Turning human waste into green fuel and fertiliser
Project Overview
Introduction
Treating human waste efficiently and sustainably can help meet net zero targets and embed circular economy principles, as well as enabling water companies to cut their carbon emissions. That’s why Severn Trent decided to expand and upgrade its sewage treatment works at Stoke Bardolph.
The aim was to increase sludge treatment capacity and maximise biogas production for export to the domestic grid, while reducing the site’s carbon footprint and improving the quality of its fertiliser byproduct.
The solution lay in a £45M new advanced anaerobic digestion facility designed, constructed and commissioned by an integrated team from our design and build business Mott MacDonald Bentley.
Thanks to innovative design and construction solutions, the project has scored significant efficiency and carbon savings. An ambitious schedule of just two and a half years from contract award to operation put the onus on collaborative working and design for manufacture and assembly (DfMA). The project included more than 100 subcontractor packages, many with multiple interfaces. This required careful coordination and early contractor engagement to achieve timely delivery – all while maintaining world class health and safety standards. The key? A relentless focus on planning and a robust delivery programme.
Renewable energy for thousands
Anaerobic digestion works by processing a feedstock – in this case, sewage sludge from wastewater treatment – inside sealed, temperature-controlled tanks known as digesters. Naturally occurring microbes break down the sludge by digesting it, producing biogas which can be captured for use as fuel. The remaining semi-solid residue, called the digestate, can be used as fertiliser for non-food crops.
A more sophisticated and effective method, known as advanced anaerobic digestion, is also available – and that’s what Severn Trent wanted for its new facility.
Advanced anaerobic digestion uses thermal hydrolysis to expose the sludge to high temperature and pressure. This makes the process more effective, enabling 15% to 20% more biogas to be produced from the same quantity of feedstock. It also kills 99.9% of pathogens, resulting in a Class A sludge cake product that is rich in nutrients and thus suitable as a fertiliser for food production.
The new advanced anaerobic digestion facility at Stoke Bardolph includes a cake import system that allows the facility to receive sludge from other wastewater treatment plants, with centrifuges that dewater the indigenous and imported liquid sludge to the correct thickness (20% to 22% dry solids) prior to treatment.
The thermal hydrolysis process (THP) plant then heats the sludge to 180°C through steam injection, driven by the steam-raising boiler house. Motor control centres power and control the THP plant, digesters and other equipment. A biogas-to-grid unit then sends the biomethane gas through a 3.2km gas export pipeline to supply up to 9000 properties in Nottingham with energy.
Clever choices cut carbon
From the project outset we prioritised conserving energy and cutting carbon emissions. Throughout the design phase we rigorously assessed the condition and performance of existing plant and structures to eliminate unnecessary new-build.
Our design lead Zach Smallbone says: “Early on we identified that we could build the new plant within an abandoned below ground activated sludge plant,” adding that this was drained and backfilled to allow for the construction of the new assets.
“By interrogating flood and groundwater levels, we reduced the backfill height, enabling a 10,500t reduction in imported fill that also minimised vehicle movements. Crushing and reusing material from the demolished structure as backfill saved a further 65t of embodied carbon,” explains Zach.
Another efficient design choice saw the conversion of existing digesters instead of building new ones, reducing the amount of concrete construction. “The four existing mesophilic anaerobic digesters at Stoke Bardolph were overloaded, but simple modifications to increase their process capacity allowed them to be converted to THP digesters, avoiding the need to demolish them,” says Zach.
We also surveyed the existing gas holder and deemed it acceptable in both volume and condition for the new installation if refurbished. The biogas system upgrades therefore only required construction of a new flare stack and pipework, rather than a complete new gas holding system. A washwater pump station, initially earmarked for replacement, was also repurposed – through an innovative approach to positioning the submersible pumps.
Maximised heat recovery
Heat recovery was another vital part of our strategy to cut carbon, this time in operational terms. The technique is applied at various stages in the new process, reducing natural gas usage and unnecessary waste.
The boiler plant recovers waste heat from the exhaust gas and water cooling of two combined heat and power (CHP) engines. The cooling system lowers sludge temperature from 80°C to 40°C after leaving the THP plant and before it is fed into the digesters. But recovering waste heat to preheat the sludge, prior to the thermal hydrolysis process, reduces the demand on the steam boiler.
Smart design saved energy in other ways. Progressive cavity pumps, which move the sludge between different processes, were equipped with a boundary layer injection system to reduce friction and thus pump pressure, lowering energy use by up to 18%. Meanwhile we raised the gantry, which holds the three coarse material separators that screen the sludge, high enough to enable free discharge into the two storage tanks. This removed the need for another set of pumps – thus reducing both embodied and operational carbon.
Streamlined design and delivery
We championed the benefits of DfMA from the outset, selecting plant elements based on a supplier’s ability to build and test major pieces offsite.
“Through DfMA, complicated offsite design and fabrication could run parallel with onsite design and build for the services and foundations, streamlining the programme,” says our design manager on the project, Shane Porter.
DfMA brought other benefits too. “Manufacturing offsite in factory conditions leads to a better-quality end product, while also reducing waste and carbon emissions and improving health and safety,” says Shane.
26 weeks productivity saving through DfMA
More than half the work packages were either fully or significantly manufactured offsite. The biggest was for the new steam-generating boiler house. Measuring 16m x 14m x 7m, this was manufactured, fully assembled and tested offsite, before being separated into modules for delivery and installation.
“Being able to conduct a full building test in a factory environment enabled any snags to be picked up prior to installation,” adds Shane. “We were also able to adopt an innovative, ‘quick-fit’ electrical connection system between boiler modules, significantly reducing onsite wiring.” This saved 26 weeks on the programme, compared with traditional construction methods.
An ultraviolet treatment rig, which will prevent contaminants being introduced into the effluent via untreated washwater after the anaerobic digestion process, was also delivered as a full offsite factory-built assembly.
Digital tools and technology keep things on track
Using building information modelling (BIM) and a common data environment ensured all data and documents were available to team members, the client and other stakeholders in real time, which proved invaluable during the pandemic lockdowns when teams were dispersed. Design review meetings and development of the project’s 3D model could continue remotely to ensure the on-time completion of programme-critical activities.
Drones were flown regularly to survey the site and monitor the project’s progress. Their aerial photography proved crucial in aiding communication between the design and construction teams that had instant access to the images.
Digital twin aids commissioning
Using the Smart Water app on Moata, our digital solutions platform, our data science team constructed a process digital twin for this project. This twin modelled the THP process and the revamped digesters. Adding thermal hydrolysis creates a different biological environment within anaerobic digesters, so converting them from conventional mesophilic anaerobic digesters to THP digesters required a careful transition.
In what is believed to be an industry first, we input daily plant readings and physical sample results into the digital twin – a virtual model of the operational process – for calibration prior to the conversion. The model then allowed multiple commissioning scenarios for the digesters to be rehearsed virtually.
Severn Trent was impressed by how our digital strategy enriched the project. “Careful design and the use of BIM, including digital twins, helped to ensure that the site layout was optimised, and interfaces carefully managed before installation of the plant commenced,” says Michael Fairlamb, the water company’s regional manager for capital and commercial services. “This reduced construction and installation timescales and improved our health and safety performance.”
A ‘sustainable, lasting legacy’
From breaking ground at Stoke Bardolph in January 2020, all significant construction work was complete by July 2021. Final commissioning was finished within two and a half years of the start of the project.
The site has been transformed into a major regional sludge processing centre and imports large quantities of biosolids from other wastewater treatment plants. It can process on average 80t of biosolids per day, with a maximum daily throughput of 120t. That is a capacity equivalent to a population of about 1.41M.
The project has also brought important environmental benefits. Improved biogas yields from the new, advanced anaerobic digestion process and the gas-to-grid facility are helping decarbonise the region’s gas supply.
For Severn Trent, the high quality of the fertiliser byproduct has cut its road haulage. The Class A sludge cake can be distributed locally to growers and farmers as it does not require transportation to other facilities for further treatment.
“The project has been very successfully delivered for the benefit of the environment and our customers,” says Severn Trent’s Michael Fairlamb. “Collaboration throughout the team has been instrumental in achieving a sustainable, lasting legacy for which the team are, quite rightly, very proud.”
Industry recognition
The project’s achievements have been acknowledged with several awards.
ICE East Midlands Merit Awards 2022
Large project (project value over £10M) award
Team achievement award
Sustainability award
British Construction Industry Awards 2022
Utility project of the year (shortlisted)
British Construction Industry Awards 2021
Industry Transformation and Innovation Champion (shortlisted)
Taking ownership of health and safety
Promoting and maintaining the highest standards of housekeeping, cleanliness and health and safety across the site was a critical objective. To this end, Mott MacDonald Bentley’s My Space initiative gave clear ownership to individuals for specific areas and welfare units, empowering them to manage material storage, waste management and access.
226,283 hours worked without any injuries
“The My Space campaign complements the great reporting culture we have at Mott MacDonald Bentley,” says project manager Jamie Beech. “Nearly 4000 positive health and safety interventions have been reported and, despite more than 220,000 hours worked on the project, it has been delivered to a world class safety standard without any lost time, medical treatment or RIDDOR reportable incidents.”
Severn Trent carried out safety audits frequently throughout delivery. External assessors from the Considerate Constructors Scheme also scored the project highly, stating: “[The] site demonstrated excellent practices and approaches to care for the occupational health and safety of the workforce and visitors and the safety of the public.”