greater dublin strategic drainage study

Greater Dublin Strategic Drainage Study

Assessment of Existing Wastewater Treatment Works

July 2002

Greater Dublin Strategic Drainage Study Assessment of Existing WwTWs _______________________________________________________________________________________________

____________________________________________________________________________________________________________ i Dublin Drainage Consultancy

Table of Contents

1 Introduction ...........................................................................................................................................1

2 Osberstown WwTW.................................................................................................................2

2.1 Introduction .........................................................................................................................2

2.2 Existing Situation ................................................................................................................2

4.2.1 Existing Treatment Process .........................................................................................2

2.3 Discharge Consent Standard..............................................................................................6

2.4 Future Development ...........................................................................................................6

2.5 Land available for expansion ..............................................................................................7

3 Balbriggan and Skerries WwTW Scheme..............................................................................9

3.1 Introduction .........................................................................................................................9

3.2 Existing Situation ................................................................................................................9

3.3 Discharge Consent Standard..............................................................................................9

3.4 Design Constraints..............................................................................................................9

3.5 Future Development ...........................................................................................................9

4 Portrane/Donabate and Rush/Lusk WwTW Scheme..........................................................11

4.1 Introduction .......................................................................................................................11

4.2 Existing Loads ..................................................................................................................11

4.2.1 Rush/Lusk ..................................................................................................................11

4.2.2 Portrane/Donabate .....................................................................................................11

4.3 Discharge Consent Standard............................................................................................12

4.4 Design Constraints............................................................................................................13

4.5 Future Development .........................................................................................................13

4.5.1 Rush/Lusk ..................................................................................................................13

4.5.2 Portrane/Donabate WwTW ........................................................................................13

4.5.3 Pumping Station Upgrades ........................................................................................14

4.6 Industrial Effluent ..............................................................................................................14

4.7 Land Available for Future Expansion................................................................................14

5 Malahide WwTW ....................................................................................................................15

5.1 Introduction .......................................................................................................................15

5.2 Existing Situation ..............................................................................................................15

5.2.1 Existing Works............................................................................................................15

5.2.2 Disinfection.................................................................................................................17



____________________________________________________________________________________________________________ ii Dublin Drainage Consultancy


6 Shanganagh and Bray WwTW..............................................................................................18

6.1 Introduction .......................................................................................................................18


6.2.1 Existing Works............................................................................................................18

6.2.2 Design Populations ....................................................................................................18



7 Swords WwTW.......................................................................................................................21

7.1 Introduction .......................................................................................................................21





8 Leixlip WwTW ........................................................................................................................24

8.1 Introduction .......................................................................................................................24


4.2.2 8.2.1 Existing Works...................................................................................................24



8.5 Industrial Loads ................................................................................................................30

9 Ringsend WwTW ...................................................................................................................32

9.1 Introduction .......................................................................................................................32


9.2.1 Existing Works............................................................................................................32

9.2.2 Existing Flows and Loads...........................................................................................33

9.2.2 Industrial Loads ..........................................................................................................33

9.3 Treatment Process............................................................................................................34

9.3.1 Inlet Screens ..............................................................................................................34

9.3.2 Stormwater Treatment................................................................................................35

9.3.3 Preliminary Treatment ................................................................................................35

9.3.4 Primary Treatment......................................................................................................36

9.3.5 Secondary Treatment.................................................................................................36

9.3.6 Disinfection.................................................................................................................37

9.3.7 Sludge Treatment.......................................................................................................38


____________________________________________________________________________________________________________ iii Dublin Drainage Consultancy



10 Small Works........................................................................................................................41

11 Summary.............................................................................................................................42

Greater Dublin Strategic Drainage Study Assessment of Existing WwTWs __________________________________________________________________________________________

______________________________________________________________________________________________________ 1 Dublin Drainage Consultancy

1 INTRODUCTION

It has been recognised that the continuing increase of the population in the Greater Dublin Area will cause a shortfall in the capacity of the existing and planned sewage treatment works.

This report details the current and planned capacities of each of the works in the study area and identifies those sites that can be readily extended to enable additional flows/loads to be treated.

Within the study area, there are five sewage works with a population equivalent (PE) above 20,000 that provide secondary treatment. Further secondary treatment works are planned at Shanganagh/Bray and Balbriggan; these sites currently provide preliminary treatment only. There is an existing secondary treatment works at Portrane but this works will be significantly extended to treat the catchments Portrane/Donabate and Rush/Lusk.

The proposed works at Shanganagh/Bray, Portrane and Balbriggan, plus the future expansion in Osberstown, will be built as design/build contracts. This type of contract will not specify the treatment process to be provided. Therefore, at these works it will be difficult to determine the available land area after this phase of construction. Similarly, introducing additional loads/flows at these sites may prove contractually difficult because the DB/DBO contract is likely to state maximum flows/loads to be treated.

Information on the existing and planned wastewater works were requested from the County Liaison Officers. To supplement the information provided, site visits were made to the WwTWs at Osberstown, Leixlip, Swords, Malahide and Shanganagh.



2 OSBERSTOWN WWTW

2.1 Introduction

Osbertown WwTW is located in Kildare County, north east of Naas. The sewage works comprises preliminary, primary and secondary treatment. There is also sludge treatment on-site consisting of thickening, digestion and sludge pressing.

The works was originally a primary treatment works, with the additional treatment (Phase 2) being recently added as a design/build contract with the contractors Earthtec Ltd.

The additional treatment was required to meet tighter effluent consent standards, and to treat wastewater from a growing population.

Phase 3 development is planned and this is discussed in Section 2.4.

2.2 Existing Situation

The existing works is designed to treat the flow from a population equivalent of 80,000. The associated design flows are shown in Table 2.1. The existing loads (based on data from July 2000 to June 2001) are shown in Table 2.2.

Table 2.1: Design flows for existing works

PE 80,000

DWF 20,000 m3/d

Maximum flow to treatment 50,000 m3/d

Within 2002 and 2003, additional connections are expected (Athgarvan, Caragh, etc.) representing an additional load of approximately 6000 PE.

The ’Load Assessment and Categorisation’ study suggested that the capacity of the existing works may be reached as early as 2004.

Table 2.2: Current loads to Osbertown WwTW

Population equivalent 54,117

BOD 3,247 kg/d

SS 4,204 kg/d

NH3 485 kgN/d

4.2.1 Existing Treatment Process

The existing sewage treatment process comprises preliminary treatment and primary settlement followed by biological/chemical treatment within sequencing batch reactors (SBRs). A schematic diagram of the existing process treatment stream is described in Figure 1.



Figure 1: Schematic of Osberstown WwTW process stream

2.2.1.1 Primary Settlement

There are two circular primary settlement tanks, each of diameter 25m and sidewall depth 2m. The primary tanks are autodesludged.

2.2.1.2 Secondary Treatment

Secondary treatment is provided by four SBR basins. The SBR basins are of the Cyclic Activated Sludge System (CASS) type, as they include a recirculation flow to the selector plug-flow compartment.

These CASS basins are designed to provide the conditions for carbonaceous oxidation, nitrification and biological phosphorus removal.

It is apparent that little biological phosphorus removal is currently being achieved. To ensure the phosphorus consent is reliably achieved, ferric dosing is also provided.

Screens Gritchambers

CASS Basins

Drumthickeners(2No.)

Digesters (2No.with heatexchangers)

Degasifier(2No.)

Beltfilterpress (2No.)

Gas storage (1No.)

Gas engine - CHP (2No.)

Boiler (2No.)

SludgeTransport

Imported Sludge

Primarysludge

Storm tanks (2No.)

Overflow

Return flow fromscreening

classifier (1No.)& grit classifier

(1No.)

Storm waterreturn

Sludgeliquor

Biogas

Biological excesssludge

Outlet toriver Liffey

Primaryclarifiers

IntermediatePumpingStation



2.2.1.3 Sludge Treatment

Primary sludge, surplus activated sludge and small amounts of sludge imported from other treatment facilities are mixed in a homogenisation tank. The blended sludge is thickened prior to being stabilised in an anaerobic digester. Gas from the digester fuels a Combined Heat and Power (CHP) unit. Finally the sludge is dewatered to approximately 20%ds using belt presses.

Table 2.3 provides detail on the existing pumping stations at the works, and also shows the extension of pumping capacity required for Phase 3. It should be noted that the CASS tanks were constructed at ground level, necessitating an intermediate pumping station.

For the Phase 3 extension, it was not considered necessary to include a new intermediate pumping station (information taken from MCOS report no. 207-501-001/Rp007).

Table 2.3: Existing and Phase 3 capacity of pumping stations

Present Flow Phase 3 Flow

Inlet pumping station 3,700 m3/h 6,000 m3/h

Intermediate pumping station 2,083 m3/h 2,083 m3/h

Tables 2.4 to 2.11 provide detail on the existing process units (all information taken from MCOS report no. 207-501-001/Rp007).

Table 2.4: Screens

Number of units 2

Size of screen aperture 6 mm

Capacity (each) 3,701 m3/h

Table 2.5: Grit chambers

Number of grit chambers 2

Volume of each grit chamber 52 m3

Flow before grit chambers 3,700 m3/h

Flow after grit chambers 2,083 m3/h

Table 2.6: Primary settlement tanks



Number 2

Diameter 24 m

Depth 4 m

Table 2.7: Sequencing batch reactors

Number of CASS basins 4

Anaerobic volume (total) 1,100 m3

Volume of each CASS basin 5,000 m3

Total volume 21,100 m3

Table 2.8: Sludge homogenisation tanks

Number 2

Volume (total) 520 m3

Total retention time 1.2 days

Table 2.9: Drum thickeners

Number 2

Capacity (total) 2200 kgds/h

Table 2.10: Anaerobic digesters

Number 2

Volume (total) 2,634 m3

Retention time (total) 29 days

Table 2.11: Belt presses



Number 2

Capacity (total) 1,100 kgds/h

Sludge production 2,115 kgds/day

Operation time 2 h/day

2.3 Discharge Consent Standard

The existing discharge consent standard is given in Table 2.12. The proposed future (Phase 3) discharge consent standard is shown in Table 2.13.

Table 2.12: Current discharge consent standard

Parameter Discharge Limit

BOD 15 mg/l

SS 35 mg/l

COD 125 mg/l

Total P 0.9 mg/l

Total N 25 mg/l

Table 2.13: Recommended (Phase 3) discharge consent standard*

Parameter Median Discharge Limit

BOD 10 mg/l 15 mg/l

SS - 35 mg/l

COD - 125 mg/l

Total P 0.35 mg/l 0.9 mg/l

Total N - 25 mg/l

Ammonia (as N) 0.9 mg/l 1.5 mg/l

*Ref.: MCOS Report No. 207-501-001\Rp007

2.4 Future Development



A study by MCOS determined that the capacity of the existing works may be reached as early as 2004. Therefore, an expansion of the works, Phase 3, is planned for 2005. This will increase the capacity to a population equivalent of 130,000. The extension will be let as a design/build contract. MCOS have provided an outline design for the extension, although the successful contractor will not be constrained by this design.

Table 2.14: Design flows for Phase 3

PE 130,000

DWF 28,500 m3/d

Maximum flow to treatment 81,240 m3/d

The extension will allow for continued growth in County Kildare for an expected period of more than 15 years.

2.5 Land Available for Expansion

Figure 2 shows a plan of Osberstown WwTW with the existing process units as well as proposed phase 3 units.

The forementioned report has estimated that the total available land area within the existing site boundary is as follows:

• 0.9ha to the north east.

• 0.16ha to the east of the entrance road.

• 0.3ha to the west.



Figure 2: Plan of Osberstown WwTW showing existing and planned units.



3 BALBRIGGAN AND SKERRIES WWTW SCHEME

3.1 Introduction

There is no WwTW at present serving the Balbriggan Skerries area. The existing sewerage system and pumping stations convey foul sewage to short sea outfalls.

The Employer’s Particular Requirements – Design/Build Work, Volume 1, Part 4 – provides the detailed requirements for Phase 1 and Phase 2 development which are summarised below.


Currently, waste water is partially screened and macerated before being discharged via short sea outfalls.

There are five existing pumping stations:

Balbriggan: Quay St. PS Isaac’s Bower PS

Skerries: Kelly’s Bay PS Rush Rd. PS Harbour Rd. PS

Design/build requirements have been documented in Employer’s Particular Requirements Design/Build Works, Volume 1, Part 4. Details of the two phases of developments are described in Section 3.5.


The future discharge consent standard is shown in Table 3.1.

Table 3.1: Design discharge consent standards


BOD 25 mg/l

SS 35 mg/l

COD 125 mg/l

3.4 Design Constraints

The odour control limit will be 1 odour unit (ou)/m3 at the site boundary on a 98% basis. This will be valid for the treatment works, pumping station and storm holding tanks.

Noise levels shall not exceed 35dB(A) at night or 50dB(A) by day at the treatment works or pumping stations.




The design/build requirements specify that a 2-phase development should be undertaken. Table 3.2 gives the estimated load for each phase. The inlet is to be designed to facilitate a third phase.

Table 3.2: Estimated flows and loads for Phases 1 & 2

Phase 1 Phase 2

Design Population 30,000 p.e. 70,000 p.e.

Hydraulic flows 230 l/person/day 230 l/person/day

Treatment plant DWF 6,900 m3/day; 80 l/s 16,100 m3/day; 186 l/s

Peak flow to treatment 3xDWF 240 l/s 560 l/s

Organic loading to treatment 1,800 kg/day 4,200 kg/day

Flow to the wastewater treatment plant in excess of 3 DWF shall be diverted to the stormwater holding tank. The stormwater holding tank shall be constructed in Phase 1, with a capacity to cater for the ultimate volume required in Phase 2. Design should be modular to facilitate future expansion.

The existing pumping stations are to be upgraded to provide the maximum flow rates indicated in Table 3.3:

Table 3.3: Proposed upgrades for existing pumping stations

Pumping Stations Max Flow (Phase 1) Max Flow (Phase 2)

Quay St., Balbriggan 160 l/s 410 l/s

Isaac’s Bower 250 l/s 565 l/s

Kelly’s Bay (installed) 56 l/s 56 l/s

Rush Rd. 175 l/s 186 l/s

Harbour Rd. PS is not included in the proposed improvements.



4 PORTRANE/DONABATE AND RUSH/LUSK WWTW SCHEME

4.1 Introduction

The Portrane/Donabate WwTW is located south west of St. Ita’s Hospital in Portrane and receives flow from Portrane, Donabate and St. Ita’s Hospital. Secondary treatment is provided by an oxidation ditch and clarifier.

The Rush/Lusk region is not currently served by a sewage treatment works. Until 2001/02, separate schemes were to be developed to treat Portrane/Donabate and Rush/Lusk.

However, a decision has now been made to combine the flows at a single treatment works to be located at Portrane. The contract for the works will be let as a design/build/operate (DBO) contract.

4.2 Existing Loads

4.2.1 Rush/Lusk

The current population equivalent for the Rush/Lusk area is some 10,500 PE (taken from a report by PH McCarthy & Partners for Fingal County Council). Industrial development is presently non-existent.

4.2.2 Portrane/Donabate

The existing works was designed to serve a contributing population of 6,000. The works comprises an oxidation ditch and secondary clarifier, as shown in Figure 3. Final effluent is discharged via a 300mm diameter outfall to the Irish Sea.

The existing oxidation ditch receives raw sewage as there are no screens or grit traps. Typically, the works floods 2 or 3 times per year. It is suspected that this is due to insufficient capacity of the outfall pipe.

A report by EG Pettit (Ref.: B6889-N-R-07-A) states that the works is currently overloaded as it is treating wastewater from a PE of 7,688. However,it could be readily upgraded to serve a PE of around 11,000 by operating the plant at a higher F:M ratio (currently 0.1 d-1) and increasing the aeration capacity.

At the time of this assessment, there are no lands zoned for development on the Portrane Donabate Peninsula.



Figure 3. Plan of Donabate WwTW


The current discharge consent standard for Portrane/Donabate WwTW is shown in Table 4.1:



BOD 20 mg/l

SS 30 mg/l

The report by EG Pettit highlights the frequent consent failures. These are partly due to the practice of wasting activated sludge to the final effluent stream.

No discharge consent standard has been confirmed for the scheme to combine the flows from Portrane/Donabate and Rush/Lusk at a single treatment works. However, it is anticipated that the effluent will need to meet the requirements of the UWWTD Regulations. Therefore, the anticipated consent is described in the Table 4.2 below:



Table 4.2 Anticipated Consent Standard

mg/l

BOD 25

SS 35

COD 125


The report by EG Pettit states that the site area is large and that consideration need not be restricted to compact treatment options.

The works is not in close proximity to existing dwellings.


4.5.1 Rush/Lusk

The ultimate PE was reported (by Fingal County Council) to be 30,000. It is anticipated that the ultimate PE will comprise a domestic load of 27,000 PE and a leachate load of 3,000 PE. The leachate term relates to the possibility of providing treatment for the proposed expansion of the landfill at Balleally.

Under the proposed scheme, all flows from the Rush/Lusk catchment are to be pumped to the Portrane/Donabate WwTW for treatment.

4.5.2 Portrane/Donabate WwTW

Table 4.2 summarises the current and future loads to be treated at the Portrane/Donabate WwTW, including the loads from the Rush/Lusk catchment.

Table 4.2: Current and projected PE loads for Portrane/Donabate WwTW

Combined Scheme PE

Existing 18,159 PE

First Stage Design 35,000 PE

Ultimate Design 65,000 PE



4.5.3 Pumping Station Upgrades

The existing wastewater collection system and pumping stations will be inadequate for the estimated future flows. The upgrades required for the 2No. Donabate pumping stations are significant and will require extra land.

Table 4.3: Pumping station details

Pumping Station New Pump Rate

Donabate No. 1 PS 460 l/s

Donabate No. 2 PS 920 l/s

St. Itas Hospital PS 18 l/s

Portrane PS 10 l/s

4.6 Industrial Effluent

There are no existing industrial contributions to Donabate WwTW. Fingal County Council Planning Department are considering a proposal to develop a science park, representing a potential contribution of 2,488PE to Donabate WwTW. Developments are unlikely to occur prior to 2006.

Other non-domestic sources reported include St. Patrick’s Boys’ and Girls’ Primary Schools and St. Ita’s Hospital.

4.7 Land Available for Future Expansion

There is the possibility of further expansion of the treatment works but the available area will be dependent on the treatment processes chosen for the new works.

Donabate WwTW is situated west of St. Ita’s Hospital. To the south-west there are fields (classified as having recreational potential) which are owned by the County Council and classified as zone ‘A’ (High Amenity) by the Council’s Planning Department.



5 MALAHIDE WWTW

5.1 Introduction

The existing sewage treatment works serves Malahide and part of its rural environs. The works, which was commissioned in 1986, was designed to treat a PE of 13,000. The current connected PE is greater than 16,000.

Malahide is a seaside town and attracts many visitors and day-trippers. Water sports are popular in the nearby estuary.

The works discharges its effluent into an estuary that is designated as a special protection area (SPA) in adherence with the European Wild Bird Directive and is also a candidate as a Special Area of Conservation under the Habitats Directive. Therefore, the waters within the estuary must meet the standards required by the Bathing Water Directive.

The works is currently being extended to provide treatment for 20,000 PE.


5.2.1 Existing Works

The works is currently being extended to provide a new inlet works, additional secondary treatment and new storm storage. The existing inlet pumping station is to be retained. Table 5.1 summarises the treatment flows:

Table 5.1: Design Flows for Malahide WwTW

DWF 5011 m3/d

FFT 15033 m3/d

Peak flow to works 25056 m3/d

The works operates as an extended aeration process without primary settlement. There are two existing biological treatment units, each with an integral settlement tank. Aeration takes place within the outer annulus, with settlement in the central settlement tank.

The annular aeration tanks have an outer diameter of 30.8m, an inside diameter of 16.8m, and a water depth of 3.2m. The total volume of each aeration tank is 6,700 m3. The aeration tanks are currently operated at a mixed liquor suspended solids concentration of 3,500 mg/l.

An additional aeration/final settlement tank has recently been constructed. However, at the time of the visit (10th March 2002) the M&E equipment had not yet been installed. The new aeration tank is being fitted with fine-bubble diffusers and the two existing aeration tanks are being converted from surface aeration to fine-bubble diffused air.

A plan of Malahide WwTW is shown in Figure 4.



Figure 4: Plan of Malahide WwTW



5.2.2 Disinfection

Some modelling work has been carried out to determine whether the required bacterial levels would be achieved at the nearest Bathing Beach without disinfection. This exercise indicated that the mandatory standard may be exceeded under certain tidal conditions.

Therefore, disinfection was recommended, and a UV disinfection plant is currently being installed.


The effluent quality standards must comply with the requirements of the UWWTD. The current discharge consent standard is shown in Table 5.2:



BOD 25 mg/l

SS 35 mg/l

COD 125 mg/l

In addition, the Bathing Water Quality Regulations apply to the receiving waters.


The site is constrained and this has caused double-handling of materials during construction. The site is bounded by the main Dublin-Belfast railway-line to the west and a boatyard to the north and east. There is a marina/apartment complex to the south.

As the current site is very constrained, the space for future development is extremely limited.



6 SHANGANAGH AND BRAY WWTW

6.1 Introduction

Currently, crude sewage is discharged from Shanganagh WwTW and Bray Pumping Station via long sea outfalls.

At Shanganagh WwTW, the flow is screened and degritted before being discharged off Killiney Beach via an outfall. The Shanganagh WwTW catchment is in the Dun Laoghaire /Rathdown County and comprises the Deansgrange sub-catchment, extending from Ballybrack to Foxrock. The catchment also includes the Carrickmines sub-catchment. The total catchment has significant undeveloped areas that are now zoned for development.

At Bray Pumping Station, the flow is screened and degritted before being pumped to an outfall. The Bray catchment consists of Bray and some contiguous areas in Co. Wicklow and Dun Laoghaire/Rathdown.

To meet the requirements of the UWWTD Regulations, secondary treatment is required at both Shanganagh and Bray. A Preliminary Report prepared by COWI-MCOS Consultants concluded that the most cost-effective solution would be to provide a combined secondary treatment works at Shanganagh WwTW, with some storm storage at Bray and the transfer of flows from Bray to Shanganagh.

A risk assessment was carried out that determined a Design/Build/Operate (DBO) contract would most likely provide optimum risk transfer and value for money.



Currently, the treatment at Shanganagh and Bray WwTWs consists of screening and grit removal before discharge through a sea outfall.

The current (2000) population equivalent served at Shanganagh WwTW is 65,700. The outfall is 1.6km in length and is 1200mm in diameter. The outfall was originally sized to take sewage from a PE of 250,000.

The current (2000) population equivalent served at Bray WwTW is 37,300. The outfall was commissioned in the early 1990s and is 1.6 km in length and 680 mm in diameter.

Therefore, the total population equivalent served at Shanganagh and Bray WwTWs is 103,000. The industrial component of the load, excluding leachate, is 8.3% of the total (8,500 PE).

It is acknowledged that the current population equivalents are based on limited sampling information and a more extensive sampling/flow monitoring exercise is planned for 2002.

There are three landfills in the catchment areas. The current overall estimate for the leachate contribution is 7,000 PE.

The small Activated Sludge plant at Corke Abbey is near the end of its useful life, and integration of this catchment is an objective of the overall scheme. Industrial loading (from Nypro) accounts for about 330 PE of the current load to this works.

6.2.2 Design Populations



Subject to the findings of the planned flow and load surveys, it is intended that the works be designed and built to treat a population equivalent of 167,400 (2020). Furthermore, the design of the works must allow for extension of the works to treat wastewater from a PE of 200,000.


The receiving waters at Shanganagh are not considered sensitive. The effluent from the works must meet the requirements of the UWWTD Regulations. The future discharge consent standard is shown in Table 6.1:

Table 6.1: Future discharge consent standard


BOD 25 mg/l

SS 35 mg/l

COD 125 mg/l

Although there is currently no secondary treatment at Shanganagh or Bray WwTWs, the water quality at Killiney Beach is generally satisfactory and has met Blue Flag standards. However, under certain conditions unsatisfactory bacteriological levels can result in the vicinity of Dalkey Island.

Historically, the water quality at Bray has not met Blue Flag standards. Although this standard has recently been met, there was a guideline standard failure in 2001.

The introduction of secondary treatment will undoubtedly reduce the total discharged bacterial load. Therefore, in the immediate future bacterial standards should improve.

However, it would be prudent to make provision within the design of the secondary treatment plant, for the future inclusion of wastewater disinfection.


The new works will be procured as a DBO type contract. The tenderers will not be restricted to a specific design or treatment process. Therefore, the land-take required for construction will not be known until the tender evaluation is complete.

The Consultants COWI-MCOS have described some process options for both sewage and sludge treatment. These options show that full secondary sewage and sludge treatment can be accommodated on the existing site at Shanganagh WwTW.

The Shanganagh catchment currently has 70 ha of developed light industrial land with a further 67 ha in Carrickmines recently rezoned and under development. Assuming a typical future light industry load equivalent of 53 PE/ha gives a PE of 7,300.

The Bray catchment currently has 83 ha of industrial development including the Southern Cross estate, which is partly developed. A further 121 ha will be developed shortly in the Fassaroe area. Using the same wastewater generation rate gives a PE of 10,800.

The expected increase in local development is from 3,000 to 3,500 PE for Shanganagh, and from 5,500 to 7,500 for Bray (these estimations are for 2005).



The Ballylogan landfill is currently scheduled to close at the end of 2002, and leachate production will decrease thereafter. The overall current leachate contribution has been increased by 2000 PE (to 9,000 PE) to account for the proposed recycling and transfer centre. However, this contribution is expected to decrease to 5,100 PE by 2020.



7 SWORDS WWTW

7.1 Introduction

Swords WwTW is located in Fingal County and is currently being extended. The original sewage works was commissioned in 1981 and was designed to treat the sewage from a PE of 22,500. The works consisted of primary settlement and activated sludge treatment.

Significant growth has occurred in the catchment since 1981 and this growth is expected to continue. The extension to the works is planned to increase the capacity to 60,000 PE during Phase 1, and to 90,000 PE during Phase 2.

The extensions will enable the works to treat higher flows and loads and also to achieve a more stringent consent standard.

The current works were let as a design-build contract to ASCON/USF Bowen. The contract documents indicated that the sewage influent contained nitrification inhibitors and that the design should make allowance for this.


Before the current extensions, the works consisted of an inlet works followed by two radial-flow primary settlement tanks. Each PST is 16.9m in diameter and 3.0m deep.

Secondary treatment was provided in two aeration tanks, each tank being 21m square and 3.9m deep. There are three final settlement tanks each 20m in diameter and 1.8m deep.

The current (1998) DWF at the works is 6,726 m3/d; the industrial flow component is approximately 10%. The current industrial load is also approximately 10% of the total load to the works.

The existing BOD load from industry is limited to 698 kgBOD/d based on existing licences, and the flow is limited to 1,084 m3/d. The resulting average BOD concentration limit is 644 mg/l.


The future discharge consent standard is shown in Table 7.1:

Table 7.1: Future discharge consent standard


BOD 25 mg/l

SS 35 mg/l

COD 125 mg/l

NH3 15 mg/l

TP 2 mg/l



Final effluent is discharged into the Broadmeadow River just upstream of its confluence with the Broadmeadow Estuary.


The contract documents required ASCON/USF Bowen to produce a Class A sludge. Class A sludges have received enhanced treatment and as a result are subject to less restrictions when applied to agricultural land.

The selected treatment consists of pasteurisation and digestion.

The estuary is defined as an area of scientific interest due to its zoological, botanical and ornithological importance. Also, the Office of Public Works has designated the Broadmeadow Estuary as a National Heritage Area.

The Broadmeadow Estuary is not designated as a Sensitive Area under the Third Schedule of the Environmental Protection Agency Act 1992 (Urban Wastewater Treatment) Regulations, 1994. Therefore, the only obligatory effluent quality standards relate to BOD, SS and COD. However, because of the high amenity value of the Estuary, Fingal County Council decided to reduce the nitrogen and phosphorus loadings into it by setting N and P effluent standards.

The works is very close to residential housing, site with some houses within 20 – 30m of the boundary fence (which is 10-15m from the inlet works). An odour limit of 2 ou/m3 has been set at the boundary of the works. The current extension to the works includes covering of the primary settlement and storm tanks and enclosing the inlet works within a building. An odour control unit treats the vented air.


Phase 1 extensions are designed to treat the flows and loads described in Table 7.2. In addition, the preliminary treatment works must be able to treat 88,000 m3/d. The Phase 1 extensions will be sufficient to meet the loads generated from the development of zoned land (1997 Swords Development Plan).

Table 7.2: Phase 1 design flows and loads

Phase 1 – 60,000 PE

DWF (m3/d) 15,179

Full treatment flow (m3/d) 45,528

Total BOD (kg/d) 3,600

Total TKN (kg/d) 782

Total P (kg/d) 130

Total SS (kg/d) 7,210

Storm flow (m3/d) 5,8615



All Phase 2 treatment stages must be designed to treat 2,734 m3/h. Estimated Phase 2 flows and loads are shown in table 7.3 below.

Table 7.3: Phase 2 design flows and loads

Phase 2 – 90,000 PE

DWF (m3/d) 21,870

Full treatment flow (m3/d) 65,616

Total BOD (kg/d) 5,400

Total TKN (kg/d) 1,157

Total P (kg/d) 192

Total SS (kg/d) 10,388

Storm flow (m3/d) 66,718

For Phase II development of the WwTW, the domestic population equivalent has been calculated to be 81,550 PE. This value is based on typical design allowances for residential development with similar figures used for industrial allowances (which are typical for dry industries with low biological loadings).

However, it is appreciated that industrial development may not be on a dry basis in terms of water usage. To allow for industrial development of a wet basis, an allowance for Phase II development of the WwTW and Main Drainage System must be made. A further 8,450 PE has been allowed for this, bringing the total to 90,000 persons for full development within the long term.



8 LEIXLIP WWTW

8.1 Introduction

Situated to east of Leixlip on the northern bank of the River Liffey, Leixlip WwTW serves the Lower Liffey Valley catchment area and receives wastewater from the towns of Leixlip, Maynooth, Celbridge and Kilcock.

Originally completed in 1971, the works was designed for a PE of 45,000 and is a conventional activated sludge process. Some of the process units were designed for a PE of 90,000 but this did not consider nutrient removal. Average loads from 1998 to 2000 indicate a PE of 56,000 with peaks of 68,000.

A second stream was completed in May 2000 to treat waste from Intel Ireland Ltd. This stream is also an activated sludge process

Sludge treatment is provided that includes thickening, stabilisation and dewatering.



Effluent is conveyed to Leixlip WwTW via 3No. sewers. The combined flow is screened (3No. screens) before being pumped to the grit trap. There are storm weirs before and after the screens. The grit trap has a capacity of 880l/s.

Downstream of the grit trap, an actuated penstock passes a proportion of the flow to the Intel Ireland Ltd. treatment stream. The remaining flow enters the main treatment stream.

The proportion of domestic wastewater that is blended with the Intel Ireland Ltd. wastewater stream is controlled so that a minimum BOD concentration of 100mg/l is maintained in the blended wastewater.

8.2.1.1 Main Treatment Stream

The flow is split between 2No. covered primary settlement tanks (PSTs). Each PST has a diameter of 26.0m, a side wall depth of 2.4m and a volume of 1,598m3. The covered PSTs are ventilated and the air is passed to an activated carbon scrubber unit.

Effluent from the PSTs is split between the 2No. aeration basins (20m x 40m x 3.5m liquid depth) each of which is aerated by 2No. surface aerators. The aeration basins were originally designed for a PE of 90,000 and to achieve the Royal Commission Standard of 20mg/l BOD and 30mg/l suspended solids.

Table 8.1: Aeration basin original design details

Total aeration volume 5,600m3

Retention @ design DWF (20,250m3/d)

6.6hrs

Reported F:M ratio 0.32



Mixed liquor from the aeration basins is pumped to the 3No. seccondary clarifiers via a splitting chamber.

Ferric sulphate is dosed to the effluent from each of the aeration basins. The storage and dosing units were constructed at the same time as the Intel Ireland Ltd. Treatment stream.

There are 2 chemical storage tanks, each with a volume of 96m3. There are independent dosing pumps for each of the treatment streams, capable of providing a maximum dose of 155mg/l.

The original works incorporated two secondary clarifiers. The third was constructed in 1989 during stage 2 development. Table 8.2 provides clarifier details:

Table 8.2: Secondary clarifier details

Original clarifiers

Volume 1,035 m3

Surface area 452 m2

Internal diameter 24.0 m

Side wall depth 1.75 m

Floor slope 7.5 degrees

Hopper volume 5.86 m3

Stage 2 clarifier

Volume 1,134 m3

Surface area 490 m2

Internal diameter 25.0 m

Side wall depth 1.8 m

Floor slope 7.5 degrees

Hopper volume 5.89 m3

Based on the design PE of 45,000, a retention of 7.6hrs is provided in the secondary clarifiers.

Effluent from the secondary clarifiers can be discharged directly to the River Liffey or passed to continuous backwash filters. The total filter surface area is 180m2.

8.2.1.2 Intel Ireland Ltd. Treatment Stream



The Intel Ireland Ltd. Treatment stream has been designed to treat discharge from Intel Ireland Ltd. along with a small domestic contribution. Table 8.3 contains details on the flow to this stream:

Table 8.3: Intel Ireland Ltd. effluent characteristics

Flow 13,000 m3/d

pH 6.0 – 9.0

BOD 100 mg/l

COD 200 mg/l

SS 75 mg/l

Sulphates 1,400 mg/l

Ammonia (NH3 as N) 25 mg/l

Nitrates (NO3 as N) 20 mg/l

Phosphorus (as P) 5 mg/l

Total heavy metals <0.5 mg/l

Temperature 30°C

There is a small domestic contribution to the effluent from Intel Ireland Ltd. Also, to maintain a minimum BOD concentration of 100mg/l in the wastewater, a controlled volume of effluent from the Leixlip catchment is added to the Intel Ireland Ltd effluent.

Table 8.4 gives details of the flow to the Intel Ireland Ltd. treatment stream.

Table 8.4: Flow to Intel Ireland Ltd. treatment stream PE of Intel wastewater 21,666

PE of domestic fraction 2,850

DWF from Intel 13,000 m3/d

DWF from domestic fraction 510 m3/d

Maximum contributing flow from main treatment stream 7,314 m3/d

* Maximum flow 23,153 m3/d

Operating temperature 10°C

* Calculation of maximum flow is based on 3 x domestic DWF and 1.1 x Intel DWF

Based on the population equivalents in Table 8.4, the total organic load from this source is 1,471kg/d.



The design of the Intel Ireland Ltd. treatment stream has allowed for an additional flow of 2,050m3/d (ie a maximum flow of 15,560m3/d), and an additional 205kg/d of organic load (ie a maximum load of 1,676kg/d).

Low lift pumps that lift flow to the inlet area are contained within the Intel Ireland Ltd. building and are reported to have been designed without considering the additional loads mentioned above.

The inlet works comprise a Jones and Attwood rotating bar interceptor screen (Model No. RB022) and Beltafine Screen (Model No. BF5012), both designed for a maximum flow of 270l/s (23,328m3/d). The screening room and wet well of the PS are ventilated and the foul air is passed through an activated carbon scrubber unit.

Screened effluent is pumped to an inlet chamber prior to the selector tank. There are 3No. pumps each capable of pumping 135l/s. Screened effluent is combined with returned activated sludge in the inlet chamber. The dimensions of the inlet chamber and selector tank are both given in Table 8.5:

Table 8.5: Dimensions of the inlet chamber and selector tank

Inlet chamber

Length 5.0m

Width 1.5m

Depth 4.9m

Volume 36.75m3

Retention at max. flow 2.3 minutes

Selector tank

Length 9.4m

Width 5.0m

Depth 8.35m

Volume 392.45m3

Retention at max. flow 24.4 minutes

Flow from the selector tank is split between two aeration basins. The basins are designed as plug flow reactors for full nitrification and partial denitrification with an anoxic zone at the inlet to the basins.

The anoxic zones contain submersible mixers (4No. ITT Flygt Ltd. Model No. SR4650.410 with 5kW motors), whilst the remaining part of the basins contain fine bubble diffusers (3No. blowers, Aerzen Model No. GM 130L with 160kW variable speed motor).

Sludge is returned within each basin at a constant rate of 1 DWF. There are 2No. recirculation pumps (ITT Flygt Ltd. Model No. PP4650.410 with 5kW motors).



The maximum flow to each of the aeration basins, including recirculated sludge is reported as 21,990m3/d. The estimated organic loading is 2,083kg/d, giving an F:M ratio of 0.072d-1.

Effluent from the aeration basins is dosed with ferric sulphate, (as previously described) and is split between 2No. secondary clarifiers. Table 8.6 summarises details of the clarifiers:

Table 8.6: Secondary clarifier details

Internal diameter 26.0m

Sidewall depth 3.5m

Volume 2361.4m3

Surface area 532m2

Retention time @ DWF of 20,837m3/d

5.4 hours

Upflow velocity time @ DWF of 20,837m3/d

0.82m/h

Retention time @ maximum flow of 23,135m3/d

4.9 hours

The secondary clarifiers are equipped with 3No. return sludge pumps (2duty, 1 standby). Each is rated at 22kW with a maximum pump rate of 433.9m3/h vs 6.62m. The pumps are arranged such that there is one dedicated pump to each clarifier with a common standby.

There is a similar arrangement for surplussing sludge. The pumps are rated at 2kW with a maximum pump rate of 18.83m3/h vs 6.62m.

Effluent from the secondary clarifier gravitates to a suite of 4No. tertiary monograde sand filters. The total surface area of the filters is 192m2 and each filter is 1m deep. At the maximum throughput of 23,153m3/d, the filtering velocity is 5.02m/h.

Occasionally the whole flow to the works may be passed through the filters, in which case the filtering velocity increases to 7.5m/h.

Methanol storage and dosing facilities have been provided. The calculated maximum rate of methanol dosing was reported to be 1,178l/d.

8.2.1.3 Sludge thickening, stabilisation and dewatering process stream

Sludge from the PSTs is pumped to the primary sludge thickeners (2No.). Each thickener has a GRP cover. The units are ventilated, with the resultant foul air being treated by an activated carbon scrubber unit.

Activated sludge from both process streams is pumped to separate drum thickeners (3No.). All thickened sludge is combined in the sludge blending tank. This tank has a retention time of 2.6 days (based on thickening the sludge to 5-6% ds).



The thickened sludge is pumped to the inlet of the anaerobic digester where it is blended with heated, recirculating, digested sludge.

The nominal retention time in the digester is 15 days at 35°C. After digestion, the sludge gravitates to a holding tank for a further 15 days prior to dewatering. The digested sludge is dewatered by centrifuge and disposed of to landfill.

Tables 8.7 to 8.10 provide detail on the sludge treatment units.

Table 8.7: Primary sludge thickeners

Number 2

Volume, total 550m3

Surface area, total 157m2

Table 8.8: Activated sludge drum thickeners

Intel sludge thickener

Capacity 200 to 600kg ds/h

Intel sludge production 1,864kg/d

Table 8.9: Sludge digesters original design parameters Number 2

Nominal capacity, total 2468.7m3

PE 90,000

Daily sludge production 7,406kg/d

Hydraulic loading from blending tank @ 4.5% 165m3/d

Total volume required @ 15 days storage 2,468.7m3

Table 8.10: Sludge holding tanks original design parameters

Number 2

PE 90,000

Daily sludge production 7,406kg/d



Daily digested sludge production 5,184kg/d

Digested sludge concentration 3%

Total volume required 2,592m3


Table 8.11: Discharge consent standard


BOD 8 mg/l

SS 15 mg/l

P 0.6 mg/l

NH4-N 9 mgN/l


It is estimated that by 2021 the works will be required to treat wastewater from a PE of up to 130,800. For the main treatment stream this translates to a hydraulic loading of 19,000m3/d, an organic loading of 6,153kg/d and a PE of 102,550. In total the works will receive a hydraulic loading of 34,560m3/d and an organic loading of 7,829kg/d.

Space has been provided within the boundary for extra units. This is situated on the northern bank of River Liffey and is surrounded by green belt. This land has been designated as a highly scenic area.

In the Water Quality Management Plan, the assimilative capacity of the River Liffey at the outfall from Leixlip WwTP is estimated at 387 kgBOD/d. The final effluent design standard for BOD5 is 8 mg/l. Based on this standard, and taking into consideration the managed low flow of 2.0 cumecs for the River Liffey, the allowable effluent flow from the treatment works at times when flow in the river falls to 2.0 cumecs has been calculated to be 400 l/s (34,560 m3/d). Therefore, the River Liffey would only be able to accept increased effluent flows if the final effluent BOD5 standard was tightened further.

8.5 Industrial Loads

A summary of the existing industrial loads, as well as the calculated loads for the medium and long term are given in Table 8.12.

Table 8.12: Summary Analysis of Industrial Loads and Calculation of Future Non-domestic Fraction



Existing Medium Term (2006) Long Term (2021)

DWF (m3//d)

Load (kgBOD/d)

DWF (m3//d)

Load (kgBOD/d)

DWF (m3//d)

Load (kgBOD/d)

Kilcock Leaf Boycetown Future Allowance

32

224.0

32

915

224 91.5

32 915 250

224 91.5 25

Maynooth Future Allowance

250

125

1000

250

Celbridge Industrial Future Allowance

5

1.2

750

187.5

3,500

525 Leixlip Intel Hewlett Packard Future (Intel) Future (other)

10,000 1,000

600 100

13,513 1,000

1,471 100

13,510 1,000 2,050 1,433

1,471 100 205 143

Total Industrial 11,037 925.2 16,460 2,199 23,690 3,034.5



9 RINGSEND WWTW

9.1 Introduction

The Ringsend WWTW is located on the Poolbeg peninsula close to the power stations and in the area of Dublin Port. The works currently provide preliminary and primary settlement of domestic and industrial flows emanating from Central and South Dublin for a population equivalent of 1,200,000. Flows from North Dublin will be introduced during the second half of 2002 and will increase the equivalent population treated to 1.7million by the year 2020. There are constraints on the flow to the works from the MainLift Pumping Station, Dodder Syphon, Dun Laoghaire PS and Sutton PS.

This upgrading of the WWTW is part of improvement in the treatment quality and capacity to ensure compliance with the following:

• The water quality standards of the Dublin Bay Water Quality Management Plan (DBWQMP);

• The Environmental Protection Agency Act, 1992 (Urban Wastewater Treatment) Regulation, 1994 (S1 419 of 1994) giving effect to Council Directive concerning urban wastewater treatment (91/271/EEC) (UWWT);

• Quality of Bathing Waters Regulation 1992 (S1 155 of 1992) giving effect to Council Directive concerning the quality of bathing waters (76/160/EEC).

For the purposes of this report, the existing works will be considered to be that related to the current design year of 2020 and the capacity for further upgrading will relate to a specified ultimate design horizon. Space (0.8 ha) has been allocated on the Site for future expansion and the Contractor is obliged to make provision in the design for increases in flow and load. These are explained further in the following text.



The existing works are those which are comprised in Contract No. 2, Ringsend WWTW. Contract No. 2 includes the incorporation of Contract No. 1, Interim SludgeTreatment, which was implemented in 1998/99 to enable decommissioning of Dublin City Council’s sludge ship and sludge disposal to sea to be abandoned.

The general principles and flow diagrams for Contract No. 2 can be found in Appendix A (Sk 3b – WWTW, Sk 5 Sludge Treatment). The works will comprise the following:

• 6mm inlet fine screening;

• Screenings handling;

• Fats, Oil, Grease and Grit (FOGG) removal for the flow to full treatment;

• Grit removal on stormwater flow;

• Stormwater settlement;

• Primary lamella settlement for treated flows;

• Intermediate pumping station;

• Secondary treatment in sequencing batch reactors (24 No.);



• Ultraviolet disinfection;

• Sludge screening;

• Thermal hydrolysis sludge treatment (Cambi);

• Anaerobic digestion;

• Thermal drying;

• Sludge gas storage;

• Sludge gas scrubbing;

• CHP, steam generation and boilers.

The WWTW receives sewage flows from four facilities:

a) Main Lift Pumping Station (via R&P, City Centre and Grand Canal Sewers);

b) Dodder Valley (gravity flow);

c) Dun Laoghaire Pumping Station;

d) New Sutton Pumping Station (for North Dublin)(in late 2002).

9.2.2 Existing Flows and Loads

The incoming flows for year 2020 are summarised in Table 9.1:

Table 9.1: Ringsend WWTW Existing Design Flows

Population Equivalent 1.7 million DWF 4.6 m3/s Average flow 5.7 m3/s Full flow to Treatment 11.1 m3/s Stormwater Flow 11.5 m3/s Peak flow to Site 22.6 m3/s

The specified design capacity assumes that between construction and 2020 the domestic population in Dublin will rise between 0.36% and 0.8% per annum. The design flows also assume a per capita wastewater discharge in 2020 of 180 l/hd/d. The domestic design average pollutant loads were estimated based on a per capita contribution of 60g BOD/hd/d, 75g TSS/hd/d., 8g AmmN/hd/d, 12g TN/hd/d and 3g TP/hd/d. The existing design loads are therefore as shown in Table 9.2:

Table 9.2: Ringsend WWTW Existing Design Loads for Raw Sewage (kg/d)

Average 75%ile 95%ile BOD5 98,400 123,000 157,600 TSS 101,100 137,496 194,300 TKN 15,600 17,940 21,400 Ammonia as N 9,500 10,925 21,800 Total Phosphorus 3,700 4,225 5,600

The design capacity of the WwTW includes for return liquors within the processes. These are not shown in Table 9.2.

9.2.2 Industrial Loads



The current estimate of the industrial load contribution to the existing Ringsend WwTW is 27,700 kg/d of BOD and 25,000 kg/d of TSS. The industrial flow is estimated to be in the range of 0.2-0.25 m3/s (averaged over a 24-hour, 7-day cycle).

Limited survey data are available for nitrogen and phosphorus loads deriving from industry. It is assumed that the trade nutrient contribution is the difference between the measured total N and P loads and the estimated domestic contribution.

Details of records provided by the Employer of imported wastes discharged at the existing Ringsend WwTW (for 1996) are given in Volume 6.

Flows from North Dublin will be introduced during the second half of 2002. The current estimate of the industrial load contribution from the North Dublin catchment is 2,800 kg/d of BOD and 2,300 kg/d of TSS. The industrial flow is estimated to be in the range of 0.1-0.15 m3/s (averaged over a 24-hour, 7-day cycle).

9.3 Treatment Process

The treatment process and flow diagrams for the works are included in Appendix A. Following preliminary treatment, primary settlement is followed by biological treatment within SBRs and UV disinfection of treated effluent. UV is only required during the bathing season (May/September).

The secondary treatment is provided in 24 no. basins. The basins can only operate in sequence mode as sets of four. Normal operation employs a 4-phase cycle over 4.2 hours; as the flow increases, the cycle time can be reduced to 2.8 hours. A 3-phase cycle is used when one of the basins is out of service.

The Contractor’s design was based on a blended effluent of 12 no. basins providing carbonaceous treatment, and 12 no. basins providing nitrified and denitrified effluent. This provides a treated flow of 11.1m3/s and an effluent standard of 25 mg/l BOD, 35 mg/l TSS, 125 mg/l COD and with 18.75mg/l AmmN on a 95%ile basis.

Additional nitrification and denitrification equipment has been fitted under a variation to the Contract and this should result in a final effluent with a total nitrogen standard of less than 10mg/l total N (measured as an annual mean). Some process control modification may be needed for this to be guaranteed. However, experience of the influent and plant performance will be required before the process control can be optimised.

9.3.1 Inlet Screens

The inlet screens are required to intercept and remove solids larger than 6mm from the incoming sewage. Seven no. screens (six duty, one standby) are required for a future maximum peak incoming flow of 23.5m3/s. Details are given in Table 9.3. One no. Rotating Bar Interceptor (RBI) is provided upstream of the inlet screens for the gravity flows received from Dodder Valley.

Table 9.3: Inlet screens No. Screens 7 no. (6 duty, 1 standby)



Design flow per screen 3.917 m3/s Channel width 2.5 m Channel Immersion 3.0 m Screen Aperture 6.0 m

Screenings removed from the influent are passed to macerators via a launder system. Three no. handling streams are provided as duty/assist/assist each capable of dealing with 12m3/h of wet screenings. The washer/dewaterers produce a final product with 35% dry solids content.

9.3.2 Stormwater Treatment

Stormwater is separated downstream of the inlet screens via two weirs. Aerated grit lanes; 30m long, 4.5m wide and 4.5m sidewall depth, are provided downstream of the weirs.

Stormwater treatment is provided for 1.5 hours retention of 11.5m3/s in six no. storm tanks, four of which are blind (see Tables 9.4 and 9.5). The blind tanks fill first. All six tanks are provided with submersible re-suspension pumps.

The effluent from the storm discharge must not exceed 3 x 106 FC/100ml on a 95%ile basis.

Table 9.4: Storm tank arrangement

No. Storm Tanks 6 no. Configuration 4 blind, 2 overflow Overflow Tanks 2 & 5 Blind Tanks 1,3,4 & 6

Table 9.5: Storm tank dimensions

Tank No. Width (m) Length (m) Volume (m3) 1 31 65 7,346 2 37 104 11,791 3 37 88 9,822 4 31 65 7,346 5 37 107 12,131 6 37 88 9,978

9.3.3 Preliminary Treatment

Screened effluent is elevated from the inlet screen outlet channel by archimedian screw pumps (see Table 9.6) to the inlet to the aerated grit and fats,oils and grease (FOGG) removal channels. The screw pumps pass forward the flow to full treatment (FFT) of 11.8m3/s.

Table 9.6: Archimedian screw pumps

No. Pumps 4 Duty, 1 Standby Maximum Flow 2.775 m3/s Maximum Lift 2.1 m



The aerated grit and FOGG removal facility comprises horizontal flow tanks with grit screws in the hopper bottoms, baffles (recently modified) and surface scraping of grease. Details of the FOGG removal units are given in Table 9.7:

Table 9.7: FOGG removal units

No. Lanes 6 Retention Time 20 mins. @ av. Flow Side wall depth 5.5 m Length 30.0 m Width 6.5 m

Grease removed from the flow is transferred to the sludge treatment, plant for mixing with sludge as it enters the Hydrolysis plant. Grit is removed and classified prior to disposal via skip.

9.3.4 Primary Treatment

Primary treatment is provided in two sets of six lamella settlement tanks. These are designed for the removal of 50% TSS and 24% BOD. The tanks are hydraulically designed for 13.8m3/s to provide sufficient future treatment capacity, but the lamella packs are only rated for 11.1m3/s at present.

Table 9.8: Primary settlement tanks No. of primary tanks 12 no. Design flow (12 tanks in service) 11.1 m3/s Design flow (10 tanks in service) 11.1 m3/s Lamella plate spacing 80 mm Project plate area per tank 2,412 m2 Total projected plate area 25,704 m2 Design loading rate per tank (12) 1.55 m3/m2.h Loading rate per tank (10) 1.90 m3/m2.h Average TSS to sludge process 53,500 kgds @3%ds 75%ile TSS to sludge process 72,500 kgds @3%ds Peak TSS to sludge process 100,800 kgds @3%ds Dimensions: Length 36.6 m Width 12.0 m Side wall depth 4.5 m

The primary tanks are flat-floored with hydraulically operated scrapers which draw the sludge to two hoppers at the end of each tank. These hoppers provide sludge consolidation before transfer to the holding/mixing tanks. The lamellas are also provided with mobile cleaning bridges (1 no. per bank).

9.3.5 Secondary Treatment

Primary settled effluent combines in the inlet channel leading to the Intermediate Pumping Station (IPS) where the sewage is pumped to the lower and upper SBR basins. The IPS has been designed with 10 no. pump stalls, with 2 no. remaining vacant for future secondary treatment upgrading. Further details are given in Table 9.9:

Table 9.9: Intermediate pumping station



No. Low Lift Pumps 4 (3 duty / 1 standby) Design Duty 1,880 l/s @ 11.4 mhd No. High Lift Pumps 4 (3 duty / 1 standby) Design Duty 1,880 l/s @ 20.9 mhd

The secondary treatment comprises 24 no. SBR basins which operate in groups of four. Four-phase cycles comprise fill, aerate, settle and decant overflow for flows arriving which are less than 6m3/s. As the flows increase, the cycles are decreased to 2.8 hours for 11.1m3/s. When one of the basins in a group is out of service, the remaining three operate on a three phase cycle; fill/aerate, settle and decant. Table 9.10 gives the dimensions of the SBR basins, as well as some of the main design criteria used:

Table 9.10: SBR basins and design values No. Reactor Basins 24 Length 52.0m Width 39.0m TWL 6.9m BWL 4.9m Total Process Volume 335,000m3 Aeration Devices Fine bubble diffused air Carbonaceous Operation Design Sludge Age 5 days (total) 4 days (aerobic) Design MLSS 2,500 mg/L @ TWL Nitrification Design Sludge Age 15-25 days (total) 15 days (aerobic) Design MLSS 4,000 mg/L @ TWL Design Flow 11.1 m3/s Design Loads 73,212 kg BOD/d 16,064 kg TKN/d 11,044 kg NH3.N/d

Each of the basins includes a pre-react zone. Submersible mixers are now provided in all of these zones, although when the basin is initially operated in carbonaceous mode there is no need for mixing. In addition, return activated sludge (RAS) pumps are provided. Returning RAS and mixing the liquor with incoming settled effluent aids the creation of nitrates, essential to compensate for low alkalinity, when operating in nitrification-denitrification mode.

9.3.6 Disinfection

Disinfection is provided downstream of the SBRs in 5 no. channels of submersed low pressure Ultra Violet (UV) lamps (one bank per channel). Details are provided in Table 9.11.

UV disinfection is provided to guarantee final effluent Faecal Coliforms (FC) during the bathing season from May to September. The channels housing the UV plant are designed for a flow of 13.8m3/s but have been reduced for the current FFT of 11.1m3/s. In addition, the channel length has been designed to accommodate a future upgrading of the coliform requirements from 100,000 FC/100ml to 10,000 FC/100ml (80%ile compliance).



Table 9.11: UV Disinfection channels

No. UV Channels 5 No. Banks per Channel 1 No. Modules per Bank 10 Each Module 2 lamps wide x 10 lamps

deep No. Lamps per Bank 200 Total No. of Lamps 1000 Design Flow Rate 11.1 m3/s UV Dose Design Rate 5.7 m3/s Effluent Quality 100,000 FC/100ml UV Transmission 45% UV Dose at end of Lamplife 16 mJ/cm2

9.3.7 Sludge Treatment

Sludge is removed from the primary settlement tanks and the SBRs to the 2 no. sludge holding/mixing tanks (7,790m3 total capacity) adjacent to each lamella bank. The average design sludge loads (assuming 50% removal of solids during primary settlement) are given in Table 9.12:

Table 9.12: Design sludge loads Design Sludge Loads (2020) Primary Sludge 53,612 kg/d @ 3%ds 1,787 m3/d Secondary Sludge 52,000 kg/d @ 0.85%ds 6,118 m3/d Combined Sludge Load 105,612 kg/d @ 1.34%ds 7,905 m3/d

Sludge is transferred from these tanks for treatment by thermal hydrolysis and mesophilic anaerobic digestion, followed by thermal drying. The final product is required to conform to USEPA Regulation 40 CFR Part 503 Class A, must have a bulk density of more than 600kg/m3, and must not be less than 92%ds.

The upstream processing of sludges comprise conventional screening (5mm), dewatering by belt thickeners and belt presses to present 101,388 kg/d of sludge (@ 20%ds) to the thermal hydrolysis plant.

The hydrolysis plant comprises two streams which begin with buffering and pre-heating of the sludge, with recycled steam, in two pulper vessels. Sludge is heated in the pulpers and also conditioned by a recirculating macerator pump. As the temperature rises, the dry solid concentration falls to about 12% ds. After transfer into the 4 no. reactors, which operate in sequence, the sludge temperature is elevated further to 165oC and pressurised to 12 bar with steam. The fill, pressurisation, depressurisation and emptying of the reactor takes approximately 1.5 hours.

The temperature of the sludge after depressurisation is approximately 100oC. This is further cooled by pre-heating boiler feed water to 40oC for feed into the digesters. Three anaerobic digesters are provided, details of which are given in Table 9.13.

Table 9.13: Anaerobic digesters



No. Digesters 3 Design Feed Rate 101,388 kg/d @ 12%ds Sludge Flow 849 m3/d Sludge Feed VM 79% VM Reduction 56% Design Gas Production 1 m3 biogas/kg VM destroyed Dimensions Side Wall 16.0 m Diameter 17.7 m Hopper Depth 2.0 m Volume 4,250 m3

After digestion, the sludge is transferred to the thermal drying plant at 6%ds. The thermal drying plant comprises three streams (A & B from Contract No. 1 and C from Contract No. 2), as shown in Table 9.14. Prior to the dryers, centrifuges dewater the feed sludge to 25-30%ds.

Table 9.14: Thermal dryers

No. Dryers 3 Design Flow Rate 52,925 kg/d Dryer Feed Rate 223 m3/d @ 30%ds Dryer Rating 4.2 te/h Final Product 63m3/d @ 92%ds


The wastewater effluent guarantees are based on the Urban Wastewater and Bathing Water Directives. The discharge consent standard is shown in Table 9.15:

Table 9.15: Discharge consent standard

Guaranteed Effluent Quality 95%ile Max. BOD 25 mg/l 50 mg/l SS 35 mg/l 87.5 mg/l COD 125 mg/l 250 mg/l Ammonia as N 18.75 mg/l 47 mg/l Faecal Coliforms (FC/100ml) 100,000 (80%ile)

The Contract has included the installation of nitrification and denitrification equipment comprising submersible mixers in the pre-react zone and RAS pumps. This is the first stage in upgrading the final effluent requirements to >10mg/l total N. However, alkalinity dosing and/or process control adjustments may also be necessary to provide these requirements.

The sludge quality standard required is shown in Table 9.16:

Table 9.16: Sludge quality standard

Final Product Sludge Micro Organisms FC < 1000 MPN/g.ds Or Salmonellae < 3 MPN/4g.ds Metals Cr - 3 mg/kg ds



Hg - 16 mg/kg ds Cu - 1000 mg/kg ds Ni - 300 mg/kg ds

Zn - 2500 mg/kg ds Cd - 20 mg/kg ds Pb - 75 mg/kg ds

Dry Solids Concentration 92%ds Bulk Density 600 kg/m3


Provision has been made in the design of the plant for future upgrading. This includes the requirement for the preliminary treatment and primary treatment to be designed for an ultimate peak flow of 23.5m3/s. FFT is specified as 11.1m3/s for the remainder of the wastewater process; however, the primary settlement tanks, whilst hydraulically designed for 13.8m3/s, will require additional lamella packs to be added. In addition, the sludge handling plant will also have to be upgraded.

The IPS has provision for additional pumps to be added to enable secondary treatment of 13.8m3/s however, additional biological basins will be required. To cater for this, 0.8ha is reserved on the site layout for additional secondary treatment.

Provision is also made for increasing the hydraulic capacity of the UV disinfection plant to deal with 13.8m3/s by removing the channel reducers. The channels can also accommodate larger equipment to enable the FC standard to increase to 10,000 FC/100ml.

With the increased flow there is likely to be a load increase which will manifest itself as sludge as a product of the process. Provision has been made for an additional reactor to be added to each of the hydrolysis process streams. Space has also been allowed for a fourth digester. Since the dryers are each 4.2 te/hr capacity, there is the potential for them to deal with 135 tds/d (with all operating).



10 SMALL WORKS

The are a number of small works in the catchment, the details of which are summarised in Table 10.1:

Table 10.1: Small works within the Greater Dublin Catchment

WwTW Current PE Future PE Treatment process

Estimated sludge production (m3/yr)

The Naul 400 Extended aeration 535

Ballyboghill 250 Extended aeration 335

Garristown 200 Oxidation ditch 316

Oldtown 500 Oxidation ditch 791

Toberbunny 640 Activated sludge 1,012

Loughshinny 1,000 ♦ 2,000 ♦ Septic tank 852

Rowlestown 100 Septic tank 51

Colecut 100 Septic tank 51

Turvey 100 Septic tank 51

Balgriffin 100 Trickling filter 80

Batterstown 100 Puriflow Package Plant

♦ Seasonal high.

All of the WwTWs in the above table are situated in Fingal County apart from Batterstown WwTW which is in Meath County. Newcastle WwTW (South Dublin) will be decommissioned.



11 SUMMARY

There is significant population growth anticipated in the Greater Dublin Area and it has been recognised that there will be a shortfall in the capacity of sewage treatment works serving this area.

This report identifies the existing sewage treatment works within the study area, describes their design capacity and identifies any planned extensions.

Within the study area there are four sewage works, Osberstown, Leixlip, Swords and Malahide that currently produce a secondary treated effluent.

Osberstown and Leixlip have recently been uprated and Swords and Malahide are currently being extended. In addition, further extensions are planned at Osberstown, Leixlip and Swords. Malahide works is situated on a very constrained site and would appear to little scope for further cost-effective development.

A secondary treatment works is currently being commissioned at Ringsend. Although the Ringsend site is constrained, which has led to the secondary units being stacked, there is a section of land available for a future extension.

Secondary treatment works are planned at Shanganagh/Bray, Balbriggan and Portrane/Donabate. Each of these works will be designed to treat a given population equivalent but the design must allow for future expansion.

There are also a number of small works within the catchment area, predominantly in Fingal County.

Table 11.1 below provides estimates of the existing population at each works/proposed works, the design capacity of these works and any allowed further expansion.



Table 11.1: Existing, Design and Ultimate PE (Totals & Trade Contribution)

WwTW Existing Design Ultimate Design

Total PE Trade PE

Total PE Trade PE

Total PE Trade PE

Ringsend 1,500,000 500,000 1,700,000 1,900,000

Osberstown 54,117 No Info 80,000 No Info 130,000 No Info

Ballybriggan & Skerries

30,000 No Info 30,000 No Info 70,000 No Info

Donabate/ Portrane/ Rush/ Lusk

18,159 NIL 35,000 NIL 65,000 5,488

Malahide 16,000 No Info 20,000 No Info 20,000 No Info

Shanganagh & Bray

103,000 17,500 167,400 No Info 200,000 No Info

Swords 30,160 3,016 60,000 No Info 90,000 8,450

Leixlip 56,000 15,420 90,000 36,650 130,800 50,575

Miscellaneous (small works)

4,490 No Info 4,490 No Info 4,490 No Info

TOTAL 1,411,926 2,186,890 2,610,290

NB Figures for Ringsend include North Dublin

The miscellaneous small works were not considered in detail and it has been assumed that the values for the existing and design population equivalents were the same.

Consideration of the existing and design PE totals indicates that there is approximately 775,000 PE spare capacity, either already constructed or soon to be so. However, it should be noted that, due to rapid growth, the current design PE for the Ringsend WwTW may actually be reached sooner than the 2020 design horizon.

In addition, consideration has been given within the existing designs to providing additional capacity of approximately 425,000 PE at the existing sites.

Greater Dublin Strategic Drainage Study Assessment of Existing WwTWs _________________________________________________________________________________________

_____________________________________________________________________________________________________ Dublin Drainage Consultancy

APPENDIX A

Flow Diagrams for Ringsend WwTW Contract No. 2


7 No. Screens@ 3.917 m3/s ea

6 No. Duty

Grit & GreaseRemoval

6 No. Lanes

LamellaPrimary

SettlementTanks

2 Banks of6 each

Intermediate Pumping Station8 No Pumps @ 1.88 m3/s ea

6 No. Duty

UltravioletDisinfection

1000 LP amps5 No Channels1 Bank in each

(Design 13.8 m3/s)

Sequencing BatchReactors

24 Basins16 No Nitrifying

8 No Carbonaceous

StormSettlement

Tanks62,100 m3

6 No.tanks4 No Blind

Grit Removal2 No. Lanes

5 No. Screw Pumps@ 2.777 m3/s ea.

4 No. Duty

MLPS 16.1 m3/s Dun Laoghaire PS 1.0 m3/sDodder Valley 3.25 m3/s

22.6 m3/s Peak23.5 m3/s Design

Emergency Overflow

FFT = 11.1 m3/sDesign = 13.8 m3/s

Storm Overflow:Ultimate = 9.7m3/sDesign = 11.5 m3/s

FFT = 11.1 m3/sDesign = 11.1 m3/s

Final Effluent = 11.1 m3/s25 mg/l BOD35 mg/l TSS18.75 mg/l Amm N

11.1 m3/s - FFT98,400 kg/d BOD101,100 kg/d TSS15,600 kg/d TKN

11.5 m3/s - SFT

Ringsend WWTWContract No. 2Existing Works

ef/NE01803/SK3b18th April 2002

Tankered Waste

Screenings

Grit

StormReturn

Sludge PlantLiquor Return

S

S

S

S

S

S

SFOG

S

SurplusSludge

52,000 kg/[email protected]% ds

N

C

S

S

Sludge S

SludgeHolding

Tank

S

53,612 kg @3 % ds

105,612 kg/d @1.34 ds


Ringsend WWTWSludge Treatment

ef/NE01803/Sk 518th April 2002

2 No. SludgeScreens

2 No. Buffer Tanks

CakeHopper

Belt Thickeners &Belt Presses 5 No. each

2 No. Streams Hydrolysiscomprising each:

Pulpers 1 & 24 No Reactors

1 No. Flash Tank

3 No. Streams AnaerobicDigestion @ m3 ea

DigestedSludge PS

2 No. SludgeHolding/mixing Tanks

BufferTank 1

BufferTank 2

Centrifuge A Centrifuge B Centrifuge C

Dryer A Dryer B Dryer C

FinalProductSilo 1

FinalProductSilo 2

2 No.Screens

Contract No. 1

Primary Settled Sludge53,612 kg/d @ 3% ds

Surplus ActivatedSludge

50%ile flow101,390 kg/d @ 11.% ds

over 20 hours

100 % Bypass105,612 kg/d @ 1.34%

Final Product63,000 kg/d @ 92% ds

Class A 600 kg/m3

75%ile Bypass31,680 kg/d @ 1.34 % ds

FO

Dryer Rating each :4.2 te/hr 45 tds/d

Centra

Filtrate

Sludge

Screenings

greater dublin strategic drainage study

Documents