DESIGN STANDARDS MANUAL
1. INTRODUCTION TO DESIGN STANDARDS MANUAL (DSM)
The Design Standards Manual is presented in a single PDF document comprising the following separate sections.
COVER SHEET GUIDANCE NOTES LIST OF CONTENTS
SECTION 1 GENERAL
SECTION 2 STORM WATER SYSTEM DESIGN
SECTION 3 SEWERAGE SYSTEM DESIGN
SECTION 4 SEWAGE TREATMENT PLANT DESIGN
SECTION 5 TREATED SEWAGE EFFLUENT SYSTEM DESIGN
SECTION 6 STANDARD AND TYPICAL DRAWINGS
The first issue for implementation of each section of the DSM will be at Revision 00. Subsequent revisions will be at 01, 02 etc. Future revisions to the DSM will be managed through annual review meetings when minor comments gained from experience of using the DSM and new technologies developed by the department/consultants will be incorporated in the DSM as discussed and agreed at the annual review meetings.
2. NAVIGATING THROUGH THE DSM PDF DOCUMENT
The DSM PDF document opens with the separate section bookmarks as identified above to the left of the screen and the DSM cover sheet at 100% magnification to the right of the screen.
The contents of each section are also bookmarked and are shown by clicking on the + sign to the left of the section bookmark. The contents can be removed by clicking on the – sign to the left of the section bookmark.
Clicking on a section bookmark or section contents bookmark will automatically take the user to that part of the DSM PDF document.
Navigation through the DSM PDF document can also be achieved by clicking on the underlined section number in the list of contents section of the document. This will take the user to the cover page of the section. By scrolling down to the table of contents page for the section and clicking on a clause within the table of contents the user will automatically be taken to that part of the DSM PDF document. To return to the list of contents click on the back to previous view arrow in the tool bar or use the document drop down menu.
The standards and typical drawings can also be accessed by clicking on the underlined drawing number in Section 6 of the DSM. To return to the DSM PDF document click on the back to previous view arrow in the tool bar or use the document drop down menu.
DESIGN STANDARDS MANUAL
LIST OF CONTENTS
Volume No. Title Revision
SECTION 1 GENERAL 00
SECTION 2 STORM WATER SYSTEM DESIGN 00
SECTION 3 SEWERAGE SYSTEM DESIGN 00
SECTION 4 SEWAGE TREATMENT PLANT DESIGN 00
SECTION 5 TREATED SEWAGE EFFLUENT SYSTEM
DESIGN
00
DESIGN STANDARDS MANUAL
SECTION 1
DOCUMENT CONTROL SHEET
Revision No. Date Revision Description / Purpose of Issue 00 Jan 2004 Updating of Design Standards Manual. 01 02 03 04 05 06 07 08 09 10
TABLE OF CONTENTS
COVER SHEET... 1
DOCUMENT CONTROL SHEET ... 2
TABLE OF CONTENTS ... 3
1.1 INTRODUCTION ... 5
1.1.1 SCOPE... 5
1.1.2 CONTENTS AND ARRANGEMENT... 5
1.2 RELATED DOCUMENTS... 5 1.3 MANUAL UPDATING... 5 1.4 DESIGN CONSIDERATIONS... 6 1.4.1 DESIGN LIFE... 6 1.4.2 DESIGN INFORMATION... 6 1.4.3 SITE INVESTIGATIONS ... 6 1.4.4 ENVIRONMENTAL IMPACT ... 6
1.4.5 CLASSIFICATION OF POTENTIALLY EXPLOSIVE AREAS ... 6
TABLE 1 – SOURCES OF HAZARDS... 7
TABLE 2 – AREA CLASSIFICATIONS... 9
1.4.6 FORMATION OF ODOROUS COMPOUNDS ... 14
TABLE 3 – ODOUR CONTROL GUIDELINES... 16
1.4.7 ENCLOSURES, COVERS AND ODOUR TREATMENT... 25
1.4.8 HEALTH AND SAFETY IN DESIGN... 27
1.4.9 VALUE MANAGEMENT AND VALUE ENGINEERING ... 28
1.4.10 COST CONSIDERATION & FINANCIAL EVALUATION ... 29
1.4.11 SPECIFICATIONS ... 31 1.4.12 DRAWINGS ... 31 1.4.13 STRUCTURAL DESIGN... 31 1.4.14 CONCRETE STRUCTURES ... 32 1.4.15 STEEL STRUCTURES... 32 1.4.16 DESIGN PRESENTATION... 33 1.5 MATERIALS... 33
APPENDIX 1 – CLIMATIC DATA... 35
APPENDIX 2 – TYPICAL SEWAGE ANALYSIS ... 36
APPENDIX 5 – TYPICAL TREATED SEWAGE EFFLUENT ANALYSIS ... 39
APPENDIX 6 – MATERIALS SELECTION ... 40
1. CONSTRUCTION MATERIALS ... 40 2. MATERIALS SELECTION... 40 3. PIPES ... 41 4. STRUCTURES... 50 5. MANHOLES ... 59 6. MANHOLE COVERS... 59
7. STEP-IRONS AND LADDERS ... 60
8. QUALITY CONTROL AND QUALITY ASSURANCE ... 61
1.1 INTRODUCTION
1.1.1 SCOPE
The Design Standards Manual (DSM) is for use by design consultants in carrying out the design of projects for the Sewerage Directorate. It presents guidelines for the design but it does not include design theories and methods of calculation but provides local practices and criteria to be adopted.
Where any deviation from these criteria is considered necessary by the designer, the Directorate shall be consulted and their approval obtained.
Copyright of the DSM in its current format is the property of the Directorate and it may not be reproduced in any format without express permission of the Directorate.
Use of the DSM does not absolve design consultants from their normal responsibilities. It is meant as a guide and should be used only by competent practitioners, with due diligence.
1.1.2 CONTENTS AND ARRANGEMENT
The DSM is divided into 6 separate sections as follows: • General.
• Storm Water System Design. • Sewerage System Design. • Sewage Treatment Plant Design.
• Treated Sewage Effluent System Design. • Standard and Typical Drawings.
1.2 RELATED DOCUMENTS
The Directorate’s companion documents to the DSM are: • Conditions of Engagement for Consulting Services. • Quality Management System.
• CAD Manual.
• Geotechnical Design Manual. • Construction Documents Manual.
• Irrigation and Landscape Re-engineering Manual. • Safety Manual.
• Operation and Maintenance Contracts Manual.
1.3 MANUAL UPDATING
working to the current issue. Any errors or omissions, or recommendations should be notified to the Directorate.
1.4 DESIGN CONSIDERATIONS
1.4.1 DESIGN LIFE
In general design life shall be as follows:
• Pipelines 50 – 60 years.
• Structures 25 – 30 years.
• Mechanical and Electrical Equipment 10 – 15 years. • Instrumentation 3 – 5 years. 1.4.2 DESIGN INFORMATION
Design information relating to: • Climatic Data.
• Typical Sewage Analysis. • Typical Ground Water Analysis. • Typical Potable Water Analysis. • Typical Treated Sewage Analysis.
is given in Appendices 1 to 5 at the end of this section of the DSM. 1.4.3 SITE INVESTIGATIONS
A description of the geology of Abu Dhabi and the requirements for site investigations is given in the Geotechnical Design Manual.
1.4.4 ENVIRONMENTAL IMPACT
The designer shall address the environmental impacts of projects in accordance with the relevant legislation.
1.4.5 CLASSIFICATION OF POTENTIALLY EXPLOSIVE AREAS
Classification of Potentially Explosive Atmospheres within sewerage systems and related operational processes are required to assess the risk of ignition in potentially explosive atmospheres and to remove or reduce them. A consistent and traceable approach shall therefore be made to each and every classification or ‘zoning’ exercise. This guidance note shall not be regarded as prescriptive, and is written to ensure that each zoning exercise complies with the relevant and current best industry engineering practice.
Each installation shall be considered individually taking into account the civil structure and the proximity of other structures and plant. Consideration shall also
be given to the consequences of an explosion when determining the subsequent classification.
Reference should be made to the harmonised standard BS EN 60010, IEC 79-10:1996 supersedes BS 5345 Part 2 which has been withdrawn. The classification and definitions of zones can be found in BS EN 60079-10
The design process shall attempt to remove or reduce the need for hazardous areas.
Guidance as to the definitions of hazardous area zones is set out in BS EN 60079.
In principle the classification of an area shall include the consideration of sources of hazards i.e. all potential releases of flammable substances. In the water industry the most common sources have been identified in Table 1 below.
TABLE 1 – SOURCES OF HAZARDS
Flammable Material Source Density Lower
Explosive Limit Petrol/Hydrocarbons Petrol station spillage into sewerage
system (Petrol tanker spillage not considered significant)
Other flammable liquids from industrial sources
Heavier than air
1.0%
Methane Infiltration from leaking gas mains Cold digestion in poorly designed sewerage system
Biogas production in digesters Geological infiltration
Lighter than air 5.3%
Hydrogen By-product of electrochlorination (OSEC) plants
Lighter than air 4.0%
Hydrogen sulphide Sewers Heavier than
air
4.3% Dust Sludge dryers and pelletisers
Powdered activated carbon (PAC) in water treatment plants
Gases and vapours are only potentially explosive when mixed with air in certain quantities. Concentrations below the Lower Explosive Limit (LEL) or above the Upper Explosive Limit (UEL) are not potentially explosive. For the purposes of this guidance, the terms ‘flammable’ and ‘explosive’ shall be considered synonymous. For an explosion to occur there must be a source of ignition. The most common sources have been recognised as follows:
• Electric arcing • Hot surfaces • Flames
• Friction and sparking from mechanical equipment and ferrous tools, manhole covers etc.
• Electrostatic discharges • Spontaneous ignition.
The classification tables listed below include common items of plant relating to surface water and waste water treatment.
The default zone classification may not necessarily be correct for every zoning exercise. Consideration shall always be given to site specific ventilation, structures and businesses that discharge (or could potentially discharge) chemicals into the sewerage system, which may change the extent of the zone or increase its severity.
TABLE 2 – AREA CLASSIFICATIONS
Item Plant / Process Area Classification Remarks
Equipment
Non-Zone 0 Zone 1 Zone 2 Hazardous 1.0 Sewerage & Storm Water
1.1 Sewers
1.1.1 Sewer !!!! Zone 1 unless solely used for domestic sewage with a low risk of flammable substance contamination
1.1.2 Manhole !!!!
1.1.3 Chamber !!!! !!!!
1.1.4 Outfall !!!! Consider flap valve
1.2 Sewer Vent Areas
1.2.1 Vent Stack !!!! !!!! 1.2.2 Air Valve !!!! !!!!
1.3 Pumping Stations
1.3.1 Wet Well !!!! !!!! Wet wells are Zone 1 or Zone 2 up to coping level. Areas above coping are deemed non-hazardous if open to atmosphere
1.3.2 Pumping Main !!!! 1.3.3 Enclosure Above Wet
Well (enclosed)
! ! !
! !!!! !!!! Similar to Enclosed Channels, ventilation dependent
1.3.4 Dry Well !!!!
1.3.5 Valve Chamber !!!! !!!! !!!! Unzoned if sealed from wet well
1.3.6 Interconnecting Paths !!!! The area between 2 sets of doors between wet well and dry well is Zone 2
Item Plant / Process Area Classification Remarks
Equipment
Non-Zone 0 Zone 1 Zone 2 Hazardous 2.0 Sewage
2.1 Preliminary Treatment
2.1.1 Sewage P.S !!!! !!!! Below coping level is Zone 1 or Zone 2 depending upon ventilation, above coping is non-hazardous if open to atmosphere
2.1.2 Screens Forebay !!!! !!!!
2.1.3 Odour Control !!!! !!!! The zoning of any ducting depends upon amount of dilution of air. Ventilation calculations required
2.1.4 Open Channels !!!! Below Coping 2.1.5 Enclosed Inlet Works !!!! !!!! !!!!
2.1.6 Screen Houses (covered)
! ! !
! !!!! !!!! Depends upon ventilation
2.1.7 Storm Tanks !!!! Storm first flush may be a source of hazard
2.2 Primary Treatment
2.2.2 Low Lift P.S. !!!! !!!! Below Coping is Zone 1 or Zone 2 depending upon ventilation
2.2.3 PST Distribution !!!! !!!! Below Coping is Zone 1 or Zone 2 depending upon ventilation 2.2.4 Primary Settlement !!!! 2.3 Secondary Treatment 2.3.1 SBRs !!!! 2.3.2 Aeration Blowers !!!! 2.3.3 Aeration Lanes/Tanks !!!! 2.3.4 Anoxic Lane !!!! 2.3.5 RAS/SAS !!!!
Item Plant / Process Area Classification Remarks
Equipment
Non-Zone 0 Zone 1 Zone 2 Hazardous
2.3.6 Biological Filters !!!! 2.3.7 Final Effluent !!!! 2.3.8 Humus Tanks !!!! 2.3.9 Final Settlement !!!! 2.4 Sludge Handling 2.4.1 Primary Sludge Transfer ! ! ! ! 2.4.2 Surplus Sludge Transfer ! ! ! ! 2.4.3 Raw Sludge Tanks !!!! 2.4.4 Digested Sludge
Storage tanks
! !!
! !!!! !!!! Open topped tank is non hazardous, however beware junction boxes etc. below coping
2.4.5 Centrifuges !!!! Indoor centrifuge locations shall be ventilated 2.4.6 Thickening Plant !!!!
2.4.7 Digesters !!!! !!!!
2.4.8 Dryers !!!! !!!! Review manufacturers risk assessment 2.4.9 Pelletisers !!!! with regard to hazardous areas
2.4.10 Bagging Plant !!!! caused by presence of combustible dusts 2.4.11 Gassifiers !!!! !!!!
2.4.12 Gas Holders !!!! !!!!
2.4.13 Flare stack !!!! !!!! Similar to vent stack when unlit, also consider this as a source of ignition
2.5 Tertiary Treatment
Item Plant / Process Area Classification Remarks
Equipment
Non-Zone 0 Zone 1 Zone 2 Hazardous 3.0 Water Treatment
3.1 Electrochlorination !!!! Electrochlorination Plants generate H2, review
manufacturers risk assessment, ventilation required 3.2 Ammoniation !!!! Where possible store NH3 containers in open air, NH3
can react with other materials to form explosive compounds, keep away from Chlorine.
Potentially Explosive Atmosphere Area Classification
Project: …... Project No. …... Date: ………. File Ref.: ………... Sheet No. …...……….. Table Revision No. …...
Plant / Process Equipment Area Classification Remarks
Zone 0 Zone 1 Zone 2 Flammable Material; Source, Ventilation, Process Conditions, Reasons,
1.4.6 FORMATION OF ODOROUS COMPOUNDS
Odorous compounds only cause a problem if they are released to atmosphere and if there are residential areas close to the site which may be affected. To ensure that development does not encroach too near to a pumping station or treatment works and lead to odour related complaints, a new site should be selected so that ideally the boundary fence should be a minimum of 400m from the nearest residential property for small works and pumping stations, and 1km for large works and large pumping stations.
Odour problems are associated with the development of anaerobic conditions (septicity) in sewage or sludge resulting in the formation of a range of malodorous compounds by the action of bacteria. The main compound associated with sewage and sludge odours is hydrogen sulphide (H2S), which is also a toxic and corrosive gas.
The amount of H2S that can be formed is dependent on the strength of the sewage (or sludge) and the retention time under anaerobic conditions. Nutrient availability and the initial concentration of sulphate limit the maximum concentration that will develop. Saline intrusion increases the sulphate concentration of the sewage, which can increase the values of sulphide developing, especially in sludges.
In sludges, other compounds such as mercaptans, dimethyl sulphide and volatile fatty acids are also formed and may be as important as H2S in adding to the total odour. The resultant lowering of pH value in sludges in the presence of volatile fatty acids enhances the release of odours.
Anaerobic digestion reduces the volatile fatty acid content of the sludge with a consequent reduction in total odour and a reduction in the potential release of sulphide. However, the digester gas produced may contain up to 3000 parts per million (ppm) of hydrogen sulphide, which, unless treated, will have an odour impact.
Oxidation of H2S and the other malodorous products of septicity to less odorous compounds will occur during aeration in activated-sludge treatment or during aerobic digestion.
Odorous chemicals present in sewage or sludge cause a problem only when they are released to the atmosphere. This typically occurs at effluent discharge points and weirs where odour containing sewage or sludge is turbulent and there is good opportunity for odours to be transferred to the atmosphere. If the odorous compounds can be retained in solution, for example by retaining in pipes they will not cause nuisance.
H2S e can be smelt at a concentration of 0.5 parts per billion (ppb) under laboratory conditions (the threshold odour concentration). Nuisance concentrations are typically 5-10 times the threshold odour value.
H2S can cause corrosion of concrete and mortar fixtures when oxidised to sulphuric acid, e.g. on moist walls of sewers and manholes. Metal work and electrical equipment is vulnerable to H2S corrosion.
• Prevent/reduce the development of septicity. • Reduce the release of odours.
• Contain and treat odours.
• Locate odorous processes as far away from potential complainants as possible.
Specific guidelines for different stages in wastewater and sludge treatment are given in Table 3 below.
TABLE 3 – ODOUR CONTROL GUIDELINES
Process stage Minimum provision Enhanced provision Comment
Sewerage system • Use gravity system rather than rising mains
• Ensure adequate velocity to prevent deposition of grit and sediments
• Minimise turbulence, sharp bends and drops
• Ensure adequate ventilation of gravity sewers
• Minimise length of siphon sections
• Minimise length of rising main sections
• Seal manholes at discharge points
• Discharge at low levels to minimise turbulent drops
• Minimise retention time in sumps
• Ensure grit and screenings can be removed from sumps (e.g. good benching, access for pumping out)
• Chemical dosing
• Seal manholes
Pumping stations • Reduce the height of hydraulic drops into sumps
• Minimise operational volume of sumps
• Provide sufficient slopes and benching so that there is no accumulation of rags or sediments
• Provide OCU if identified
problem •
Pumping stations can be a source of odour release due to turbulence, and odour formation if sumps are oversized or if
Process stage Minimum provision Enhanced provision Comment • Allow intermittent drain down to clear
rags and sediments
• Where rags and screenings accumulate, include regular cleaning out in operational procedures
• Do not use screw pumps
• Avoid turbulence of flow in channels and at the discharge
• Cover wet well Inlet discharge – rising
main/septic •• Do not locate near sensitive boundary Minimise turbulence at discharge points, including at intermediate pumping stations and all downstream locations prior to secondary treatment stage
• Cover channels, sumps, detritors, screens receiving pumped sewage
• Ensure materials below covers are resistant to sulphide/sulphate attack
• Chemical dosing to upstream sewerage system, nitrate salts, or STW, iron salts
• Minimise turbulence of discharge
• Cover channels and sumps
• Ensure materials below covers resistant to sulphide/sulphate attack
• Vent from below covers to OCU
• Consider using gravity sewerage system with lift stations rather than long rising mains
• Sewage can become very septic in rising main sewers with consequent impact on odours at intermediate pumping stations and the discharge point. Sulphide will also cause corrosion and will pose a health and safety risk to workers
Process stage Minimum provision Enhanced provision Comment
Inlet discharge - gravity • Install away from sensitive boundary
• Avoid cascades and other areas of turbulence
• Keep channels non turbulent, minimise bends
• Ensure liquor/returned storm sewage/imported wastes discharged at low level to reduce splashing
• Minimise turbulence of discharge
• Covers and OCU
• Sewage smells even when fresh and draws air along the sewer which may be unpleasant. Turbulence exacerbates release of odour
Imported wastes and
sludges • Discharge at low level to covered sumpor use close coupling
• Locate tanker discharge point away from sensitive boundary
• Treat displaced air in OCU
• Connect tanker vents to OCU if air mixing employed
• Imported wastes are generally odorous
Grit removal • Do not select aerated grit channels
• Ensure grit is washed •
Do not select aerated grit channels
• Cover unit
• Ensure grit is washed
• Enclose grit conveyor and classifier
• Aerated grit channels can lead to a significant release of odours
Inlet screens and
screenings handling • Provide local covers and minimiseturbulence as far as possible
• Ensure materials below covers are resistant to sulphide/sulphate attack
• Wash screenings
• House screens in a building actively vented to OCU
• Provide local covers and minimise turbulence as far as possible
Process stage Minimum provision Enhanced provision Comment • Ensure materials below covers
are resistant to
sulphide/sulphate attack
• Provide a gas alarm system as high levels of H2S could
accumulate if ventilation system fails
• Discharge washed screenings to enclosed skips
• Do not store on site Inlet channels • Ensure a reasonable slope so that there
is no grit deposition but not so much that there is turbulence
• Avoid drops and sharp bends
• Minimise height of discharges for example of return liquors, to reduce splashing
• Cover
• Ensure materials below covers are resistant to
sulphide/sulphate attack
Storm/balance tanks • Ensure overflow weir is upstream of any liquors or tanker discharge
• Discharge to base of storage tank to minimise splashing
• Use an effective cleaning system such as rotating jets. Operation should be stopped when the jet is exposed
• Use an automatic system of return
• Discharges into tanks release odours unless at low level. The impact is increased if the sewage discharging to the tank contains odorous wastes or liquors
Process stage Minimum provision Enhanced provision Comment • Design to ensure tank and associated
channels and pipelines can be
completely drained of sewage, sludges, sediments and debris
• Use an effective cleaning system such as rotating jets
• Use an automatic system of return
• Return storm/balanced flows downstream of the overflow weir and are at low level in the channel to minimise splashing
• Ensure associated feed and return channels can drain back
• Cover with air displaced during filling vented to odour control
• Storm/balance tanks can cause problems if sludges accumulate or if sewage is retained for excessive periods
• Cleaning is important, but jet cleaners can cause odour release when the jet is exposed
Primary tanks • Provide close-coupled pumped desludging to avoid exposure of sludges to the atmosphere
• Desludge frequently and remove sludges at a low concentration to avoid excessive retention
• Design arrangement so that tanks can be removed from operation at times of low flow to avoid excessive retention of sewage
• Minimise the height of drop over weirs to reduce splashing
• Design without a primary sedimentation stage or
• Provide covers vented to odour control. Ensure materials below covers are resistant to
sulphide/sulphate attack
• Primary sedimentation is a very odorous stage
allowing septicity to develop in sewage and sludges if retained for excessive periods with release mainly at PST weirs and
downstream channels and from sludge withdrawal handling and treatment
Process stage Minimum provision Enhanced provision Comment
Lamella separators • Minimise need for manual cleaning of plates
• Do not select systems that incorporate sludge thickening within the unit
• Provide covers vented to odour control. Ensure materials below covers are resistant to
sulphide/sulphate attack
• The level of septicity and odours developing is in proportion to the retention time Activated sludge/ membrane Bioreactors/ sequencing batch reactors
• Ensure adequate aeration and mixing
• Fine bubble aeration systems are preferred to mechanical surface aeration systems
• Use submerged or non-turbulent inlet and outlet arrangements
• Cover distribution chambers, inlet channels and anoxic zone areas
• Minimise the loading rate
• At normal loadings, activated sludge has a low odour level, decreasing as the loading rate decreases
• FBDA systems release less aerosol and odours than mechanical surface aerators. There also is less risk of septic pockets developing
Conventional biological
filters •• Ensure operating correctly Minimise the height of drop between distributor and media surface
• Use recirculation if signs of ponding
• Ensure adequate ventilation
• Can be a cause of odours if overloaded and ponding
Submerged biological aerated filters fixed or fluidised media
• Fluidised media preferred to fixed media
• Avoid turbulence at inlet and during backwashing
• Cover and vent to odour control • Septic areas can develop, particularly in fixed media systems
Process stage Minimum provision Enhanced provision Comment
High rate biological
filters • Cover and vent filter and effluent sumpto odour treatment system. Draw air from the base of the filter
• Do not co-settle sludge
• Replace with an alternative system or
• Cover and vent by drawing air down to the base i.e. in the same direction and the sewage flow. Treat the vented air
• Ensure materials below covers are resistant to
sulphide/sulphate attack
• Can be a significant source of odours due to the
development of thick biofilms with release of odours from the top of the filter in the ventilation
Final sedimentation, tertiary sand filter, UV treatment
• Recycle backwash waters from sand filters without storage
Picket fence thickeners
and raw sludge storage • Cover and vent tanks to OCU, passivemay be sufficient. Toxic levels of hydrogen sulphide will develop below covers
• Site away from sensitive boundary
• Non-turbulent low-level inlet, outlet and supernatant discharge
• Locate motors for mixers outside tanks, use external pumps
• Mix at low, rather than high, speed
• Minimise the number of times that sludge is handled before thickening
• Replace with mechanical thickeners
• Active venting to OCU
• Odours in sludges and sludge liquor strength increase with storage
• PFTs can be a significant source of odour formation with release of odours from: the surface of the PFT, the overflow weir, the sludge liquor drainage system and from
subsequent handling of the sludge
Process stage Minimum provision Enhanced provision Comment • Minimise retention time prior to
thickening, digestion and dewatering stages
Secondary sludge
storage •• Minimise retention prior to thickening Aeration may be used to maintain condition of sludge
• Cover tank, vent to OCU • Biological sludges are odorous if they become anaerobic
Mechanical sludge thickening and dewatering
• Ensure that there is more than sufficient capacity, including standby, so that raw sludge does not back up in the system
• Minimise turbulence of liquor discharge e.g. below belts, into sludge liquor system
• Enclose, vent covers to OCU
• Minimise retention time of raw or secondary sludges prior to thickening, treatment and dewatering stages
• Locally enclose and actively vent to OCU
• A building may be required
Sludge liquors • Discharge at level to reduce odour emission
• Balance flow and composition
• Return to secondary treatment, not primary or inlet, if imported sludges on site
• Chemical dosing e.g. using permanganate or iron salts, may be used to reduce sulphide release
Process stage Minimum provision Enhanced provision Comment
Anaerobic sludge
digestion • Ensure that there is more than sufficientcapacity
• Cover tanks, feed, mixing and take-off points
• Ensure the gas handling system is fully operational. Whessoe valves, gas storage flare stack, CHP units and/or gas engines
• If gas is not required for heating or engines, it should be flared
• Chemical dosing of sludge with iron salts to reduce sulphide level in off-gas
• Capacity is required to prevent the risk of sludge backing up in the system causing upstream odour problems
• Digester gases can contain significant levels of H2S
which is oxidised by flaring or burning
Aerobic digestion • Ensure that there is more than sufficient capacity
• Cover feed, mixing and take-off points
• Cover tanks and ventilate to
OCU •
Odours will be released during aeration of raw and secondary sludges.
Thermal treatment
processes and drying • Odour control, possibly by thermaloxidation
• Tall stack
• Volatilisation of a range of organic compounds may occur to due the high temperature
1.4.7 ENCLOSURES, COVERS AND ODOUR TREATMENT
In some instances covers, or an enclosed building, to contain and collect odours will be the only way to ensure that odour release can be controlled. If processes are enclosed within a building, additional local covering is likely to be necessary to ensure that the working atmosphere is safe. Processes that are commonly provided with local covering are:
• Inlet works (may also be within a building). • High rate filters.
• Sludge storage tanks.
• Sludge thickening and dewatering processes (may also be within a building).
• Sludge liquor sumps. • Sludge import facilities.
Provision of covers will create a confined space where high concentrations of potentially hazardous gases may develop, requiring appropriate measures in terms of zoning (including for ventilation fans and/or odour treatment) and personnel access. Fan assisted ventilation may be needed to:
• Convey odours to an odour treatment system.
• Prevent the accumulation of high levels of odours that could be displaced during operations.
• Reduce the level of corrosion below covers. • Reduce condensation and consequent corrosion.
• Prevent the accumulation of high levels of potentially hazardous chemicals. • Ensure that working conditions meet Health and Safety requirements.
• Choice of materials for covers will need to take into account: strength and thickness, durability, weight, cost, aesthetics, supplier and operational requirements. Covers must be resistant to corrosion, both from external forces such as weathering and UV radiation, as well as internal chemical attack due to the hydrogen sulphide, sulphuric acid or organic acids below covers.
• Fibre reinforced plastic (with appropriate choice of resin, UV absorbers and light stabilisers) and aluminium are commonly used. Vinyl ester resin is considered to have excellent corrosion resistant properties. Aluminium with the correct choice of alloy is also corrosion resistant, although susceptible to corrosion if splashed with sewage.
• Covers should withstand wind loadings and static loads.
• Materials for covers and supports, and any equipment below the cover should be resistant to corrosion. Where possible motors etc should be located outside the cover.
• Suitable platform access and walkways should be provided to any equipment. In general facilities to allow access of personnel onto covers should not be provided.
• Inspection and access hatches will be required for repair and maintenance purposes. Alternatively cover sections may be designed to be removable. • Where possible, design should be such that equipment below covers can be
easily and quickly removed to minimise time when covers need to be opened.
• Covers should be sealed as far as possible. Inspection /access hatches should be sufficiently durable so that they continue to be effectively sealed for the design life of a piece of plant.
• Overflow and discharge pipes should be designed and constructed to prevent a route for air under covers being discharged to the atmosphere. All buildings containing sewage or sludge processes will need some form of ventilation to avoid build up of potentially hazardous (explosive or toxic) atmospheres. Where housing is close to the STW, this ventilation air will require odour treatment.
Design of the ventilation and odour control system may need to take in to account the handling of potentially hazardous gases, and the zone requirements of the area in which it is installed.
Odour releasing units (such as screens or belt presses) within a building should be locally enclosed, and a proportion of the required ventilation air drawn from the body of the building towards the odorous unit to ensure odours do not escape into the body of the building.
The siting of stacks and emergency vents should be away from potential complainants.
The choice of odour treatment process and the number of treatment stages depends on:
• Flow rate of air to be treated.
• The strength and composition of the incoming air and whether intermittent or continuous.
• The percentage removal required (the standard of odour treatment required to avoid an odour problem can be derived from odour dispersion modelling). • Space availability and zoning requirements of proposed location.
Design of odour control unit, ductwork, chemical storage and associated equipment should take into account expected temperatures:
• 25-35oC sewage. • 0-50oC ambient.
• 85oC maximum radiating temperature (surfaces).
• up to 30oC air vented from the sewerage system to an OCU. Preferred odour treatment technology is:
• Wet chemical scrubbing employing as a minimum single stage treatment using alkali/oxidant such as sodium hypochlorite with sodium hydroxide. • Polishing treatment or treatment at small or remote sources (such as
pumping stations) using activated carbon. In most cases carbon regenerated using alkali (caustic soda or potash) is preferred.
Post treatment of vented air or lightly odorous air by ducting to the activated sludge process should be considered.
Post treatment of ventilation air for example using carbon should be considered at sensitive sites and at pumping stations.
The same technology should be used throughout a site, for ease of operation. At existing sites, existing technology should be duplicated.
Several sources should be combined to a single odour control unit, possibly providing more than one stage of treatment.
Odour control equipment should be designed to remove the range of odorous compounds expected. The factors influencing the treatment process, the number of units provided on a site and the number of stages of treatment would be:
• Odours from buildings housing sewage processes (e.g. screening) will include:
a) Hydrogen sulphide (typically up to 10 ppm in ventilation air).
b) Lower concentrations (typically less than 1ppm) of other sulphurous and nitrogen compounds (such as ammonia).
c) Trace levels of solvent type odours.
d) Odours from below vented covers could be ten times these values. Odours below unvented covers could be one hundred times these values.
• Odours from buildings housing sludge processes will include: a) Hydrogen sulphide (typically 3 to 50 ppm in ventilation air). b) Mercaptans (typically 3 to 50 ppm in ventilation air).
c) Dimethyl sulphide (typically 3 to 50 ppm in ventilation air) and similar organic sulphides.
d) Ammonia if handling digested sludge/sludge liquors or if lime addition employed.
e) Polyelectrolyte breakdown products (amines).
f) Odours from below the covers of vented sludge storage tanks could contain ten times the above concentrations. Concentrations below the covers of un-vented tanks could be one hundred times these levels (toxic levels).
The designer must be aware of all his responsibilities in the design and detailing of a project and shall:
• Ensure that any design he prepares and which he is aware will be used for the purposes of construction work includes among the design considerations adequate regard to the need:
a) To avoid foreseeable risks to the health and safety of any person at work carrying out construction work or cleaning work in or on the structure at any time, or of any person who may be affected by the work of such a person at work.
b) To combat at source risks to the health and safety of any person carrying out construction work in or on the structure at any time, or of any person who may be affected by the work of such a person at work.
c) To give priority to measures which will protect all persons at work who may carry out construction work or cleaning work at any time and all persons who may be affected by the work of such persons at work over measures which only protect each person carrying out such work. • Ensure that the design includes adequate information about any aspects of the project or structure or materials (including articles or substances) which might affect the health and safety of any person at work carrying out construction work or cleaning work in or on the structure at any time or of any person who may be affected by the work of such a person at work. • Co-operate with the planning supervisor and with any other designer who is
preparing any design in connection with the same project or structure so far as is necessary to enable each of them to comply with the requirements and prohibitions placed on him in relation to the project by or under the relevant statutory provisions.
1.4.9 VALUE MANAGEMENT AND VALUE ENGINEERING
Value management (VM) can be defined as “A service which maximises the functional value of a project by managing its development from concept to completion and commissioning through the examination of all decisions against a pre-defined value system.”
Value engineering is defined as the “The application of VM techniques within the design process”.
The principles of value management are: • Agreeing clear objectives.
• Agreeing value criteria.
• Ensuring that they are understood by all parties. • Generating ideas for options.
• Identifying value enhancements on the selected option. Value = Satisfaction of needs / Resources used. Value management:
• Provides a structured approach to decision making. • Provides a common focus on value and objectives. • Limits misunderstandings & misinterpretations.
• Delivers cost benefits by eliminating unnecessary work.
• Increases team building, shared knowledge and understanding.
• Ensures that the project outcome will correspond to the Client’s needs and aspirations.
• Enhances the value of projects.
Value management reviews should be carried out throughout the project life cycle, as set out in the value management plan, and the number required will depend on the project complexity. The reviews should generally follow the following sequence:
• VM1 Project definition.
• VM2 Concept design.
• VE1 Preliminary design and engineering.
• VE2 a,b,c Detailed design.
• VM3 Procurement and contract strategy.
• VM4 Post project feedback.
The ability to add value is at it’s highest during the early stages of a project and reduces rapidly as decisions are taken and work implemented.
The cost of adding value is at it’s lowest at the outset but increases rapidly as the project progresses. The aim should be to focus on the 20% of the project that accounts for 80% of the total project cost.
Value management should be carried out through structured value engineering workshops, as well as being an integral part of the day to day design development. Formal workshops should be run by a trained facilitator and include all stakeholders including the design team; the client project team; the operations and maintenance teams who will operate and maintain the works on project completion.
1.4.10 COST CONSIDERATION & FINANCIAL EVALUATION
Robustness and redundancy is an essential part of the design of works as obtaining spare parts at a later date can be problematic. Minimising capital expenditure resulting in a works that will require a lot of maintenance is not what is required.
Civil works cost estimates may be built up using local rates and allowing further margins for overhead and profit of any overseas involvement.
Major mechanical and electrical equipment cost estimates can be obtained from international suppliers of equipment. Allowances must be made for shipping costs, installation, overhead and profit and local agents’ costs. These can more than double the base price.
Cost estimates should also allow for the consideration of: • Project complexity.
• Levels of competition.
• Current and international workload. • Unusual project scope.
• Operations expenditure.
Operations expenditure (Opex) covers the following aspects: • Labour.
• Power. • Chemicals. • Sludge disposal.
• Maintenance and spares.
Currently labour costs are generally low therefore high manning levels are acceptable.
Chemicals can be difficult to obtain and can be expensive. They will be required for certain processes but if their use can be avoided it is desirable.
Maintenance costs should be based on 1% of the capital value of the plant costs. Net Present Value
Net present value (NPV), or discounted cash flow (DCF), calculations are a method of comparing capital and operations costs over a period to determine which has the lowest overall value. In essence all costs are reduced back to present day prices.
Capital costs for expenditure in the first year, year 0, are the actual costs whereas costs for future capital expenditure e.g. phased construction or replacement of plant are represented by the sum which invested now would build up to the capital sum needed in the future.
Operations costs are represented by the present day sum that invested now will enable the annual running costs to be paid and reduce to zero at the end of the term.
A discount rate is chosen on which to base the assessment. Normally this is between 3%-7%. A sensitivity analysis can be done at different discount rates if required. Inflation need not be considered as all sums are reduced to present day values.
The period of the NPV calculations should be at least 20 years. Replacement of items of computer hardware should be allowed for every 5 years, machinery 15 years and civil structures 30 years. In all but the most sensitive calculations there is no need to consider residual values i.e. the remaining value of the item at the end of the term under consideration.
The NPV may be calculated according to the following equation: NPV = Cost/(1-r)n
Where n= number of years and r = discount rate 1.4.11 SPECIFICATIONS
The basic specifications for use on projects are the General Specification for Civil Works and the General Specification for Mechanical and Electrical Works. Where used in contract documents they shall remain unaltered and may be referred to without the need to incorporate as hard copies into all documents.
1.4.12 DRAWINGS
A complete list of standard and typical drawings is given in Section 6 of the DSM. The standard drawings should be used in their original format without alterations. Where used in contract documents their numbers shall remain unaltered and may be referred to without the need to incorporate as hard copies into all documents. Typical drawings are presented as an indication of standard format and quality. These may be used as the basis of individual contract drawings but must be renumbered and edited accordingly for specific projects.
1.4.13 STRUCTURAL DESIGN
Structural design calculations shall be submitted to the Municipal Engineer’s Department of Abu Dhabi Municipality for approval.
Design should be generally in accordance with their publication “Building Regulations & Recommendations for Structural Design & Concrete Practices”. The structural design submission shall include a separate design information sheet which contains the following:
• Imposed loadings.
• Clear cover to main reinforcement. • Concrete properties.
• Protective methods used for concrete. • Reinforcement properties and coating.
• Safe allowable bearing capacity of soil, soil report to be attached. • Pile foundation arrangement where appropriate.
• Types of structures. • Dewatering requirements. • Concrete curing methods. • Formwork removal notes. 1.4.14 CONCRETE STRUCTURES
Calculations should satisfy the requirements of ACI 318-63 or ACI 318-83, BS 8007 or BS 8110 or any equivalent and acceptable international code of practice. For serviceability limit state the following apply:
• Partial safety factor for all loads is 1. • Factor of safety against flotation is 1.1. • Design crack width is 0.2mm.
• Liquid level to be the working top water level.
• Allowable steel stress in direct or flexural tension is 130N/mm2. For ultimate limit state the following apply:
• Partial safety factor for earth and water pressure is 1.4.
• Allowable anchorage bond stress is 1.6N/mm2 and 2N/mm2 compression. • Maximum sheer stress is 4.75N/mm2.
Other principal factors are:
• Characteristic strength of concrete is 40N/mm2.
• Yield strength of steel is 460N/mm2 for high yield defined bars.
• Minimum reinforcement is 0.35% of the cross section in each direction and in both faces.
• Maximum bar spacing is 300mm or the thickness of the section. 1.4.15 STEEL STRUCTURES
In general the design of structural steelwork shall be in accordance with AISC -Manual of Steel Construction or BS 5950 or other equivalent and acceptable international standard.
Both the working stress and the ultimate stress methods of calculation are acceptable but it should be in accordance with the recognised standard.
Steel should be A36 to ASTM & AISC and grade 43 to BS.
Consideration must be given to fire and corrosion protection and appropriate methods applied.
1.4.16 DESIGN PRESENTATION
All calculations are to be presented on standard A4 size calculation sheets. All information contained on the sheets is to be printed and the title blocks are to be filled in completely. All pages are to be numbered and sketches used as required to clarify the calculations. All assumptions, references, units and calculations are to be clearly stated. The originals of all calculations are to be indexed and bound for submittal.
Drawing format as specified in the CAD Manual shall be adopted for all design projects.
All drawings are to be signed by a professional engineer and two initials of the draughter, designer and checker must be included as appropriate in the title block.
All design dimensions shall be expressed in metric units only.
Drawings should generally be presented in the following arrangement: • Cover sheet.
• Index of drawings. • Location plan. • Project drawings. • Standard drawings.
The Consultant has total responsibility for the accuracy and completeness of the plans, calculations and related documents as required under the scope of work. Prior to final design submittal, the Consultant is expected to perform an internal quality control review carried out by engineers experienced in the appropriate disciplines to ensure a product of neat appearance, technically and grammatically correct and checked and signed by the draughter, designer and checker where appropriate.
1.5 MATERIALS
Materials shall be chosen which result in the least maintenance and are not prone to decay by weathering or corrosion causing structural deterioration, leakage and infiltration.
Established International Standards and guides such as ASTM, BS, EN, ISO and WIS should be followed in the selection of and specification for construction materials. Ideally the material product should be covered by an established ISO 9000 Quality Control system and wherever possible a third party quality assurance scheme.
A discussion on materials selection is given in Appendix 6 for the designers information.
APPENDIX 1 – CLIMATIC DATA
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Temperature Degree C
Mean Dry Bulb for Month 19.8 20.7 23.1 27.1 29.4 32.0 30.9 33.5 30.9 28.9 25.3 22.0 Daily Ave. Max. 24.4 25.3 28.5 33.4 36.7 38.5 41.3 39.2 37.3 35.7 30.1 26.3 Daily Ave. Min. 15.2 14.5 19.6 22.0 23.6 27.4 31.1 29.8 26.7 23.0 20.3 16.9 Absolute Max. 30.1 32.7 39.8 41.7 41.9 44.6 44.8 45.5 43.0 39.6 36.7 31.3 Absolute Min. 12.0 10.5 13.2 15.6 18.6 24.5 26.5 26.0 24.7 21.0 15.4 13.3 Ground Min. Daily Ave. 13.2 13.0 15.7 19.7 21.3 25.6 30.2 28.9 24.9 21.0 18.7 15.1 Absolute Ground Min. 9.0 8.3 11.6 13.3 15.7 22.8 24.5 24.6 21.8 17.8 13.9 10.4 Earth 50cms. Daily Ave. 25.2 24.4 25.8 29.4 31.5 34.7 36.8 36.8 36.5 34.1 30.8 27.5 Earth 100cms. Daily Ave. 26.8 25.7 26.1 28.6 30.5 33.2 34.9 34.9 35.8 34.2 31.8 29.0
Relative Humidity %
Mean RH for Month 68 65 56 50 55 58 57 66 67 65 63 68
Daily Ave. Max. 83 88 82 71 79 80 79.3 84.7 85 86 78 85
Absolute Max. 100 100 100 84 97 91 92 92 94 100 9 10
Daily Ave. Min. 51 43 36 28 28 33 32.8 42.2 41 29 45 5
Absolute Min. 37 15 10 13 11 17 17 13 13 16 1 27
Wind Speed (46' above ground) Knots
Mean Wind for Month 7.2 6.5 7.4 8.2 7.3 8.5 8.9 8.8 7.9 7.3 8.1 6.9
Absolute Max. (for at 24.0 32 31 27 25 27 26 25 22 19 23 32
least 10 mins)
Highest Gust 30.0 45 39 39 32 33 30 34 30 25 29 30
Precipitation (Rainfall) mm
Total Amount Nil 20.1 0.8 2.7 TR Nil TR Nil Nil Nil Nil 0.3
Max. for any one day Nil 10.5 0.7 2.1 TR Nil TR Nil Nil Nil Nil 0.3
No. of days with Rain Nil 5 2 4 2 Nil 3 Nil Nil Nil Nil 1
Atmospheric Pressure mbs (MSL)
Mean for Month 1019 1018 1014 1010 1006 999 997 997 1005 1012 1016 1019
Cloudiness-Oktas (Eighths of Sky)
Total Cloud-Mean for Month
1.8 2.6 1.8 2.7 0.6 0.8 2.1 1.4 0.8 0.3 1.0 1.6
Low Cloud (8000) - Do 1.4 1.5 0.8 0.5 0.1 Nil 0.3 0.2 0.4 0.3 0.4 1.0
Evaporation mm (Standard Piche)
Total for Month 464 413 517 404 482 N/A 456 382 316 339 30.9 24.8
Max. for 24 hours 28.1 28.0 29.0 25.0 28.5 29.0 24.2 19.3 16.5 16.10 19.8 17.0
Solar Radiation mls
Total Distillation of Water 306 354 506 501 607 637 537 515 507 487 363 313 Maximum Distillation of
Water for 24 hours
14.2 16.6 22.4 21.2 24.5 21.6 22.8 20.1 19.3 23.2 15.2 16.0
Visibility (vis 1000m in 24 hours)
APPENDIX 2 – TYPICAL SEWAGE ANALYSIS
The following is typical sewage analysis, as recorded at Mafraq sewage treatment works inlet, during the period January to December 1998.
pH TSS BOD NH3 COND No. of samples 64 64 64 64 121 Mean 7.0 182 233 27 3192 Maximum 7.2 259 340 30 3800 Minimum 6.9 123 170 24 3000 Standard deviation 0.1 28.2 39.4 1.4 214 Legend to Appendix 2
TSS Total suspended solids (mg/l) BOD Biochemical oxygen demand (mg/l) NH3 Ammoniacal nitrogen (mg/l)
COND Electrical conductivity (µ S/cm)
APPENDIX 3 – TYPICAL GROUNDWATER ANALYSIS
The following are typical analyses of groundwaters that have been encountered in Abu Dhabi.
Location pH Chloride (g/l) Sulphate (g/l)
Abu Dhabi Island 6.5 - 8.3 13 - 120 2.2 - 6.1
Mussafah 7.0 - 7.2 148 - 188 2.6 - 4.6
Khalifa City A 6.3 - 7.5 40 - 210 1.5 - 6.8
APPENDIX 4 – TYPICAL POTABLE WATER ANALYSIS
The following is typical potable water analysis, based on data from Umm al Nar desalination plant.
Parameter Typical values Units
PH 8.3 - 8.8
-Conductivity 250 - 500 µ S/cm
Total alkalinity as CaC03 20 - 30 mg/l
Hardness as CaC03 20 - 35 mg/l Chloride 60 - 120 mg/l Sulphate 5 - 7 mg/l Residual chlorine 0.4 - 0.8 mg/l Calcium 8 - 15 mg/l Magnesium 4 - 7 mg/l
APPENDIX 5 – TYPICAL TREATED SEWAGE EFFLUENT ANALYSIS
The following is typical sewage effluent analysis, as recorded at Mafraq sewage treatment works outlet, during the period January to December 1998.
pH TSS BOD NH3 CL2 No. of samples 64 64 64 64 121 Mean 6.9 3.3 0.9 0.4 1.5 Maximum 7.2 9.4 2.4 2.1 2.1 Minimum 6.7 1.0 0.2 0.1 1.1 Standard deviation 0.1 1.3 0.4 0.3 0.3 Legend to Appendix 2
TSS Total suspended solids (mg/l) BOD Biochemical oxygen demand (mg/l) NH3 Ammoniacal nitrogen (mg/l)
CL2 Total residual chlorine (mg/l)
APPENDIX 6 – MATERIALS SELECTION
1. CONSTRUCTION MATERIALS
As materials on storm water and groundwater control, sewerage and treated sewage effluent contracts can constitute up to 60% of the capital costs of a project it is essential that suitable materials are selected for the long term benefit of the Client. The design principles adopted for a particular system may reduce the number of options on material selection either from cost or geological/geographical standpoint.
Established International Standards and guides such as ASTM, BS, ISO, WIS, and WRC should be followed in the selection of and specification for construction materials. Ideally the material product should be covered by an established ISO 9000 Quality Control system and wherever possible a third party quality assurance scheme.
In selecting standards to specify materials it should be noted that European Standards are normally written for temperate climates whereas American Standards can reflect the diversity of climates experienced within the American continent e.g. Alaska to California in order to achieve materials/products that will perform under Abu Dhabi climatic and geophysical conditions. It may be necessary to combine standard specifications with technical data on testing of materials at temperatures equivalent to those experienced in Abu Dhabi.
2. MATERIALS SELECTION
General
In order to determine if a material is suitable for inclusion in storm water and groundwater control, sewerage and treated sewage effluent projects, several factors have to be considered. These include:
• Suitability for intended purpose. • Availability of material locally and cost.
• Capital cost of selected material offset against reduction or elimination of maintenance costs.
• Capital cost of installation by Non Destructive Methods (NDM) or Microtunnelling offset by reduction in disruption to traffic etc.
• Quality of the medium being transported. • Ground conditions (strata and groundwater).
• Difficulties in handling, transporting and installing the material.
• Environmental conditions within the network such as high temperature, poor ventilation, high levels of corrosive products and significant sand accumulation.
• Future use of land.
The selection of materials should strive to maximise all options available to provide the lowest total installed cost of the system without compromising the long term performance.
The conveyance of sewage, combined with poor ventilation and high temperatures creates anaerobic conditions resulting in the creation of hydrogen sulphide gas, which has the familiar rotten egg smell. This in turn will convert to sulphuric acid which is highly corrosive to cementitious and ferrous materials. Hence, if the practice of discharging sewage into the stormwater network is practised, materials must be selected to withstand such an environment. Caution should be exercised in industrial areas where dumping of neat waste into sewerage or drainage networks, in the absence of local legislation, may result in abnormal high concentrations of corrosive products in specific locations of the network.
The accumulation of sand and silt in storm water systems is a frequent occurrence of a predominately arid climate such as in Abu Dhabi. Wind blown sand and silt can easily enter the system. The lack of vegetation gives higher overland flows and allows more material to be washed off the open areas than otherwise would be the case. The pipes and culverts are sized for peak design flows, which occur infrequently and as a result self cleansing velocities are not achieved and the sediment cannot be flushed away regularly. Accordingly even with a correctly designed system, maintenance of the network and removal of sand and debris is necessary and has to be carried out on routine basis.
In reviewing possible materials for inclusion in storm water and groundwater control, sewerage and treated sewage effluent, materials have been considered which are currently included in the existing networks as well as some newer materials which are now available to the local and regional construction industry. The two largest volumes of material utilised on storm water and groundwater control, sewerage and treated sewage effluent systems are concrete and pipeline materials. This appendix concentrates on the major items.
3. PIPES
Good guidelines to follow are WRC Pipe Materials Selection Manual and EN 1295-1:1997 Structural design of buried pipelines under various conditions of loading.
The material for a pipeline must be selected to suit the liquid being conveyed and the installation conditions.
General guidelines on the selection of pipe materials and the properties of pipe materials are given in the tables below.
SUITABILITY OF PIPE MATERIALS FOR USE IN STORM WATER, SEWERAGE AND TREATED SEWAGE EFFLUENT
Pipe Material
Class Storm Water Sewerage Treated Sewage Effluent Manufacturing Base
Relative Cost per m Gravity Pressure Gravity Pressure Gravity Pressure
Lined and coated RC
Rigid Yes No No No Yes No UAE
Saudi Arabia
Medium
DI Semi rigid No Yes No Yes No Yes Europe
USA
High
GRP Flexible Yes Yes Yes Yes Yes Yes UAE Medium
HDPE Flexible Yes Yes Yes Yes Yes Yes UAE Medium
MDPE Flexible Yes No Yes No Yes No UAE Medium
PVCu Flexible Yes No Yes No Yes No UAE Low
SUMMARY OF PROPERTIES OF PIPE MATERIALS
Property DI GRP HDPE PVCu VC
Specification ISO 2531 ISO 8179 Coating BS EN 545 BS EN 548 ASTM C128 ASTM D3262 AWWA C400 BS 486 BS 5480 ISO R160 ASTM D1447 ASTM D3035 DIN 8074 ISO 4427 ASTM D1784 ASTM D1785 ASTM D2241 ASTM D2665 BSEN 1452-2 ISO 11922-1 ASTM C700 BS 65 BS EN 295 DIN 1230 Maximum operating pressure Maximum 25 Bar Maximum 25 Bar 2.5 Bar to 30 Bar 16 Bar 10 Bar Structural type
Semi rigid Flexible Flexible Flexible Rigid Standard
length
5/6m length 6m max 100m coil up to 110mm dia > 12m length above 110mm dia 6m 2m
Jointing Push fit spigot and socket, Flanged joints Push fit rubber gasket collar joint, Spigot and socket with gasket, Slip on collar flange Butt fusion welding, Electrofusion , Flange Push fit spigot and socket, Solvent welding
Push fit with rubber gasket
Anchor blocks
Required Required Not required on welded
lines
Required Required
Fittings DI fittings GRP HDPE
fabricated fittings, Standard mechanical joints PVCu VC limited
Property DI GRP HDPE PVCu VC Deflection
allowed
Only on long length
Max 50 More than 50 35D 1 to 50 Up to 200mm 2.90, For 1000mm 0.60 Trench required
Wide trench Wide trench Narrow trench
Wide trench Wide trench Installation Overground,
Underground
Underground Can be laid overground /underground on slopes Underground Underground Corrosion Effected by certain soil chemicals Resistant to soil corrosion, Chemically inert Resistant to soil corrosion, Chemically inert Resistant to soil corrosion, Chemically inert Resistant to soil corrosion, Chemically inert Weight Heavy Lightweight Lightweight Lightweight Heavy Handling Cam be damaged by heavy handling Careful handling, cracks if badly handled Easy handling, not easily damaged Careful handling because of brittle nature Careful handling, can be easily damaged Hydraulic properties High frictional loss, High pumping cost Low frictional loss, low pumping cost Low frictional loss, low pumping cost Low frictional loss, can be susceptible to fatigue, surge failures have been known Low frictional loss Abrasion resistance Lining suspect to abrasion
Good Good Limited Good
Breakage Damaged due to heavy impact loads Impact loads cause cracks Impact resistant, unbreakable Damaged by impact Damaged by impact
Property DI GRP HDPE PVCu VC Installation Easy installation, larger sizes need craneage Careful installation, required Easy installation, Less time required, Only very large sizes need craneage Easy installation but subject to poor installation methods Easy installation, many joints due to small length Bedding requirements As dug material As dug material, important to support along entire length must be self compacting Selected as dug material, target 90% standard Proctor Selected as dug material or processed granular materials, target 90% standard Proctor Selected as dug material or processed granular materials, target 90% standard Proctor Supports, clamps Supports required Not applicable, Above ground installation not possible No support required, frequent clamping Not applicable, Above ground installation not possible Not applicable, Above ground installation not possible Maintenance Low maintenance Low maintenance No maintenance No maintenance No maintenance Leakages Frequent if corroded
Normal No Leakage Normal allowances
for push fit joints
Normal
Surge head High surge pressure Medium surge pressure Low wave velocity, Less surge pressure Medium surge pressure Not suitable for pumping
Water supply Depending on water quality cement internal lining is provided Common Recommend ed all over world Commonly used in distribution Not applicable
Property DI GRP HDPE PVCu VC Sewage Common in
pump mains
Common in Middle East
New to UAE Common in UAE
Common throughout
World Test pressure times
operating pressure 1.5 times operating pressure 1.5 times operating pressure 1.5 times operating pressure Air/gravity test Design life 35 years
depending on environment 50 years 50 years minimum 50 years (without brittle failures) 50 years Deterioration with time Corrosion encrustation etc Joints deteriorate encrustation etc Nil Joint deterioration Joint deterioration
Availability Imported UAE Local up to 1200mm dia UAE Commonly available up to 400mm dia Saudi Arabia
UV light Not affected Deteriorates in UV
Stabilised Deteriorates in UV
Not affected
GENERAL USE OF MATERIALS IN PIPELINES Material Size Trunk 300mm up to 2400mm dia Distribution ≥50mm, generally300mm to 800mm dia Services ≤50mm dia Pumping Stations
Storm water Lined and coated RC, GRP, PVCu, VC Lined and coated RC, GRP, PVCu, VC Not applicable DI
Sewage Lined and
coated RC, DI, GRP, HDPE, VC GRP, HDPE, PVCu, VC Not applicable DI Treated sewage effluent GRP, HDPE, MDPE, VC HDPE, MDPE, PVCu, VC
HDPE, MDPE DI, MDPE, HDPE
Structural Behaviour & Classification of Pipes
A buried pipe and the soil surrounding it are interactive structures. The extent of the interaction and hence the magnitude of the pipe loads arising depends on the relative stiffnesses between the pipe and the pipe bedding and native soil. Pipes are generally classed into rigid, semi-rigid or flexible, depending on the degree of this interaction.
Rigid pipes are those where due to the nature of the pipe material, only very small diametrical deflections are possible before fracture occurs at a well defined limiting load. These deflections are too small to develop significant lateral passive pressure in the pipe zone fill material due to external vertical loading. Thus all the external load is taken by the pipe itself and bending moments are induced in the pipe wall. The design of rigid pipes is based upon the concept of a maximum loading at which failure occurs. Some examples of rigid pipe are reinforced concrete pipe (RC), vitrified clay pipe (VC) and asbestos cement pipe (AC).
Semi-rigid pipes are capable of being distorted sufficiently without failure to transmit a part of the vertical load to the pipe zone fill material, thus mobilising a measure of lateral passive support from the surrounding soil, with the pipe wall continuing to take the remainder of the load in bending. Resistance to vertical loading is thus shared between the pipe wall itself and the lateral support from the pipe zone fill material, the proportions of this distribution depending upon the relative stiffnesses of the pipe and the soil surround. Some examples of semi-rigid pipe are ductile iron (DI) and cylinder type pre-stressed concrete.
Flexible pipes are capable of being distorted sufficiently without failure to transmit virtually all vertical load to the surrounding pipe zone fill material for lateral support; the proportion of the load resisted by the pipe wall itself is very small. Flexible pipes are designed on the basis of maximum acceptable deflection, or strain induced in the pipe wall and resistance to buckling under load. The ability of the pipe zone material to provide support is a function of its stiffness, or modulus of reaction. Some common flexible types of pipe are un-plasticised polyvinyl chloride pipe (PVCu), polyethylene pipe (PE), glass reinforced plastic pipe (GRP) and glass reinforced epoxy pipe (GRE).
Pipe Bedding
The selection of the proper type of bedding and surround material is important in the long-term integrity and performance of both rigid and flexible pipes.
Although rigid pipes support vertical loads mostly through their inherent strength and little support is generated by the horizontal soil reaction, nonetheless the selection of an appropriate pipe bedding installation can significantly increase its load bearing capacity by ensuring a more even distribution of vertical loads onto the pipe itself and also by transmission of the load by the pipe to the trench formation beneath.
There is a much greater interaction between flexible pipes and the pipe zone material. The integrity of a flexible pipe is therefore critically dependent on the width and degree of compaction of the pipe bedding material and the stiffness of the native soil. A flexible pipe should be totally surrounded with granular bedding material. Sufficient trench width each side of the pipe is essential to allow correct placement and compaction of the granular bed and surround. Incorrect placement will lead to distortion of the pipe walls. A geotextile membrane is often employed to avoid loss of fines from the native soil and/or to stiffen up the pipe zone material.
Due care should be exercised during placement of aggregates so as not to damage any of the pipes, especially the flexible types which are more susceptible to such type of damage. Flexible pipes may require import of backfill if the existing material is too coarse and contains large amounts of sharp pieces.
Joints
Joints are an essential component of any pipeline system providing continuity between individual pipes. The number and type of joints can considerably affect cost and timescales for a particular pipeline.
Flanged joints for rigid connections are normally employed for above ground use and within pumping stations. Cautionary notes should accompany any joints between GRP and DI flanged pipes/fittings as the correct bolt tightening sequence should be followed to prevent damage. Nuts, bolts and washers should be specified to suit the prevailing conditions e.g. stainless steel in wet and/or corrosive environments.
For buried pipelines it is important to allow for some movement of the pipeline which occasionally occurs through differential settlement of the soil. There are three principal types of flexible joint:
• Spigot and socket. • Sleeve coupling. • Bolted coupling.
Push fit spigot and socket joints comprise a belled end integrally formed at one end of the pipe. This has a slightly enlarged internal diameter sized to receive the spigot end of the next pipe. Sealing of the joint is achieved with flexible elastomeric gaskets which allow a limited degree of angular rotation and longitudinal movement without risk of leakage or fracture.
A sleeved coupling comprises a short cylinder into which the machined ends of the two pipes are inserted. Sealing is affected by two elastomeric gaskets, one for each end of each pipe, which also allow movement of the joint. The sleeve can have a raised ring, or central locating register on the inside to ensure that the pipes are correctly inserted.
