new thermal advanced digestion plant at ......2015/07/21 · using alfa laval’s latest generation...

17th

European Biosolids and Organic Resources Conference

www.european-biosolids.com

Organised by Aqua Enviro Technology Transfer

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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mg

/l),

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ALI

NIT

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mg

/l),

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(m

g/l

)

DATE

Digester Number One - Early Process Performance Data

Dig No1 AmmN mg/l Dig No1 TA mg/l Dig No1 VFA mg/l Dig No1 %DS %DS Dig No1 pH

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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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Digester Number Two - Early Process Performance Data

Dig No2 AmmN mg/l Dig No2 TA mg/l Dig No2 VFA mg/l Dig No2 %DS %DS Dig No2 pH

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Digester Number Three - Early Process Performance Data

Dig No3

AmmN mg/l

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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

0.0%

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30.0%

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20-Jun 10-Jul 30-Jul 19-Aug 08-Sep 28-Sep 18-Oct

%C

H4

DATE

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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European Biosolids a

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.




Summary energy balance based on measured performance during October



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


has now been fully commissioned with performance exceeding contract

biogas production appears to be 15% higher than expected, and this plant appears to


ed performance during October



programme. GTM (a

project team to

luding tight site, working on

May, using digested sludge cake

20 June and the sludge load was


has now been fully commissioned with performance exceeding contract

his plant appears to


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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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