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NEW THERMAL ADVANCED DIGESTION PLANT AT NORTHUMBRIAN WATER’S
HOWDON WASTE WATER TREATMENT WORKS
Rawlinson, D.1, Oliver, B.2. 1Northumbrian Water, 2 Imtech Process
Corresponding Author Tel. 01543 496600 Email [email protected]
Abstract
Northumbrian Water’s latest Advanced Digestion plant at Howdon Waste water Treatment
Works has now been completed and is in full operation, and generating more than 100 MWh/d
of renewable power. This is a key part of NW’s strategy to process all sludge through Advanced
Digestion in order to maximise renewable power generation and move towards power self-
sufficient wastewater service.
The new Advanced Digestion plant has been successfully delivered by GTM (a joint venture
between Galliford Try and Imtech). This plant builds on the improvements achieved with recent
projects and lessons learned from installation of an earlier plant at Bran Sands.
The new plant will process up to 40,000 tDS/y of sewage sludge and includes raw sludge
screening, centrifuge dewatering, cake reception, Cambi thermal hydrolysis, sludge cooling,
three 7000m3 digesters, three high efficiency 2MWe CHP units, and three composite steam
boilers. Enhanced quality treated sludge is dewatered and recycled to local farm land.
Process commissioning of the new plant commenced on time in May 2012, and process start-up
progressed smoothly. The plant has now been operated reliably under full-load conditions and
early results show higher than expected biogas production with a best recorded efficiency level
of 1.2 MWh/tDS. Some innovative features have been incorporated in order maximise re-use of
available heat and provide a fully integrated and optimised solution.
The team is now supporting further optimisation of the Cambi Advanced Digestion plants at
both Howdon and Bran Sands.
Northumbrian Water (NW) are considering further improvements including biogas into grid and
co-digestion of food waste to maximise renewable power generation. This programme puts NW
at the forefront of renewable power generation and sustainable wastewater service.
Keywords
Advanced Anaerobic Digestion, Thermal Hydrolysis, Operational experience, Carbon reduction,
Fertiliser value of enhanced sludge quality, power self-sufficient operation.
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Introduction
NW constructed the UK's largest thermal sludge drying facility at Bran Sands in 1995, and for 10
years sludge treatment was focussed on sludge drying at Bran Sands, Middlesbrough, and
dewatering with lime stabilisation at Howdon, Newcastle.
In 2006 a strategic review identified that the installation of Thermal Hydrolysis (TH) Advanced
Anaerobic Digestion (AAD) at both of these sites was a much more sustainable solution that
would generate multi-million savings in annual operating costs and significant environmental
benefits including:
• 2,000,000m³ of liquid sludge reduced down to 150,000m³ of sludge cake,
• Sludge cake being a Class A biosolid,
• 50% reduction in solids requiring disposal,
• 10 MW of renewable power from biogas production,
• 20% of NW’s total electricity demand being met by renewable sources,
• All of NW sewage sludge being used to produce renewable electricity,
• The facilities include cake imports to reduce the impact of sludge transportation,
• 20% reduction in carbon footprint,
• Provides NW with a more sustainable strategic sludge management solution,
• Existing assets used where possible, and
• Bran Sands driers and the ability to lime stabilise to remain as a strategic
contingency
This strategy was agreed and two guaranteed performance design and construct contracts
(totalling over £60m) were awarded to:
• Aker Solutions (now Jacobs E&C) to install AAD at Bran Sands in 2007
• GTM (a joint venture between Galliford Try and Imtech Process) to install AAD at
Howdon STW in 2010.
The plant at Bran Sands has been operating successfully since August 2009.
Howdon Advanced Digestion
GTM, a joint venture between Galliford Try and Imtech, has delivered NW’s latest thermal AAD
plant at the Howdon STW. This plant has built on the lessons learned from Bran Sands and
similar plants built for other water companies in recent years, including Dŵr Cymru Welsh
Water’s Cardiff and Afan plants.
Plant Description
The new TH plant at Howdon has a design capacity of over 40,000 tDS/annum and can process
all the indigenous sludge arising at Howdon and imported sludge cake from satellite sites. Figure
one shows a simplified flow diagram of the Howdon Advanced Digestion plant. The plant
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includes a new strain press installation to screen all liquid sludges. Elsewhere, screening has
been found to significantly improve plant availability, reduce downstream maintenance, and
improve renewable power generation. Liquid sludge is conditioned with polymer and dewatered
using Alfa Laval’s latest generation of high speed dewatering centrifuges. The sludge cake is
conveyed directly into one of two sludge cake storage silos.
Screened
Indigenous
Sludge
Imported
Sludge
Cake
2 THP
Feed
Silos
2 stream
Thermal
Hydrolysis
Coolers
3 Nr
Digesters
2No Gas
Holders
3 Nr
2MWe
CHP
Engines
ELECTRICITY3 Nr
Boilers
Exhaust
Gases
Existing
Centrifuge
Steam
LTHW
Mixing and
Recirculation
Boiler water
Treatment
2 Nr
CentrifugesFinal Effluent
UV
Natural gas / oil
~28%DS
Bio-solids
to agriculture
Centrate to
existing STW
Potable water
Figure one: Process Flow Diagram of Howdon Advanced Digestion plant
The plant also includes sludge cake reception facilities comprising a fully-enclosed building with
vehicle access, two cake reception hoppers, and automatic transport of sludge cake into the
sludge cake silos. This plant was delivered by Agrivert using the well proven design developed by
Hunning. All raw sludge reception facilities are fully enclosed with dedicated odour control
facilities to ensure no odour nuisance. The odour control plant is based on lessons learnt at
Anglian Water and includes a first stage biological scrubber and second stage chlorine dioxide
impregnated carbon which can reliably treat high levels of Mercaptans.
Sludge cake is conveyed from the storage silos and diluted to approximately 17%DS before
being pump transferred to the TH plant. Hot water recovered from the high efficiency CHP units
is used for cake dilution in order to optimise the heat balance and minimise the use of support
fuel. The TH plant is a two-stream installation with each stream comprising a pulper, four
reactors, a flash tank, and vapour condensing skid. This technology is well proven, with heat
recovery and mixing in the pulper, TH within the reactors (165oC for 30 minutes) using direct
steam injection, and balancing of the thermally hydrolysed sludge in the flash tank, allowing
continuous feed to each digester. Condensed vapours and inert gas is transferred forward to the
digesters to ensure effective odour control.
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TH sludge is diluted using UV disinfected final effluent (to avoid re-contamination), mixed with
recirculating digested sludge, cooled by final effluent using specialised heat exchangers and fed
to each digester via the pump mixing system to ensure effective dispersion. Three post-
tensioned concrete digesters have been installed, each of approximately 7000m3 capacity and
including specialist pump mixing and integrated sludge coolers to allow the optimum digester
temperature of approximately 40oC to be maintained. The anaerobic digesters provide optimum
conditions for the digestion of hydrolysed sludge and included lessons learnt from other plants
including increased free-board and facilities for foam management.
Biogas flows from the digesters through large diameter pipework specially inclined to a number
of large condensate traps to allow cooling and automatic removal of condensate. The biogas is
used primarily by the three 2MW high efficiency CHP units, allowing more than 40% of the
available energy to be converted to renewable power. In addition, the exhaust gas is transferred
from each CHP unit to one of three composite steam boilers. These boilers can use biogas and
natural gas or fuel oil as support fuel to reliably and efficiently produce the steam required for
TH.
The digested liquid sludge is displaced from the digesters and flows by gravity to a covered
buffer tank before polymer conditioning and centrifuge dewatering. Liquors are returned to the
main treatment works for further clean-up.
Lessons learnt
Lessons learnt from other recent installations, in particular Bran Sands, have been widely used
in the design, construction and commissioning of the new plant. These include:
• Use of sludge screens and strain presses to fine screen all liquid sludges, reducing
downstream maintenance and improving overall availability
• The design includes multi-stream processes with sufficient standby provisions to ensure
no single point of failure
• Well proven equipment has been selected wherever possible
• Plant flexibility including liquid and imported sludge cake facilities improves NW’s
overall resilience for sludge treatment, renewable power generation and recycling to
agriculture
• Full containment of raw sludge operations with extraction to a proven two-stage odour
control system using biological first stage and chlorine dioxide impregnated carbon in
second stage for improved removal of mercaptans
• Use of well proven twin stream TH with standardised modular design and factory pre-
assembly for improved quality control and safe and lean site installation
• Optimised heat balance including, control of TH feed DS, cake dilution with hot water,
use of surplus hot water for office heating, use of CHP exhaust for producing steam, and
triple fuel composite steam boilers for full flexibility
• Optimised anaerobic digester design including jet mixing, increased free board and foam
management
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• The use of reverse osmosis plant for improved boiler make-up
• Installation of large integrated high efficiency CHP units (3x2MWe) for optimum
utilisation of biogas and maximum economic benefit
• Fast process start-up, use of process start-up which was successfully developed for
Cardiff, allowing the full load conditions and optimum economic benefit to be achieved
within six weeks of seeding with acclimatised sludge cake
• Design flexibility to accommodate future expansion.
Energy balance
The optimised energy balance for the Howdon Advanced Digestion plant is presented as a
sankey diagram in Figure two. This diagram shows that there is almost 12MW of energy
available in the biogas at the design throughput of 40,000 tDS/d, with almost 5MW of
renewable power generated by the CHP units. Support fuel provides approximately 1.4MW of
additional energy, which combines with the CHP exhaust gas to generate almost 4MW of steam
for thermal hydrolysis. The heat balance was optimised through the addition of economisers
and use of the hot water for cake dilution, boiler feed water heating and space heating of site
offices.
Figure two: Sankey diagram showing the design energy balance of the Howdon AD plant at
full design throughput and including the addition of Waste Heat Economisers,
and use of low temperature hot water (LTHW) for pre-heating FE for pre-TH
cake dilution and heating boiler feed water, and heating the admin building.
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Project delivery
Following competitive tendering the contract for delivery of the thermal advanced digestion
plant was awarded to GTM in October.
Detailed design and delivery of the contract presented many challenges. However, through
working closely within a co-located project office at Howdon the team built on both GTM and
NW’s delivery experience and lessons learnt from the delivery of similar schemes, and this
contract has been delivered within programme. The key challenges associated with this project
included construction within a confined area, dealing with excess spoil, construction within an
existing wastewater treatment works, site transport restrictions and interface with existing
assets and other contract work. Also local residents were sensitive to historic odour issues,
which required proactive consultation and management throughout the project. Third party
issues including planning consultation, Environment Agency, natural gas supply and electrical
network improvements had to be proactively managed by the team.
Detailed review of the design, materials of construction, equipment selection, programming
construction activities, HAZOP studies, maintenance reviews and detailed commissioning
strategy helped to overcome all the challenges associated with this project. Although there was
considerable concern over the digesters following difficulties at Bran Sands, at Howdon there
were no issues with construction and all digesters were successfully water tested within
programme. Construction of the new steam boiler plant was particularly challenging and
required active management and design modification to allow for the use of two different
support fuels, which in turn mitigated any programme delay.
Regular liaison with the local operations team helped keep them informed of progress
throughout the construction period and a comprehensive training programme was provided
ahead of process commissioning. Unforeseen issues including excessive screenings, final effluent
supply, natural gas supply, and delays in third party approvals were quickly identified and
proactively resolved in order to maintain programme. Achieving the programme despite the
restricted site and unforeseen issues is a credit to the positive proactive approach of the
combined project team, including operations.
Plant commissioning
The commissioning strategy for Howdon was developed taking account of previous ‘right first
time’ commissioning experience at similar plants such as the Cambi Advanced Digestion plant
delivered at Cardiff in 2010 for Dŵr Cymru Welsh Water. Following initial testing, process
commissioning started at Howdon on 28 May 2012 when digested sludge cake was imported
from the Bran Sands Advanced Digestion plant. This acclimatised seed was diluted to
approximately 5%DS with disinfected water. Also, sodium bicarbonate was added to increase
the alkalinity level of the liquid seed sludge to over 4000 mg/l. This diluted seed was transferred
to digester number one and once a liquid level of over 10 m was achieved then the digester
mixing systems were commissioned and the seed sludge was bought up to temperature by
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recirculation through a special steam injection heating unit. Once the required volume of seed
sludge had been transferred to digester one, diluted seed sludge was transferred to digester
two, and the sequence was repeated, followed by digester three.
Digester one was filled with the required volume of seed sludge, heated to a temperature of
over 37°C and reliably mixed by 20 June 2012. This allowed the Cambi plant to be started and
hydrolysed raw sludge to be fed to digester one at a controlled rate. Regular sampling and
analysis confirmed good process stability and the loading plan was therefore continued. Early
performance data for digesters one, two and three are presented as Figures three, four, five and
six. The performance data confirmed good process stability throughout the commissioning with
VFA levels rapidly reducing and then being maintained within the range of 1000-1500 mg/l.
Following commissioning the alkalinity level in all three digesters steadily increased from around
4000 mg/l up to approximately 8000 mg/l and the ammonia concentration increased from
around 800 mg/l to around 2600 mg/l, showing clear evidence of stable digestion. Similarly
biogas production has exceeded expectation with methane content normally averaging around
63% by volume.
Figure three: Early process performance data for digester one
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Figure four: Early process performance data for digester two
Figure five: Early process performance data for digester three
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Dig No3
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Figure six: Biogas methane content during start-up of the Howdon Advanced Digestion
Plant
Early performance results
Following successful process commissioning, all of the advanced digestion plant at Howdon has
now been fully commissioned, including indigenous sludge screening and dewatering, imported
raw sludge cake reception, Cambi thermal hydrolysis, anaerobic digestion, three high efficiency
2MWe CHP units and an integrated steam heating system.
Raw sludge dewatering
The new high speed centrifuges have been fully commissioned and are reliably dewatering
indigenous raw sludge to a sludge cake dry solids content of approximately 22%.
Raw sludge cake reception
Raw sludge cake is now being imported from NW’s satellite sites including Hendon, Birtley,
Streshome and Morpeth. The cake reception facility is fully enclosed within a building complete
with extraction to a new odour control plant, to ensure that there is no odour nuisance
associated with this activity. The dry solid content of the imported sludge cake normally ranges
between 22-25% DS.
Cambi Thermal Hydrolysis
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%C
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Howdon Advanced Digestion - Biogas Methane Content
Dig No1 - %CH4 Dig No2 - %CH4 Dig No3 - %CH4
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Performance tests have demonstrated a maximum throughput exceeding 42,000 tDS/year
compared to the contract requirement of 40,000 tDS/year. The raw sludge cake has been
reliably diluted to approximately 16-17% DS before thermal hydrolysis. Also, following thermal
hydrolysis the thermally hydrolysed sludge is diluted using UV disinfected final effluent to a dry
solids content of 8-10 % before anaerobic digestion. Early results show a steam demand of less
than 1 tonne/tDS.
Anaerobic digestion
The new digesters have operated reliably since initial seeding in May 2012 and feeding with
hydrolysed sludge from 20 June. The feed rate of raw sludge was steadily increased with close
monitoring of the digestion process. Full load conditions were achieved within a period of 50
days. The digesters are now operating at a temperature of approximately 40-41°C with an
ammonia concentration in the range of 2500 – 2700 mg/l, VFA range of 1200 – 1400 mg/l and
alkalinity level of 7500 – 8000 mg/l.
Biogas production
Biogas production appears to be approximately 15% higher than expected and early results
indicate that the biogas yield is in excess of 0.5 m³/kg DS, which supports the volatile solids
destruction level of over 60%. The methane content of the biogas is typically in the range of 61
– 64% by volume.
Renewable power generation
All three high efficiency CHP units are now in operation and renewable power generation is
exceeding 100 MWh/day. Early results suggest that this is the most efficient advanced digestion
plant to date, with specific renewable power generation exceeding 1.2 MWh/tDS. Previous
performance levels were typically around 1 MWh/tDS, reducing to around 0.85 MWh/tDS for
advanced digestion of mainly secondary sludge at Cardiff.
Energy balance
Figure seven shows a summary energy balance based on measured performance during October
2012. This energy balance shows that with the higher than expected biogas production the
energy within the biogas is approximately 270 MWh/day. The high efficiency CHPs use this
biogas to generate approximately 110 MWh/day of renewable power, approximately 80
MWh/day of steam and approximately 60 MWh/day of hot water. Natural gas is used as
supplementary fuel, providing an extra 40 MWh/d of energy to help generate the full quantity of
steam required for the Cambi plant. Current performance data indicates that this support fuel
required for this site is less than 0.4 MWh/tDS.
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Figure seven: Summary energy balance based on measur
2012
Pathogen destruction
Early results have confirmed that the dewatered sludge cake arising from the new plant at
Howdon achieves enhanced quality with the absence of salmonella and coliform levels of less
than 10/g.
Conclusions
The Howdon AAD plant has been successfully completed within a tight
joint venture between Galliford Try and Imtech) worked closely with
jointly overcome the challenges associated with this contract inc
existing works, third party approvals and interfaces with other contracts.
Process commissioning commenced earlier
from Bran Sands. Feeding of hydrolysed sludge commenced on
steadily increased. Stable process conditions allowed full load conditions to be demonstrated
during August.
The complete plant has now been fully commissioned with performance exceeding contract
requirements.
Measured biogas production appears to be 15% higher than expected, and t
be the most efficient yet, with specific renewable power generation in excess of 1.2 MWh/tDS.
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Summary energy balance based on measured performance during October
Early results have confirmed that the dewatered sludge cake arising from the new plant at
Howdon achieves enhanced quality with the absence of salmonella and coliform levels of less
plant has been successfully completed within a tight programme
joint venture between Galliford Try and Imtech) worked closely with the NW project team to
jointly overcome the challenges associated with this contract including tight site, working on
existing works, third party approvals and interfaces with other contracts.
commenced earlier than planned, on 28 May, using digested sludge cake
from Bran Sands. Feeding of hydrolysed sludge commenced on 20 June and the sludge load was
steadily increased. Stable process conditions allowed full load conditions to be demonstrated
has now been fully commissioned with performance exceeding contract
biogas production appears to be 15% higher than expected, and this plant appears to
be the most efficient yet, with specific renewable power generation in excess of 1.2 MWh/tDS.
ed performance during October
Early results have confirmed that the dewatered sludge cake arising from the new plant at
Howdon achieves enhanced quality with the absence of salmonella and coliform levels of less
programme. GTM (a
project team to
luding tight site, working on
May, using digested sludge cake
20 June and the sludge load was
steadily increased. Stable process conditions allowed full load conditions to be demonstrated
has now been fully commissioned with performance exceeding contract
his plant appears to
be the most efficient yet, with specific renewable power generation in excess of 1.2 MWh/tDS.
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The successful delivery of this project is a credit to the positive collaboration between GTM and
NW and the numerous design consultants working with NW on AAD.
Next Steps for NW’s Sludge Strategy
The Howdon Advanced Digestion plant has now been successfully commissioned, allowing NW
to treat all its sludge by Advanced Digestion and generate approximately 80 GWh/y of
renewable electricity, equivalent to 40% of the energy needs for wastewater treatment.
The whole Howdon waste water treatment works is almost power self-sufficient, with any
excess renewable electricity being exported to grid.
NW is both innovating and leading the industry in utilising AAD:
• Employing AAD using novel TH, which produces 50% more biogas than is possible using
conventional anaerobic digestion,
o 30,000,000m3 biogas produced from NW’s sludge (per annum),
• 100% of its sewage sludge used to produce energy. No other UK water company uses all
of its sludge production to generate power, and
• Design includes the innovative re-use of waste heat from the AAD process to service
adjacent facilities on site.
The innovation continues and NW is currently investigating:
• Further re-use of any remaining waste heat from the AAD process,
• Cleaning and directly injecting the biogas into the national gas grid,
• Continuous optimisation of the process so that operational performance can be
maximised, and
• Introducing alternative feedstocks (e.g. food waste) to increase operational
performance even further.
Wider environmental benefits include:
• Sharing benefits from lower energy use and government incentivisation schemes for
renewable energy with customers - keeping customer bills low,
• Fewer process emissions released into the atmosphere,
• NW’s sludge treatment follows UK Waste Hierarchy priorities (Reduce-Reuse-Recycle-
Recover),
• Produces a Class A enhanced biosolid with a high nutrient value - more attractive for use
in agriculture,
• Increased self generation acts as a buffer against volatile world energy markets,
• Lower consumption of fossil fuels,
• A strategic sludge management solution to NW,
• Negligible odour impact,
• Re-using existing assets where practicable, and
• Significantly reducing sludge transportation across NW’s region. 2,000,000m3 of liquid
sludge ultimately reduced down to 150,000m3 digested sludge cake.
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Acknowledgements
The authors wish to thank Northumbrian Water for its support and assistance in the
development of this article. Imtech Process, as part of the GTM joint venture with Galliford Try,
was pleased to be involved in helping Northumbrian Water to deliver its industry leading
Advanced Digestion strategy. The successful delivery of this programme was only possible
through the excellent commitment and performance of the project and local operations team.
We wish to thank everyone involved in the successful delivery of this project.
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