Supply Chain - Renewables & Energy Management - Thermal Hydrolysis
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Gas to grid process diagram - Courtesy of Costain[/caption]
Thermal hydrolysis plant process diagram - Courtesy of Costain[/caption]
(left) Inflated gas holder and (right) biogas upgrade unit and carbon storage silos - Courtesy of Costain[/caption]
(left) Sludge transfer pumps and (right) elevated strain press in front of the buffer tanks - Courtesy of Costain[/caption]
Dewatering
The screened sludge is dewatered from approximately 3% to 20% dry solids for transfer to the THP feed silos. 3 (No.) centrifuges work in a duty/duty/standby arrangement and backwash with final effluent (FE) as required.
THP & boilers
The cake is diluted with FE to 16% dry solids and fed into one of the 2 (No.) THP streams which process up to a maximum of 60 tonnes of dry solids per day (tds/day) per stream.
The sludge is heated to around 95°C in pulper to become ‘homogenised’. This sludge is then pumped sequentially through each of the streams 4 (No.) reactors for batch treatment. Steam from the boilers is used to heat the sludge between 50-180°C where it is held at 6-bar pressure to undergo pasteurising hydrolysis for around 30-45 minutes. After hydrolysis, sludge is passed into the flash tank, where the pressure and temperature of the hydrolysed sludge are decreased by flashing steam back to the pulper. The flashtank provides the necessary thermal buffer capacity to release the excess energy contained in the sludge before entering the downstream pumps.
The process kills all pathogens and, in turn, increases the potential gas yield of the sludge.
2 (No.) boilers, procured from Italy, produce 192°C steam from potable water, operate at 12bar and can run on either natural gas imported from the grid or biogas produced from the gas to grid plant. 2 (No.) CHP engines are situated locally to the boilers and the exhaust gas heat is recovered by boilers to optimise performance, these CHP engines can also run on either natural or biogas. The boilers are operated by BOAS accredited operators who were transitioned across from Costain to Severn Trent upon handover.
The warm hydrolysed sludge is then blended with UV FE to reduce the temperature to around 42°C where it is fed back into the digesters via a bank of 8 (No.) sludge return pipes traversing 500m across the site footprint.
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(top left) Centrifuges, (top right) THP silos, (bottom left) the boiler and (bottom right) the boiler house - Courtesy of Costain[/caption]
UV treatment and sludge cake
A UV plant was constructed to ensure no pathogens were introduced to the sludge post-THP which would normally be present in non-UV treated FE. This ensures that the biosolids product produced by the works is ‘enhanced’ quality and can be sold to farmers to fertilise fields growing a wider range of crops than those which can have ‘standard’ biosolids. A pair of Bollfilters with 25µm strainers from Bollfilter UK Ltd screen the FE prior to UV treatment to ensure that stringent disinfection standards are achieved and to prevent particulates damage to the plant.
Other THP highlights
The THP installation also included a polymer dosing plant, FE and potable break tanks, odour control unit, deep drainage and centrate pump stations with 700m of buried return pipework to head of works, a new FE pump station and a 1200m3 cake bunker.
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THP reactors - Courtesy of Costain[/caption]
(left) Cake bunker cofferdam and (right) cake bunker - Courtesy of Costain[/caption]
CHP engines- Courtesy of Costain[/caption]
Muck shift in progress - Courtesy of Costain[/caption]
Costain worked collaboratively with Severn Trent to draft plans for the reuse of the muck in the proposed BIA and began the process of sampling and testing the muck. A comprehensive Materials Management Plan approved under the CL:AIRE regulations was produced detailing the proposal. Upon approval from all parties, the muck shift operation began. Costain operated three 30 tonne articulated dumpers (ADTs) to transport the muck across the site. Each dumper averaged 25 trips per day and the full operation was completed in just a little under two weeks.
The overall benefits were:
Finham THP & G2G - Courtesy of Costain[/caption]
Odour model emission contours for selected solution - Google Maps[/caption]
Phase 1 odour control unit from ERG (Air Pollution Control) Ltd - Courtesy of Galliford Try[/caption]
(top & bottom left) BIM models showing the THP plant area and (bottom right) Phase 2 THP area looking north-west - Courtesy of Galliford Try[/caption]
Digester feed pumps are used to pump hydrolysed sludge from the flash tank to the digesters. Disinfected process water is added to the suction side of the digester feed pumps to dilute the hydrolysed sludge to achieve the target digester feed sludge dry solids content.
Diluted hydrolysed sludge is combined with recirculated digested sludge before being cooled while passing to the two existing digesters.
The new digested sludge dewatering stage comprises new duty/standby centrifuges and an associated polymer preparation/dosing system. Enhanced treated cake is conveyed to the new cake barn.
There is an existing 800 kW CHP engine. High grade waste heat from the CHP engine exhaust gas is recovered in a new composite steam boiler. Low grade heat is also recovered from the CHP engine jacket cooling water and used to pre-heat the boiler feed water. These heat recovery systems help to minimise the consumption of supplementary fuel (biogas or diesel) required to meet the THP steam demand.
A new final effluent (FE) pumping station supplies cooling water to the sludge coolers. All applications requiring dilution or carrier water are supplied with filtered FE that has been UV-disinfected, ensuring that an enhanced treated cake is produced.
The schematic diagram below shows the process stages with the additional odour reduction measures undertaken in Phase 1 (in green) and the main changes to the sludge treatment process for Phase 2 (in orange).
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Schematic showing the process stages - Phase 1: Additional odour reduction measures (green) and, Phase 2: The main changes to the sludge treatment process (orange)[/caption]
View looking at Serpecon conveyors and pre-THP silo feed pumps - Courtesy of Galliford Try[/caption]
THP area looking north east - Courtesy of Galliford Try[/caption]
This approach has proved successful throughout the first three years of the current programming AMP7 period (2020-2025) as it affords greater flexibility and control. Wherever possible the existing structure was retained and re-purposed to speed up the construction, and to reduce overall costs and the carbon footprint of the scheme. Before reusing the foundations, detailed in situ and pull-out testing was undertaken. The Cambi B2 unit and associated ancillary equipment are all located within the original cake drying beds.
In parallel to the cake bay works, construction was undertaken within the existing plant to facilitate the installation of the other process units. This required close collaborative working between the contractor and the onsite operations team, to minimise disruption to day-to-day activities and the running of the treatment plant. Throughout the scheme, this close cooperation has continued, ensuring all stakeholders are kept up to date with activities and progress.
Field Systems Design Ltd was engaged by Galliford Try to undertake the detailed design and installation of the complex interconnecting cabling and mechanical piping systems throughout the scheme. This included high voltage cabling and transformer upgrade works, low voltage power cables, signal cables and fibre optic networks. The scope of the mechanical supply included all piping and access arrangements, steam lines and pressure networks.
In total, 48 kilometres of cable, 2.5 kilometres of pipes and associated support systems have been installed. These systems are predominantly placed above ground for ease of future maintenance, and to mitigate the risks of burying equipment within an already congested plant. Collaborative planning and design management have been undertaken between the site team and Galliford Try, to ensure the smooth flow of the mechanical and electrical phase of the works.
Overall, the scheme is now (June 2023) approaching 200,000 hours worked without any lost time or other significant incidents.
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View looking north-east towards the Saxlund pre-THP cake silo - Courtesy of Galliford Try[/caption]
(top left) BIM model showing the steam boiler system, (top right) the rear of the steam boiler building and emissions stack, (bottom left) the front of the steam boiler building and emissions stack, and (bottom right) the Dunpy boiler package - Courtesy of Galliford Try[/caption]
Mini CS & Company News
- Strongford STW
Strongford STWSevern Trent’s Strongford Sewage Treatment Works recently completed a major upgrade to improve wastewater processing for a population of around 350,000 people across Newcastle-under-Lyme and Stoke-on-Trent. A key part of the upgrade project was introducing Thermal Hydrolysis Process (THP) technology, a method that treats sludge (the solid byproduct of wastewater) so it is safer and more efficient to process. THP helps break down the sludge, making it easier to produce biogas — a renewable energy source — and improving the plant’s overall efficiency.
PBJ Engineering’s role
PBJ Engineering's team was on the ground at Strongford every day for around 18 months, running the steam boilers and training Severn Trent’s staff until they were ready to earn their Boiler Operations Accreditation Scheme (BOAS) certification. Our job didn’t stop there, though. During this period and beyond, we supplied boiler testing kits, managed stock resupplies, maintained detailed logbooks, and offered ongoing support, including coverage during management leave. We also provided and installed a hired reverse osmosis (RO) unit, currently still in operation on site. This unit improves boiler performance by reducing total dissolved solids (TDS), which lowers the frequency of boiler blowdowns and minimises operational issues. It was a proactive measure to help ensure smoother day-to-day operations while the client develops a permanent solution.Challenges and solutions
Keeping everything running smoothly meant being prepared for any unplanned downtime. The PBJ team provided round-the-clock support and quick-response services whenever needed. We supplied specialised testing equipment and designed a comprehensive logbook to track and simplify daily maintenance — making sure nothing slipped through the cracks. The reverse osmosis unit was also key to maintaining efficiency, preventing disruptions by improving boiler reliability and performance. This combination of technical expertise and ongoing support ensured that Strongford’s upgrades moved forward without a hitch. [caption id="attachment_18128" align="alignnone" width="1200"]
Boiler area - Courtesy of PBJ Engineering[/caption]
Value & impact
With our support, Strongford’s upgrades ran safely and efficiently, helping Severn Trent meet their environmental goals. By reducing downtime and boosting reliability, we contributed to a more sustainable treatment system that benefits the entire community. In the bigger picture, the THP technology has transformed Strongford into a central sludge treatment hub, now capable of handling up to 94 tons of sludge per day and supporting nearby sites by taking on additional processing. The upgrades also boost Severn Trent’s sustainability efforts: the plant now produces an extra 16.34 GWh of biogas per year, which improves energy self-sufficiency and reduces carbon emissions by 3,000 tons annually, alongside producing cleaner Class A sludge that’s safer for reuse or disposal. One thing to note: while our THP Operator services were not required on this project, it is an option we can offer in the future. If that’s something you’re considering for your next project, get in touch to discuss your requirements.
For more information contact PBJ Engineering: +447758 120432 | http://www.pbjengineering.co.uk/
