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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
15th
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Organised by Aqua Enviro Technology Transfer
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
15th
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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.
15th
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
European Biosolids and Organic Resources Conference
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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 the delivery team.
In order to ensure effective integration of design, engineering and construction activities the full
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 including the
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 factory construction, pre-assembly and testing of
both Cambi plants before disassembly and transport to site. This allowed mechanical
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
schemes, with special provisions for early proving of key plant areas and ensuring that the
An innovative feature of both projects was the decision to implement DCC’s automation
strategy as part of these projects . This included the selection of intelligent starters,
standardised instrumentation and Enterprise SCADA (Seimens PVSS open structured information
e delivery team.
In order to ensure effective integration of design, engineering and construction activities the full
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.
ncluding the digesters
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
assembly and testing of
both Cambi plants before disassembly and transport to site. This allowed mechanical
(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.
15th
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
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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 savings. This strategy included
more focus on expediting off site manufacture and testing and initial proving of key process
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
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
cake handling, dilution and pumping plant, including the proving of innovative, new
instrumentation and control facilities. Detailed design of the plant included specialist facilities
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 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. Issues often include
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 optimum seeding plan and start up rate. This range of tests
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
ngs. This strategy included
more focus on expediting off site manufacture and testing and initial proving of key process
process commissioning. The steam boiler plant was initially proved using natural
lysis plant to be tested and proved using water. Similarly
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
cake handling, dilution and pumping plant, including the proving of innovative, new
instrumentation and control facilities. Detailed design of the plant included specialist facilities
designed to allow for
start up of the steam boilers in July, allowing initial proving of Cambi in
using natural gas which, in turn,
allowed early G59 testing and connection agreements. Early completion of PM5 facilities
Issues often include
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
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
0
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Date
Hydrolysed Sludge Feed to Digesters (m3) Sludge to Belt Presses (m3)
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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pH
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VFAs Alkalinity Ammonia %DS pH
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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
0
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Hyd
rog
en
Su
lph
ide (
pp
m)
Co
mp
osit
ion
(%
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