qatar drainage spec. foul sewerage vol. 2

State of Qatar -Public Works Authority Drainage Affairs

Volume 2 Foul Sewerage Page i

1st Edition June 2005 - Copyright Ashghal

CONTENTS

1 Sewerage Systems Design ....................................................................................... 1

1.1 Standards ..................................................................................................................... 2

1.2 Sources of Information .................................................................................................. 2

1.3 Estimation of Flows ....................................................................................................... 3

1.3.1 Domestic ....................................................................................................................... 5

1.3.2 Industrial ........................................................................................................................ 8

1.3.3 Commercial ................................................................................................................... 8

1.3.4 Institutions such as Schools, Health Centres, Hospitals and Mosques ........................ 9

1.3.5 Infiltration ....................................................................................................................... 9

1.4 Peaking Factors .......................................................................................................... 10

1.5 Hydraulic Design ......................................................................................................... 13

1.5.1 Formulae ..................................................................................................................... 13

1.5.2 Minimum Pipe Sizes and Gradients ............................................................................ 16

1.5.3 Minimum and Maximum Velocities .............................................................................. 16

1.6 Septicity in Sewage, Odour Control and Ventilation ................................................... 17

1.6.1 Explosion and Combustion Risk ................................................................................. 18

1.6.2 Corrosion ..................................................................................................................... 18

1.6.3 Impact on Subsequent Treatment Processes ............................................................. 18

1.6.4 Odours ......................................................................................................................... 18

1.6.5 General Design Guidelines for Odour Control in Sewerage Systems ........................ 19

1.7 Pipeline Materials and Jointing ................................................................................... 24

1.8 Pipe Bedding Calculations for Narrow and Wide Trench Conditions .......................... 24

1.8.1 Bedding Design for Rigid Pipes .................................................................................. 25

1.8.2 Bedding Factors .......................................................................................................... 26

1.8.3 Design Strength........................................................................................................... 26

1.9 Manhole Positioning.................................................................................................... 27

1.10 House Connections..................................................................................................... 28

1.11 Construction Depths ................................................................................................... 28

1.12 Manholes, Chambers, Access Covers, and Ladders .................................................. 30

1.12.1 Inspection Chambers .................................................................................................. 30

1.12.2 Sewer System Manholes ............................................................................................ 30

1.12.3 Elements of Design ..................................................................................................... 30

1.13 Industrial Wastes ........................................................................................................ 31

1.14 Septic and Sewage Holding Tanks ............................................................................. 31

1.14.1 Design of Septic Tanks and Soakaways ..................................................................... 32

1.14.2 Sewage Holding Tanks ............................................................................................... 32

1.15 Oil and Grease Interceptors ........................................................................................ 32

1.16 Flow Attenuation Methods .......................................................................................... 32

1.16.1 Flow Controls .............................................................................................................. 33

1.16.2 Attenuation Storage Tanks and Sewers...................................................................... 33

1.17 Abandonment of Sewers............................................................................................. 39


Page ii Volume 2 Foul Sewerage


2 Pumping Stations .................................................................................................... 39

2.1 Standards ................................................................................................................... 39

2.2 Hydraulic Design ......................................................................................................... 39

2.2.1 Hydraulic Principles .................................................................................................... 40

2.2.2 Pump Arrangements ................................................................................................... 41

2.3 Rising Main Design ..................................................................................................... 42

2.3.1 Rising Main Diameters ................................................................................................ 42

2.3.2 Twin Rising Mains ....................................................................................................... 42

2.3.3 Economic Analysis ...................................................................................................... 42

2.3.4 Rising Main Alignment ................................................................................................ 43

2.4 Maximum and Minimum Velocities ............................................................................. 43

2.5 Pipe Materials ............................................................................................................. 43

2.6 Thrust Blocks .............................................................................................................. 43

2.7 Air Valves and Washout Facilities .............................................................................. 44

2.7.1 Air Valves .................................................................................................................... 44

2.7.2 Vented Non-return Valves .......................................................................................... 44

2.7.3 Wash Outs ............................................................................................................... 44

2.7.4 Isolating Valves ........................................................................................................... 45

2.8 Flow Meters ................................................................................................................ 45

2.8.1 Application and Selection ........................................................................................... 45

2.8.2 Magnetic Flowmeters .................................................................................................. 45

2.8.3 Ultrasonic Flowmeters ................................................................................................ 46

2.9 Surge Protection Measures ........................................................................................ 46

2.10 Screens ....................................................................................................................... 48

2.11 Pumping Station Selection .......................................................................................... 49

2.12 Pumps and Motors ...................................................................................................... 52

2.13 Sump Design .............................................................................................................. 53

2.14 Suction/Delivery Pipework, and Valves ...................................................................... 55

2.15 Pumping System Characteristics ................................................................................ 56

2.16 Sump Pumps and Over-Pumping Facilities ................................................................ 59

2.17 Power Calculations including Standby Generation ..................................................... 59

2.17.1 Introduction ................................................................................................................. 59

2.17.2 Load Type ................................................................................................................... 59

2.17.3 Site condition .............................................................................................................. 60

2.17.4 Generator set operation and control .......................................................................... 60

2.17.5 Type of installation ...................................................................................................... 60

2.17.6 Type of Control Panel ................................................................................................. 60

2.17.7 Ventilation system ....................................................................................................... 60

2.17.8 Fuel system ................................................................................................................ 60

2.17.9 Starting method .......................................................................................................... 61

2.17.10 Service facility ............................................................................................................. 61

2.17.11 Generator set sizing .................................................................................................... 61

2.18 Switch Gear and Control Panels ................................................................................. 65

2.18.1 Typetested and partially type tested assemblies (TTA and PTTA) .......................... 65


Volume 2 Foul Sewerage Page iii


2.18.2 Total connected load ................................................................................................... 65

2.18.3 Short circuit level ......................................................................................................... 65

2.18.4 Type of co-ordination .................................................................................................. 66

2.18.5 Form of internal separation ......................................................................................... 66

2.18.6 Bus Bar rating.............................................................................................................. 67

2.18.7 Type of starter ............................................................................................................. 67

2.18.8 Protection device ......................................................................................................... 68

2.18.9 Interlocking facility ....................................................................................................... 70

2.18.10 Accessibility ................................................................................................................. 70

2.18.11 Cable entry .................................................................................................................. 70

2.19 PLCs SCADA/Telemetry ............................................................................................ 70

2.19.1 PLC ............................................................................................................................. 70

2.19.2 RTU ............................................................................................................................. 71

2.19.3 SCADA and Telemetry Systems ................................................................................. 72

2.20 Lighting ....................................................................................................................... 73

2.20.1 Light Fitting Selection Criteria ..................................................................................... 73

2.21 Maintenance Access ................................................................................................... 77

2.22 Gantry Cranes and Lifting Facilities ............................................................................ 77

2.23 Ventilation, Odour Control and Air Conditioning ......................................................... 78

2.23.1 Ventilation .................................................................................................................... 78

2.23.2 Odour Control .............................................................................................................. 79

2.23.3 Air Conditioning ........................................................................................................... 80

2.24 Structural Design ........................................................................................................ 81

2.24.1 Substructures .............................................................................................................. 81

2.24.2 Superstructures ........................................................................................................... 90

2.25 Site Boundary Wall/Fence .......................................................................................... 97

2.26 Site Facilities ............................................................................................................... 97

3 Documentation ........................................................................................................ 98

3.1 Reference Standards .................................................................................................. 98

3.2 House Connection Survey .......................................................................................... 98

3.3 Building Permit ............................................................................................................ 98

4 Health and Safety .................................................................................................... 99

5 Trenchless Technologies ..................................................................................... 100

5.1 Alternative Techniques ............................................................................................. 100

5.1.1 Pipe jacking (Open/Close Face) ............................................................................... 100

5.1.2 Microtunnelling (Closed Face) .................................................................................. 102

5.1.3 Directional drilling ...................................................................................................... 104

5.2 Planning and Selection of Techniques...................................................................... 104

5.2.1 Initial Planning ........................................................................................................... 105

5.2.2 Selection Criteria ....................................................................................................... 110

5.2.3 Factors Affecting Choice Of Method ......................................................................... 110

5.3 Geotechnical Investigations ...................................................................................... 110


Page iv Volume 2 Foul Sewerage


5.3.1 Geological Strata Overview ...................................................................................... 110

5.3.2 Groundwater Regime ................................................................................................ 110

5.3.3 Soil/Rock properties .................................................................................................. 111

5.3.4 Indicative Scope of Interpretative Reporting ............................................................. 113

5.4 Design ....................................................................................................................... 113

5.4.1 Feasibility Study ........................................................................................................ 113

5.4.2 Pipe Design .............................................................................................................. 113

5.4.3 Shaft Design ............................................................................................................. 114

5.4.4 Ground Movements .................................................................................................. 115

5.5 Environmental Assessment ...................................................................................... 117

5.5.1 Vibration .................................................................................................................... 117

5.5.2 Noise ......................................................................................................................... 117

5.5.3 Dust ........................................................................................................................... 118

5.6 Approvals Procedures and Formats ...................................................................... 118

5.6.1 Guidance for Design Check ...................................................................................... 118

5.7 Risk Assessment ...................................................................................................... 118

5.8 Trenchless Construction References ........................................................................ 122

5.9 Trenchless Construction Glossary ............................................................................ 123

6 References ............................................................................................................. 125


Volume 2 Foul Sewerage Page 1


1 Sewerage Systems Design

This volume of the Manual covers the design of new and existing sewerage systems, detailing the design standards, parameters and approaches to be adopted. However, this information should not be regarded as prescriptive in all situations, as each design needs to be prepared, reviewed and approved by appropriately skilled and experienced staff, both within the designers and the Drainage Afairs (DA) organisations.

The sewerage systems in Qatar are separate in that foul sewage, comprising domestic, commercial and industrial effluent is collected in a separate system to that which collects stormwater runoff and ground waters.

The sewerage system for Qatar collects foul flow discharges from premises, located within the developed areas of its towns and cities, and directs the collected flows to the Sewage Treatment Works (STW).

Sewage flows discharge, generally by gravity, into the sewerage system through house connections to the sewer pipelines and manholes outside the property boundary. This network of branch and trunk sewers directs flows by gravity to pumping stations, which pump flows to the STW.

The flat topography of Qatar discourages long lengths of gravity sewer due to the resulting great depths of construction that would be required. The sewerage systems therefore include many pumping stations, with the result that sewage flows will often be pumped several times before arriving at the STW.

The major sewerage systems and STWs are located in Doha, with similar systems in the smaller towns such as Al Khor.

The Doha Catchments

The Doha sewerage system is contained within three catchments, being the Doha West Catchment Area, the Doha South Catchment Area and the Industrial Area. The system in each catchment is similar, in that it comprises networks of sewers and

manholes, directing flows to numerous pumping stations. The flow from each catchment is then pumped to either the Doha South or Doha West STW.

The Doha South Catchment can be broadly defined as that part of Doha being southeast of the Salwa Road and east of the Industrial Area, along with the central business district within the B Ring Road. The Catchment extends southwards to include Abu Hamour, the Airport area and onwards as far as Wakrah, as well as including Wukair and areas to the north and east of the Abu Hamour area.

The extent of the system and the considerable distances over which sewage is transferred across flat terrain, necessitate some 52 sewage pumping stations. The layout of the network results in foul sewage from certain locations being pumped through as many as six or seven pumping stations before reaching Doha South STW.

Development in the catchment is of predominantly low to medium density, with higher densities in the central business district. In total, some 415km2 of land falls within the catchment that it is predicted will be sewered to Doha South STW. In broad terms, only one quarter of this area is presently developed.

The Doha West Catchment comprises some 250km2 of western and northern Doha. The area also includes North Doha, Rural and Urban Rayyan and the Umm Slal Planning Areas of Qatar. The Catchment lands rise from sea level in the east, to some 35m above sea level in the west. The ground level at Doha West STW is about 45m above sea level.

The sewerage network in the Doha West Catchment is served by a terminal pumping station (PS 32) at the south-west edge of the built-up area, from which sewage is delivered in two parallel rising mains to Doha West STW.

Development in the Catchment is of low to medium density, with some areas completely undeveloped. The major future development area is located at the northern end of Doha Bay, where high-density residential and commercial development is planned.

In order to minimise construction, operation and maintenance costs for pumping stations, new designs should use gravity for the movement of


Page 2 Volume 2 Foul Sewerage


sewage flows. However there are the following practical considerations:

Depth of trench excavation should generally not exceed 5.0m, or 7.5m maximum in extreme cases, dictated by excavator access and pipeline strength, where possible. It is acknowledged that greater depths are often necessary in Doha, but these should be avoided because of the danger of deep excavations and the difficulty of achieving good compaction in the backfill;

Gradients should not be flatter than the minimum stated herein, to minimise siltation and septicity.

In theory a separate sewerage system should exhibit no increases in flows from rainfall. However, all systems suffer from infiltration to some extent due to faults and openings in the fabric of the system and illegal connections of stormwater collection systems. Many sewerage authorities deal with such flow increases by incorporating overflows which divert foul flows to watercourses at times of rainfall. However such arrangements are impractical for Qatar due to the lack of watercourses operating all year round, and the resulting unacceptable pollution which would result from discharge of foul flows to wadis with little or no flow.

The extent of infiltration is not fully understood in Qatar, but knowledge will improve with ongoing studies and Drainage Area Plans. In the meantime the sewerage system should avoid the need for overflows, with any increased flows being contained within the sewerage system.

The only overflows permitted are for emergency use only, and only to be located at pumping stations. These emergency overflows are only to operate on failure of pumps, through mechanical or electrical breakdown. Pumps are to be rated to pump all flows expected to be received at the station.

All elements of the sewerage system, including pipelines, manholes, chambers, are to be located on publicly owned lands. Pumping stations and associated facilities shall be on DA owned land. Ideally, access for operation and maintenance of the sewerage system should also be located on publicly owned lands. If not, wayleave agreements should be in place to facilitate such access.

1.1 Standards The following standards are of interest to designers in surface water and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1, Section 1.5 also contains the complete list of references for all manuals.

BS EN 752 Drain and sewer systems outside buildingsi. This supersedes BS 8005ii, which is withdrawn, and part of BS 8301iii.

Part 1: 1996 Generalities and Definitions

Part 2: 1997 Performance Requirements Part 3: 1997 Planning Part 4: 1998 Hydraulic Design and

Environmental Considerations

Part 5: 1998 Rehabilitation

Part 6: 1998 Pumping Installations

Part 7: 1998 Maintenance and Operations

BS EN 598: 1995 Ductile iron pipes, fittings, accessories and their joints for sewerage applications Requirements and test methodsiv.

BS EN 1610: 1998 Construction and testing of drains and sewersv.

Sewers for Adoption 5th Edition (WRC)vi.

BS EN124: 1994 Gully tops and manhole tops for vehicular and pedestrian areas Design requirements, type testing, marking, quality controlvii.

1.2 Sources of Information The following publications are of interest to designers in surface water and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1 Section 1.5 also contains the complete list of references for all manuals.




Department of the Environment National Water Council Standing Technical Committee Reports, 1981,

Design and analysis of urban storm drainage -

The Wallingford Procedure, National Water Council UK.

State of Kuwait Ministry of Planning & Hyder Consulting, 2001, Kuwait Stormwater Masterplan Hydrological Aspects - Final

Report. Cardiff, (AU00109/D1/015), Hyder Consulting.

Highways Agency, 2002, DMRB Volume 4 Section 2 Part 5 (HA 104/02) Geotechnics

and Drainage. Chamber pots and gully tops for

road drainage and services: Installation and

maintenance, London, Highways Agency.

Water Research Council, 1997, Sewerage Detention Tanks A Design Guide, UK, WRC.

Construction Industry Research and Information Association, 1996, Report R159: Sea Outfalls construction, inspection and

repair, London, CIRIA.

Building Research Establishment, 1991, Soakaway Design, BRE Digest 365, BRE Watford UK.

HR Wallingford DC Watkins, 1991, Report SR271 -The hydraulic design and performance of soakaways, Wallingford UK.

Construction Industry Research and Information Association, 1996, Infiltration Drainage Manual of Good Practice, London UK, CIRIA.

Chartered Institution of Water and Environmental Management, 1996, Research and Development in Methods of Soakaway

design, UK, CIWEM.

Construction Industry Research and Information Association, 2000, C522 Sustainable Urban Drainage Systems Design

Manual for England and Wales, London UK, CIRIA.

Construction Industry Research and Information Association, 2001, C523 Sustainable Urban Drainage Systems Best

Practice Manual for England, Scotland, Wales, and Northern Ireland, London UK, CIRIA.

Velocity equations for the hydraulic design of pipes Wallingford Research.

HR Wallingford and DIH Barr, 2000, Tables for the Hydraulic Design of Pipes, Sewers and

Channels, 7th Edition, Trowbridge, Wiltshire, UK Redwood Books.

Ministry of Municipal Affairs and Agriculture, 1997, Qatar Highway Design Manual, January 1997, Qatar, MMAA.

Construction Industry Research and Information Association, 1996, Design of sewers to control sediment problems, Report 141, London CIRIA.

Clay Pipe Development Association Limited, 1998, Design and construction of drainage and sewerage systems using vitrified clay pipes, Bucks, UK, CPDA.

Report for the hydraulic design of pipes Wallingford Research.

Construction Industry Research and Information Association, 1998, Report 177, Dry Weather Flows in Sewers, London, CIRIA.

Water Research Council, 1994, Velocity equations, UK, WRC.

Bazaraa, A.S., Ahmed, S., 1991. Rainfall Characterization in an Arid Area, Engineering Journal of Qatar University, Vol. 4, pp35-50.

1.3 Estimation of Flows The flows in a foul sewerage system are made up of contributions from a number of different sources, including: domestic properties; commercial areas; industrial facilities; institutional contributions from hospitals, schools, etc.; groundwater infiltration; and surface run-off. The contributions to the system from each of these sources must be determined before the required hydraulic capacity of the sewerage can be established. Each of these contributions will follow a different diurnal pattern, with flows varying over a 24-hour period. The design of the system must take these fluctuations into account and be




capable of catering for the peak flows likely to be encountered in any 24- hour period. Diurnal flow patterns will be different on working days, from the patterns on rest days.

The starting point for the design of foul sewerage should be the estimation of the average flow rate or the Dry Weather Flow (DWF). This is calculated from the following formula:

DWF = PG + I + E Equation 1.3.1

DWF = dry weather flow (litres/day)

P = population served

G = average per capita domestic water consumption (l/hd/day)

I = Infiltration (l/day)

E = average industrial effluent discharged in 24 hours (l/day)

The process for establishing flow rates should follow the sequence set out below:

1. Define catchment and sub-catchment boundaries for the area under consideration. This should include all the properties and establishments that contribute to the system and may include future developments as well as existing. The catchment represents the entire upstream area contributing to a point or node in the sewerage system. Generally, catchments are taken to contribute to trunk sewers, while sub-catchments contribute to branches. Thus a catchment may comprise a number of sub-catchments.

2. Determine the numbers and types of dwellings within the catchment and from this, determine the existing and future contributing domestic population and hence the flows from that population to the network. Section 1.3.1 gives detailed guidance on this process. Establish the diurnal flow pattern for the domestic contribution.

3. Identify any existing and proposed industrial establishments in the catchment, together with their daily contributing flow and diurnal flow pattern. Section 1.3.2 gives guidance on this.

4. Identify any existing and proposed commercial establishments within the catchment, together with their working populations and diurnal variations. Section 1.3.3 provides detailed guidance on this.

5. Identify any existing and proposed institutional establishments such as schools, health centres, hospitals and mosques that are within the catchment boundary. Determine the usage of these institutions and derive a diurnal flow pattern for them. Section 1.3.4 provides details of this process.

6. Determine infiltration rates into the sewerage system using the methods described in section 1.3.5. These may increase with time or it may be proposed to rehabilitate the system to reduce infiltration.

7. The flows that are likely to occur in the sewerage system can now be estimated. This is done by adding together the total daily contributing flows from each upstream source to any given point in the network. This is usually done sub-catchment by sub-catchment working down the trunk sewer. It can be done graphically and will establish the maximum likely flow that has to be catered for at the given location. The total daily flow from each contributing source is calculated and summed to give a total daily flow through a given point. This flow is then averaged for a 24-hour day to give an average Dry Weather Flow or DWF. The peak flow for design purposes in upper catchment areas can be taken as 6xDWFviii. From the peak flows the required pipe sizes can be determined. However, it should be noted that the peaking factor would decrease in downstream catchment areas (see section 1.4 for information on peaking factors). Hydraulic design is described in section 1.5.




Figure 1.3.1 -Typical chart showing diurnal

variations in domestic sewage

flows

0.00

0.50

1.00

1.50

2.00

2.50

00:00 04:00 08:00 12:00 16:00 20:00 00:00

Hours

Flo

w,

l/s

Where sewerage systems are very long and the time of flow from top to bottom is significant, peak flows will be heavily attenuated. This is because, for example, locally generated domestic flows in the lower parts of the catchment will have passed downstream by the time the flows arrives from the upper areas. This has the effect of smoothing out the peaks in flows.

1.3.1 Domestic

Domestic flows form the largest proportion of flows in foul sewers. They derive from normal domestic appliances such as sinks, basins, toilets, showers, washing machines, baths, etc., and are dependent on the number of persons in a dwelling. In order to determine suitable domestic contributions to the sewerage system, it is necessary to make certain assumptions. For example, each property is assumed to house a certain number of persons, and this will vary from one type of property to another. The assumption is made that all properties of a given type will contain a given number of persons.

Butler and Daviesix suggest that between 75% and 85% of water used in a dwelling in the Middle East is returned to the sewerage system. Thus, if a property is metered, a good assessment of return to sewer flows can be obtained.

Table 1.3.1 below gives the discharge rates that should be used for the design of foul sewerage systems. Discharges in the table below are averaged over 24 hours in the determination of DWF because the application of peaking factors allows for the diurnal profile.

Table 1.3.1 Typical Daily Discharges in the ME

Development

type

Discharge

l/day

Unit

Domestic 170 Litres/head/day

Domestic low density high value properties

250 Litres/head/day

Average Infiltration 100 Litres/jhead/day

Infiltration range 0- 250 Litres/head/day

The figures in this table provide general guidance for the design of foul sewerage systems.

The figure to be used for design purposes in Qatar where there is no better information is 270l/h/d, comprising 160 l/hd/day or sewage and 110 l/hd/day infiltration.

Where the area to be served is low density palaces and villas consideration should be given to the use of 200 l/head/day. If the catchment is inland and the ground water table level is low then the infiltration allowance can be reduced or even eliminated.

Design populations of the existing and proposed properties are based on the plots indicated on the Action Plans that can be obtained from the Land Information Centre and the occupancy levels given in Table 1.3.2. The number of discharge units per property is then allotted based on BS 8301, as shown in Table 1.3.2.




Table 1.3.2 Indicative Occupancy Levels (from

BS 8301)

Plot Description Occupancy

Levels

Discharge

Units

For plots less than 1225m2 6 people 14

For plots equal to and between 1225 and 2500m2

9 people 21

For plots greater than 2500m2

Small Palaces

15 people 35

Larger Palaces

25 people 58

The dry weather flow is then obtained from Figure 1.3.2, which has been reproduced from BS 8310, Figure 2. Where no Action Plan plot or housing information is available, the future area can be assumed as developed at an average of the existing planned plot density.




1

10

100

1 10 100 1000 10000Discharge Units

Flo

w (l/s

)

Figure 1.3.2 Conversion of Discharge Units to Flow Rates




1.3.2 Industrial

Estimation of daily discharges from Industrial areas will be dependent on the type of industry occupying the area. The majority of industries in Qatar are dry industries such as warehousing and workshops. These will have lower consumption rates than wet industries such as concrete or paper manufacture. If possible, metered water consumption rates should be used in design but where these are not available or are impractical to use, the values in Table 1.3.3 can be applied.

Table 1.3.3 Design Allowance for Industrial

Wastewater Generation

Category Volume (l/s/ha)

Conventional Water - Saving

Lightx 2 .5

Mediumx 4 1.5

Heavy 8 2

Category Volume

Slaughterhousexi 6600 l Per tonne of produce

Drink Productionxi

8400 l Per cubic metre of produce

Laundryxiv 1500 2100 l/d Per machine

Tanneryxii 30 35 m3 Per tonne of produce

Tanneryxi 7600 l Per tonne of produce

In the above table, light industry may be taken as dry industries which generally handle materials and goods which do not include washdown facilities. Heavy industries will include factories with washdown facilities and using water in the unit processes. These figures are to be used only in initial land usage planning, and developers must obtain confirmation from end users before final design.

1.3.3 Commercial

Most significant developments include a degree of commercial activity and this should be included in the assessment of discharges to the foul system. This activity can range from a single small office or shop, up to major shopping, hotel or office complexes. Each development type needs to be assessed.

Commercial activities include all those listed above and each may have its own characteristic discharge profile, which will inevitably be different from the standard domestic profile.

Table 1.3.4 gives an indication of the likely discharges from various types of commercial activity.

Table 1.3.4 Typical flows from commercial premises

Development type Discharge

l/day

Per

Commercial Centresxiii

50 Customer per 12 hour day

Airportxiv 11 - 19 Passenger

Hotelsxv 150-300 Bed

Restaurantsxvi 30-40 Customer

Social Clubsxvii 10 20 Customer

Cinemaxviii 10 Seat

Officesxix 750 100m2

Shopping Centresxx 400 100m2

Department Storexxi 2000 Per toilet

Recreationalxxii Centres

80 Customer per 6 hour day

Commercial premisesxxiii

300 100m2

Where possible, the above discharge rates should be checked using installed water supply meters for existing developments. Proposed developments




should be assessed using the figurers given in the table above.

Diurnal profiles should be derived for each type of commercial development and applied to the daily discharge rate from the table.

1.3.4 Institutions such as Schools, Health Centres, Hospitals and Mosques

Table 1.3.5 contains typical values of discharges from various types of institutional premises.

Table 1.3.5 Typical Institutional Discharges

Development

type

Discharge

l/day

Per

Educational Centresxiii

70 Pupil per 8 hour day

Day schoolsxxiii 50 - 100 Pupil per 8 hour day

Residential schoolsxxiv

150-200 Pupil

Mosquexiii 100 Worshiper per 12 hour day

Sports Centrexxiii 10 30 visitor

Retirement Homexxiii

250 Bed

Nursing Homesxxiii 300 - 400 Bed

Assembly Hall 11 - 19 Guest

Prison 300 - 570 Inmate

Hospitalsxxiii 500-750 Bed

Each category of premises will have a different diurnal discharge profile, with day schools only contributing during the school day, and hospitals likely to contribute flows for much of the waking day.

As with other types of development, metered water supply records should be consulted wherever possible to provide an indication of actual consumption figures. A suitable return to sewer factor should then be applied to the results. Sometimes, it may be possible to determine diurnal

profiles by reading water meters at say, hourly intervals throughout the day. The resulting profile is then applied to the daily consumption.

1.3.5 Infiltration

Infiltration describes flows in the foul system, which are not legitimate discharges. Infiltration comprises two components:

inflows from faulty manhole covers, cross-connections from storm and groundwater control systems, and tidal sources. Inflows can also come from the illegitimate practice of lifting manhole covers to drain surface water during and after storms;

infiltration of groundwater through displaced and open pipe joints, cracks, fractures and breaks in the fabric of the main sewers and lateral connections, manholes and chambers.

Infiltration causes reduced capacity for legitimate sewage flows, increased requirements for pumping and sewage treatment, and possible structural damage.

Infiltration into foul sewerage systems can be problematic. It generally derives from groundwater entering the pipe network through: poor joints in the pipes; cracks or fractures; defects in manholes; or through private drainage connections. Infiltration generally occurs in areas with a high water table. In coastal areas, infiltration can be saline which can have a detrimental effect on sewage treatment processes and can cause corrosion of metalwork in manholes and pumping stations.

It is normal to allow a figure of 10% of DWF for infiltration. Infiltration should be excluded from the calculation of flows using peaking factors. Thus for a peaking factor, Pf, peak design flow would be given by the equation:

Q = Pf (PG + E) + I

Equation 1.3.2

Where: Q = Peak Design Flow (l/d)

Pf = Peaking Factor

P = Population




G = Daily per capita flow (l)

E = Daily Industrial Flow (l)

I = Daily infiltration flow (l)

A sample calculation sheet for sewers using the above formula is included in Volume 1 Appendix 1

Where local conditions indicate that the figure of 10% DWF for infiltration is too low, then a higher figure may be adopted. However, this must be justified by supporting information, such as the analysis of flow survey results. At the time of drafting this manual, DA suggest that for G in the above formula, an overall figure of 270l/hd/day be used for all domestic flows. This will be revised when flow survey results become available .

Conversely, where the water table is known to be well below the level of the sewerage system, the allowance for infiltration will be less significant locally.

Infiltration is often associated with exfiltration, which is the leakage of foul flows due to faults and openings in the pipework, manholes and chambers. Exfiltration of foul flows results in contamination of the surrounding soils and possible pollution of groundwater.

Since both infiltration and exfiltration involve flows passing through physical defects in the sewerage system fabric, they often occur together in conjunction with fluctuating groundwater levels. This continuing flow mechanism can result in erosion of the surrounds and foundations to pipes and manholes. In serious cases, failure of the asset or ground subsidence has resulted.

The Sewer Rehabilitation Manual provides a detailed explanation of the factors involved in infiltration.

Two CIRIA reportsxxv,xxvi describe various methods for estimating base-flow infiltration. Inflow of stormwater runoff is estimated from the area of development contributing to the flow monitor. Estimation of both components relies on detailed flow and rainfall monitoring, combined with hydraulic modelling to understand the relative contributions of the components in wet and dry weather.

The Infiltration Reduction Procedure contained in the Sewerage Rehabilitation Manual should be

followed, where infiltration is to be reduced. This is an iterative approach to successively focus on sources of excessive infiltration, and to ensure that reduction measures are cost-effective.

It is very evident that removal, or more realistically, significant reduction of infiltration, is a time-consuming and expensive process. It is far more cost-effective to avoid its occurrence in the first place. This can be done by strictly controlling the quality of new and renovated sewerage installations, and by ensuring that best quality materials and construction techniques are used, to provide a long-lasting leak-free system. Such standards should be applied to both private and public sewerage. Property connections should also be correctly made, and abandoned sewers and septic tanks properly sealed.

1.4 Peaking Factors As described in section 1.3, the rate of discharge of sewage from a given property to the sewerage system will vary during the day. The sewerage system must be able to cope with the highest flows likely to occur in the day. Different contributors to the system will have different discharge profiles. For example, shopping areas will generally only contribute flows during the periods when the shops are open, and then the flows will be in proportion to how busy the shops are through the day.

Domestic properties generally show marked morning and evening peaks, which coincide with peak domestic activity. This suggests that foul sewers should be designed to cope with higher than the average, or dry weather flow (DWF), and a common way of designing systems is to cope with a flow of up to six times DWFviii. While this approach may be satisfactory for the smaller sewers at the head of the system, it will tend to over design the larger sewers and ignores the attenuation effects as the flows move downstream.

At the head of a sewerage system, discharges tend to be pulsed, with individual pulses of flow being the discharge from individual appliances. As the pulses flow along the pipe system, the peaks tend to become attenuated and as the flows progress down the system, these pulses combine to form a more consistent flow. The peaking factor will depend on the upstream population and the distance the




sewage has travelled. A number of different ways of determining the peak factor have been proposed which take account of the attenuation downstream with increasing population. There are several formulae for calculation of peaking factorix, for which the Babbit formula is most representative in Qatar

The Babbit Formual (1952) is;

5

5P

PF = ,

Where PF represents the peaking factor, and P is the population in thousands.

However, the formula is not representative at low populations.

Therefore, the upper limit for peaking factors shall be taken as six for populations up to and including 500.. For populations over 500 the Babbit formula shall be used. The minimum value of peaking factor shall be 3.

It is considered that values in excess of six, and below three, are unrealistic for conditions in Doha, but these figures may be revised after a detailed flow survey is carried out (see section 1.3 above).

The variation of peaking factors with population is shown graphically in Figure 1.4.1, which follows




Peaking Factors

0.001.002.003.004.005.006.007.00

100 200 500 1000 2000 5000 10000 20000Population

Fac

tor

Babbit BSEN 752

Minimum value 3

Maximum value 6

Figure 1.4.1 Plot of Peaking Factors v Population




1.5 Hydraulic Design The hydraulic design of sewerage systems involves achieving a balance between pipe size, pipe gradient and pipe depth, such that self-cleansing velocities are achieved without surcharge, but with the most economical combination of size and depth. Wherever possible, pipe depths should be used that avoid the need for concrete bed and surround.

General Principles

The general principles of foul sewer design are as follows:

Pipe size should not generally decrease downstream;

Sewers should be designed to convey peak flows without surcharge;

Sewers should achieve self-cleansing velocity at least once per day. Note that half-pipe velocity is numerically the same as full-pipe velocity.

To allow for ventilation of the system, maximum design depth of flow should not exceed 0.75 x pipe diameter or d/D 0.75.

Where there is a chance of heavy construction plant tracking over new sewers laid during construction of a site, the minimum depth of cover should be measured from the formation level of the site above the sewers;

Self-cleansing velocities increase with pipe size (see sections 1.5.1 and 1.5.2 below);

At manholes, all pipes should be laid such that their soffits are at the same level. Pipes in manholes should not be laid with the inverts level, as this can promote the deposit of solids in minor branches leading to odour problems and blockages;

Junctions should not enter a sewer at right angles but should enter at an angle of 45 to the direction of flow of the main sewer;

Sewers should commence at minimum depth upstream and follow ground profiles if possible to minimise excavation. However, it is recognised that in Qatar, due to flat topography, depths will gradually increase

downstream in order to maintain minimum gradients (see section 1.5.1 below). Trunk sewer sections serving larger catchments are likely to become very deep (but see also section 1.11);

Backdrop manholes should be used where there is a difference >600mm in level between a branch/rider sewer and the main sewer. Backdrops (see also section 1.12.2 below) should not be used to reduce gradients on main sewer lines.

Design Tools

Hydraulic computer models are invaluable tools for understanding the performance of sewerage systems. Hydraulic models are of particular value for:

Understanding the performance of the complete system, in particular attenuation of flows;

Understanding the flow regime of complex and interdependent systems, such as those with bifurcations and loops;

Understanding the flow characteristics of multiple pumping systems, as found in Doha;

Readily understanding the effects of changes in development on existing systems;

Simulating modifications to the construction and/or operation of the system.

Hydraulic computer models should use InfoWorks CS software, and be verified against flow and depth measurements carried out on the actual system.

1.5.1 Formulae

1.5.1.1 The Colebrook-White Equation

The Colebrook-White equation allows calculation of velocity of flow in a gravity drain flowing full for any given gradient, diameter, and roughness coefficient, as follows;




( ) ( )

+=

gDSDDk

gDSv s251.2

7.3log22

Equation 1.5.1

Where g = acceleration due to gravity, m2/s

D = diameter, m

S = slope or headloss per unit length

sk = roughness coefficient, mm

= kinematic viscosity of water (m2/s).

Thus, for a 400 mm diameter pipe with sk = 1.5 , and slope 1 in 157, flow temperature 15oC, the velocity will be 1.33 m/s

Using the relationship:

Q=AV

Equation 1.5.2

Where:

Q = flow in the pipe (m3/s)

A = Cross-sectional area of flow

V = velocity of flow

This allows the pipe full discharge to be calculated where:

A=piD2/4

Equation1.5.3 Thus, for the above pipe at full flow, the capacity will be 167 l/s

A sample calculation sheet for sewers using the above formulae is included in Volume 1 Appendix 1

Tables are available from hydraulic research giving values for a wide range of pipe sizes at a range of gradients for various values of ks.

Tables 1.5.1 and 1.5.2 below give recommended values of ks and . Both are taken for the Slimed sewers category from Wallingford design tablesxxvii.

Table 1.5.1 - Pipe Roughness ks Values

Material ks Value (mm)

Normal Poor

Concrete (Precast + O Rings) 3.0 6.0

Concrete (Steel Forms) 3.0 6.0

DI (PE Lined) 0.6 1.5

GRP 0.6 1.5

VCP 1.5 3.0

Further values can be obtained by direct reference to Appendix 1 of the Wallingford design tables.

Caution should be exercised in the use of the Wallingford tables. It should be noted that the quality of pipes can vary considerably from one manufacturer to the next, and that condition of pipes can vary with time. Designers should avoid using the optimistic values quoted by some plastic pipe manufacturers, as these invariably refer to new pipes under laboratory conditions. The figure to be used for design purposes shall be 1.5 in all cases

Table 1.5.2 - Kinematic Viscosity Values Temperature, 0C Viscosity, m2/s x 10-6

15 1.141

25 0.897

35 0.727

For detailed sewage modelling applications, the viscosity should be varied for a range of temperatures, but for routine applications, a conservative approach will be to use the lower temperature of 150C.

A graph for proportional velocity and discharge in part-full circular sections is reproduced in Figure 1.5.1. This illustrates the relationship between depth of flow, and velocity. It can be used for estimating the velocity of flow in partially full pipes, and should be used to check velocities for self cleansing velocities at low flow (see table 1.5.4)




Figure 1.5.1 - Proportional Velocity and Discharge in Part-Full Circular Sections




1.5.1.2 Mannings Equation

Mannings equation is an empirical formula for uniform flow in open channels. Mannings equation is:

v=(1/n)R2/3S0

Equation 1.5.4

Where: n is Mannings roughness coefficient S0 is bed slope R is the hydraulic radius of the flow

The equation may be useful in pumping station approach hannels and elements of sewage works. However, all pipe calculations must use Colebrook White

Typical values of Mannings n are given below.

Table 1.5.3 - Typical values of Mannings n

Channel Material n range

Cement 0.010-0.015

Concrete 0.010-0.020

Brickwork 0.011-0.018

Mannings equation is only valid for rough turbulent flow conditions.

1.5.2 Minimum Pipe Sizes and Gradients

CIRIA Report R141xxviii defines self-cleansing sewers as follows:

An efficient self-cleansing sewer is one having a sediment-transporting capacity that is sufficient to maintain a balance between the amounts of deposition and erosion, with a time-averaged depth of sediment deposit that minimises the combined costs of construction, operation and maintenance.

Foul sewers should be at least 200mm diameter and laid to a gradient of 1 in 60 or 1.67%. This gradient can be relaxed to 1 in 150 (0.67%) where several dwellings are connected to the head of the sewer, and the standard of workmanship during construction is high. Peak flow velocities of

0.75m/sec can be assumed to be self-cleansing in pipes of 150mm diameter.

As sewer sizes increase, so too do self-cleansing velocities, with the result that very large foul sewers require velocities to exceed 1.5m/sec to be self-cleansing. Such velocities in large diameter pipes pose a safety hazard and facilities must be provided to prevent operatives being washed downstream in these sewers.

1.5.3 Minimum and Maximum Velocities

CIRIAxxvi recommends that sewers should be designed to:

1. transport a minimum concentration of fine particles in suspension.

2. transport coarser granular material as bed load.

3. erode cohesive particles from a deposited bed.

In order to minimise the maintenance requirements of any given length of sewer, it is normal to design the sewer to be self-cleansing. This means that the sewer is designed to achieve a velocity at least once per day that will carry all solid deposited material along the pipe and not leave any materials deposited in the invert of the sewer.

Table 1.5.4 is based on the simplified CIRIA method of assessing self-cleansing velocities in foul sewers. Surface water sewers require generally higher self-cleansing velocities because of the higher particle densities.

Table 1.5.4 Approximate Self-Cleansing

Velocities for Foul Sewers

Pipe size

(mm)

Approximate self- cleansing

velocity (m/sec)

200300 0.75

400 0.77

500 0.82

600 0.86

700* 0.87




Pipe size

(mm)

Approximate self- cleansing

velocity (m/sec)

800 0.88

900* 0.88

1000 0.92

1200 1.03

*700 and 900 are non preferred sizes

Where large diameter sewers (over 1.0m diameter) are laid to steep gradients, very high flow velocities occur. For example,; a 1000mm pipe laid to 1:100 gradient with a depth of flow of 750mm will have a discharge velocity approaching 3.4m/sec, which is unacceptable in foul sewers. The designer should implement energy dissipation measures in such cases. It should be emphasised that scour in pipes at these velocities is not a significant problem with modern materials, but if velocities become very high, odour emissions can be increased and noise can become a problem.

As a general rule, it is preferable to aim to achieve self-cleansing velocity at least once per day. The designer should aim to achieve a velocity at the design flow (i.e. peak flow) of between self-cleansing and 2.0m/s, with 2.5m/s as an upper limit.

In small sewers, less than 600mm diameter, it is not necessary to include measures to limit flow velocity. The use of backdrop manholes for this purpose is discouraged. However, backdrop manholes may be justified where there is a significant difference in level between a branch sewer and trunk sewer. In this case, the economics may justify the construction of a backdrop to minimise excavation for the branch sewer trench. The discharge from a backdrop into a manhole requires careful design to prevent flows from washing over the benching.

Backdrops for large diameter sewers are complex structures that may involve the creation of vortices to dissipate energy, for which specialist design is required. These are often purpose-made in stainless steel. A typical example is included in the standard drawings, Volume 8.

1.6 Septicity in Sewage, Odour Control and Ventilation

In rising mains and shallow gravity sewers, respiration of bacteria in wastewater and slimes present on submerged sewer walls rapidly depletes any dissolved oxygen or nitrates causing anaerobicity (septicity)xxix. One of the main impacts of septicity is the formation of sulphide by the bacterial reduction of inorganic sulphate present in the wastewater. Some of the sulphide will combine with metals in the sewage. The remainder will be present in ionised and unionised form, as below.

S2- HS- H2S

Only the un-ionised form is released to the atmosphere. The proportion of sulphide present in the un-ionised form is dependent upon the pH value of the sewage and is about 50% at a pH value of 7. For example, a liquid concentration of 2mg/l of sulphide at pH 7.0 would be in equilibrium with a gaseous H2S concentration of 300ppm (ml/m3). At a pH value of 8.0 this would decrease to about 60ppm.

Septicity can have an impact on health and safety, corrosion, subsequent treatment processes and odours. Hydrogen sulphide is a toxic gas. WHO guidelines for dose-effect relationships for H2S are given in Table 1.6.1xxx.

Table 1.6.1 - Health Impacts of Hydrogen

Sulphide

H2S Level

(ppm)

Health Impact

1000-2000 Immediate collapse with paralysis of respiration

530-1000 Strong central nervous system stimulation, followed by respiratory arrest

320-530 Risk of death

150-250 Loss of olfactory sense

50-100 Serious eye damage

10-20 Threshold for eye irritation




UK Occupational exposure limit (OEL) concentrationsxxxi of hydrogen sulphide and other gases associated with septic conditions are given in Table 1.6.2.

Table 1.6.2 - Exposure Limits for H2S and Other

Gases

Gas Long term OEL (8-hour) (parts per million)

Short term OEL (15

minute) (parts per million)

Hydrogen sulphide 5 10

Methyl Mercaptan (methanethiol)

0.5 -

Ethylmercaptan (ethanethiol)

0.5 2

Ammonia 25 35

Methylamine 10 -

Ethylamine 10 -

Dimethylamine 10 -

1.6.1 Explosion and Combustion Risk

The WRC report Enclosed Wastewater Treatment Plantsxxxii considers the potential risk of the development of flammable concentrations of gases arising in a STW. The possible gases considered are given below in Table 1.6.3.

The lower explosive limit for hydrogen sulphide is 40000ppm. This concentration is unlikely to be achieved under normal operation, and risk is therefore minimal.

Table 1.6.3 - Flammable Gases in Sewers

Gas Lower explosive limit % v/v in air

Upper explosive limit % v/v in air

Carbon Monoxide

12.5 -

Hydrogen sulphide

4.0 (40000 ppm) 46

Petroleum 100 ppm

Methane 5.3 15

Spontaneous combustion of sulphur around the edge of lifted manhole covers has been reported in Doha. In this instance, the reaction of hydrogen sulphide with iron oxide at the manhole cover has led to its catalytic oxidation to sulphur, which is spontaneously combustible. Operational procedures may be required to reduce this risk. Although these are beyond the scope of normal design functions, it is important that the designer is aware of such issues and to include mention of them in the design HARAs.

1.6.2 Corrosion

Hydrogen sulphide is associated with the corrosion of concrete and mortar as the result of its bacterial conversion to sulphuric acid. High levels of hydrogen sulphide may develop below covers, with consequent impact on the structure of the tank or manhole as has been found at a number of sites. Metal work and electrical equipment is also vulnerable to H2S corrosion.

Selection of construction materials for tanks, manholes, covers, and fittings should take into account the potential for corrosion.

1.6.3 Impact on Subsequent Treatment Processes

The discharge of septic sewage can increase the rate of development of sulphide in the primary sedimentation stage sewage and sludges.

High levels of septicity have been associated with poor settleability of activated sludges.

High levels of septicity or sulphates have been associated with poor gas yields from mesophilic anaerobic digestion processes.

1.6.4 Odours

The discharge of septic sewage can be a significant source of odours at the discharge point, whether to an intermediate pumping station or to the inlet of a STW. Threshold levels for various odours are listed in Table 1.6.4.

The odour threshold level of hydrogen sulphide measured in a laboratory is about 0.5 parts per billion (ppb). The level above which odour problems




can occur is typically ten times this value. Background H2S levels are often in the range 2-5ppb.

Table 1.6.4 Odour Threshold Levels

Gas Odour threshold (parts per billion)

Hydrogen sulphide 0.5

Methyl Mercaptan (methanethiol)

0.0014-18

Ethylmercaptan (ethanethiol)

0.02

Ammonia 130-15300

Methylamine 0.9-53

Ethylamine 2400

Dimethylamine 23-80

1.6.5 General Design Guidelines for Odour Control in Sewerage Systems

The design of sewerage systems to reduce the development of septicity is the subject of a number of guidesxxxiii. Guidelines include:

Rising mains

Minimise lengths of pumping mains, and use lift pumps rather than long rising mains to minimise retention under anaerobic conditions( there is no satisfactory minimum length of rising main which can be quoted for design purposes. Even a retention time of 30 minutes is sufficient to develop anaerobic conditions. );

Minimise turbulence at the discharge point;

Discharge into the gravity sewerage system at low level to avoid turbulence and consequent release of odours;

Location of discharge point should NOT be immediately prior to hydraulic drops or sharp bends;

Manhole covers at discharge points may need to be sealed.

Pumping stations

Minimise turbulence at inlet to sump. Use submerged, rather than overflow weirs;

Use level detectors to minimise the volume of sump used under normal flow conditions;

Use frequent pumping regimes to minimise retention time in sump, and also spread odour load more thinly over the day;

Maximise benching to give self-cleansing conditions and ensure no accumulation of grit. Guidelines are given in BS 8301xxxiv;

Ensure any screenings or grit can be removed, or are washed back into main flow of sewage;

Active/passive odour control unit may be required depending on the sensitivity of the site, size of installation, and other factors such




as degree of septicity of sewage under normal flow conditions.

Gravity sewers

Maintain self-cleansing velocities;

Avoid turbulent flow (including sharp bends and drops);

Minimise length of siphons (which will act as rising mains);

Ensure there is ventilation of the sewer (by provision of vents);

Design to ensure no accumulation of grit or debris.

Storage Tanks and Shafts

Minimise turbulence of discharges to tanks and shafts (discharge at low level);

In sensitive areas (i.e. next to houses) odour control may be needed to treat displaced odours when levels rise.

Refer also to section 2.23 of this volume, and section 1.5 of volume 5

The formation of sulphide in rising mains and gravity sewers has been the subject of extensive studies xxxv, xxxiii.

The concentration can be estimated from the following equationxxxvi:

Cs=K tCOD[(1+0.004D)/D]1.07(T-20)

Equation 1.6.1

Where:

Cs = concentration of sulphide (mg S/l) Kc = constant, usually taken to be 0.00152 t = anaerobic retention time (mins) D = diameter of rising main (cm) T = temperature of sewage (C) COD = COD of sewage (mg/l) In gravity sewers, there is a balance between sulphide formation when flow is stagnant, and sulphide release and oxidation during turbulent flow. In practice, little sulphide should be formed in a well-ventilated, self-cleansing, partially-filled gravity sewer used for domestic sewage. Where problems do occur, they are typically associated with sewers of shallow gradients where accumulation of grit, silt

and slimes causes localised septicity at points where turbulence is insufficient to remove such debris.

An indicator of the likelihood of septicity in a gravity sewer is the Z formula with the effect of different values of Z as indicated in Table 1.6.5.




Z as calculated below is a dimensionless parameter that indicates the potential for sulphide generation.

Z = 3(EBOD) x P

S0.5Q0.33 b

Equation 1.6.2

Where:

EBOD = 5 day BOD (mg/l) multiplied by a temperature factor 1.07 (T-20)

T = sewage temperature (co) S = slope of sewer (m/100m) Q = wastewater flow (l/s) P = wetted pipe wall perimeter (m) b = surface width of the stream (m)

Table 1.6.5 - Values of Z, Indicating Sulphide

Generation Potential

Value of Z

Likely condition




Measures are also required to ensure that chemicals do not deteriorate during storage, e.g. due to exposure to sunlight, moisture, or heat.

The chemicals most commonly used for septicity control in the sewerage system or receiving wastewater treatment works are:

Calcium Nitrate is used widely to prevent septicity in sewerage systems. Micro-organisms present in the sewage and in the slimes on the sewer wall will use nitrate as an alternative oxygen source under anoxic conditions. Sodium nitrate can also be used.

High doses of nitrate can be added at the start of the sewerage system, and there will be no loss along the sewerage system. Excess nitrate may lead to rising sludge due to denitrification in the primary sludge.

Some oxidation of sulphide in sewage and sludges may also be achieved by nitrate addition. A required ratio of between 10:1 to 30:1 of nitrate to sulphide has been reported. Addition of nitrate with anthroquinone has recently been proposed to oxidise sulphide in sludges.

Nitrate salts are supplied and stored as a liquid and dosed as a liquid to the pump sump at the start of a rising main. Average daily dose rates are calculated from the aerobic respiration of the sewage, but assume that the rate of nitrate uptake is 40% of that under fully aerobic conditions. The amount of nitrate required for rising mains of different diameters is given in Table 1.6.6. These values are derived assuming that the demand for nitrate nitrogen is 40% of that derived previously; that 2.85 grams of oxygen are available for every gram of nitrate nitrogen, and that calcium nitrate is supplied at a concentration of 110.6g/l N.

The uptake of nitrate results in a slight reduction in BOD. If sufficient nitrate is provided, the sewage will remain fresh.

Table 1.6.6 - Nitrate Dosing Requirements for

Different Pipe Diameters

Diameter(mm)

Nitrate required per 1000 m length

(kg/d as N) l/d assuming 110.6gN/l

350 11.0 99.9

500 18.2 164.2

1000 52.2 471.9

Iron nitrate acts in the same way as calcium nitrate when dosed at the start of a rising main. The iron component also combines with sulphides as they form, and hence dosage rates in practice may be approximately half that calculated for calcium nitrate.

Iron nitrate is, as with other iron salts, an acidic chemical requiring appropriate storage and handling.

Iron salts (sulphate, chloride and nitrate) have been used very effectively to control odours. Iron salts combine with sulphide in the sewage to form a number of insoluble iron sulphides (FeS, Fe2S3, Fe3S4 and FeS2). Ferric salts are more effective than ferrous salts. However a mixture of ferric and ferrous salts in the presence of dissolved oxygen may be the most effectivexxxv:

Fe2_ + 2Fe3+ + 4HS- Fe3S4 + 4H+

The required dose rate decreases with increasing pH value and increases at acidic pH values, with little effect expected at pH values much below 6. At pH 7.0, the dose rate is 2.4mgFe / mgS.

Iron salts are added as a liquid at the discharge point of a rising main or to a septic flow, such as sludge liquor prior to return to the sewage flow. Dosage rates for rising mains containing sewage at pH 7.0, temperature 30oC, with a COD of 600mg/l are given in Table 1.6.7.

Table 1.6.7 Dosing Rates for Iron Salts

Diameter (mm)

Iron required per 1000m length, assuming 2.4mg/l as Fe

(kg/d as Fe)

350 19.4

500 29.2

1000 68.2

Iron salts are acidic and corrosive and require care with storage and handling. Iron salts attack metals,




and appropriate materials are required for bunded tanks, dosing pumps and pipework, together with appropriate safety equipment such as safety showers and eyewashes.

Although effective at precipitating sulphide, iron salts have no impact on the concentration of other odorous chemicals such as volatile fatty acids or on the degree of septicity of the sewage. They therefore may be less effective than septicity prevention systems for reducing odour.

Addition of iron salts to sewage may:

Increase the mass, volume and thickness of primary sludge;

Reduce concentration of phosphate below the required concentration for secondary treatment;

High doses may adversely affect the settleability of the primary sludge;

Give some solids deposition within the sewer;

Affect subsequent ultraviolet disinfection processes;

Increase the combustibility in subsequent thermal drying processes.

Oxygen supplied and stored as a liquid and then dosed into a side stream of sewage as a high purity gas has been used in rising mains and sewers elsewhere in the Middle East to prevent septicity. However, the amount that can be dosed is limited by the saturation concentration of dissolved oxygen, being about 34mg/l at 30oC. The injection of excess oxygen or air into rising main sewers can give rise to gas pockets, which may adversely affect pump regimes. Excess oxygen also exacerbates microbiologically induced corrosion.

Under aerobic conditions, sulphide will be oxidised (predominantly by microbial action) to thiosulphate and sulphuric acid, with some chemical oxidation to sulphur. The rate of oxidation in the sewage stream depends on the numbers of oxidising bacteria present in the sewage and can be in the range of 1 (fresh sewage) to 15mgS/l.h. Some reduction of BOD and COD is seen. Oxidation can occur within the sewage stream, where it will reduce the risk of subsequent odour problems. Where the oxidation

to sulphuric acid occurs in the slimes on exposed sewer walls or sumps, corrosion of the sewer fabric can occur.

The uptake of oxygen results in a corresponding reduction in BOD. If sufficient oxygen is provided, the sewage will remain fresh.

Dose rate is calculated to match the respiration rate of micro-organisms in sewage (typically 12mg/l.h) and wall slimes (assumed to be 1.9g/m2.h at 30oC). This can be calculated for a length of rising main of radius r metres and length, L metres:

gO2/h = ((2prLx 1.9)+ (pr2Lx12))1.07(T-30)

Equation 1.6.3

Overall respiration rate (mg/l) of sewage and slimes in rising mains of different diameters is given in Table 1.6.8.

Table 1.6.8 - Respiration Rates of Sewage and

Slimes

Pipe

diameter

(mm)

Respiration rates (mg/l.h at 30oC)

Total DO demand rate mgO2/l.h Slimes Wastewater

350 22.1 12 34.1

500 15.5 12 37.5

1000 13.7 12 15.7

The amount of oxygen required per 1000m for mains of different diameters using the above respiration rates is given in Table 1.6.9.

Table 1.6.9 - Oxygen requirements

Diameter (mm)

Oxygen required per 1000m length (kg/d)

350 78.7

500 129.4

1000 371.9




The concentration of oxygen at the injection point is determined by dividing the daily oxygen requirement by the daily flow rate of sewage. The maximum amount of oxygen that can be injected is limited by its saturation concentration (about 34mg/l at 30oC and atmospheric pressure).

Alkali addition, such as lime or caustic soda, can be used to increase the pH value of sewage or sludge. At raised pH values, the release of H2S, other acidic sulphurous compounds and volatile fatty acids will be suppressed. Addition is generally to pH 8.0 or 8.5, as higher values can lead to a significant release of ammonia. Use of alkali will become less effective if dosed sewage is diluted with neutral sewage downstream.

Chlorine/hypochlorite acts as an oxidant and also as a bactericide, reducing oxygen demand, and slime growth, but is not widely used because of the high chlorine demand of the sewage, and the reluctance to add chlorine to the sewage flow.

Hypochlorite is used as an oxidant in wet-scrubbing of odorous air:

HS- + 4Cl2 + 4H20 SO42- + 9H+ + 8Cl-

HS- + Cl2 S + H+ + 2Cl-

Hydrogen peroxide oxidises previously formed sulphide to sulphur and water, and provides dissolved oxygen.

Peroxide dosed at the inlet to the rising main provides dissolved oxygen to satisfy the oxygen demand of the sewage and slimes. Dosage rates can be calculated as for oxygen, with available oxygen calculated as 0.48gO2 per gH2O2.

Peroxide can be dosed at the top of the rising main to oxidise sulphide present in the sewage. Dose rate is assumed to be 1 gram of H2O2 per gram of S oxidised at a pH value of less than 8.5.

Chlorine dioxide has been used as an oxidising agent, mainly with sludges and sludge liquors.

Potassium permanganate has been used successfully as an oxidising agent to reduce sulphide levels in sludge liquors and sludges.

Enzyme based chemicals have been promoted for septicity control. These appear to act by reducing fat levels or by inhibiting the activity of sulphate reducing bacteria. Their effectiveness is very site specific, and long-term effectiveness is not known.

1.7 Pipeline Materials and Jointing

The preferred material for use in gravity foul sewers (in Qatar) is vitrified clay pipe (VC), up to 1000mm diameter.

VC pipes are manufactured to 1200 dia in the Middle East. However, Glass Reinforced Plastic (GRP) is preferred for diameters in excess of 1000mm.

uPVC is not acceptable on DA projects.

HDPE is not preferred, but may be used as a sliplining where trenchless methods (see section 5) are necessary for installation, using concrete jacking pipes. Such instances may occur because the high strength concrete pipes necessary for withstanding jacking forces do not have adequate chemical resistance to withstand the aggressive nature of the sewage. The concrete pipe thus provides the required strength, and the lining is chemically resistant.

All materials and jointing should be specified in accordance with QNBS. See also Volume 1 section 4.3.

1.8 Pipe Bedding Calculations for Narrow and Wide Trench Conditions

Pipes can be categorised as rigid, flexible and intermediate:

(a) Rigid pipes support loads in the ground by virtue of resistance of the pipe wall as a ring in bending;

(b) Flexible pipes rely on the horizontal thrust from the surrounding soil to enable them to resist vertical loads without excessive deformation;

(c) Intermediate or semi-rigid pipes are those pipes that exhibit behaviour between those in (a) and (b).

Vitrified clay pipes are examples of rigid pipes while steel, ductile iron, UPVC, MDPE and HDPE pipes may be classified as flexible or intermediate pipes,




depending on their wall thickness and stiffness of pipe material.

The load on rigid pipes is concentrated at the top and bottom of the pipe, thus creating bending moments. Flexible pipes may change shape by deflection and transfer part of the vertical load into horizontal or radial thrusts, which are resisted by passive pressure of the surrounding soil. The load on flexible pipes is mainly compressive force, which is resisted by arch action rather than ring bending.

The loads on buried gravity pipelines are as follows:

(a) The first type comprises loading due to the fill in which the pipeline is buried, static and moving traffic loads superimposed on the surface of the fill, and water load in the pipeline;

(b) The second type of load includes those loads due to relative movements of pipes and soil caused by seasonal groundwater variations, ground subsidence, temperature change and differential settlement along the pipeline.

Loads of the first type should be considered in the design of both the longitudinal section and cross section of the pipeline. Provided the longitudinal support is continuous, has uniform quality, and the pipes are properly laid and jointed, it is sufficient to design for the cross section of the pipeline.

In general, loads of the second type are not readily calculable and they only affect the longitudinal integrity of the pipeline. Differential settlement is of primary concern especially for pipelines to be laid in newly reclaimed areas. The effect of differential settlement can be catered for by using either flexible joints or piled foundations. If the pipeline is partly or wholly submerged, there is also a need to check the effect of flotation of the empty pipeline.

The design criteria for the structural design of rigid pipes is the maximum load at which failure occurs while those for flexible pipes are the maximum acceptable deformation and/or the buckling load. The approach for rigid pipes is not applicable to flexible pipes. For all DA projects, the designer must refer in the first instance to the manufacturers literature, to ensure that the design is in compliance with recommendations.

Please refer to Volume 1 Appendix1

1.8.1 Bedding Design for Rigid Pipes

The design procedures for rigid pipes are outlined as:

(a) Determine the total design load due to:

the fill load, which is influenced by the conditions under which the pipe is installed, i.e. narrow or wide trench conditions;

the superimposed load which can be uniformly distributed or concentrated traffic loads; and

the water load in the pipe.

(b) Choose the type of bedding (whether granular, plain or reinforced concrete) on which the pipe will rest. Apply the appropriate bedding factor and determine the minimum ultimate strength of the pipe to take the total design load;

(c) Select a pipe of appropriate grade or strength.

Details of design calculations, tables, etc, are contained in Appendix 1, Volume 1 - General.

1.8.1.1 Narrow Trench Conditions

When a pipe is laid in a relatively narrow trench in undisturbed ground and the backfill is properly compacted, the backfill will settle relative to the undisturbed ground and the weight of fill is jointly supported by the pipe and the shearing friction forces acting upwards along the trench walls. The load on the pipe would be less than the weight of the backfill on it and will be determined as in narrow trench conditions.

1.8.1.2 Wide Trench Conditions

When the pipe is laid on a firm surface and then covered with fill, the fill directly above the pipe yields less than the fill on the sides. Shearing friction forces acting downwards are set up, resulting in the vertical load transmitted to the pipe being in excess of that due to the weight of the fill directly above the pipe. The load on the pipe will then

qatar drainage spec. foul sewerage vol. 2

Documents

bedding design

sewerage systems design

hydraulic design

design strength

bedding factors

general design guidelines

odour control

pipe bedding calculations