adssc sewage treatment works-version 1.0

ABU DHABI SEWERAGE SERVICES COMPANY (ADSSC)

DESIGN GUIDELINES

SECTION 4

SEWAGE TREATMENT PLANT DESIGN

ADSSC/DG Design Guidelines

Section 4

Sewage Treatment Plant Design Rev: 01 April 2008 of 15

Design Guidelines Abu Dhabi Sewerage Services Company

(ADSSC)

DOCUMENT CONTROL SHEET

Revision No. Date Revision Description / Purpose of Issue

01 April 2008 First Issue

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TABLE OF CONTENTS

4.1 GENERAL REQUIREMENTS ............................................................................4 4.1.1 SCOPE ...................................................................................................4

4.2 PERFORMANCE REQUIREMENTS..................................................................4 4.2.1 INFLUENT DATA....................................................................................4 4.2.2 EFFLUENT STANDARDS ......................................................................5 4.2.3 TSE STANDARDS..................................................................................5 4.2.4 MARINE DISCHARGE STANDARDS ....................................................6 4.2.5 SLUDGE STANDARDS..........................................................................6 4.2.6 STORM OVERFLOW STANDARDS ......................................................6

4.3 DESIGN REQUIREMENTS ................................................................................6 4.3.1 GENERAL...............................................................................................6 4.3.2 SITE SURVEY ........................................................................................7 4.3.3 LAYOUT .................................................................................................7 4.3.4 HYDRAULIC DESIGN ............................................................................7 4.3.5 DISCIPLINE STANDARDS.....................................................................7 4.3.6 CIVIL ENGINEERING.............................................................................8 4.3.7 STRUCTURAL ENGINEERING .............................................................8 4.3.8 MECHANICAL ENGINEERING ..............................................................8 4.3.9 ELECTRICAL ENGINEERING ...............................................................9 4.3.10 GEOTECHNICS .....................................................................................9 4.3.11 PROCESS SELECTION.........................................................................9 4.3.12 SCREENINGS HANDLING ..................................................................10 4.3.13 GRIT AND GREASE REMOVAL ..........................................................10 4.3.14 ENVIRONMENTAL PERFORMANCE..................................................10

4.4 OPERATIONAL REQUIREMENTS..................................................................11 4.4.1 GENERAL.............................................................................................11 4.4.2 MAINTENANCE AND FAILURE PROVISION......................................11 4.4.3 HEALTH & SAFETY .............................................................................12 4.4.4 MAINTENANCE REQUIREMENTS......................................................12 4.4.5 OPERATIONAL RESOURCE REQUIREMENT ...................................13 4.4.6 AUTOMATIC OPERATION ..................................................................13 4.4.7 TELEMETRY REQUIREMENTS ..........................................................13

4.5 DELIVERABLES ..............................................................................................13 4.5.1 DRAWINGS ..........................................................................................13 4.5.2 DOCUMENTS.......................................................................................13

APPENDIX 4.1 – PROCESS SELECTION MATRIX.................................................14


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4.1 GENERAL REQUIREMENTS

4.1.1 SCOPE

All Sewage Treatment Plant design must:

a) Be robust b) Meet irrigation quality, marine discharge quality and other regulatory

standards

c) Be demonstrated that it is the option with the lowest whole life cost 4.2 PERFORMANCE REQUIREMENTS

4.2.1 INFLUENT DATA

a) The design for sewage treatment plants shall be based on average daily domestic flow of 275 l/cap. Industrial wastewater flows and allowances for infiltration water are in addition to this and need to be identified and incorporated into the design flow data. The peak daily flow entering the works should be calculated as the sum of the peak domestic, industrial and infiltration flows.

b) The variation of flow and load to the works throughout the year also

needs to be established. For example, certain residential developments experience large variations in flow and load during the summer months and holiday periods, when residents travel to visit relatives and occupancy rates can fall. Certain industrial processes are seasonal and can place large variations in load on the works at certain times of the year. These factors need to be identified and quantified so that accurate figures for the typical and peak flows can be established.

c) The diurnal variation of flow and load to the works must also be

established. Flows during the night can be considerably lower than daytime flows, leading to the risk of the works being underloaded during the night and overloaded during the day. Either of these scenarios can lead to process failure.

d) It is therefore recommended that the following data be gathered before

commencement of the design phase:

i. A two-week flow and load survey of the works influent, incorporating hourly composite samples of the influent and flow measurements taken at least every twenty minutes. If there is a period of significant variation of the flow and load entering the works due to periods of low occupancy rates, seasonal industrial activity etc., then a separate survey should be carried out to model this variation.

ii. A register of the mean and maximum flows, COD loads, suspended


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solids loads and ammonia loads from all significant traders in the catchment.

iii. A register of significant variations in population and occupancy rates in the catchment.

iv. For new developments, details of the land use and development matrix from the Master Plan should be obtained, together with a phasing plan for the development. The phasing plan should cover the period from construction to full occupancy.

v. An accurate estimation of the volume of groundwater infiltration and its variation.

e) Allowance should be made for recycled works liquors within the

proposed plant or tankered wastes from septic tanks. The design of the works should therefore be sized to include an allowance of 10% additional hydraulic and biological capacity to allow for the contribution of returned liquors.

4.2.2 EFFLUENT STANDARDS

a) The required degree of wastewater treatment shall be based on the effluent requirements and water quality standards as decided by ADSSC with respect to potential for re-use. The effluent must meet the quality standards required for irrigation water and marine discharge.

b) The quality standards required of any specific discharge should meet or

exceed the requirements of the Environment Agency, Abu Dhabi (EAD). 4.2.3 TSE STANDARDS

a) The following effluent standards apply to all treated sewage effluent that is offered for reuse as irrigation water. The standards are maximum values (except in the case of pH, residual free chlorine and dissolved oxygen), hence the discharge must be of a quality that is equal to or better than the stated value.

i. BOD 10 mg/l. ii. SS 10 mg/l. iii. TDS < 2,000 mg/l. iv. Coliforms < 250cfu/100ml. v. Salinity < 1,000 mg/l. vi. Chloride < 140 mg/l vii. pH 6.5 – 8.5 viii. Ammoniacal nitrogen 2 mg/l ix. Turbidity 2 NTU x. Dissolved oxygen >3.0 mg/l xi. Residual free chlorine > 0.5 mg/l xii. E. Coli <100 cfu/100ml xiii. Phosphorus 2 mg/l xiv. Plus various limits upon metal content etc.


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4.2.4 MARINE DISCHARGE STANDARDS

a) The following standards apply to all sewage effluent that is discharged into the marine environment:

i. BOD 30 mg/l. ii. COD 50 mg/l iii. SS 30 mg/l. iv. Coliforms < 1000 colonies/100ml. v. Ammonia 5 mg/l. vi. Turbidity 75 FTU vii. Phosphorus 2 mg/l viii. Plus various metal limits as above

b) The designer should contact the EAD to establish/confirm the latest

standards prior to the commencement of the sewage treatment works design.

4.2.5 SLUDGE STANDARDS

ADSSC will issue guidance which should meet or exceed the appropriate EAD Standards on the quality parameters of sludge produced at a works at an appropriate time. The limits applied in these standards will be in addition to the appropriate EAD standards, which must also be met. Such parameters are likely to include (but not be limited to) toxic metal content, pH, pathogen count, coliform content, % dry solids content and % volatile solids content. ADSSC may also apply conditions relating to the type of sludge treatment and the type of sludge thickening applied at the site.

4.2.6 STORM OVERFLOW STANDARDS

All discharges of sewage effluent to the marine environment must be compliant with the EAD Marine Discharge standards applicable at the time of the design.

4.3 DESIGN REQUIREMENTS

4.3.1 GENERAL

a) All designs for new sewage treatment works and for alterations to existing sewage works in the Emirate of Abu Dhabi shall be issued to ADSSC prior to the commencement of any site works.

b) All design is to be the current best practice utilising the latest methods

and design software, if applicable.

c) The design life of the various items of plant should be specified by the Contractor. In general, these should be as detailed in Table 4.3.1.


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Table 4.3.1 Design Life

Type of Equipment Civil Structures Pumps Other M&E ICA Minimum Design Life 30 years 15 years 15 years 5 years

4.3.2 SITE SURVEY

A site survey shall be carried out in addition to the Flow and Load surveys described earlier in this document. This site survey will include (amongst other items which shall be determined by ADSSC on a site-specific basis) a topographic survey, a geotechnical assessment, a contaminated land assessment, a topographical survey, a groundworks assessment and an ecological assessment.

4.3.3 LAYOUT

a) The proposed site layout shall be submitted to ADSSC prior to the commencement of the construction phase.

b) The proposed layout should minimize cut-and fill operations and should

minimize land loss.

c) The designer should liaise with ADSSC and the relevant planning authorities to ensure that the proposed landscape design is appropriate.

4.3.4 HYDRAULIC DESIGN

a) The proposed hydraulic design should avoid or minimise interstage pumping, minimize headloss, eliminate the possibility of in-channel deposition and should make the best use of existing gravity falls across the site.

b) All liquid-retaining structures should be designed with sufficient

freeboard to ensure that there is no spill under any circumstances.

c) All flow-splitting is to be even except where agreed otherwise with ADSSC.

d) All devices that control and modify flow through the plant should be

subject to automatic control except where agreed with ADSSC. 4.3.5 DISCIPLINE STANDARDS

a) The design shall meet the requirements of the appropriate discipline specifications for Civil, Structural, Mechanical and Electrical work.

b) The discipline standards detailed below do not include all propriety

treatment systems such as those copyrighted by suppliers (these are subject to a separate approval process) but covers the main civil,


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structural, process, hydraulic, M&E, instrumentation, geotechnical and architectural design aspects of a WWTP.

4.3.6 CIVIL ENGINEERING

a) The design of pressure pipelines thrust blocks shall comply with “Guide to the Design of Thrust Blocks for Buried Pressure Pipelines” ref: CIRIA Report 128: 1994

b) The design of pump sumps shall comply with best international practice

and/or Prosser’s “Design Guides for Pump Sumps and Intakes”

c) The selection of materials for applications shall be informed the degree of corrosiveness associated with that environment.

d) All design loadings (dead & live loads, wind loads, thermal loads,

dynamic loads from process equipment) shall either be in accordance with specified design standards, or to the relevant UK codes or an equivalent Eurocode.

4.3.7 STRUCTURAL ENGINEERING

a) Structural works – concrete, steel, masonry, foundations and jointing (construction joints, expansion joints, and movement joints) shall either be in accordance with specified design standards, or to the relevant UK codes or an equivalent Eurocode, or a recognized Good Practice Guide.

b) All structural analysis to use the latest software and methods.

c) Structural concrete shall comply with BS 8110: Structural use of

Concrete and with BS 8007: Design of Concrete Structures for Retaining Aqueous Liquids

d) Structural designs shall comply with the requirements of the Design

Guidelines, Section 1: General, Parts 1.4.11 to 1.4.14

4.3.8 MECHANICAL ENGINEERING

a) All mechanical, electrical and instrumentation equipment is to be compatible.

b) All equipment is to be uniquely tagged and numbered. The tagging

system is to be discussed with ADSSC prior to the commencement of the construction phase. A full list of equipment tag numbers (in electronic format) is to be passed to ADSSC prior to takeover of the plant.


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4.3.9 ELECTRICAL ENGINEERING

a) All cabling lengths are to be minimized.

b) Lightning protection systems are to be designed in line with the best international regulations and practice.

c) All automatic equipment is to be made available for local and remote

control, and to be available for remote monitoring via a SCADA system. 4.3.10 GEOTECHNICS

All geotechnical works should take account of the risks of; sliding, overturning, groundwater issues, flotation, seismic disturbance, settlement and bearing capacity. An appropriate factor of safety should be included. Geotechnical works should be designed so that cut/fill operations are minimized and that the best use is made of the local topography.

4.3.11 PROCESS SELECTION

One of the most important decisions that must be made when designing a sewage works is the selection of the appropriate treatment process. Certain processes are appropriate for large populations but are less appropriate for smaller populations. Table A4.1 (in Appendix 4.1) has been produced to assist developers in their selection of appropriate sewage treatment processes.

a) The contractor will provide robust arguments, including whole life cost

calculations, to justify their process selections.

b) Whole-life cost estimates, including a full breakdown by treatment element, are to be provided.

c) Design shall allow for the expansion and upgrading of facility necessary

to accommodate the envisaged increases in flows and loads over a ten year period.

d) All process design shall be to best current practice. Where innovative

designs are suggested, these are to be justified in terms of cost, performance, operability and environmental performance.

e) Provision for an emergency bypass should be incorporated for all

process units.


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4.3.12 SCREENINGS HANDLING

Crude sewage contains gross solids which can interfere with downstream processes and which pose a significant threat to human health and must be dealt with appropriately. The following two procedures have been shown to be successful in alleviating the risks posed by gross solids:

a) In-line maceration. This is acceptable if the downstream process is an

aeration lane or PST, but is unacceptable if the downstream process is a SAF or an oxidation ditch.

b) Screenings may be removed in the normal way and then passed to a liquid separator (LISEP) plant. This plant dewaters the screenings to approximately 50% dry solids content, producing a material that is solid, compact and easily handled. The compacted screenings are then passed to a skip and are periodically removed from site.

4.3.13 GRIT AND GREASE REMOVAL

The UAE has a predominantly sandy surface geology and consequently crude sewage in the UAE can contain a significant amount of grit. In addition domestic activities such as cooking and washing will produce a significant quantity of grease. All sewage treatment works serving a population of more than 1,000 people should therefore contain appropriately-sized grit and grease removal systems. (Note: these two systems can easily be combined in the same unit.)

4.3.14 ENVIRONMENTAL PERFORMANCE

a) The proposed design shall include estimates of:

i. Chemical consumption ii. Energy consumption iii. Washwater consumption iv. Noise v. Odour vi. Visual impact etc.

b) An Environmental Impact Assessment shall be carried out (in

accordance with EAD guidelines.)

c) These estimates are to be submitted to ADSSC prior to completion of the design phase.

d) The designer should seek to minimise the carbon footprint of the plant

during its lifetime (i.e. the plant should have a minimum Whole Life Carbon Cost (WLCC)). The designer shall supply appropriate justifications for any areas of plant design that do not comply with this principle.


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e) All architectural aspects of the finished plant are to be sympathetic to

surrounding environment, as specified by the appropriate planning authorities.

f) The plant should be compliant with all relevant noise control legislation.

The designer shall produce a model to demonstrate that the plant shall not produce noise at the plant’s boundary that shall be deemed a nuisance by the appropriate regulatory authorities.

g) The design should include provision for offices, stores and hygiene and

washing facilities for site operatives and visitors.

h) The plant should be compliant with all relevant odour control legislation. The designer shall produce a model to demonstrate that the there will be no nuisance odour detectable at the plant’s boundary.

i) All plant consumables are to be identified and quantified.

j) The designer shall calculate the capital and operational cost of the plant.

This cost is to be expressed in cost per day and cost per cubic metre of sewage treated. This cost is to include all costs relating to power, consumables, manpower, spare parts etc.

4.4 OPERATIONAL REQUIREMENTS

4.4.1 GENERAL

a) Suitable H&S and operational signage is to be provided.

b) Emergency lighting is to be provided. Such lighting shall comply with relevant H&S requirements.

c) Fire detection and control systems are to be provided.

d) Site services, lighting, telecommunications etc are to be provided.

e) Building works – include furniture, fully serviced and air-conditioned,

workshop are to have all tools required for maintenance.

f) A fully-equipped laboratory is to be provided, unless agreed otherwise with ADSSC.

g) Energy-saving measures are to be incorporated into the designs.

4.4.2 MAINTENANCE AND FAILURE PROVISION

a) The contractor will provide sufficient capacity and numbers of streams of process units that the works can cope with the failure of all items in the main process stream. Furthermore, it should be possible to remove any


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unit from the operational stream (eg for maintenance purposes) whilst maintaining the capability of treating 100% of the design mean flow.

b) All pumps, blowers, tanks and other major process units are to be

operated in a duty/standby or duty/assist/standby mode. Duty-only operational systems can only be installed with the prior agreement of ADSSC.

c) Standby power generation is to be provided and is to be able to provide

sufficient power to allow for whole plant operation under standard flow conditions.

d) All equipment to be standardised as much as possible to limit spares

requirements.

e) All spare parts to be quantified and sources of them identified prior to handover.

f) A list of critical spares shall be issued to ADSSC for comment prior to

construction. All critical spares are to be identified and provided on-site, unless alternative arrangements are agreed with ADSSC in advance.

4.4.3 HEALTH & SAFETY

a) All systems are subject to a Health and Safety HAZOP during the design phase.

b) Health and safety is to be considered and prioritized in all design

including for construction and operation phases.

c) A full Potentially Explosive Atmosphere Zoning (PEAZ) assessment of the site is to be carried out during the design phase. A report shall be issued to ADSSC identifying the various zoned areas and the reasons for such classifications.

d) The designer shall seek to minimize the number and zoning

classification of confined spaces within the plant, and identify them on drawings.

4.4.4 MAINTENANCE REQUIREMENTS

a) All process items should have appropriate and safe access that will allow normal operational and maintenance activities to be carried out.

b) All process units should be designed so that all items of equipment can

be maintained whilst the plant is operational. Certain operations are of high but not immediate importance to the sewage treatment process (such as the desludging of primary tanks.) The operation of such processes is vital to the performance of the works but can be delayed for moderate periods. Maintenance of equipment in these process streams


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can therefore be carried out when the process is taken off-line for a short period.

c) Maintenance of equipment in vital process streams can be done in one

of two ways: either by bypassing the relevant item of equipment, or by transferring the flow through an alternative process stream.

4.4.5 OPERATIONAL RESOURCE REQUIREMENT

a) The designer shall specify the operational and maintenance resource requirements (in terms of man-hours per week) for the plant.

b) The design shall seek to achieve minimal operator intervention.

4.4.6 AUTOMATIC OPERATION

All standard processes involved in operating the sewage works shall be automated unless prior agreement to the contrary has been reached with ADSSC. Signage shall be included that shows the relevant operational needs.

4.4.7 TELEMETRY REQUIREMENTS

The designer shall prepare a schedule of data to be recorded and monitored on-site. This schedule shall be issued to ADSSC during the design phase. ADSSC will then issue a schedule of telemetry signals that will be required of the plant.

4.5 DELIVERABLES

4.5.1 DRAWINGS

a) The designer shall produce a full set of contract drawings to ADSSC. These drawings shall include (but not be limited to) General Arrangements and P&IDs for all major processes and hydraulic profiles, plus location plans, single line drawings for major electrical items and other drawings.

b) Any amendments to contract drawings or documents shall similarly be

submitted to ADSSC for comment. This shall happen either during the design phase or as soon as possible afterwards.

4.5.2 DOCUMENTS

The designer shall supply a full set of contract documents. These shall include (but not be limited to) O&M manuals, the User Requirement Specification, Functional Design Specification (FDS), a list of spares containing information on the likely replacement intervals and costs for such items, all process calculations, hydraulic & structural calculations and electrical power calculations.


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APPENDIX 4.1 – PROCESS SELECTION MATRIX Table A4.1 – Process Selection Matrix

Population Equivalent

Consent Standard (BOD/TSS /Ammonia) 0 – 100 101 – 1,999 2000 – 9,999 10,000 – 49,999 > 50,000

15/20/5

nSAF

nRBC Or Package ASP Or Trickling Filter

Oxidation Ditch Or Trickling Filters + Tertiary Or BAFF

ASP Or Trickling Filters + Tertiary Or BAFF

ASP Or BAFF Or SBR

10/10/5

nSAF + Tertiary

nRBC + Tertiary Or Package ASP + Tertiary Or Trickling Filters + Tertiary

Oxidation Ditch + Tertiary Or Trickling Filters + Tertiary Or BAFF

ASP + Teriary Or Trickling Filters + Tertiary Or Multi-stage BAFF

ASP + Tertiary Or Multi-stage BAFF Or SBR + Tertiary

5/5/2

No process

No process

MBR

MBR

MBR

Legend: SAF – Submerged Aerated Filter STP – Package Sewage Treatment Plant ASP – Activated Sludge Plant

RBC – Rotating Biological Contactor BAFF – Biological Aerated Flooded Filter SBR – Sequence Batch Reactor MBR – Membrane Bioreactor n – nitrifying

There are a number of issues that affect the interpretation of the above table. These include:

1. The sampling basis upon which the consent is based e.g. annual average, daily average, daily composite 95%ile or 95%ile spot. This will have significant implications regarding the performance expected from the various processes identified in the matrix.

2. The above matrix is based upon Process Guarantees which refer to standards

achieved under typical Western conditions. These standards may not be achieved under conditions typical in the UAE (stronger sewage, higher temperatures, and possible septicity.)

3. High salinity can have a serious effect upon plant performance. In cases

where the influent may be contaminated with significant quantities of saline


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water, the performance of the plant cannot be guaranteed. ADSSC recommend that the PSM is not applied in these cases.

4. Dosing of the final effluent with chlorine (which is fairly ubiquitous in the UAE)

will certainly reduce ammonia concentrations. However, there is a risk of the formation of significant concentrations of chloramines. Should the discharge be subject to tight chloramine consent then the developer should either consider including a separate ammonia removal stage prior to chlorination, or alternatively introduce activated carbon filtration downstream of the chlorination to remove the chloramines.

5. Skilled maintenance is required for both BAFFs and MBRs. This may or may

not be readily available and would certainly incur an additional operating cost. 6. It is very difficult to achieve consistently high discharge quality with small

treatment plants, even with the addition of tertiary treatment. However, the pollution load they produce is relatively small. ADSSC therefore recommend that discussions with the EAD be undertaken with respect to such discharges, with a view to securing a more generous discharge consent for such plants.

7. Only an MBR can consistently achieve a 5/5/2 standard, although an extended

aeration oxidation ditch equipped with tertiary sand filtration would come close. Again, ADSSC recommend that discussions with the EAD be undertaken with respect to such discharges.

END OF SECTION


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