Avonmouth Gas to Grid

Wessex Water appointed Sweco to carry out the concept and detailed design of a new Biomethane upgrade plant, including the integration of specialist package equipment and the overall control system at their Avonmouth Waste Water Treatment Works (WwTW) site. When it was commissioned in 2014, the Avonmouth scheme was the largest and most complex gas-to-grid project in the UK. Since then, we have completed two other projects of the same size – Five Fords G2G (DCWW) and Howdon Biomethane Upgrading (NWG).

Key Facts

Client: Wessex Water

Date: 2013 – 2014

Location: Avonmouth, Bristol

Commissioned in Nov 2014


Gas-to-grid is an established technology used throughout in Europe. The process acts to upgrade raw biogas (produced by the digestion of organic waste such as sewage sludge or food) to a quality that is equivalent to natural gas which can be injected and exported into the gas grid. A water washing and filtration process is used to remove impurities and carbon dioxide from the biogas before injection into the grid.

Avonmouth WwTW is Wessex Water’s principal wastewater treatment works. As well as providing secondary treatment for approximately 1.1 million people, the works accounts for more than 50% of the region’s sludge treatment.


In addition to the installation of the upgrade plant, Sweco designed and oversaw the installation of a bulk propane store and transfer plant, and the grid entry unit which monitors the quality of the exported gas and communicates with the gas network operator.

The team also produced a specification designed to integrate the various individual packages into a working installation. The specification contained information to enable an installation contractor to accurately price and subsequently execute the works, and included comprehensive drawings and schedules covering the civil, mechanical and electrical infrastructure works, as well as the ICA and SCADA modifications that would be required.

As the gas to grid technology was still in its infancy in the UK, Sweco assisted the client in overcoming various regulatory and procedural challenges associated with obtaining the various consents (planning and environmental) needed to allow the gas into the public grid network.

Benefit to the Community

The project is the largest gas-to-grid scheme currently in operation in the UK and generates renewable gas for industrial and domestic use in the Bristol area. At the time, the plant produced fuel for road vehicles such as ‘Bio-Bus’, the UK’s first ever bus to run on food waste and organic sludge from the plant.

The plant, which was commissioned in November 2014 injects up to 1,500 cubic metres of renewable, low carbon, biogas per hour into the National Gas grid, which is sufficient to replace the natural gas consumption of 3,000 homes.

At the time of the commissioning, Andy King, Sweco’s Director for Energy said: “Supporting Wessex Water’s aims of decarbonising the gas grid through biomethane injection is an incredible achievement and one of which we can be proud."


  • Concept Design
  • Vendor Prequalification, Tendering and Selection
  • Mechanical Engineering Design
  • Electrical Engineering Design
  • Civil & Structural Engineering Design
  • ICA Design and Integration
  • Site Investigation and
  • Geotechnical Engineering Design
  • Contract Administration
  • Project Management
  • Production of Planning
  • Drawings
  • Environmental Permitting