Cost-s: a whole-life costing approach to sewerage (2003-2005)

Funding body: Engineering and Physical Sciences Research Council (EPSRC)

This project was undertaken collaboratively with the University of Sheffield and University of Bradford. In respect of water services, and hence sewerage provision, in England and Wales the Regulator seeks to ensure effective and efficient expenditure by service providers on service delivery whilst protecting the environment, meeting standards, and achieving social goals. This is against a background of seeking to minimise the cost to customers and maintain reasonable levels of shareholder returns (to guarantee financial viability for the businesses). However, in respect of the investment in the renewal, maintenance and operation of the underground sewerage assets there is a need for better economic justification.

At the same time there is an emerging requirement to include sustainable development objectives in the decision making process and increasingly a balance has to be struck between purely economic socio-environmental criteria. Hence, for the sewerage network, there is a need to develop a tool that takes account of changing attitudes to social and environmental responsibilities within an appropriate economic framework. This project addresses this need.

The project aimed to provide a framework that allowed for the impact of management decisions in sewerage to reflect the holistic costs, the associated affect on asset performance (the cost driver) and, provides a key component contributing to assessment of the impact on sustainability of different options. Also, the research sought to make the whole life costing (WLC) approach applicable to both existing systems and first time sewerage. For example, some 4% of properties in the UK are not connected to public sewers but are connected to alternative wastewater disposal systems. The Water Industry Act (1991) requires that, subject to a number of conditions, sewerage undertakers should provide a public sewer to replace such domestic private systems. To do this requires consideration of both the technical and economic factors and hence the proposed WLC approach will implicitly address these issues.

The proposed WLC methodology builds on our previous work and is based on absolute values (in monetary units) that require robust performance and accounting modules to be in place in order to assign (cost) consequences to actions. WLC has the advantages that:

  • the costs themselves are being used, so that there is a universal understanding,
  • the process is fully auditable to external reporters,
  • it provides a holistic and robust rationale for asset management,
  • it considers both costs and performance over extended periods,
  • it incorporates sustainability issues related to resource use and impacts on the environment and society.

The aims of the project were:

  • To derive a tool to minimise the whole life costs of sewerage systems based on constrained service delivery targets.
  • To develop appropriate criteria and integrated performance models for new, existing and unsewered systems.
  • To establish a framework that allows the impact of management decisions for sewerage to reflect the holistic costs and link these to performance (the cost driver).
  • To develop a methodology to define in what circumstances first time sewerage is viable.
  • To provide an approach for the assessment of existing and new sewerage systems in terms of management options thereby contributing to an assessment of the sustainability of these sewerage systems.
  • Implicit in these aims is the need to incorporate the linkages between decision making and the economic, social and environmental aspects of system performance and operation in real value terms.


To develop a methodology and software tool to assist management decisions in order to provide acceptable performance at a minimum cost over the whole life of the sewerage system.

Cost-S Framework

Network Definition describes the sewerage system and its performance. Hydraulic and asset performance models have been developed, these output KPIs to describe system performance. The accounting module assesses costs based on the predicted performance and chosen management strategy.

Network Definition

Hydraulic modelling

Hydraulic modelling is used to assess system performance in terms of:

  • Dry Weather Flows
  • Wet Weather Flows
  • Sedimentation

Dry Weather Flow - Sewerage Available to Transport

The SATT score is calculated based on peak flows and durations from the diurnal flow profile.

Wet weather flow - HKPI

Wet weather events of varying return period and duration are specified. KPIs are aggregated for the hour which includes the peak flows.


Sedimentation is assessed during dry weather flow velocities as specified in CIRIA Design Manual Report 141.

Asset performance modelling

Asset performance modelling currently comprises blockage, collapse and deterioration. These models are derived from historic data, thus the primary limitation is quantity and quality of data.


Deterioration is based on a Markov transition at each timestep. Transition probabilities (pj,k) have been derived from repeat CCTV data.

Blockage and collapse

Numbers of blockages and collapses are predicted within pipe sub-groups over the catchment, rather than actual locations of incidents.

Cost accounting

The accounting module considers all costs, whether direct or indirect, arising from operation, maintenance and management of the system and uses an activity based costing approach. Costs are user definable within the accounting spreadsheet.

Software tool

DST builder

The Cost-S software tool brings together the network performance models and the cost accounting framework in a package which allows system performance to be assessed at user defined time intervals over a variable whole life period.
Models are run at each timestep to calculate costs and KPIs. These can then be viewed, selected and aggregated using the list of indices and the GIS interface.

Based on the predicted performance, the user is able to implement a range of intervention sets. The effect of each different intervention set on costs and performance can then be assessed by re-running the models.

Policies explorer

The policies explorer allows the user to assess the cost and performance associated with different intervention sets over both a single time step and multiple time steps.

The future

The research work carried out to date can be characterised as proof of concept. Although a variety of tools have been developed and integrated into the software tool, it is very much a research based tool that has not been fully bench tested. It is clear that a good case can be made for the further development of the work in order to refine the tools and provide a more user friendly and ultimately an industry useful & attractive application tool. To do this would require its application to actual case studies, through which better user interfaces can be developed and customised to more closely fit industry standards and requirements.

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