 6
Medium density 130 to 160 6 to 6
Low density 160 to 200 6 to 7
Agricultural holdings Including irrigation < 2 670 > 26
Domestic water only < 2 670 > 26
Golf estate - excluding golf course water requirements < 2 670 > 26
Retirement village 400 to 670 10 to 13

Notes :
#1 - Assumed net area factor = 0.8 x gross area (20% allowance for roads, servitudes and open spaces)

#2 - Calculation based on a square shape stand/plot

Worked example S4 – Groundwater infiltration

For the proposed development comprising 50 medium-density residential units and 30 business/commercial units(as was used in previous examples), the total sewer pipe length should be estimated from Table K.11 as follows:

For the medium-density residential category, estimate the pipe length from Table K.11 = 15m x 50 units = 750 m

For the business/commercial units category, estimate the pipe length (assumed length) = 250 m
Total pipe length = 1 000 m

Assume that all reticulation is 150 mm Ø with no further allowance needed for bulk or collector outfalls. The groundwater infiltration can thus be calculated using the following formula with the results as indicated in Table K.12:

Infiltration flow (L/s) = Infiltration rate (L/min/m/m) × Pipe length (m) × Pipe diameter (m) ÷ 60 sec

K.56

Table K.12: Worked example S4 – Groundwater infiltration
Input data
Pipe length (m) #1 1000  
Pipe inside diameter (mm) #1 150  
Infiltration rate (L/min/m pipe/m Ø) #2 0.04  
Calculations - Infiltration
     
  Peak flow (L/s) Daily flow (kL/d)
Groundwater infiltration flow 0.100 8.6

Notes:
#1 - Example input data (See Table K.11)
#2 - See Table K.10 for recommended groundwater infiltration

The groundwater infiltration flow should be calculated as follows:

Groundwater infiltration flow = 0.04 L/min/m/m x 1 000 m x (150 mm ÷ 1 000) ÷ 60 sec
  = 0.100 L/s
  = 0.10 L/s x (24 h x 60 min x 60 sec ÷ 1 000
  = 8.6 kL/d

K.4.2.3 Allowance for stormwater ingress

Gravity sewers should be sized not only to accommodate the peak flow during dry weather conditions (IPDWF incl. infiltration), but a spare capacity allowance should be provided for to accommodate stormwater ingress as was illustrated in Figure K.16. The percentage spare capacity allowance criteria for stormwater ingress are to be obtained from the local authority. In the absence of such local criteria, use 30% as recommended for reticulation, and between 15% and 30% as recommended for outfall sewers. Refer to the following section for the calculation.

K.4.2.4 Design flow calculation

The calculation of the design flow or IPWWF (instantaneous peak wet weather flow), as detailed in preceding sections, can be summarised as follows:

Step 1: Calculate the IPDWF (instantaneous peak dry weather flow) excluding groundwater infiltration (Section K.4.2.1)
Step 2: Calculate the groundwater infiltration (Section K.4.2.2)
Step 3: Calculate the IPDWF (instantaneous peak dry weather flow) including groundwater infiltration as follows:
  IPDWF (L/s) = IPDWF (excl. Infiltration) (L/s) + Infiltration flow (L/s)
Step 4: Calculate the design flow or IPWWF (instantaneous peak wet weather flow) as follows:
 
Design Flow or IPWWF (L/s) =
IPDWF incl infiltration (L/s)/ (1 – spare capacity for stormwater ingress)

K.57

K.4.3 Hydraulic design guidelines for waterborne sanitation systems

K.4.3.1 Hydraulic spare capacity calculation

The different spare capacity types are illustrated in Figure K.22.

Figure K.22: Absolute and relative spare capacity

K.58

The ‘spare capacity’ for a regular pipe in a gravity system, which is unaffected by upstream pumps, is defined as follows:

Absolute spare capacity (%) =
Full flow pipe capacity (L/s) - Max flow (IPDWF) (L/s)/ Full flow pipe capacity (L/s)

The relative spare capacity is the spare hydraulic capacity expressed as a percentage of the relative capacity, which is the capacity of the pipe less the total upstream continuous pump flow rate.

If there are pumps upstream that pump at a continuous rate, it is necessary to consider the relative effect of these pumps on the spare capacity in the downstream pipes. Part of the capacity should cater for the continuous pumpflow. Any spare capacity should be expressed as a percentage of the remaining available capacity, i.e. the relativecapacity of the pipe, which is the total capacity less the effect of the upstream pumps.

Relative spare capacity (%) =
Full flow pipe capacity (L/s) - Max flow (IPDWF) (L/s)/ Full flow pipe capacity (L/s) - Upstream continuous pump rate (L/s)
X 100

It should be noted that in the case of variable speed pumps, the amount of flow that flows into the pump structure is pumped out, unless the flow is more than the capacity of the pump; then it overflows. For continuous speed pumps,the pump flow rate is constant, regardless of the inflow.

Worked example S5: Hydraulic capacity

The example below shows that spare capacity is below 30%, assuming this is the minimum hydraulic spare capacity allowance for stormwater ingress. Thus the pipe is too small and should be upgraded.

If the upstream continuous pump flow rate is excluded, the relative spare capacity is more than 30%. Thus no upgrade is required.

Table K.13: Worked example S5 – Hydraulic capacity
Input data
Pipe’s full-flow capacity (L/s) #1 32.0
Max flow (IPDWF) (L/s) #1 28.0
Total upstream continuous flow rate (L/s)#1 20.0
Spare capacity required#1 30%
Calculations
Absolute spare capacity  
Spare capacity (L/s) 4.0
Available 12.5%
Relative spare capacity  
Spare capacity (L/s) 4.0
Max flow (IPDWF) excluding continuous pump flow rate (L/s) 8.0
Available 33.3%

Note:
#1 - Example input data

K.59

K.4.3.2 Velocity calculation

(i) Gravity sewers

The following flow formulae are used for the calculation of velocity and flow in sewers pipe sections during normal depth conditions (slope of the water surface and channel bottom is the same and the water depth remains constant).

Table K.14: Flow formulae
Formula name Formula Roughness constant
Mannings (n = 0.012)
Chezy (KS = 0.600)
Colebrook-White
Kutter (n = 0.012)

Where:

A =    Cross-sectional area of flow/conduit (m2)
R =    Hydraulic radius (m)
S =    Gradient (assuming uniform flow)
n =    Manning’s roughness coefficient – dependent on material type
kS or 3 =    Absolute roughness of conduit (m)
C =    Chezy roughness coefficient
f =    Darcy-Weisbach friction factor
DH =    Hydraulic diameter (m)
Re =    Reynolds number

These formulae are used assuming full flow in the pipe. Any of the above formulae can be used as long as they produce values approximately the same as the equivalent Colebrook-White formula that uses a KS of 0.6. For modelling purposes, the general Manning roughness coefficient is 0.012, but it is dependent on the pipe material and condition.

For partially full pipes, the partial flow diagram (see Figure K.23) can be used to calculate the flow and velocity based on proportions of the full-flow velocity and discharge, as well as the depth of flow.

K.60

 

Figure K.23: Partial flow diagram

(ii) Rising (pumped) sewers

For pumped sewers flowing full, the pipe velocity is calculated as follows:

V = Q/A

Where:

V = Pipe Velocity (m/s
Q = pumped flow rate (capacity) in m3/s
A = Cross-sectional area of the sewer (m2)

K.61

For circular sewers it is calculated as follows:

A = π Ø2/4

Where Ø = pipe diameter (m)

K.4.3.3 Gravity sewer system

The following design criteria are recommended for gravity sewers:

(i) Gravity main – minimum and maximum flow velocities and gradients

Sewers may follow the general slope of the ground, provided that a minimum full-bore velocity of 0.6-0.7 m/s is maintained at the minimum gradient in all gravity mains. This is to ensure that sufficient scouring of the mains takes place.

The maximum flow velocity under full-flow conditions should be not more than 2.5 m/s to prevent damage to the pipelines, although a higher flow velocity of up to 3.5 - 4.0 m/s may be acceptable over short pipe lengths and for short periods. The maximum pipe velocity should be checked with the pipe manufacturer. Too high velocities should be avoided due to separation and abrasion.

Table K.18 shows absolute minimum gradients for different diameter pipes required to achieve the minimum full-bore velocity of 0.65 m/s. If flatter grades and lower velocities are considered, it is essential that a detailed cost-benefit study be conducted. The cost of the regular, systematic maintenance and silt/sand removal that will be required when flatter grades and lower velocities are used, will need to be compared to the additional capital cost required to maintain the above minimum grades at full-bore velocity of 0.6 - 0.7 m/s

The diameters in Table K.15 are for illustrative purposes only. The actual Manning coefficient of the pipe should be obtained from the pipe manufacturer to calculate the minimum gradient to achieve the required minimum velocities of 0.65 m/s.

Table K.15: Minimum gradients for ±0.65 m/s full flow velocity (more than 21 units#1)
Pipe diameter Class Material (general) Minimum gradient #1(Manning n = 0.012) Flow
Nominal
(mm)
Outside
(mm)
Inside
(mm)
@ 70%
(L/s)
110 110 104 34 uPVC 1 : 120 4
160 160 151 34 uPVC 1 : 200 8
200 200 188 34 uPVC 1 : 250 13
250 250 235 34 uPVC 1 : 350 20
315 315 297 34 uPVC 1 : 500 32
355 355 334 34 uPVC 1 : 600 40
450 533 416 100D Concrete 1 : 700 51
525 616 534 75D Concrete 1 : 800 62
600 699 611 75D Concrete 1 : 1 100 103
675 787 685 75D Concrete 1 : 1 300 136

K.62

Table K.15: Minimum gradients for ±0.65 m/s full flow velocity (more than 21 units#1)
Pipe diameter Class Material (general) Minimum gradient #1(Manning n = 0.012) Flow
Nominal
(mm)
Outside
(mm)
Inside
(mm)
@ 70%
(L/s)
750 870 762 75D Concrete 1 : 1 500 171
825 946 830 75D Concrete 1 : 1 800 208
900 1 041 913 75D Concrete 1 : 2 000 247
1 050 1 194 1 066 50D Concrete 1 : 2 300 297
1 200 1 365 1 219 50D Concrete 1 : 2 800 407
1 350 1 524 1 372 50D Concrete 1 : 3 400 529
1 500 1 689 1 523 50D Concrete 1 : 4 000 668
1 650 1 878 1 700 50D Concrete 1 : 4 600 823
1 800 2 019 1 803 50D Concrete 1 : 5 300 1 028
#1 When the number of upstream units exceeds 21, the minimum slope as provided above should be used for the corresponding diameter when assuming the Manning coefficient is 0.012. For pipes servicing fewer than 21 units, the gradients as shown in Table K.16 should be used.

The sewer pipes should have a steeper gradient closer to the upper end of the sewer network to ensure the pipes are cleared and that settlement is avoided, as the pipes do not flow full regularly and low-flow conditions can occur (depth of flow less than 20% of the diameter). Minimum gradients based on the number of upstream units are listed in Table K.16 to ensure sufficient pipe flushing. House connections should be laid at a minimum slope of 1:60 for 110 mm nominal diameter pipes.

Table K.16: Preferred and minimum gradients for upper end of sewer network (less than 21 units)
Number of units 110 mm – Nominal diameter 160 mm – Nominal diameter
Preferred gradient Minimum gradient Preferred gradient Minimum gradient
1 to 10 1 : 60 1 : 75 1 : 80 1 : 100
11 to 20 1 : 75 1 : 100 1 : 100 1 : 140
21 and more 1 : 90 1 : 120 1 : 120 1 : 200

(ii) Gravity main – minimum size/diameter

The minimum permissible diameter for gravity sewer pipes in a municipality should be at least 150 mm inside or nominal diameter, but the absolute minimum diameter of the pipe in sewer reticulation should be 100 mm (connections to properties).

A minimum pipe diameter of 200 mm (outside) is recommended for CBD developments. This is to provide some spare capacity for future densification, because of the difficulty of installing additional services in the CBD.

K.4.3.4 Pumped sewer system

Sewer pumping stations should only be considered where absolutely necessary, and where a gravity alternative is not feasible. A sewer pump station should consist of a sump to receive incoming sewage, and pumps that pump the sewage through a rising main into a downstream stilling chamber. The design recommendations for each of these four components are provided below.

K.63

(i) Sizing of sumps

The sump receives the sewage flow and acts as a storage vessel from where sewage is periodically pumped. The sump comprises an active and emergency storage volume. The active volume is defined by the operating levels of the sump. The emergency storage volume provides additional safety when the pumps fail, in that it provides time for the maintenance crew to make repairs before an overflow happens. The calculation of the emergency and active sump volume is detailed below.

a. Emergency storage

A minimum emergency storage capacity should be provided representing a capacity that is equivalent to four to six hours’ flow at the design flow rate, over and above the capacity available in the sump at normal top-water level (i.e. the level at which the duty pump cuts in). This provision applies only to pump stations serving less than 250 dwelling units and where no backup power for pump stations is supplied. The aim is to contain any sewage spillage.

For pump stations serving larger numbers of dwelling units, the sump capacity should be subject to special consideration, in consultation with the local authority concerned. Emergency storage may be provided inside or outside the pump station. Emergency sump volume is calculated using the following formula:

VE = q × TE

Where:

TE = minimum emergency storage time (specified by local authority – generally 4 to 6 hours)
q = average raw sewage inflow rate (ADDWF)
VE = sump emergency storage volume (m3)

Some emergency storage capacity might also be available in the up-stream gravity lines and manholes.

b. Active sump volume

Active sump volume is calculated using the following formula:

T =
VA/ q
-
VA/ (Q – q)

Where:

T = Minimum cycle between pump starts (time to fill + time to empty)
VA = Sump active volume (m3/s)
Q = Pumping rate
q = Sewage inflow rate

The total sump volume is the sum of the active and emergency volumes:

V = VA + VE

K.64

c. Buoyancy calculations

Ensure that the structure will not float when subjected to high groundwater levels.

(ii) Sizing of pumps

Pumps are mechanical equipment used to transfer sewage from the sump to a higher location within the sewer system. The selection of the pumps depends on the hydraulic requirements they must meet and the level of safety the design requires.

a. Design flow

The capacity of the pumping station should equal or exceed the instantaneous peak wet weather flow (IPWWF) that arrives at the pumping station to allow for stormwater ingress. In the case of a 30% stormwater allowance, the pump should have a capacity equal to the design flow, generally:

IPDWF/ (1– 0.3)
=
IPDWF/ 0.7
=1.43 x IPDWF

b. System hydraulics

The pumping station should be designed to operate under the full range of projected system hydraulic conditions. The system should be designed to prevent a pump from operating for long periods of time beyond the pump manufacturer’s recommended normal operating range. Start/stop cycles should not exceed the pump motor manufacturer’s recommendation.

The pump station should be designed in such a way that the pumps operate a maximum of two duty cycles per hour during average flow conditions and not more than six cycles per hour during instantaneous peak wet weather flow.

c. Efficiency

Pumps should be selected to ensure that the operating point is near the maximum efficiency point on the pump performance curve, within the pump’s recommended operating range, and within the manufacturer’s recommended limits for radial thrust and vibration.

d. Standby pumps

Pumping stations should be designed to accommodate instantaneous peak wet weather flow (IPWWF), with at least one reserve pump. At least two pumps should be installed, each capable of pumping at a flow rate more than the peak wet weather flow (for emergency purposes), but at the same time, taking care not to provide excessive standby capacity. The standby pump should come into operation automatically if a duty pump or its driving motor fails.

Where three or more pumps are selected, they should be designed to fit actual flow conditions and must be so designed so that with any one pump out of service, the remaining pumps will have the capacity to pump the IPWWF. Pumps should be designed in such a way that one pump can empty the sump plus handle the average inflow in less than 30 minutes.

K.65

e. Hydraulic influence of pump stations

Although sewer pump stations operate intermittently, their flows can influence the hydraulics of the downstream pipes at any time during the day. It is therefore advised to model the pumps as ‘continuous’ pumps that pump for 24 hours per day when sizing downstream gravity sewers.

f. Surge analysis

Consider hydraulic surges and transients (water hammer) during the design of pump stations and pumping mains.

g. Cavitation

Ensure that the Net Positive Suction Head (NPSH) available is higher than the NPSH required to avoid cavitation damage to the pump.

h. Backup power for pump stations

Emergency power supply should be provided to pumping stations to ensure continuous operation when primary electrical supply is out of service (standby generator). Larger pump stations should have permanent diesel-oilfuelled, engine-driven generator units with automatic transfer switches to transfer the electrical feed from the primary to the standby unit when a power failure is detected by the instrumentation and control system, sized to operate all electrical components. For smaller pump stations, where a dedicated backup generator is not available, a portable generator should be available. A manual transfer switch and an emergency plug-in power connection to the station, for use with the portable generator, should in these cases be installed. A standby generator should be provided to supply the pump station with full load power for at least 6 hours.

i. Pump sizing and design

The appropriate pump should be selected by considering the pump system curves. The pump system curves indicate the interaction between the pump performance and the pumping main used to deliver the discharge. The pumping system curves should be determined in the selection of the pumps to ensure an appropriate and efficient pumping system.

Every pump manufacturer has a pump performance curve for every pump. The pump performance curve shows the discharge relative to the pressure for a particular pump and impeller size (diameter). It also shows the efficiency, as illustrated below. The system curve (which represents the static head and the friction losses of the pumping main)should be used in conjunction with the pump performance curve to specify the most appropriate pump that can accommodate the flow and provide the required head at the desired efficiency. The operating (duty) point is wherethe pump performance curve and system curve intersect. The duty point should be near the pump’s best efficiency point (BEP), as is shown in Figure K.24.

K.66

Figure K.24: Pumping system curve

The following should be considered when selecting the correct pump:

  • The pump should be selected to ensure that the duty specified falls well within the stable range of the head/ quantity characteristic curve of the pump
  • The pump should have a non-overloading power curve.
  • Maximum suction lift should not exceed the pump manufacturer’s recommendations and should be based on a net positive suction calculation with an allowed factor of safety

(iii) Rising mains

Rising mains should be designed to take care of the following:

  • Minimum and maximum flow velocities: The minimum velocity of flow in a rising main should be 0.6 m/s. Flow velocities must be limited to protect pipeline coatings and reduce the effects of water hammer. The preferred maximum allowed is 1.5 to 1.8 m/s, but an absolute maximum of 2.5 m/s is acceptable where only intermittent peak flows occur
  • Minimum size/diameter: The minimum internal diameter of a rising main should be 100 mm, except where amacerator system is used, in which case the diameter can be reduced to 75 mm.

Other issues to consider when designing a rising main include:

  • Where possible, the rising main must have a positive grade with no low points or high points so as to avoid possible gas release and grit deposition.
  • Scour valves and air valves must be avoided at all cost.
  • Protect the pipeline against hammer and surge forces (analyse and provide protection).
  • Turbulence must be avoided to prevent the release of H2S gas at the outlet.
  • Provide protection against unbalanced forces (thrust) where necessary (thrust blocks and support).

K.67

(iv) Stilling chambers

Stilling chambers should be provided at the heads of all rising mains, and should be designed so that the liquid level always remains above the soffit level of the rising main where it enters the chamber. Stilling chambers should preferably be ventilated.

K.4.4 General design guidelines for waterborne sanitation systems

K.4.4.1 Sewer pipes

(i) Location

Sewers should be sited to provide the most economical design, taking into account the topography (i.e. in road reserves, servitudes, parks, other open spaces, etc.). The minimum clear width to be allocated to a sewer in the road reserve should be 1.5 m.

a. Siting

Sewer pipes should be located in open areas, road reserves or municipal land where they may be easily accessed at all times, preferably on the lower side of the road. In road reserves, sewers should be installed between the stormwater drain and the plot boundary where applicable. In built-up areas, sewer pipes should preferably be located 1.2 to 1.5 m from the plot boundary. The positioning of infrastructure in municipal areas is often guided by municipal specifications and standards.

Mid-block sewers should be avoided as far as practically possible. Where the mid-block system is unavoidable, the sewer connections should not be installed deeper than 2 m and the main sewer should not be installed deeper than 3 m. If these depths are to be exceeded, a double system must be used. In cases where decision-making is difficult, a comparative estimate of costs with the double system must be made. When designing a double system, it is essential that close attention is paid to where other services, particularly stormwater drains, are crossed. Special permission is required for mid-block sewers if they cannot be avoided. Mid-block sewers are not recommended in townships with individual stands of less than 400 m2 in area. The following aspects should be considered when routing sewers:

  • The sewer should follow the natural fall of the ground.
  • The sewer should be laid next to those properties that will benefit most directly from the sewer.
  • Road crossings should be kept to a minimum.
  • All other municipal services should be taken into account when installing a new sewer.
  • There should be minimum interference with existing structures.

b. Road crossings

Where a road crossing is unavoidable, consider using the following:

  • Existing crossings, such as culverts and bridges, to avoid excavation and pipe jacking for ground crossings.
  • Pipe jacking where applicable and acceptable.
  • Encasement of sewers that cross under surfaced roads (existing tar roads) in concrete.
  • Backfilling of trenches in accordance with relevant construction specifications. (The selected materials should be hand-compacted to a depth of at least 300 mm above the top of the pipe).

K.68

Postal address
The Department of Human Settlements
Private Bag X644
Pretoria
0001

Call centre
0800 146873

Website
www.dhs.gov.za

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The Neighbourhood Planning and Design Guide
Creating Sustainable Human Settlements

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