United Utilities - Mathematical Modelling of Treatment Operating Costs

Sweco were commissioned to assist with United Utilities’ Unit Cost of Treatment project. The objective of the project was to calculate the theoretical and actual operational expenditure relating to energy and chemical use so as to inform the budgets for the AMP6 Business Plan.

Key Facts

Client – United Utilities
Location – North West England
Fee Value – £350k


Sweco were commissioned to create a modular mathematical modelling tool using MATLAB/Simulink to enable United Utilities operational staff to select individual process components of a site, connect them, and populate them with site-specific parameters such as tank dimensions, and control settings, etc.

We created fluid dynamic process models for both clean water and wastewater treatment works to calculate a theoretical cost of treatment. These costs were then compared with the actual costs. The model was designed to link with site telemetry to model the asset and cost performance of the site.

In Phase 1, we developed computational dynamic models of the highest cost components at four sites. We used site telemetry to monitor the influent conditions and create a robust cost model. We worked as part of a team consisting of UU’s engineering staff and an independent contractor.

Following the success of Phase 1, we extended the roll-out of the models by creating both site-specific and generic components comprising a comprehensive modelling library for UU to use on their own. Our work was related to a separate project aimed at updating UU’s Asset Data Hierarchy (ADH) and data visualisation systems. We worked with two other consultancies in addition to the Phase 1 stakeholders to deliver an updated suite of dynamic models which were capable of both reading telemetry data directly from the new data systems and providing results.


For the Phase 1 study, Sweco created dynamic process models using MATLAB/Simulink of the highest cost components at four sites. We utilised static equations developed in partnership with UU and an independent contractor. The innovative models included a wide range of available telemetry and lab results to set a moving target. This provided a more accurate cost estimate than UU’s original internal calculations, which could not account for varying influent conditions or the impact of one process unit on another downstream process unit. UU used our OPEX costs with their own CAPEX estimates to calculate the TOTEX impact and assess the favourability of proposed interventions at the sites modelled.

The success of this work led to Phase 2 as UU asked us to model several additional sites and to develop a generic software tool using MATLAB/Simulink which would allow UU to build models of other sites themselves. We coordinated with the other stakeholders to develop a production plan working with the ADH project. While Sweco designed and implemented the underlying architecture for the software tool, initial site-specific information (including telemetry, process equations and ADH) was gathered and assessed by the other stakeholders. This information was provided to Sweco, and we checked and validated the data, designed and built the site-specific process models, verified the results and returned the completed site model to UU. We worked with the other stakeholders to integrate the model with the new data system which was then under development.


The models developed by Sweco were used to identify opportunities to reduce chemical dose rates and energy use, while continuing to meet consent levels. A total of £500k in annual savings was identified.

Examples of the savings identified included:

• Refurbishing and optimising the chemical dosing in the coagulation stages at Clean Water Site A would save in excess of £140k per year.
• Refurbishment of the inter-stage pumps at Clean Water Site B would save £30k per year while refurbishing just the worst performing pump would save £10k annually. A delay in repairing a broken ammonia sensor at Wastewater Site C was found to have incurred costs of £14k over a 70 day period.
• Improved control of the blowers on Wastewater Site D's BAFF plant would save at least £3k annually.

In addition to these savings, United Utilities received a modelling library which will allow them to build their own models, monitor the performance of their sites using telemetry data, and identify additional opportunities for TOTEX savings from interventions on energy and chemical use.