Project Overview

Brentwood Sewage Treatment Works (STW) is a medium‑sized treatment facility serving West Brentwood, Warley and Pilgrims Hatch in Essex. With population growth forecast to increase demand from 36,500 to over 41,000 residents by 2026, Thames Water required a significant upgrade to ensure continued compliance with increasingly stringent discharge consent standards. Joseph Gallagher was appointed by SMB JV - an alliance between Skanska, MWH Treatment and Balfour Beatty - as a design and build subcontractor under the Eight2o AMP6 framework.

Scope of Works

Joseph Gallagher was responsible for the design and construction of two new 32‑metre‑diameter percolating filter tanks, each approximately 3 metres deep. The scope included:

Design and delivery of temporary works
Shaft construction and groundworks
In‑situ reinforced concrete structures
Washout and collection chambers
All associated drainage and ducting

The new filters were designed to operate in parallel with existing assets, increasing hydraulic capacity and significantly improving overall biological treatment performance.

Key Challenges

Delivering major civils within a fully operational treatment works presented a number of challenges:

Managing safety and logistics during large‑scale bulk excavation
Identifying and working around previously uncharted services
Coordinating significant plant movements within restricted space
Maintaining programme certainty despite seasonal weather risks

During excavation, several unknown services were uncovered, requiring additional enabling works. These were safely integrated into the programme without disruption to live operations.

To reduce vehicle movements and minimise materials brought onto site, Joseph Gallagher worked collaboratively with follow‑on civils contractors, supplying a crane mat at no cost to support efficient site operations and reduce environmental impact.

Project Delivery

Groundworks and construction were carefully phased to maintain operational continuity at the treatment works. Due to the predominantly buried nature of the structures, the filter bases and walls were constructed using in‑situ reinforced concrete, designed and built entirely by Joseph Gallagher.

The larger and deeper filters enable higher flows to be treated, improving resilience and long‑term compliance. All civil, mechanical, electrical and ICA works were completed by June 2020, with successful commissioning achieved ahead of the regulatory output date.

JGL at Brentwood Sewage Treatment Project

Outcome

The upgraded assets met the revised discharge permit requirements by March 2020, as planned. The new process solution not only met but exceeded performance expectations against initial models, delivering a robust, reliable and future‑proof treatment process. A strong collaborative relationship with Thames Water’s Operations Team throughout the project ensured swift decision‑making, effective risk management and a smooth transition into operational use.

Conclusion

The Brentwood STW upgrade demonstrates Joseph Gallagher’s ability to deliver complex infrastructure projects safely, efficiently and collaboratively within live operational environments - providing long‑lasting performance improvements for essential utilities.