advanced digestion at cardiff and afan; dwr cymru welsh water … · 2019-03-22 · 15 th european...

15th

European Biosolids and Organic Resources Conference

www.european-biosolids.com

Organised by Aqua Enviro Technology Transfer

ADVANCED DIGESTION AT CARDIFF AND AFAN; DWR CYMRU WELSH WATER DRIVE

FOR LOWEST SUSTAINABLE COST OF SLUDGE TREATMENT AND 15% REDUCTION IN

CARBON FOOTPRINT

Bowen, A.1, Evans, B.¹ Oliver, B.2, Evans, R.² Merry, J² 1Dwr Cymru Welsh Water, 2 Imtech Process

Corresponding Author Tel. 01543 496600 Email [email protected]

Abstract

Dwr Cymru Welsh Water has developed an innovative sludge strategy for AMP5, moving away

from energy intensive thermal drying and lime stabilization to Advanced Digestion with a

programme to process 75% of its sludge production across four key sludge treatment centres.

The core of this strategy is the development and delivery of Advanced Digestion plants at both

Cardiff and Afan. These sites will process 50,000 tDS/y using Cambi thermal hydrolysis plants

and new concrete digesters, prior to belt press dewatering, storage and recycling enhanced

sludge cake to agriculture. This sustainable approach to sludge treatment has been encouraged

by Welsh Assembly Government, Regulators, local planning departments and local communities,

with no objections to planning proposals.

Detailed design and delivery of the projects has progressed smoothly through DCWW’s capital

delivery partners Imtech Process and Morgan Sindall. The projects have been delivered ahead of

programme, within budget and will generate more than 4.5 MW of renewable power, reduce

operating costs by over £7M/y and reduce DCWWs operational Carbon footprint by

approximately 15%.

Design and delivery experience together with early commissioning experience is presented in

this paper. The next goal is power self sufficient wastewater service at Cardiff.

Keywords

Advanced Digestion, Cambi Thermal Hydrolysis, High Efficiency CHP, enhanced sludge quality,

power self sufficient operation, operational savings, and sustainability

Introduction

DCWW sewage sludge treatment and recycling management plans have been progressively

developed over previous AMP periods to ensure sewage sludge is effectively treated and

recycled to agriculture with the original main emphasis being consistent outlet.

With the potential onset of new legislation early in AMP3 aimed at tightening controls on the

agricultural recycling, the sludge strategy at that time focused on rationalising the number of

sludge treatment centres and establishing effective pathogen reductions processes at strategic

locations throughout Wales. It was confirmed that agricultural recycling was the most

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sustainable outlet and therefore sewage sludge was treated to meet the standards agreed in the

Safe Sludge Matrix. The following sludge treatment processes were then established as part of

the sludge management plan.

• Mesophilic Anaerobic Digestion (MAD) followed by batch storage achieving the

‘conventionally’ treated standard.

• Lime Stabilisation by either hydrated lime or quick lime dosing achieving an ‘enhanced’

treated standard.

• Thermal drying of sewage sludge at high temperatures to produce an ‘enhanced’

treated product with a very high Dry Solids (DS) content.

The AMP4 sludge management plan maintained the serviceability of the established sludge

treatment assets however in the early years of AMP4 the following factors highlighted the need

for a more significant strategic change to the sludge management plan for future years.

• Likely significant increases in energy prices

• Climate change and the need to reduce greenhouse gas emissions

• Reductions in sludge volumes for transport and recycling

• Opportunities for the production of ‘Green Energy’

• Higher than frontier sludge treatment and recycling costs.

Notwithstanding the need for longer term consistent outlet routes, the opportunities to recover

energy from sewage sludge by maximising the generation of green power was identified as a

major opportunity for DCWW to invest in sustainable sludge treatment processes. This would

address all the above factors and importantly significantly reduce operating costs and carbon

emissions for sewage sludge processing and recycling.

The benefits of this approach were demonstrated by the comparison of current and projected

DCWW operating costs for sludge treatment and recycling. This was presented in a Roadmap

format to show a ‘do nothing’ scenario compared with an investment in energy recovery

processes, see chart below.

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Figure 1: Roadmap to Optimise Future OPEX Tends

Sludge production in Wales at the end of AMP4 was 85,000 tonnes dry solids (tds)/year and is

expected to increase to 88,000tds/year by 2015. At the end of AMP4 this was nominally split

into 25,000tds/year in North Wales, and 60,000tds/year in South Wales.

Throughout AMP4 sludge drying plants operated at the Cardiff, Afan and Nash WwTWs in South

Wales and processed between 25,000 and 30,000 tds/year. The OPEX associated with these

plants was high and very sensitive to energy prices. In North Wales the Five Fords WwTW

sludge treatment centre was the largest sludge treatment centre that processed circ

10,000tds/year by lime stabilisation.

The nominal split of sludge treatment processes throughout AMP4 was.

30

25

20

15

10

5

0

AMP4 AMP5 AMP6

Eign

£0.5m

Five Fords

Phase 1

£0.5m

STC Optimisation

£0.5m

Cardiff/Nash

£4.8m

Afan

£2.0M

DCWW optimum trend for

OPEX with full realisation

of savings

UK Best

Practice

Frontier

AMP4 Operational

Improvement Plan

Do Nothing

Five Fords

Phase 2 £0.5m

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Figure 2: AMP4 Sludge Treatment Processes

With energy recovery from sludge and longer term sustainability as the main focus, approval

was granted for feasibility work to confirm the optimum energy recovery processes that would

match the specific DCWW future sludge strategy objectives. This concluded that the provision

of incineration processes was highly likely to be problematic in getting planning approval and

the technological development of gasification processes was less well progressed. It was

therefore further concluded that the main Water Industry opportunity for the future would be

investment in Advanced Anaerobic Digestion (AAD) processes. These would produce larger

quantities of bio-gas to generate green electrical power via CHP plants thereby leading to

significant OPEX reductions.

An initial assessment of AAD technologies relative to sludge treatment centre size was carried

out and a realistic minimum viable plant capacity of approximately 10,000 TDS/year established.

All DCWW sludge treatment centres were reviewed and 5 sites identified for the possible

provision of AAD plants namely Cardiff, Newport (Nash), Afan and Hereford (Eign) WwTW in

South Wales and Five Fords WwTW, Wrexham in North Wales. Technical and financial

assessments were carried out and it was concluded that the optimum investment plan was for

the provision of AAD plants as follows.

Hereford, Eign WwTW: 10,000tds/year AAD plant

Cardiff WwTW: 30,000tds/year AAD plant with capacity to treat sludge

imported from Nash WwTW

Afan WwTW: 20,000tds/year AAD plant

Five Fords WwTW:

AMP5: 10,000 tds/year conventional AD plant

AMP6: Conversion to a 15,000tds/year AAD plant

In adopting this plan the expected change in sludge treatment process is shown below.

36,951

18,245

30,880

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

100,000

End of AMP4 Process Split

DryingLimingDigestion

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Figure 3: Proposed Sludge Treatment Processes AMP4 to AMP5

Approval was granted by DCWW in 2008 to progress with the delivery of the Eign WwTW AAD

plant, prior to the commitment to the other plants. This scheme comprised the provision of a

hydrolysis plant in front of the existing conventional digestion plant. The Monsal Enhanced

Enzymic Hydrolysis technology was selected for this site mainly due to the plant size and

integration with the existing digestion facilities.

Notwithstanding this, the main cost benefit for the company was identified as the provision of

the AAD plants at Cardiff and Afan WwTW. This would allow the de-commissioning of the high

OPEX sludge drying plants at Cardiff, Nash and Afan WwTW and maximise the OPEX reduction

for the processing of 50,000tds/year. The reduction in carbon emissions for the schemes would

also a make a significant 15% contribution to the overall company carbon reduction targets for

AMP5.

A major potential risk and constraint to the plan for the Cardiff and Afan plants was the UK

Government’s revised bandings for processes that benefit from Renewable Obligation

Certificate (ROC). This stated when published in 2007 that for future schemes that generated

power from sewage sludge gas the ROC benefit would be de-classified to 50% of the ROC value

unless the following timescales for project delivery detailed were met. If these were met the

process would retain the full ROC benefit for 20 years as long as:

• Preliminary Accreditation by Ofgem was obtained with an effective date before 1 April 2009.

To achieve this accreditation Planning Approval had to be obtained for the project by this

date.

• Full Accreditation by Ofgem was obtained before 31 March 2011 based on the current

legislative timetables. The plant had to be operational and producing power for full

accreditation.

Meeting this criteria was essential for the Cardiff and Afan schemes in particular therefore,

outline design was progressed for both schemes and Planning Applications submitted. Planning

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

90,000

AMP 4 AMP 5

Thermal drying

Advanced Digestion

Conventional Digestion

Lime Pasteurisation

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Approval was granted for both schemes prior to the 1 April 2009. This met the first condition

for the retention of the 1 ROC benefit and Preliminary Accreditation was consequentially

granted by Ofgem.

The overall AAD programme was presented to OFWAT as part of the overall PR09 sludge

management plan however prior to the AMP5 Final Determination approval was granted by

DCWW to progress with the delivery of the Programme. This demonstrated DCWW’s

commitment to the strategy and was also critical to progressing the programme to meet the

conditions to retain the 1 ROC benefit for the Cardiff and Afan schemes in particular.

Table 1: The high level objectives for the plan as presented to OFWAT were:

Site Projected Base

OPEX Saving

£M

Carbon Footprint

Reduction

T/year CO2 equiv

Green Power

Generation

MW

Eign 0.5 2,790 1.0

Cardiff/Nash 4.8 29,541 2.7

Afan 2.1 5,204 1.8

Five Fords 0.4 25 0.5

Total 7.8 37,115 6.0

Design and delivery experience

With the focus on the full construction and delivery of the Cardiff and Afan schemes to meet the

April 2011 ROC deadlines, DCWW selected delivery contractor partners from their existing

Alliance partners via a specific Request For Proposal (RFP) process. This resulted in Morgan

Sindall being selected for the delivery of all Civil works and Imtech Process being selected for the

delivery of all Process & M&E Engineering work

Critical to the overall success of the delivery of the two schemes was the final selection of the

AAD technologies for these sites.

A full technical and commercial assessment of available AAD technologies and suppliers was

undertaken via a tender process. The main AAD processes consider were Enzymic Hydrolysis

and Thermal Hydrolysis.

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Table 2: The key higher level comparison factors for selection were:

Comparison factor Thermal Biological Comments

Unit Process cost √ Lower CAPEX for EEH

Volatile Matter (VM)

destruction √

Proven higher VM destruction and hence

biogas production

OPEX √ Higher OPEX savings due to more biogas

Supplementary Fuel √ Higher Supplementary for Thermal

Final Cake DS √ Thermal proven to produce higher DS

SAS Processing √ Proven experience

Suitable for cake

imports √

Dewatered sludge cake fed to thermal

process

Suited to liquid imports √ Thickened liquid sludge fed to process.

In addition the following OPEX sensitivity factors were closely reviewed for each process and

cost models developed using a range of data for each factor.

Table 3

� Bought in electrical power price

� Export electrical power price

� Gas/Thermal energy price

� ROC value

� Sludge availability & throughput

� Sludge type (Primary/SAS)

� VS Destruction

� Hydrolysis plant energy demand

� Maintenance requirements

� Gas Yield

� Gas Quality

� CHP engine efficiency

� CHP plant availability

� Final sludge cake quality

� Final Sludge cake DS content

� Final Dewatering plant Polymer

� Consumption

This resulted in the Cambi thermal hydrolysis being selected for Cardiff and Afan schemes for

the following reasons

� Best overall economic solution relative to DCWW objectives

� Proven technology experience at the required scale

� Proven delivery capacity and capability

� Process flexibility for various sludge types and quantities

� Proven treatment of SAS

� Process proven on sludge trials

� Higher VM destruction

� Reduced volumes of sludge to agriculture

� Good operational integration with the existing plants

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Detailed Design and Delivery

Following outline design and planning application the programme progressed into detailed

design phase. This included a risk and value review with operations and key technology

providers to ensure that the best whole life cost solution was developed at both the Cardiff and

Afan schemes. The risk and value review confirmed sufficient raw sludge cake storage to ensure

a resilient long term operation together with optimum sizing of the Cambi Thermal Hydrolysis

plants and storage of treated sludge cake to allow reliable recycling to agriculture.

Detailed design development included adopting 3D CAD to allow optimum design and

standardisation across both plants together with integration of key supplier design details.

A key challenge was the design and construction of the new digesters at each site. At Cardiff

there were particular site constraints arising from the historic use of the site for steel works

operations, the presence of large waste ingots led to the selection of a raft design to support the

two new 7500 m3 digesters at Cardiff. At Afan there were different constraints including very

limited footprint, here post tension design was selected consistent with successful recent

application at Dublin.

A key aspect of detailed design was the challenge to maximise cake dry solids and improve the

recovery facilities in order to minimise the amount of support fuel required for the Cambi

process.

An integrated design team was used for both the Cardiff and Afan projects working closely with

key suppliers and operations and standardising design wherever possible. Detailed design

activity schedules were developed, integrated with procurement activities and programmed to

ensure timely delivery.

Given the challenge to deliver both Cardiff and Afan before April 2011 to gain full ROC

accreditation a challenging design and delivery programme was developed, with the key target

of completing both projects six months early. This was reinforced through active management

including a proactive approach to risk management developing risk mitigation plans wherever

required and fully resourcing the project to allow effective expediting of key technology

packages during off-site manufacture. The key technology suppliers were incentivised to

achieve the accelerated project programme but also focus on right first time.

To ensure effective liaison and understanding of operational constraints a senior manager from

the operations team was assigned to the capital delivery team, coordinating regular design and

operational appraisals of all aspects of the new works and integration with existing assets.

The selected Capital delivery partners, Morgan Sindell and Imtech Process worked very closely

with DCC setting up a JV arrangement to ensure active management and single project focus to

drive optimum delivery decisions. Continued challenge of best practice, time, cost and risk

management, with the overarching theme of health and safety throughout.

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European Biosolids a

The detailed delivery programme included learning from issues which led to delays

schemes, with special provisions for early proving of key plant areas and ensuring that the

design included features to allow commissioning right first time.

An innovative feature of both projects was the decision to implement DCC’s automatio

strategy as part of these projects . This included the selection of intelligent starters,

standardised instrumentation and Enterprise SCADA (Seimens PVSS open structured information

management system) with dedicated software engineers forming part of th

In order to ensure effective integration of design, engineering and construction activities the full

project team were co-located on site at the earliest opportunity.

The key dates associated with delivery of this programme was planning approval in December

2008, formal agreement of the target cost in January 2009 and start on site in March 2009.

Following initial enabling works construction of major civil structures i

commenced in May 2009. Through active focus of construction issues the digesters at both sites

were completed on programme before April 2010. M&E installation commenced in January

2010. A particular feature of delivery included fa

both Cambi plants before disassembly and transport to site. This allowed mechanical

installation to be completed within one week at both Cardiff and Afan.

Process Description

A simplified Process Flow Diagram

Figure 4.

Figure 4: Process Flow Diagram (PFD) of Cardiff Advanced Digestion plant




The detailed delivery programme included learning from issues which led to delays


design included features to allow commissioning right first time.

An innovative feature of both projects was the decision to implement DCC’s automatio



management system) with dedicated software engineers forming part of the delivery team.


located on site at the earliest opportunity.



Following initial enabling works construction of major civil structures including the



2010. A particular feature of delivery included factory construction, pre-assembly and testing of


installation to be completed within one week at both Cardiff and Afan.

iagram (PFD)of the Cardiff Advanced Digestion plant is shown in

Process Flow Diagram (PFD) of Cardiff Advanced Digestion plant

The detailed delivery programme included learning from issues which led to delays on previous


An innovative feature of both projects was the decision to implement DCC’s automation



e delivery team.




ncluding the digesters



assembly and testing of


(PFD)of the Cardiff Advanced Digestion plant is shown in

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The existing wastewater treatment works at Cardiff serves a population of approximately

900,000 and consists of inlet pumping station, fine screening and degritting, and a series of

Sequential Batch Reactors (SBR) with treated effluent discharged via a sea outfall with a

pumping station to overcome tide. Surplus activated sludge (SAS) is transferred to a buffer tank

before centrifuge dewatering and thermal drying. The plant also includes facilities for importing

sludge cake from other sites together with a series of odour control plants for the inlet works

and reciprocating thermal oxidisers (RTOs) for the dryers.

The new advanced digestion plant has been designed to replace the thermal driers but is

integrated with the existing works to deliver a sustainable and capital efficient solution.

Strain presses have been fitted to effectively screen the works SAS and ensure reliable operation

of the downstream works. The screened sludge is dewatered using the existing centrifuge

dewatering facilities and the sludge cake at approximately 20% DS is transferred by PC pumps to

two silos each with a capacity of 400 m³. The plant includes cake reception hoppers to allow

sludge cake to be imported from other works including Newport Nash. The imported cake is

transferred by PC pumps to the new silos, including in line facilities to automatically dilute the

cake dry solids to approximately 22% using hot water.

Raw sludge cake is transferred to the first vessel of the Cambi Hydrolysis plant by specialised PC

pumps including automatic cake dilution to approximately 16-18% DS using hot water and

controlled by in line dry solids monitors. The Cambi Thermal Hydrolysis plant is two stream

with three pressure reactors per stream. Raw sludge cake is effectively mixed and pre heated in

the pulper before being transferred to one of the pressure reactors. Within the pressure reactor

high pressure steam is injected to raise the sludge temperature to approximately 165°C and the

pressure of 6 bar. Sludge is held at these conditions for a period of approximately 30 minutes.

The pressure is then released back to the pulper to recover heat and then the hydrolysed sludge

is transferred forward to the flash tank, which operates at a temperature of just over 100°C.

Malodorous steam vapour is passed through a cooler to remove any condensable gases and the

remaining gas is compressed before being injected into the digester feed line to ensure odour

free operation of the plant. Hydrolysed sludge is automatically diluted to a DS concentration of

approximately 9-10% and is continuously fed forward to the digesters at a controlled rate.

Digested sludge is recirculated from the digester and combined with the hydrolysed sludge

before passing through a concentric tube heat exchanger and then into the digester. Final

effluent is pumped through the outer annulus of the heat exchanger at a controlled rate in order

to maintain the digester operating temperature at approximately 39-40°C .

The two anaerobic digesters at Cardiff are of reinforced concrete design each with an operating

capacity of 7500 m³ and include four pump mixing units with multiple nozzle arrangements at

low and high level. Each digester is fitted with hydrostatic overflow arrangements, and facilities

to monitor temperature, level and foam.

Digested sludge is displaced into a buffer tank and is then conditioned with polymer and

transferred forward to one of three belt filter presses. The digested sludge cake is discharged by

gravity into a collection bay at a dry solids content of approximately 30%. The collection bay

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stores the sludge cake for a period of up to two days with potentially odorous air transferred to

a dedicated odour control plant. The sludge cake is transported by front loader into a covered

storage area which provides storage for up to 14 days, allowing the dry solids to increase to

approximately 35% before transport to agriculture.

Liquors arising from the dewatering of digested sludge are collected in the liquor balance tank

and transferred back to the works inlet at a controlled flow to ensure treated effluent

compliance.

Digester gas flows from the digesters into two gas holders, each of 2000 m³ capacity. The gas

pipelines are fitted with automatic condensate traps to allow safe release of condensate as the

gas cools. Under normal operation digester gas is used in three high efficiency CHP engines,

each rated at 1.56 MW electrical output and with an electrical conversion efficiency of over

40%. The exhaust gas from the CHP engines is routed through composite boilers in order to

raise steam for the Cambi plant. Natural gas is also used in the composite boilers in order to

produce additional steam for the Cambi plant. Hot water is recovered from the CHP engines

and used for sludge cake dilution, sludge cake heating and boiler water preheating in order to

optimise the overall heat balance of this plant.

Final effluent is screened to 300 micron and used for cooling the thermally hydrolysed sludge.

Also, final effluent is further screened to 50 micron and UV disinfected before being used for

dilution following thermal hydrolysis and polymer makeup and washing of the belt filter presses.

This facility eliminates the risk of recontamination and ensures that enhanced quality sludge

cake is recycled to agriculture.

Energy balance

The energy balance of the Cardiff advanced digestion plant is summarised in the Sankey diagram

presented in fig 5. This shows that when operating under normal design conditions the energy

available in the biogas will be approximately 180 MWh/d. The high efficiency CHP engines will

convert this energy to 73 MWh/d of electricity and 40MWh/d of hot water. CHP exhaust gases

and natural gas will be used in the composite boilers to generate 61MWh/d of steam which will

be used by the Cambi plant. The overall specific efficiency of this plant is 0.9 MWh/tDS, which is

very high considering the sludge is mainly surplus activated.

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European Biosolids a

Figure 5: Sankey diagram of Cardiff advanced digestion plant

Commissioning and early results

The commissioning strategy for the Cardiff and Afan Advanced Digestion plants took account of

previous experience at other full scale plants which had often led to extended commissioning

periods, project delays, increased costs and reduced operational savi

more focus on expediting off site manufacture and testing and initial proving of key process

items ahead of process commissioning. The steam boiler plant was initially proved using natural

gas, allowing the Cambi Thermal Hydro

high efficiency CHP units were initially operated using natural gas in order to prove the overall

system. Special tests were planned to allow early commissioning of high risk areas such as

sludge cake handling, dilution and pumping plant, including the proving of innovative, new

instrumentation and control facilities. Detailed design of the plant included specialist facilities

to allow right first time commissioning. For example, facilities were

digester seeding and heating prior to start

Key dates included start up of the steam boilers in July, allowing initial proving of Cambi in

August. Also, this coincided with early start

allowed early G59 testing and connection agreements. Early completion of PM5 facilities

allowed remote automatic operation of the steam plant.

A particular risk associated with similar plants has been process start up.

severe foaming of the digesters on start up and delays to allow process acclimatisation and the

onset of reliable gas production. In order to minimise this risk a series of bench scale tests were

carried out in order to identify the o




Sankey diagram of Cardiff advanced digestion plant

Commissioning and early results



periods, project delays, increased costs and reduced operational savings. This strategy included


process commissioning. The steam boiler plant was initially proved using natural

gas, allowing the Cambi Thermal Hydrolysis plant to be tested and proved using water. Similarly



cake handling, dilution and pumping plant, including the proving of innovative, new


right first time commissioning. For example, facilities were designed to allow for

digester seeding and heating prior to start-up of the Cambi process.

start up of the steam boilers in July, allowing initial proving of Cambi in

August. Also, this coincided with early start-up of the CHP units using natural gas which, in turn,


allowed remote automatic operation of the steam plant.

A particular risk associated with similar plants has been process start up. Issues often include



carried out in order to identify the optimum seeding plan and start up rate. This range of tests



ngs. This strategy included


process commissioning. The steam boiler plant was initially proved using natural

lysis plant to be tested and proved using water. Similarly



cake handling, dilution and pumping plant, including the proving of innovative, new


designed to allow for

start up of the steam boilers in July, allowing initial proving of Cambi in

using natural gas which, in turn,


Issues often include



ptimum seeding plan and start up rate. This range of tests

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included seeding with either conventionally digested sludge or thermally hydrolysed digested

sludge from an existing full scale plant, with a series of different start up feeding regimes. The

results of these tests showed clear advantages in starting the digesters with acclimatised seed

sludge. Using this sludge there was no evidence of foaming, even with high start up rates. The

final agreed digester start up plan was to transport digested sludge cake from the Cambi

advanced digestion plant at Cotton valley, Milton Keynes down to Cardiff. Freshly dewatered

acclimatised sludge cake was transported over a two week period, At site, the sludge cake was

blended down to approximately 10% DS with water, sodium carbonate was added to increase

alkalinity, and fed forward direct to the digesters. Each digester was purged before adding seed

sludge and then filled to a volume of approximately 3000 m3. The seed sludge was recirculated

and heated to a temperature of 40°C by injection of steam. The digester mixing system was

commissioned and then the Cambi thermal hydrolysis plant was started and thermally

hydrolysed sludge fed to the digesters at an initial loading rate equivalent to a hydraulic

retention period of 60 days. The feed rate was increased at 5% per day, subject to the results of

daily process monitoring of the digesting sludge and biogas production and quality.

Start-up of the digesters proceeded, as planned, right first time and without incident at both

Cardiff and Afan. The start-up plan and early results for the Cardiff site are presented in fig 6.

Fig 6 shows the start up plan for the Cardiff AD plant. Initially the digesters were filled to a

volume of approximately 3000 m³ with diluted seed sludge at approximately 5% DS and heated

to a temperature of approximately 40°C. The increase in feed rate of hydrolysed sludge to the

digesters is presented together with biogas production and anticipated electrical power

generation, leading to the filling of the digesters and start up of the filter press dewatering

plant.

Figure 6: Cardiff Digester Seeding & Start-up

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Total Volume in Digesters (m3) Total BioGas Production (Nm3/d)

CHP generated power (kW) CHP generated power (MWh)

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Fig 7 shows the digester operating conditions during start up. Using sodium bicarbonate to

increase the alkalinity of the seed sludge ensured that on start up of the hydrolysis plant and

increase in volatile fatty acids (VFA) concentration the pH of the seed sludge was maintained at

7.4 and above. The VFA concentration has not increased above 3000 mg/l demonstrating

stability of the digestion process. Similarly the ammonia concentration has started to increase

consistent with the high proportion of SAS in the feed sludge.

Figure 7: Cardiff AD Digester No1

Fig 8 shows the composition of biogas during start up. The methane content of digester gas

quickly increased to above 50% within five days of feeding the digesters with hydrolysed sludge.

Subsequently, the methane content has increased to above 60% and stabilised.

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Figure 8: Cardiff AD Digester No1 Biogas Composition

In summary commissioning at Cardiff followed the schedule in Fig 6, allowing the CHP units to

be operated on biogas and full ROCs accreditation within the first month of process

commissioning. The ramp up rate was particularly impressive, allowing the first drier stream to

be taken out of service within two weeks of starting the Cambi plant and the complete drier

installation to be taken out of service within one month.

Operational Savings

During the development and detailed design of the Cardiff AD plant the operational and

maintenance costs of the existing sludge treatment plant were monitored and the O&M costs of

the new treatment facility was reviewed and agreed with the local operational team, supported

by information from key technical suppliers and actual costs at other full scale plants. The

agreed operational savings are approximately £7 million/year. The capital delivery partner will

support DCWW to optimise plant performance and maximise savings over the first two years of

operation.

Carbon Savings

Carbon modelling of the existing thermal drying plant and the new advanced digestion plant has

been undertaken including fuel and power requirements and emissions associated with

transport operations. The carbon benefits of the advanced digestion plant include significantly

reduced natural gas usage, reduced power consumption and renewable power generation.

Overall, the operational carbon saving from advanced digestion at both Cardiff and Afan is

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(%

CH

4,

%C

O2,)

Date

% CH4 % CO2 H2S

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35,000 tonnes CO₂ equivalent, which represents an operational carbon saving for Dwr Cymru

Welsh Water of 15%.

Towards Power Self sufficient service

Power self sufficient wastewater service has been achieved at other advanced digestion plants

including Kings Lynn, Great Billing and Eign. However, achieving power self sufficient service at

Cardiff is particularly challenging due to high power requirements to lift the sewage into the

inlet works, for aeration of the SBRs and pump transfer of final effluent to sea. Although the

performance of the existing SBRs has been optimised to minimise the power required for

aeration, the average power use is approximately 100 MWh/d which exceeds the expected

power output from the Advanced Digestion plant at approximately 70 MWh/d. Further

optimisation of the overall works will continue in order to drive towards power self sufficient

service. Improvements under consideration include importing additional sludge cake, and

providing settlement upstream of the SBRs.

Results and Conclusions

1. The strategic development and implementation of the Advanced Digestion programme

has been an excellent example of purposeful collaborative working with the delivery of

both the Cardiff and Afan projects safely, within a challenging time-scale, and right first

time through learning from previous experience at other sites.

2. The strategy to deliver sustainable sludge treatment facilities, reduce energy use and

the operational Carbon Footprint, including the generation of renewable power and a

drive towards power self sufficient service, has received unanimous approval from Dwr

Cymru’s stakeholders.

3. The “Joint Venture” arrangement between Morgan Sindall and Imtech Process has

proved to be a very successful and purposeful delivery team working closely with Dwr

Cymru.

4. Both the Cardiff and Afan Advanced Digestion projects have been designed,

constructed, and commissioned within two years of formal award in January 2009.

5. The process commissioning plan took account of issues and delays associated with other

projects and has proceeded quickly, right first time.

6. Early results confirm the forecast operational savings and carbon footprint reduction of

the Cardiff and Afan advanced Digestion plants.

Acknowledgement

The authors wish to thank Dwr Cymru Welsh Water for its support and assistance in the

development of this article. Imtech Process, as part of the joint venture with Morgan Sindall,

was pleased to be involved in helping DCWW to develop the sustainable sludge strategy for

AMP 5 and its ultimate decision to invest in advanced digestion at both Cardiff and Afan. The

successful delivery of this programme was only possible through the excellent commitment and

performance of the project team and we wish to thank everyone involved in the successful

delivery.

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Photo 1: The completed Cardiff AD plant

Photo 2: The digesters under construction

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Photo 3: Cambi Thermal Hydrolysis plant being installed

Photo 4: Under construction

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Photo 5: CHP installation

Photo 6: CHP installation

advanced digestion at cardiff and afan; dwr cymru welsh water … · 2019-03-22 · 15 th european...

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