Otterbourne WSW (2023)

Phase 2 capital maintenance project - upgrade of disinfection systems
By: Isa Mohammed and Richard Froom
Published: November 18, 2023
Otterbourne WSW (2023)
Construction progress (July 2023) - Courtesy of GTb

Otterbourne Water Supply Works (WSW) is a surface water supply works that supplies a large number of customers in Southampton, Winchester and the surrounding areas. The site abstracts and treats raw water from two different sources; these include raw water from boreholes and surface water from the River Itchen. Each raw water source is treated separately prior to feeding into the distribution system. The ground and surface water streams are licenced to abstract 58 MLD and 45 MLD respectively. Both treatment streams are aged assets, so the upgrade will prevent any significant risk to water quality and supply in the future.

Phased approach

As part of Southern Water’s commitment to ensuring a wholesome water supply to customers, a programme of capital maintenance work has been initiated. This will take place using a phased approach to replace the existing assets. The phases include:

  • Phase 1: AMP6 Intervention works to extend the life of existing assets until replacement is complete.
  • Phase 2: AMP7 project to replace the existing disinfection process (made up of two separate streams) with a new common disinfection system.
  • Phase 3: AMP7 project, further intervention works to extend the pre-disinfection process residual life and power resilience until the Phase 4 project is commissioned.
  • Phase 4: AMP8 replacement of the existing pre-disinfection process with modern technology.

This article focuses on Phase 2 of the programme, aimed at replacing the existing two stream disinfection processes with a common system that treats both ground and surface water, to the dates agreed between Southern Water and the Drinking Water Inspectorate.

Google Earth image of Otterbourne WSW - Courtesy of GTb

Google Earth image of Otterbourne WSW - Courtesy of GTb


The design was carried out by an integrated team comprising Southern Water Engineering Technical Solution (ETS) comprising Southern Water engineers and its strategic solution partners, led by Stantec UK, and the delivery partner GTb, a partnership between Galliford Try and Binnies.

GTb is leading the detailed design, construction and commissioning of the works. This project has been ongoing since early AMP7 and planned for completion by July 2024.

Otterbourne WSW - Phase 2: Supply chain: key participants
  • Client: Southern Water
  • Principal contractor & designer: Galliford Try Binnies JV
  • Ground investigation: Quantum Geotechnic
  • Telemetry survey: Multiform Technologies
  • SCADA upgrade: MCS Control Systems Ltd
  • Mechanical installation: Powerrun Pipe-mech Ltd
  • Electrical contractor: Field Systems Design
  • HV installation: Smith Brothers
  • Turbidity monitor installation: Linton Electrical
  • Yewhill linestop: UTS Engineering
  • Concrete surveys & repairs: Concrete Repairs Ltd
  • SED tank refurbishment: DD Engineering
  • Chemical dosing: Richard Alan Engineering Ltd
  • Chemical dosing: Colloide
  • Kiosks: Morgan Marine

  • Surge vessels: Quantum Engineering Developments Ltd
  • Valves: AVK UK Ltd
  • Blowers: Howden
  • Abstraction pumps: ERIKS
  • Backwash tanks: Hayes GFS
  • UV plant: Trojan Technologies
  • Large pumps: SPP Pumps Ltd
  • Contact tanks: Cookson & Zinn
  • HV switchgear: BRUSH Group
  • WWR panel: Saftronics
  • Generators: Aggreko
  • Steel flooring & security fencing: Steelway
  • Earthing: BEST
  • Scaffolding: Formula Scaffolding
Intermediate pumping station - Courtesy of GTb

Intermediate pumping station - Courtesy of GTb

Existing disinfection system (separate systems)

The surface water treatment stream abstracts raw water from the River Itchen, via an intake eel screen and low lift pumping station. The principal process stages are clarification (sedimentation and accentrifloc type clarifier), filtration (rapid gravity sand filters) and chlorine disinfection in a dedicated contact tank, before feeding the distribution network. The groundwater source abstracts raw water from a combination of boreholes and wells, with the abstracted water being treated via a microfiltration plant and chlorine disinfection in a dedicated contact tank, before feeding the distribution network.

The existing chlorine contact tanks are below ground concrete tanks, each having two cells.

New disinfection system (combined flows)

An optioneering exercise was carried out following the principles of the Risk & Value (R&V) process. The outcome of this determined that the best option on the balance of risk mitigation and whole life cost was to install a common 91 MLD offline disinfection system to treat both ground and surface water flows.

High lift pumping station - Courtesy of GTb

High lift pumping station - Courtesy of GTb

Scope of works

The scope of the Otterbourne Phase 2 Capital Maintenance Project, with an estimated project budget of £37m, includes the design, provision, installation, commissioning, testing and handover of the following components:

  • New interstage pumping station and dedicated interstage balance tank.
  • New TrojanUVFlex® ultraviolet disinfection treatment process and ancillaries from Trojan Technologies.
  • New combined above-ground chlorine contact tanks.
  • New high lift pumps and dedicated treated water balance tank.
  • New sodium hypochlorite storage and dosing system sized for 10% (w/w) chemical, including carrier water system, and decommissioning of existing disinfection sodium hypochlorite dosing system as required.
  • New orthophosphoric acid dosing skid, including carrier water system.
  • New pre-contact tank inlet run to waste facility.
  • Run to waste tank and run to waste transfer pumps.
  • New service water system.
  • Final water quality monitors and appropriate control functions.
  • Interface new plant items control with existing assets.
  • Implement appropriate control and shutdown control.
  • Decommissioning of existing contact tanks and high lift pumps.
  • New SCADA system.
  • New communication networks.
  • Upgrade of some existing PLCs.
  • Modifications and upgrades of the site power distribution system.
  • Decommissioning of any facilities made redundant because of this project.
Contact tanks from Cookson & Zinn - Courtesy of GTb

Contact tanks from Cookson & Zinn - Courtesy of GTb

Modular solutions

Modular solutions were prioritized. The chemical dosing building, contact tanks and ultraviolet units were constructed offline and off-site, which meant minimal disruption to the operation of the existing dosing systems. Off-site and offline production and construction also provided a better working environment enhancing the health, safety and wellbeing of the construction team. It also enabled lean manufacturing techniques to be employed, delivering savings in time and cost, as well as reducing the burden on client resources and making a positive impact on the carbon footprint.

The TrojanUVFlex® system from Trojan Technologies is a new method of UV treatment, and this is one of the first units to be deployed. Each system was fully tested and commissioned before being transported to site for final assembly. This offered further programme efficiencies as well as reducing the logistics requirement.

The design also takes into consideration the long-term interface requirements for the water resource management plan and power upgrade, by allowing adequate space and tie-ins for easy integrations of any future works.

(left) Otterbourne WSW under construction and (right) new RGF clean backwash storage tanks from Hayes GFS - Courtesy of GTb

(left) Otterbourne WSW under construction and (right) new RGF clean backwash storage tanks from Hayes GFS - Courtesy of GTb

Current progress

At the time of writing (August 2023), Construction works are completed, the project is now progressing with dry commissioning and the wet commissioning is planned to start by the end of August 2023.

The target for benefit realisation is July 2024.

The editor and publishers would like to thank Isa Mohammed, Senior Project Design Lead and Richard Froom, Senior Project Manager, both with Southern Water, for providing the above article for publication.

3D View of proposed solution - Courtesy of GTb

3D View of proposed solution - Courtesy of GTb