Severn Trent is expanding its advanced sludge treatment
capability at Strongford with a second thermal hydrolysis plant
by Matthew Frost BSc CEng MICE & Scott Taylor
(published October 2018)
north-east at the sludge dewatering, cake handling and THP
area. Final preparations being made for the cake silos base
slab concrete pour - Courtesy of Severn Trent Water
Strongford STW is located at the north west end of Severn
Trent’s region and covers a significant catchment in and around
Stoke-on-Trent with a population equivalent of 350,000. The
Strongford digestion plant currently processes 34 tonnes dry
solid per day (tds/day) indigenous; and 14 tds/day liquid
imports which includes 7 tds/day from the recently closed
digestion plant at Checkley. Going forward the plant will
receive a further 19 tds/day as 23% dry solids (ds) cake from
the digestion plants planned for closure at Coalport, Four Ashes
(Cannock) and Brancote (Stafford). This gives a total base load
on the plant of 66tds/day. There is scope within the local
authority planning permission for this to rise to 80tds/day (no
net change in traffic movements). Exploiting the full capacity
of the proposed THP would require a different way of importing
sludge and/or exporting digestate.
Benefits of the
thermal hydrolysis plant (THP)
hydrolysis as part of the sludge recycling process helps Severn
Trent Water meet a number of strategic and tactical goals
recycling land bank security with enhanced sludge product.
Reducing operation costs.
The Strongford THP team
is regularly tapping into knowledge and experience being gained
as Severn Trent Water’s first THP plant at Minworth STW
(Birmingham) goes into operation.
Undertakings and budget
The budget for the whole
project is £33.6m. This includes £27.4m for the work at
Strongford STW and £6.2m to install thickening and dewatering
facilities at three satellite sites where existing single
digester plants will be closed and sludges exported to
Strongford. Doosan Enpure Ltd is acting as CDM principal
designer and principal contractor for the works at Strongford.
Existing STW and sludge
The existing STW consists
of inlet, primary and storm settlement, a mix of carbonaceous
and nitrifying ASPs and NSAF. The final effluent consent in mg/l
is BOD:12/SS:30/ NH3
An existing mesophilic anaerobic digestion plant consists of 4
(No.) 2500m3 and 3 (No.) 3000m3
conventional mesophilic anaerobic digesters. The plant has a
throughput of up to 48 tonnes dry solid (tds/day). In practice,
this is up to 1000m3 of sludge per day. The primary
heating source is 2 (No.) 1000mW Jembacher biogas fired CHP
engines with supplementary biogas only boilers available to
assist. All additional gas is passed to a Malmberg gas upgrade
plant from where it is exported to grid.
image from 3-D model of sludge dewatering, cake handling
and THP area (looking north-west) - Courtesy of Severn
Proposed work at
imported (liquid) sludges - imported sludge cake: At the
front end, handling of indigenous sludges and liquid imports
will remain largely unchanged. Low percentage dry solids sludges
will be thickened (primary and SAS separately) to target 6% dry
solids before being mixed with imports, strain pressed and
pumped to the new sludge handling facilities.
Here sludges will be
dewatered to target 23% dry solids in one of 2 (No.) Alfa Laval
G3 centrifuges (duty/duty) and stored in one of 2 (No.) CTM
Systems 300m3 silos. Imported cake will be delivered
to a below ground 31.5m3 bunker and transferred by 2
(No.) Mono Pumps model No. C21D (duty/duty) into the silos.
Silos are dedicated one per THP stream. Sludges are transferred
from the silos with 2 (No.) Mono Pumps model No. W1BC
(duty/standby) and re-watered to a target 16% dry solids using
primarily final effluent, but with an option to supplement with
liquid imported trade waste.
The thermal hydrolysis
plant: After some considerable work with potential suppliers the
team has settled on Cambi for the supply of the THP plant. It
will consist of 2 (No.) streams of its B4 model (with a combined
maximum throughput of 94 tds/day).
cake reception equipment being installed - Courtesy of
Severn Trent Water
The THP process heats and holds sludge at 160°C at 6 bar
pressure for 45 minutes. The heating medium is steam. Steam for
the THP will be generated by 1 (No.) Steamatic Fire Tube steam
boiler (model FT 400/15/Biogas–CH4) and 1 (No.) heat recovery
steam boiler (model FTR 400/15/BIOGAS-CH4/ Combi) both with
capacities of 3.080kg/h generation.
incorporating CIB-Unigas burners, are supplied by Thorne
Industrial Boiler Services. In addition, boiler water feed will
be pre-heated using:
Jacket and exhaust heat
recovered from a new Jembacher 420 biogas fuelled combined heat
and power (CHP) engine supplying electricity into HV ring main
Water loop used to cool
sludges exiting the THP.
The CHP engine is
supplied and installed by Clarke Energy Ltd contracted directly
to Severn Trent. The civils and all other infrastructure to
agreed battery limits is provided by Doosan.
anaerobic digesters (MADs): After THP Process, thermally
hydrolysed sludges will be passed forward to the MADs which will
have been refurbished. During the transfer, sludges will be
cooled to 55° and thickness reduced to target 10.5% using a
combination of HRS heat exchanges and mixing with micro-strained
and UV treated final effluent. UV treatment will ensure digested
sludge retains its enhanced status. The UV treatment plant is
being designed and delivered by Doosan.
and inspection: Six out of the 7 (No.) digesters will be
refurbished and upgraded to receive THP sludges. Digesters will
be taken out of service, one at a time, for refurbishment. The
task of emptying purging and cleaning is outside Doosan’s scope.
This work is being undertaken by GTS Maintenance Ltd managed by
ST Digester Cleaning Team.
Empty digesters are then
force vented and handed over to Doosan. Digesters are inspected
using a camera fitted to a drone. At this stage it will be
determined whether a full birdcage scaffold will be needed to
facilitate repair/replacement of the gas space lining or a
single tower scaffold to enable installation of gas mixing
manifold feed pipes only.
Completed sludge silo base with bottom section of silos
fitted - Courtesy of Severn Trent Water
modifications/refurbishment: Digester refurbishment
comprises stripping out the existing gas mixing equipment then
re-fitting with new. Mixing will remain a continuous unconfined
type. A single ring of diffusers in the digester base is to be
replaced with two concentric rings of leaf-spring diffusers
connected by dedicated down-pipes from a manifold on the
The duty-only gas
compressors at ground level on each digester will be replaced.
All this work is being undertaken by Utile Engineering.
probes, new radar level detection, foam detection and the
facility for retro-fitting foam suppression later, are all
installed at this stage. Inlet and outlet pipework is to be
modified to take on board lessons from Thames Water published in
a paper by Fountain and De Macedo.
Existing digesters are refurbished to deal with THP’d
feed sludges. Roofs are fitted with radar level
measurement, foam detection and foam suppression
equipment - Courtesy of Severn Trent Water
Modifications to the
digester outlet will lower the top water level and increase the
gas space to mitigate the potential effects from low density
mousse-like sludges often found in the upper portions of post-THP
digesters. Digester refurbishment is being undertaken by North
Midland Construction or one of their suppliers under a
sub-contract agreement with Doosan.
Gas system changes:
The completed plant will produce significantly more biogas than
the plant does at present. The gas system modelling by Utile
Engineering has identified the need to modify and reinforce some
of the gas pipework. Further amendments are required to ensure
the plant matches Severn Trent’s design manual recommendations.
A new gas holder.
A new gas flare and vent.
Re-orientation of some
pipework to separate gas producing and gas consuming elements in
relation to the gas holder.
post-digestion dewatering liquors: MADs fed with
thermally hydrolysed sludge produce sludge and post-digestion
dewatering liquors with significantly higher levels of ammonia
than conventionally fed MADs.
currently return to the head of Strongford works without any
special treatment and contain, on average, 700kg/ammonia per
day. Going forward it is expected that the liquors will contain
up to 2,700kg of ammonia per day. The project team is working
through a number of options on how to provide the best TOTEX
solution for treating these liquors both in the short, medium
and long term.
A solution using the
lanes and final tanks of an abandoned ASP is likely. Historical
and ongoing calcite issues in centrate lines has precluded the
use of lime to provide alkalinity in more traditional approaches
(eg: ASP or oxidation ditch).
The sludge cake
reception, sludge dewatering, THP equipment including the
boilers and CHP, will be sited as close as possible to the
existing sludge thickening and digestion plant. The plot
allocated is a restricted area on all sides. It is bounded by an
elevated air pipe to existing ASPs, site road and the abandoned
Two final settlement
tanks (FSTs) have to be demolished to make way for the new
equipment, whilst 2 (No.) FSTs will be retained for possible
incorporation into the liquor treatment solution.
Delivery of THP, CDM
principal designer and principal contractor
Doosan Enpure Ltd
Contracted directly with
Civils construction and
Cake reception plant
CTM Systems Ltd
Cake reception pump
Mono Pumps Ltd
Thermal hydrolysis plant
Cambi UK Ltd
Alfa Laval Ltd
Sludge storage silos
CTM Systems Ltd
make-up and dosing
Richard Alan Engineering
Boiler Services Combustion Ltd
Heat exchanger sludge
HRS Group Ltd
Sub-contracted to North
Utile Engineering Company
TCL Structures Ltd
At the time of writing
(June 2018) work to clean out and refurbish the existing MADs
has begun - three digesters have been emptied, cleaned and
inspected. In each case no work has been required to gas space
liners. Not having to install complex scaffolding has saved time
and money on this section of the work. New mixing equipment and
inlet and outlet pipework has been installed; and the first two
digesters have been put back into service.
way through 190m3 concrete pour of the sludge
cake silos base
Courtesy of Severn Trent Water
finishes being applied following concrete pour at CAMBI
base No. 1
Courtesy of Severn Trent
Elsewhere on the site the
two of the four redundant FSTs have been demolished. This has
made way for the deepest chamber on the site to be constructed.
This structure will contain the import sludge cake reception bin
and cake handling equipment. Completion of this structure has
allowed backfilling of this area which releases work areas for
sludge silo bases, and very soon after that areas for boiler,
CHP engine and THP equipment itself.
Construction work is
programmed to finish December 2018; with the first sludges
thermally hydrolysed in February 2019. Plant handover is due in
July 2019. Work on satellite sites will follow on from so the
plant will be running at the intended 66tds/day throughput by
and publishers would like to thank Matthew Frost, Programme
Manager with Severn Trent Water, and Scott Taylor, Senior
Project Manager with Doosan Enpure Ltd, for providing the
above article for publication.