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JK Fabrications Ltd

Didcot STW – Picket Fence Thickener

Design, supply, and installation of a new picket fence thickener (PFT)
By: JK Fabrications Ltd
Published: October 30, 2025

Picket Fence Thickener – Didcot Sewage Treatment Works

Background

As part of a programme of upgrades to improve sludge treatment efficiency and operational reliability at the Didcot Sewage Treatment Works, Thames Water required the installation of a new Picket Fence Thickener (PFT) system.

JK Fabrications was appointed to design, supply, and install the new PFT in full compliance with Thames Water Asset Standards. The objective was to deliver a robust and low-maintenance thickening solution capable of handling the site’s demanding operational conditions while ensuring safe, long-term performance.

Objectives

The main goals of the project were to:

  • Deliver a fully compliant thickening system in accordance with Thames Water standards.
  • Provide a durable, corrosion-resistant structure suitable for a wide range of process conditions.
  • Ensure safe access and ease of maintenance for operations staff.
  • Complete the works within a tight delivery window while minimising disruption to ongoing site operations.

Scope of Works

JK Fabrications’ turnkey package covered the complete design, supply, and installation of the PFT system, including:

  • Glass-fused steel tank (8.539 m diameter x 4.78 m high) providing a total capacity of 245.08 m³.
  • GRP roof system with a beam and infill design and 12 access hatches for cleaning and inspection.
  • Grade 316L stainless steel outlet launder with V-notch weirs and scum boards, supported from the internal tank wall.
  • Peripheral walkway and bridge structure fabricated in galvanised mild steel.
  • Central support column designed and coated to WIMES 4.01.
  • PFT mechanism components including thickening gate, launders, scum boards, and diffusion drum in grade 316 stainless steel.
  • Local Control Panel (LCP) featuring hand/off/auto control, local isolation, and push-button operation.
  • Hydrostatic testing of the completed tank assembly.

The tank and associated components were designed for liquids with a specific gravity of 1.05, a pH range of 2–11, and operating temperatures up to 44°C. The external finish was supplied in Midnight Green (BS4900 12-B-29) to align with site aesthetics.

Project Challenges

Working within a live wastewater treatment site required careful coordination to maintain operations and safety. The site presented limited access and confined working areas, necessitating precise logistical planning for the delivery and assembly of large prefabricated components.

JK Fabrications collaborated closely with Thames Water’s site management team to schedule works during low-flow periods, ensuring minimal disruption. The use of prefabricated steel and GRP elements reduced on-site construction time and improved installation efficiency.

Outcome

The project was successfully completed within the planned six-month timeframe, covering design, fabrication, installation, and full commissioning.

Hydrostatic testing and mechanical inspections were completed on first attempt, confirming compliance with Thames Water’s technical standards. The new Picket Fence Thickener now provides reliable sludge thickening performance, with enhanced access for inspection and maintenance and improved operational resilience.

Client feedback highlighted the quality of fabrication, clear communication, and safe working practices demonstrated throughout the project.

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