arrangement - Courtesy of Stantec Treatment
STW is Severn Trent’s largest sewage treatment works and serves
a population equivalent of 1.75 million from Birmingham with
full flow to treatment of 1,070Ml/d. The works treats sludge
arising from a population equivalent of 2.3 million which is
made up of indigenous sludge plus imported sludge from the
nearby Coleshill STW and various works in south Staffordshire
and North Warwickshire. As part of their PR14 submission to
OFWAT, Severn Trent has made commitment to cut their carbon
footprint by 10% through AMP6 and to produce an enhanced product
for 25% of all of their biosolids. In order to achieve these
drivers a thermal hydrolysis plant (THP) has been constructed at
Minworth designed to treat peak 89,060 tDS/year and average
71,175 tDS/year. The THP has been designed and constructed by
Severn Trent’s framework contractor Stantec Treatment in
collaboration with key technology providers.
Primary sludge screens
Primary sludge is
screened with 6 (No.) Huber SP4 Strainpresses designed for ease
of maintenance with service wheels to allow the body to be split
for clear access to the auger. The Strainpresses were factory
pre-assembled in six modules complete with the electro-pneumatic
local control panels which were fully cabled and tested. They
were installed on an elevated platform in just 1 week saving
site time and reducing the risks associated with working at
height. The screenings capture rate is impressive and will
protect the THP from blockages and excessive wear.
indigenous thickened surplus activated sludge (SAS) and screened
primary sludges are blended in existing reception tanks then
transferred utilising the existing ram pumps to new buffer tanks
with air mixing. New DI pump delivery manifolds were installed
in the digester pump station with twin 300mm diameter feed
pipelines running 400m to the new plant.
A key design
consideration was to ensure a consistent feedstock so the sludge
inventory was monitored to confirm sludge characteristics. The
commissioning team worked with STW Operations to verify the
primary/SAS ratio of 70:30.
THP constructed on redundant tanks and integrated with
existing digester and renewable energy assets - Courtesy
of Stantec Treatment
3 (No.) Alfa Laval G3-125
centrifuges dewater the sludge to 20-22% dry solids with
associated Richard Alan polyelectrolyte dosing plant. These
decanters feature innovations to reduce the power consumption,
increase capacity and achieve precise control of the
differential speed between centrifuge bowl and scroll conveyor
using the back-drive controller.
The ‘two touch’ advanced
electronic centrifuge controller ensures that a constant quality
of cake is produced despite variations in feed. Cake-transfer
Mono Pumps with boundary layer injection transfer the sludge to
3 (No.) 300m3 CTM silos equipped with live screws.
Cake is then diluted to 16.5% +/- 1.5% and pumped into the 3
(No.) parallel Cambi THP streams. Interconnecting pipework with
manual valves was included to increase operational flexibility
The cake import bunker
receives imported Coleshill cake which is blended with
indigenous sludge in the THP feed silos. The delivery of a cake
load involves a 30m3 HGV reversing into position and
tipping the cake load into the hopper. Imported cake with a
thickness of 20-22%DS will be pumped into the cake silos. It is
important that the dry solids content of the imported cake
remains consistent to ensure that there will be no over or under
dilution of the sludge or stratification in the THP feed silos.
The THP is capable of
treating a maximum of 250tDS/day or an average of 198tDS/day.
The Minworth THP consists of 3 (No.) Cambi B6(4) streams, each
of which feeds a block of 4 (No.) digesters. Each stream
operates independently in three process stages:
Sludge is fed into the
pulper from a dedicated THP feed silo. The sludge is heated to
100°C using recycled steam from the reactors and flash tank.
sludge is then pumped sequentially to each reactor, for batch
treatment. Sludge is heated by live steam injected directly via
‘sparge’ pipes. The sludge is heated to 165°C, >6 barg pressure,
and held for 30 minutes for hydrolysis to take place and ensure
it is pasteurised. Cycle time can be reduced to 22 minutes to
The sludge is blown down
to the flash tank using the pressure in the reactor. The flash
tank operates at ambient pressure, so the sludge experiences a
sudden de-pressurisation which ruptures the cell walls, ensuring
complete pathogen kill and maximising the digestibility of the
12 (No.) digester
recirculation pumps are located within the existing digester
galleries. Each pumps from the digester and combines with its
digester pair via a recirculation main, through the heat
exchangers, where hydrolysed sludge is blended at a 1:3 ratio
and cooled to its target temperature set point (normally
42-44°C) then fed back into the pair of digesters. The flow to
each digester is controlled by a modulating plug valve and
dedicated flowmeter with a flushing cycle to optimise
Cambi THP and sludge coolers - Courtesy of Stantec
The recirculation system
dilutes the hydrolysed sludge prior to cooling, thereby raising
pH of the combined stream and lowering cooler inlet temperature.
The pumps are Landia chopper-type equipped with an integral
cutting blade to macerate the digester contents and deal with
the legacy of high rag content experienced during digester
The use of Augmented
Reality 3D modelling facilitated the detailed design and enabled
the optimisation of the installation of numerous pipelines into
a busy operational area. The main stainless steel recirculation
pipe systems were incorporated into high level rack systems
which were prefabricated and factory tested by MEPS as part of
the DfMA strategy.
Final effluent and UV
The existing final
effluent pumping station (FEPS) feeds the Klampress units in the
secondary sludge dewatering plant. Higher duty final effluent
pumps have been installed to serve the increased demand from the
additional users in the THP area, with new strainers and a UV
treatment package to ensure pathogens are not introduced into
the post THP processes such that enhanced product status of the
cake is compromised.
A set of 3 (No.) 300µm
coarse strainers serve the heat exchangers, dewaterer polymer
carrier water, washwater, cake import dilution and pre-THP
dilution. The Bollfilter auto-backwash strainers feature a
Foulex coating to prevent the build-up of biofilm in the
strainer elements. A further stage of 50µm fine strainers and
Xylem UV treatment system is installed to treat the FE supplied
to the THP digester feed pumps, THP process gas cooling and to
the secondary sludge dewatering plant for Klampress washwater.
Pressure sustaining valves ensure that the FE distribution
network remains pressurised at all times.
Biogas system and recirculation pipework - Courtesy of
Digester surging was
identified as a risk to the project because the uplift in biogas
production from THP has more capability to create a sludge
density differential. To mitigate this risk, the sludge in the
spill columns is continuously aerated using an air lift system
to prevent the consolidation of sludge and to date there have
been no surging events.
Biogas is collected
beneath the floating roofs of the digesters and transferred to
the biogas-to-grid (B2G) plant, the 8 (No.) CHP engines and 3
(No.) boilers. In anticipation of a greater biogas yield from
the THP, 2 (No.) new biodomes (4000m3 each) will
provide additional storage capacity and improved biogas pressure
stability. The renewable energy team proactively manage the
biogas utilisation to maximise production.
Biogas boosters - Courtesy of Stantec Treatment
New biogas boosters will
elevate the system pressure downstream of the biodomes to 33mb
feeding the CHP’s and steam boiler siloxane plant.
2 (No.) Uniflare waste
gas burners (duty/assist) have been provided, capable of burning
1.5 times the anticipated peak gas yield, (7338m3/hr).
This safety device has a variable burn rate to prevent a release
of biogas through the digester pressure relief valves (Whessoe),
in the rare event that more biogas is being produced than
Waste heat ducting and steam pipework in
Courtesy of Stantec Treatment
Dunphy dual fuel burners
Courtesy of Stantec Treatment
Steam generation is
provided by 3 (No.) 6T/h boilers (duty/assist/hot standby)
operating at 11.5 barg and 191.6°C (saturated). The combination
steam boilers incorporate a ‘fired side’ which burns biogas or
natural gas and a ‘waste heat side’ to recover high-grade heat
from the CHP exhausts at 450°C through economisers. In order to
maximise energy recovery, 5 (No.) CHP engines are connected to
procedures for the steam boilers are complex and potentially
dangerous; only personnel with BOAS accredited qualifications
have the authorisation to operate the boilers. The high
integrity control system allows the boilers to be unmanned for
Dunphy won the
prestigious Combustion Engineering Association Lord Ezra Award
2017 for the design and manufacture of the high capacity, low
NOx, prefabricated, modular energy centre.
Dunphy Boilerhouse and CHPs with high grade waste heat
Courtesy of Stantec Treatment
A critical commissioning
activity involves converting the digesters from conventional to
hydrolysed sludge. This transition of sludge type alters the
biological activity and conditions inside the digesters so must
be carefully monitored and controlled to reduce the risk of
instability. Hydrolysed sludge is fed into the digesters and
existing contents are gradually displaced over a 3-month period.
The rate of ramp-up is
adjusted daily in small increments. Each THP stream and its
block of 4 (No.) digesters was converted in sequence to manage
sludge processing site wide.
The loading of the B/C/D
block digesters was phased to maintain digester stability:
Initially 32-52 tDS/day @
8-9%DS (conservative due to the pre-start conditions).
Interim 53-66 tDS/day @
9%DS design load per THP stream.
80-84tDS/day maximum throughput per THP stream.
The team of experienced
THP process commissioning engineers and operators were able to
utilise their knowledge to minimise the conversion timeline.
Recirculation pipework in digester gallery
Courtesy of Stantec Treatment
Siloxane plant carbon filters
Courtesy of Stantec
The health of the
digesters determines the sludge feed rate so samples of the
digested sludge are taken on a daily basis to monitor for % dry
solids content, % volatile solids content, volatile fatty acids,
total alkalinity, pH, ammonia content and % methane content in
the biogas. The analysis of these parameters is monitored
against pre-determined target figures (traffic light system)
which is recorded for comparison with the ‘golden measures’
If the daily monitoring
shows signs of digester volatility then the digester feed is
reduced to the ‘sick’ digester until it returns to stable
conditions. The feed is diverted to the A-block non-THP
digesters, or to the other ‘healthy’ converted digesters. An
anti-foam dosing skid was installed to manage foaming events in
the digesters which were experienced during conversion.
The effectiveness of the
digester mixing influences conversion rates so the existing gas
compressors had to be adjusted to run continuously and will be
upgraded in the future.
Smart ICA benefits
THP electrical design
collaboration with the Severn Trent ICA Standards Group has
successfully produced the following innovative approaches:
First project to commit
to the smart ICA ethernet MCC standard developed by the ICA
framework suppliers (TAP & Cema).
First commitment to the
new ACQ 580 ABB variable speed drives.
4 (No.) ME1D approvals
for complex package plant panels; approved in 24 hours.
Back to back ICA section;
saving 5m MCC length.
Use of compact IP54 ABB
VSDs on booster package
Pre-built wiring looms to
The MCC system integrator
TAP and electrical installation contractor S&R worked
collaboratively to provide a power and control system of the
highest quality within tight time constraints.
THP motor control centre - Courtesy of Stantec Treatment
10km of interconnecting
pipework were required which was one of the largest programme
risks, so the scope was split into five subcontract packages: (i)
Alpha Plus (ii) ABC Stainless (iii) Franklyn Yates (iv) MEPS and
(v) SWT. This engagement with the supply chain unlocked
production and installation resources and together with
concurrent working enabled us to achieve key dates due to the
determination of the mechanical supervision team.
Extensive application of
Stantec’s innovative digital delivery strategy has upskilled the
team and generated efficiency. 4D Digital Rehearsals enhanced
visualisation and prevented unplanned site activities which was
recognised with two synchro awards. BIM has unlocked the power
of constructive collaboration and allowed 40 supply chain
partners to visually engage, integrate and sequence.
UK manufacturing delivers
A supply chain with
proven performance on THP to ensure resilience has been utilised.
Design for Manufacturing and Assembly (DfMA) on 85% of equipment
has saved time, improved quality and reduced H&S risks. Key
process equipment was manufactured in the UK which will ensure
that STW get operational support during the plant life.
Minworth THP project has
been delivered ahead of schedule achieving an early completion
bonus which is testament to the high performing collaborative
Severn Trent and Stantec team. The attention to detail and
lessons learnt from other schemes has delivered a fully
integrated design based on operational experience. The
integration of the new assets with the existing process units
has been extremely complex but the interface manager has
excelled in the planning and coordination of the works with the
Operations & Maintenance teams to ensure a smooth transition
with minimal disruption.
Geese inspect the THP! - Courtesy of Stantec Treatment
The site management team
has demonstrated strong leadership to achieve a world class
safety performance with zero accidents and Considerate
Constructors Gold Award 2018. Process commissioning has been
well executed and STW are realising the benefits of the advanced
digestion upgrade with increased biogas and improved
dewaterability of hydrolysed sludge. Performance testing is
ongoing with completion forecast for November 2018.
and publishers would like to thank David Nyul, Programme
Manager with Severn Trent Water, and Richard Thomson,
Project Director with Stantec, for providing the above
article for publication. The authors thank the following for
their input: Andrew Bennett, Interface Manager, Darren
Chiles, Principal Electrical Engineer, Neil Horne, Project
Engineer, and Marina Parker-Dennison, Process Commissioning
Engineer, all with Stantec, and Severn Trent Water Staff and