Plymouth Central Sewage Treatment Works

Helping Plymouth make the most of mayflower 2020, to celebrate the 400th anniversary of the sailing of the Mayflower from Plymouth UK to Plymouth Massachusetts. 

Pell Frischmann are working with South West Water and collaborating with Arcadis to improve combined sewer overflows (CSOs) within two bathing water zones to achieve excellent water quality for residents, and tourists around the world.

Starting with an optioneering phase with Arcadis, we spent 18-months modelling the catchments upstream of both Plymouth Hoe East and Plymouth Hoe West to find the best solutions for reducing the number of storm overflow spills that contaminated the local bathing water.

The process involved looking at three major treatment work catchments, with HR Wallingford employed to undertake hydrodynamics and dispersion modelling. This allowed us to assess how the CSO spills impacted on the rivers, and how the major pollutions contributed to the Bathing Water quality. With this information, we were able to control the CSO spills to minimise impact on the Bathing Water to maximum benefit.

With over 30 combined sewer overflows and a need to maximise use of available resources, we decided to focus on the ten biggest overflows whilst still providing screening on all combined sewer overflows. Through this, we could significantly reduce spill volumes, and provide better improvements than focusing on all areas. With this information, the Environment Agency (EA) agreed to the betterment solution. This has led to working on the big spillers such as Plymouth Central Sewage Treatment Works, where we are putting in an Ultraviolet (UV) disinfection system on the storm overflow to treat the storm water rather than storing and treating dilute storm water.

To make sure we were on top of all combined sewer overflows in our modelling phase, we collaborated with Arcadis to peer review each other’s work. This created an open and honest environment - something vital for creating excellent solutions.

Created an open and honest environment - something vital for creating excellent solutions

Another challenge within this scheme was managing expectations of multiple partners and beneficiaries, and attempting to keep all relevant stakeholders happy. To maintain this, a consistent communication line with all stakeholders was created, making sure progress and changes within the scheme were appropriately communicated with everyone.

Further challenges saw the need to cater for contactors that joined the scheme later in the project. We were responsible for making sure they understood the task in hand, the implications of the programme, its time constraints, and our reasons for making particular decisions. This saw the use of our expertise for technical support throughout the project.

As a diverse and highly collaborative project that involved thorough liaison with South West Water, the EA, HR Wallingford, and MWH, we have been able to share valuable resources that has enabled us to develop a strategy to truly transform Plymouth’s bathing water. 


  • icon-arrow2 - bathing water zones
  • icon-arrow18 - month modelling period
  • icon-arrow30 - combined sewer overflows

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