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JK Fabrications Ltd

Horley STW – ROTANOX Tank System

Design, manufacture, and installation of the new ROTANOX Tank System.
By: JK Fabricaations Ltd
Published: November 5, 2025

Horley STW – ROTANOX Tank System

Background

JK Fabrications Ltd was engaged by the Eight2O Alliance to undertake the replacement and refurbishment of Rotanox Tank No. 5, which had reached the end of its operational life.The works formed part of Thames Water’s wider infrastructure upgrade programme to ensure the long-term efficiency and reliability of its wastewater treatment assets.

The project required the complete decommissioning, dismantling, and reconstruction of the existing Rotanox tank system, including both the inner Final Settlement Tank (FST) and the outer oxidation ditch.

Objectives

  • Safely decommission and dismantle the existing Rotanox Tank
  • Design, manufacture, and install new glass-fused steel tanks for both the FST and oxidation ditch
  • Replace and modernise all mechanical equipment, including the scraper system and FBDA aeration system
  • Reuse and reinstall existing walkways where possible to optimise cost and efficiency
  • Deliver a fully tested, commissioned, and operational tank system ready for reintegration into the treatment process

Scope of Works

JK Fabrications’ turnkey scope covered all mechanical design, fabrication, supply, and installation elements associated with the refurbishment, including:

  • Drain down and clean-out of the existing Rotanox Tank No. 5, including handling, pumping, and removal of residual sludge in accordance with site procedures
  • Dismantling and removal of the existing inner and outer tank walls and the obsolete FBDA system

Design, manufacture, and installation of new Glass Fused Steel (GFS) tanks:

  • Inner FST tank: 15.5 m diameter × 3.6 m high
  • Outer oxidation tank: 28.9 m diameter × 4.78 m high

Replacement of the FST scraper bridge, including:

  • Main bridge structure, tripod, slewing ring, scraper system, and scum skimmer
  • Drive motor, gearbox, slip ring collector, handrail system, and new cablin
  • Flooring clips compliant with Thames Water asset standard
  • Replacement of the outlet launder system with new stainless steel weirs and scum board
  • Installation of new FBDA aeration system
  • Hydrostatic testing of both tanks to verify structural integrity and watertightness

Tank Design and Materials

  • Glass Fused Steel tanks
  • External tank colour: Midnight Green
  • Internal tank colour: Moss Green / Conifer
  • Central tripod support fabricated in galvanised mild steel and painted to WIMES 4.01 – Table 4A
  • Scraper system, diffusion drum, outlet launders, and scum boards fabricated in Grade 304 Stainless Steel

Electrical and Control Works

  • New local isolation station with e-stop for the scraper bridge
  • Supply and installation of new bridge cabling and slip ring collector
  • Integration with existing site control systems managed by the main contractor

Project Challenges

The refurbishment of Rotanox Tank No. 5 required extensive coordination with the civil and electrical contractors under the Eight2O framework.

Working within an existing tank structure presented challenges related to space, lifting operations, and sludge handling, which demanded precise sequencing and planning.

JK Fabrications’ site and engineering teams developed detailed method statements and risk assessments to ensure all operations were conducted safely and efficiently.

Additionally, the integration of the new FBDA system with existing infrastructure required close collaboration with the main contractor’s process design engineers.

Outcome

JK Fabrications successfully completed the replacement of Rotanox Tank No. 5, delivering a modernised, high-performance glass-fused steel tank system fully equipped with new mechanical and aeration components.

Following installation, the tanks and scraper system underwent comprehensive hydrostatic and functional testing, confirming full compliance with Thames Water and WIMES specifications.

The upgraded system has since provided enhanced process reliability, ease of maintenance, and extended service life, supporting the continued performance of the treatment works.

This project highlights JK Fabrications Ltd’s capability to manage complex refurbishment works within operational treatment environments — delivering safe, high-quality solutions that align with Thames Water’s engineering standards.

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