Joseph Gallagher Ltd

Brentwood Sewage Treatment Works

Construction of two new percolating filters to increase treatment capacity and support future compliance requirements.

By: Joseph Gallagher Ltd

Published: August 12, 2026

In‑situ concrete works for a new percolating filter at Brentwood STW, delivered by Joseph Gallagher Ltd

Project Overview

Brentwood Sewage Treatment Works (STW) serves the communities of West Brentwood, Warley and Pilgrims Hatch in Essex. With the population within the catchment forecast to increase from approximately 36,500 to over 41,000 residents by 2026, Thames Water identified the need to increase treatment capacity and improve process performance to meet future demand.

To ensure continued compliance with increasingly stringent discharge consent standards, Thames Water commissioned an upgrade to the site under its Eight2o AMP6 programme. Joseph Gallagher was appointed by SMB JV (Skanska, MWH Treatment and Balfour Beatty) as the design and build subcontractor for the civil engineering works.

Construction of one of the new 32m diameter percolating filter tanks at Brentwood STW, featuring the reinforced concrete structure and underdrain system prior to commissioning.

Scope of Works

The project involved the design and construction of two new percolating filter tanks, each measuring 32m in diameter and approximately 3m deep. Joseph Gallagher’s scope of works included:

Design and delivery of temporary works
Bulk excavation and groundworks
Shaft construction
In situ reinforced concrete structures
Washout and collection chambers
Associated drainage and ducting infrastructure

The new filters were designed to operate alongside the existing treatment assets, increasing hydraulic capacity and enhancing biological treatment performance.

Construction progress on two new percolating filter tanks at Brentwood Sewage Treatment Works, designed to enhance treatment resilience and support future population growth.

Key Challenges

Construction took place within a fully operational sewage treatment works, requiring careful planning and coordination to maintain uninterrupted site operations throughout the programme. Key challenges included:

Managing safety and logistics during large-scale excavation works
Working within a constrained operational site
Coordinating significant plant movements
Managing programme risks associated with seasonal weather conditions
Identifying and accommodating previously uncharted underground services

During excavation works, several unknown buried services were discovered. Through close collaboration with Thames Water and the wider project team, these services were safely incorporated into the programme without impacting operational performance or project delivery.

Joseph Gallagher also worked collaboratively with follow-on contractors to improve efficiency and reduce environmental impact by providing a crane mat for subsequent civils activities, helping to minimise material imports and vehicle movements.

Project Delivery

Due to the predominantly buried nature of the structures, the percolating filter walls and bases were constructed using in-situ reinforced concrete. Joseph Gallagher was responsible for both the design and construction of these elements, ensuring a durable and robust long-term solution.

Works were carefully phased to accommodate ongoing treatment operations and restricted site access. Detailed logistics planning enabled the safe management of construction activities while maintaining operational continuity across the treatment works.

The larger and deeper filters provide increased treatment capacity, allowing higher flows to be processed while improving overall site resilience and future operational performance.

All civil, mechanical, electrical and ICA works were completed by June 2020, with successful commissioning achieved ahead of the required regulatory output date.

Outcome

The upgraded assets met the revised discharge permit requirements by March 2020, as planned. The enhanced treatment process delivered performance that exceeded original modelling expectations and provided Thames Water with a robust, resilient and future-proof solution.

The scheme increased treatment capacity, improved biological treatment efficiency and strengthened the site’s ability to accommodate future population growth while maintaining compliance with environmental standards.

Strong collaboration between Joseph Gallagher, SMB JV and Thames Water Operations ensured effective decision-making, proactive risk management and a smooth transition into operational use.

Conclusion

The Brentwood STW upgrade demonstrates Joseph Gallagher’s ability to successfully deliver complex wastewater infrastructure projects within live operational environments. Through careful planning, collaborative working and technical expertise, the project provided significant improvements to treatment capacity and process performance while maintaining uninterrupted site operations.

The completed scheme delivers long-term benefits for Thames Water and the communities served by the treatment works, ensuring reliable and compliant wastewater treatment for years to come.