Arley STW – AMP7 Quality and Growth Improvements

To comply with stringent water quality targets under the Water Framework Directive (WFD), and improve the local aquatic environment, Severn Trent Water needed to upgrade Arley Sewage Treatment Works. In addition to achieving tighter permit conditions, improvements were required to provide additional treatment capacity for the 10% catchment growth and future-proof the works until 2033. The scheme comprised chemical dosing and tertiary solids removal plant to reduce chemical concentrations in the final effluent.

What we did

Arley STW scheme exemplifies Pell Frischmann’s multi-disciplinary capabilities working seamlessly across our UK offices. Pell Frischmann’s Process MEICA Team was responsible for the outline design of the scheme and our experts provided services such as:

  • Gap Analysis;
  • Ground Investigation Survey Specification;
  • Outline Design Development;
  • Structured what-if analysis;
  • Buildability reviews to produce Design & Build contract data for client acceptance.

Pell Frischmann addressed the environmental targets by promoting a structured design process to optimise the balance between improving the existing aged works infrastructure where practical, whilst integrating innovative technology package plants and equipment installations.


As well as providing an outline design to meet the future permit requirements, our team addressed a few health and safety and maintenance issues with the existing works, such as improving access for the inlet works, repurposing the biological filter clarifier to a humus tank, and optimising the primary settlement tank desludging arrangements. By adapting and repurposing the existing works infrastructure in this way, it contributed to reducing the scheme’s carbon footprint.

A new tertiary solids removal plant and chemical dosing rig was promoted to integrate effectively with the existing works infrastructure. With their primarily offsite build and standard solution design, this also contributed to reducing the carbon footprint with a lower whole-life cost solution than alternatives.

Mike Hicks, Pell Frischmann’s Framework Manager on the scheme, said:

The Arley STW design team developed a high-quality, reduced carbon, cost-effective design by optimising the existing works infrastructure and integrating an off-site package plant. This addresses the Water Framework Directive water quality targets and provides a successful blueprint for subsequent projects in Severn Trent’s investment programme.

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