DESIGN GUIDELINES
WASTEWATER NETWORKS – GRAVITY FOUL SEWERS
SYSTEMS
Contract No:
17293-2
Contract Title:
Design Guidelines: Wastewater Networks – Gravity Foul Sewers Systems
Date:
15 April 2021
Strictly Confidential
All Rights Reserved
Document Reference
Issue Date
NEOM Data Owner
Technical Lead(s)
Document Author
Client accepted
Document Location
Reason for Issue
NEOM-NWA-TGD-2021-021
15/04/2021
Joao Pitta
N.R. Orman, P. Tzinaetis
N.R. Orman, P. Tzinaetis
N Burnett
17293-2\Wastewater Networks\Technical Reports
For Approval
CONTENTS
1. VERSION CONTROL .................................................................................................................... 3
2. GLOSSARY OF TERMS ............................................................................................................... 3
3. INTRODUCTION ........................................................................................................................... 4
4. COMPLIANCE WITH STANDARDS ............................................................................................. 4
Document Change Request .............................................................................................. 4
Waivers ............................................................................................................................. 4
Health and Safety .............................................................................................................. 4
Confidentiality .................................................................................................................... 5
5. NATIONAL & INTERNATIONAL STANDARDS .......................................................................... 5
6. DESIGN GUIDELINES .................................................................................................................. 5
General .............................................................................................................................. 5
6.1.1.
Design consideration ......................................................................................................... 5
6.1.2.
Site considerations ............................................................................................................ 6
Hydraulic design ................................................................................................................ 6
6.2.1.
Design flow ........................................................................................................................ 7
6.3
Minimum pipe size ............................................................................................................ 7
6.4
Layout & Access ............................................................................................................... 7
6.4.1
Layout ................................................................................................................................ 7
6.4.2
Access points .................................................................................................................... 8
6.5
Structural design ............................................................................................................... 8
7 DESIGN BEST PRACTICE ........................................................................................................... 8
7.1
General .............................................................................................................................. 8
7.2
Burial depth ....................................................................................................................... 8
7.3
Hydraulic design ................................................................................................................ 9
7.3.1 Depth of flow ......................................................................................................................... 10
7.4
Sewer connections .......................................................................................................... 11
7.4.1 Property Connections .......................................................................................................... 11
8 REFERENCES ............................................................................................................................ 12
Annex A – Domestic and commercial water demand ................................................................. 13
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General Guideline Document
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1. VERSION CONTROL
Reference
NEOM-NWA-TGD-2021-021
Description
For Approval
Department
Date issued
15/04/2021
Actions
2. GLOSSARY OF TERMS
Terms & Acronyms
Access point
Provision to access to a sewer or drain for testing, inspection, maintenance and
removal of debris and includes any manholes and inspection chambers.
Drain
Pipeline, usually underground, designed to carry wastewater and/or surface water from a
source to a sewer.
Drainage
Group of technologies, techniques, and actions to achieve satisfactory hygiene and public
health relating to wastewater, including both foul wastewater and surface water.
Inspection
Chamber on a sewer with working space at ground level only, used to introduce
chamber
equipment for testing, inspection, and maintenance.
Layout
Configuration of components of drain and sewer systems in both horizontal and vertical
planes.
Manhole
Chamber with working space at sewer level used for entry of personnel and equipment.
Sewer
Pipeline or other construction, usually underground, designed to carry foul water,
wastewater, surface water or other unwanted liquids.
Sewer system
Network of pipelines and ancillary works which conveys wastewater and/or surface water
from drains to a treatment works or other place of disposal.
Surface water
Water from precipitation, which has not seeped into the ground and is discharged to the
drain or sewer system directly from the ground or from exterior building surfaces.
Trade effluent flow A fluid discharge, with or without matter in suspension, resulting wholly or in part from any
manufacturing, industrial or commercial process, including farm, research institution hospital
and nursing home effluent.
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3. INTRODUCTION
This Design Guidelines set out the standards and guidelines required for design of NEOM gravity foul
sewers.
4. COMPLIANCE WITH STANDARDS
Document Change Request
It is a requirement to comply with the Design Guidelines and there shall be no departures from these
guidelines except where formally confirmed by NEOM; such departures being technically justified or
representing advances in knowledge or technology.
If the designer has good reason not to comply with this standard and identifies alternative standards or
guidelines, a change request shall be submitted to the NEOM Wastewater Engineer, with evidence for
change.
The Engineer will approve or reject the request and their decision is final.
Waivers
NEOM is committed to using new and innovative technologies where they, in NEOM’s opinion, represent
the best technical solution, provide low life cycle costs and value for money. All technologies will be
considered for use by NEOM providing they have been proven in terms of performance, quality, and cost.
NEOM will approve plans for new systems, extensions to new areas or replacement sewers only when
designed upon the separate system, and when they meet the requirements of these guidelines. NEOM
reserve the right not to adopt any system that fails to meet the minimum standards of these guidelines.
If the designer identifies alternative technical solutions to those stated in this standard or recently
developed or innovative products which properties and performance can be determined to align with their
intended duty and design life, a design waiver request shall be submitted to the NEOM Wastewater
Engineer, with evidence for change.
The Engineer will approve or reject the request and their decision is final.
Calculations shall clearly state which standard has been used with a full justification for its use. Further,
this shall be supported by the collection of data and site-specific evidence to validate the proposal. Design
consultants shall also need to refer to International industry guides and codes of practice such as British
Water Code of Practice for Flows & Loads for the guidance of flow rate calculations.
NEOM emphasise that all sewerage design shall be done in accordance with accepted international
Standards. Where deviation from Standards is required it shall be fully justified on a case-by-case basis.
Health and Safety
The designer shall comply with all NEOM Health and Safety Regulations and shall endeavor to design
out any identified hazards, or if impractical, to identify control measures to minimise risk, following
guidance in the UK CDM Regulations 2015.
The designer is responsible for ensuring that the design eliminates, reduces, or controls foreseeable risks
to the health and safety of all people during construction and at all other stages of the asset life including
during inspection, cleaning, repair, maintenance, and decommissioning.
Considerations in design to mitigate risks will include but not be limited to:
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• The designer shall develop designs that preclude the need to enter confined spaces wherever
possible.
• Safe access shall be provided to all plant requiring maintenance.
• All above ground systems shall be fenced off and be inaccessible to the public.
• Craneage or mobile lifting facilities shall be provided for all heavy equipment.
• Stairways shall be equipped with hand railing and toe plates in accordance with ISO 14122-3 and
BS 4592.
• All hazards shall be signposted.
• Lighting: Luminaires, is to be provided wherever access is required.
• Welfare facilities shall be provided to allow operatives to clean up after maintenance work.
• Access points (i.e., manholes and inspection chambers) shall be equipped with covers that are
secure yet can be easily removed for maintenance purposes.
• Provision of flow isolation facilities.
Confidentiality
Only employees and contractors of NEOM have a right to see this document and reproduction of the
whole or part shall not be made without permission of NEOM.
5. NATIONAL & INTERNATIONAL STANDARDS
BS EN 752:2017 – Drain and sewer systems outside buildings - sewer system management
BS EN 12056-2:2000 – Gravity drainage systems inside buildings. Sanitary pipework, layout and
calculation
BS EN 16933-2:2017 – Drain and sewer systems outside buildings - design. Hydraulic design
BS 4592 – Industrial type metal flooring, walkways and stair treads
ISO 14122-3 – Safety of machinery — Permanent means of access to machinery — Part 3: Stairs,
stepladders and guard-rails
prEN 16933-1 – Drain and sewer systems outside buildings - Design - Part 1: Layout principles (under
development)
SASO GSO EN 752 – Drain and sewer systems outside buildings
6. DESIGN GUIDELINES
General
6.1.1. Design consideration
Separate foul, trade effluent and surface water systems shall be provided.
Gravity foul sewer network design shall take account of the following considerations:
• Best use of available resources shall be made to ensure economy of design.
• Sewer depths shall be sufficient to accommodate not only all existing properties but also any future
properties likely to be erected within the area. In certain cases, the depth of basements may need
to be borne in mind.
• Where main sewers are laid at considerable depths, it may be more economical to lay shallow
rider sewers to receive the local house connections and to connect the riders at a small number of
convenient points into the main sewer.
• Consideration should be given to the likely form and method of construction as a consequence of
depth and other factors such as nature of ground, groundwater and the proximity of foundations,
services, etc.
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• Buoyancy of sewers shall be considered, and flotation of the pipe shall be prevented with
appropriate means where high groundwater conditions are anticipated. (See Design Guidelines –
Sewerage Structures – NEOM-NWA-TGD-2021-019 for structural design).
• All sewers shall be designed to prevent damage from superimposed live and dead loads. Proper
allowance for loads on the sewer shall be made because of soil and potential groundwater
conditions, as well as the width and depth of trench. Where necessary, special bedding, haunching
and initial backfill, concrete cradle, or other special construction shall be used to withstand
anticipated potential superimposed loading or loss of trench wall stability. (See Design Guidelines
– Sewerage Structures – NEOM-NWA-TGD-2021-019 for structural design).
• Sewers shall generally be kept as short as possible and unproductive lengths avoided.
• Sewer gradients shall be chosen to ensure velocities are high enough to prevent deposition of
solid matter in the invert. Gradients shall be maintained without sudden changes.
• Where a scheme is to be developed in phases, consideration shall be given to the likely flows
following the initial stages of construction so that self-cleansing velocities are attained at times of
peak flow each day.
• The route and depth of a new sewer shall take account of land where there is a possibility of future
development.
• Steep gradients/high velocities shall be avoided to reduce problems of turbulence and the
consequent gas/odour release and increased corrosion potential.
• Adequate access provision for maintenance, sewers shall be laid out in straight lines, as far as is
as practical.
• Consideration shall be given to such aspects as:
- The position of other existing or proposed services.
- The proximity of existing buildings and their foundations.
- The nature of the road construction.
• The impact of the construction of the sewer and subsequent maintenance activities upon road
users and the public.
• When areas are being improved or redeveloped the possibility of replacing the existing sewerage
system shall be considered with a view to its relocation to a more suitable layout.
• Septicity development shall be avoided as far as possible.
6.1.2. Site considerations
Prior to design, information on topography, belowground conditions, existing services, service
reservations, future development, etc. shall be collected.
The positions of all existing services shall be ascertained as accurately as possible and physically
checked by exploratory holes if necessary.
Ground investigation shall be considered in the light of the knowledge of site conditions already gained
and of the probable disposition and depths of excavation.
Hydraulic design
Hydraulic design of foul sewers shall be in accordance with BS EN 16933-2 [3].
Friction losses shall be calculated using the Colebrook-White method.
In foul sewers, a biofilm is created at the inner surface of the pipe which largely determine roughness
conditions, therefore, for all sewer material type, the Colebrook-White hydraulic roughness value, ks, shall
have a default value of 1.5 mm. If the designer wishes to use a different hydraulic roughness value in the
design, evidence for change shall be submitted to NEOM for approval. The NEOM wastewater engineer
will approve or reject the request and their decision is final.
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6.2.1. Design flow
Wastewater pipelines shall be designed so that there is sufficient capacity to convey the peak design
flow from the area they service without surcharge. The peak design flow provides an allowance in the
pipe network sizing calculations to ensure that the network has capacity for less-frequent, but particularly
large spikes, in dry weather flow that can occur from time-to-time.
The basis of gravity-effluent network design is the assumption that cleansing velocity is achieved once
per day within a network. Continual cleansing is not necessary and cannot generally be achieved due to
the solids within the effluent.
6.2.1.1. Less than 500 persons contributing population
The peak design flow rate from tributary areas with contributing population less than 500 persons shall
be calculated in accordance with BS EN 12056-2 System II [2] as recommended in BS EN 16933-2 [3].
For commercial developments, domestic design flow shall be calculated in accordance with BS EN
12056-2 [2] System II and BS EN 16933-2 [3] and for industrial developments trade effluent design flow
derives from metered discharge from similar premises.
6.2.1.2. Contributing population equal to or greater than 500 persons
In areas with contributing population equal to or greater than 500 persons and up to 90,000 persons,
based on water consumption usage a peaking factor shall be applied. Annex A provides indicative values
of domestic and commercial water demand. Design flow calculation shall consider 10% infiltration
allowance in non-coastal areas and 20% in coastal areas.
In areas with light and heavy industries may not be feasible to establish guidelines for wastewater
generation (being industries with extensive water use for industrial production). In those circumstances,
the designer shall interview the industrial user or look at historic data of water use in similar industries.
It is anticipated that 90% of water usage returns to the sewer system.
The peak design flow rate shall be calculated by applying a peaking factor for all average sewerage flows
from a known or assumed tributary population. The peaking factor (PF) is based on the Babbitt Formula,
as follows:
𝐹=
5
1
[(𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛) × 10−3 ] ⁄6
Foul sewers and drains should be designed to operate at no more than 75% of pipe full capacity.
6.3
Minimum pipe size
Minimum nominal internal diameter for foul sewers should be:
•
•
•
100mm for pipes serving more that 10 dwelling units; and
150 mm for pipes serving more than 10 dwelling units; and
200 mm for primary trunk sewers.
Service connections (laterals) 200 mm and larger shall be made only at a manhole.
6.4
Layout & Access
6.4.1
Layout
Layout of foul sewers shall be in accordance with the guidelines set out in BS EN 752:2017 [1], 8.3,
Annex D and NA.6.
Note: A new standard prEN 16933-1 [4] is under development which will replace BS EN 752:2017 [1]
Annex D.
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6.4.2
Access points
6.4.2.1. General
The designer shall reference to NEOM-SP-AS_004_rev03 for the minimum requirements for design,
materials, fabrication, testing, inspection and delivery of the manholes, chambers, and other
miscellaneous appurtenances such as frame and covers, steps, ladders, vent shafts standard specific.
Inlet and outlet pipes shall be joined to the access point with a gasketed flexible watertight connection
that allows differential settlement of the pipe and access point wall to take place.
6.4.2.2. Provision
Access shall be provided in accordance with BS EN 752:2017 [1], D.2.3 and NA.6.4.2.
As minimum access points shall be provided:
•
•
•
•
•
•
•
•
at every change of alignment, gradient, pipe material or pipe size;
at the head of all sewers;
at the connection of two or more lines;
maximum every 100 m for manholes;
maximum every 45 m for inspection chambers;
where the depth from cover level to soffit is greater than 3 m (manhole);
where the diameter of the largest sewer is greater than or equal to 450 mm DN/ID (manhole); and
where the chamber contains equipment (e.g., penstock or flow control device) that will require
maintenance (manhole).
6.4.2.3. Dimensions and layout
The dimensions and layout of access points shall be in accordance with BS EN 752:2017 [1] NA.6.4.
6.4.2.4. Drop type
A drop pipe shall be provided for a sewer entering a manhole at an elevation of 600 mm or more above
the manhole invert. Where the difference in elevation between the incoming sewer and the manhole
invert is less than 600 mm, the invert shall be filleted to prevent solids deposition.
6.5
Structural design
Materials and structural design of gravity foul sewers and associated structures shall be in accordance
with Design Guidelines – Sewerage structures – NEOM-NWA-TGD-2021-019
7. DESIGN BEST PRACTICE
7.1. General
Drainage design should be considered at the early design stage and should take into account aspects
such as topography, geology, street layout, soil remediation and ecology in a holistic manner.
Design of the site should consider the risk of flooding that might occur as a result of blockages for which
purpose the ground levels around the site and flood routes shall be considered.
Where further future development is expected, it is unwise to select oversized pipes to address future
needs. This may lead to settling out of solids, long retention periods, blockages and build-up of septicity.
7.2. Burial depth
Sewers shall be laid sufficiently deep to receive sewage from the lowest floor level of adjacent
development. The minimum burial depth to the top of pipe shall be:
• 1.5 m – under roads and parking areas; and
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• 0.9 m – under other locations not specified above.
Where the minimum burial depth requirements cannot be met, then additional protection (such as full
concrete encasement for rigid pipes or the use of concrete protection slabs spanning the pipe trench for
flexible pipes) shall be provided. In special areas with heavy loading and highways specific protection
may be required based on structural evaluations and a design check shall be carried out.
For protected pipes, the minimum cover required to the top of pipe shall be 0.5 m.
In all cases, the pipe minimum and maximum depth should conform to the pipe manufacturers’
recommendations. However, designs shall be carried out to ensure that pipes are no deeper than 6
meters, unless specific written approval is provided by NEOM.
Minimum clearance required between the existing services is 0.3 m.
7.3. Hydraulic design
Sewers should be designed to be self-cleansed. The minimum flow velocity should be 0.75 m/s to achieve
a self-cleansing regime. Maximum flow velocity shall be 3.5 m/s.
Minimum self-cleansing flow 0.75 m/s shall be achieved at least once a day.
As an additional design parameter to the minimum velocity for gravity sewers, the recommended
minimum pipe slopes depending on the internal pipe diameter shall be deemed. To achieve the required
minimum velocity in sewer lines, pipes should be designed by observing the minimum gradients listed in
Table 7.1.
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Table 7.1 Recommended minimum slope of gravity foul sewers
Internal Diameter (mm)
Gradient (%)
100
1.25*
150
1.00**
200
0.50
250
0.40
300
0.30
350
0.24
400
0.20
450
0.18
500
0.15
550
0.14
600
0.12
700
0.10
Note:
*This should be increased to 2.5% where no WC’s are connected.
**At least 10 properties connected
In the case of upstream sewer sections where the flow could be too small to achieve the self-cleansing
flow of 0.75 m/s and the criteria for the pipe to be half full at peak flow, the pipe laid at minimum gradient
shall be deemed to satisfy the design requirements specified in Table 7.1 if the minimum gradient is
adopted provided that at least two industrial plots or six residences are connected. Calculations should
be submitted to NEOM confirming the likely actual flow. Consideration should be given to using smaller
diameter pipes for the initial run of pipe work.
It should be noted that it may be necessary to provide for flushing the sewers at the beginning of their
design life, in cases where velocities may be too low to secure self-cleansing.
7.3.1 Depth of flow
The following Table 7.2 shows the recommended depth of flow in gravity sewer lines. The ratio “d/D” is
the ratio of the flow depth (d) to the nominal pipe diameter (D). Where minimum diameters are not
achievable (such as at the heads of lines), then the actual estimated flows and depths shall be analysed
and cleansing velocity confirmed.
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Table 7.2 Minimum and maximum depth of flow in sewers at peak flows
Description
Trunk sewer lines
Main and lateral sewer lines
7.4
Maximum (d/D)
0.75
0.85
Sewer connections
Service connections (laterals) 200 mm diameter and larger shall be made only at manholes or inspection
chambers.
For service connections (laterals) less than 200 mm diameter the connections to main pipe should have
a vertical angle of no more than 60°.
For connections to sewers of nominal internal diameter of 300 mm or less, connections should be made
at a 45° angle, or 90° angle using curved square junctions. For sewers of nominal diameter greater than
300 mm:
• for connecting pipes of diameter less or equal than half the diameter of the sewer pipe - the
connection should be made using a preformed saddle fitting; and
• for connecting pipe of diameter greater than half the diameter of the sewer pipe - an access point
should be constructed.
Note: Connections with saddle fittings should be made in accordance with the manufacturer’s
instructions.
7.4.1 Property Connections
Where a connection from a property joins the main sewer an inspection chamber shall be provided as
near as possible to the point where the drain crosses the boundary of the property to mark the change
of ownership. Typical layout is shown in Figure 7.1 below.
Figure 7.1
Typical layout – Position of inspection chamber marking change of
ownership
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8 REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
BS EN 752:2017 – Drain and sewer systems outside buildings - sewer system management
BS EN 12056-2:2000 – Gravity drainage systems inside buildings. Sanitary pipework, layout
and calculation
BS EN 16933-2:2017 – Drain and sewer systems outside buildings - design. Hydraulic design
prEN 16933-1 – Drain and sewer systems outside buildings - Design - Part 1: Layout
principles (under development)
BS 4592 – Industrial type metal flooring, walkways and stair treads
ISO 14122-3:2016 – Safety of machinery — Permanent means of access to machinery —
Part 3: Stairs, stepladders and guard-rails
Design and Construction Guidance for foul and surface water sewers offered for adoption
under the Code for adoption agreements for water and sewerage companies operating wholly
or mainly in England (“the Code”).
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ANNEX A – DOMESTIC AND COMMERCIAL WATER DEMAND
Table A.1 below shows indicative average daily flows for various establishments (e.g., institutional,
recreational, commercial etc.). The values provided are for information only.
Table A.1 Average daily flows for various establishments
Use
Average daily flow
Domestic
250 L/capita
Mosque
10 L/worshipper
Small Business
7 L/m2
Shopping centre
37 L/m2
Restaurant
32 L/customer + 57 L/employee
Coffee house
22 L/customer + 57 L/employee
Hotel
200 L/bed
Kindergarten
76 L/pupil + 50 L/employee
Secondary School
95 L/pupil + 50 L/employee
Offices
9 L/employee
Hospital
200 L/bed
Sports Stadium
20 L/seat
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