April 2008: Integrated modelling for strategic planning: a case study based on sewage sludge disposal

Introduction

Strategic planning for sustainability requires complex systems to be modelled. Because of this complexity, it is unrealistic to develop new and comprehensive models for each of the numerous possible situations likely to be encountered. Many models already exist for component parts of a sustainable system, especially the technical components. Unfortunately, being developed at different times by different organisations, these component models exist in different forms and formats and are not especially designed to work together as components in a bigger system.

Objective of this work

The aim of this research work was to demonstrate how a flexible Integrated Modelling Environment (IME) can be used to build strategic planning models through a systems engineering approach.

Fig 1. Integrated modelling of complex systems

Owing to the complexity of the interactions in any practical strategy for sustainability, the modelling of sustainable systems is a difficult undertaking. One approach has been to develop large, sophisticated representations of the real world such as global climate models. At the other end of the scale, simple spreadsheet based models can be developed with relative ease, but with less sophistication, and often for application to specific local issues.

... design and modelling ... encompasses the entire system lifecycle.

In addition, the design and modelling of a complex system of sustainable technologies encompasses the entire system lifecycle. Typically, such a system will be developed by multi-disciplinary teams of engineers. As a consequence, design freedom can be severely restricted by the unknown effects that design changes made by individual team members may have on overall system capabilities.

This presents designers with a problem of conceptualising their work at the overall system level since it is difficult to effectively integrate the diverse modelling applications that each individual design engineer uses. By developing an IME where the modelling of a system can itself be simulated at the conceptual stage, difficulties such as these can be overcome.

In this research, a powerful integrated meta modelling environment has been developed that allows external models to be built into larger systems using a ‘plug and play’ approach. Features of the modelling environment such as property tracing and capture of design rationale allow a comprehensive exploration of a system to be carried out. In particular, the approach offers a powerful and flexible mechanism for modelling strategies for sustainable development.

The Integrated Modelling Environment (IME)

The IME was originally developed for the aerospace industry to model systems ranging from fleet availability, deployment and performance down to the level of component systems such as an aircraft fuel pump. IME supports the integration of component models into complex systems. This approach permits the monitoring of emerging systems properties, which can be viewed and assessed, and also gives designers a means of recording their rationales. Central to the approach is a traceability capability which works throughout the system hierarchy from high level emerging properties down to low level properties.

As a demonstration of capability, the IME has been used to model sewage sludge disposal options. The modelling framework which was developed provides a strategic managerial tool to find an optimum solution.

Disposal of sewage sludge

The EU’s Urban Water Directive [91/271/EEC] banned the dumping of sewage at sea from 1999. Subsequently much sewage went to landfill or was sprayed onto agricultural land. Neither of these routes is sustainable due to the decreasing number of available landfill sites and increased health concerns associated with spraying.

Recent technological solutions include drying, using the hot exhaust from a gas turbine driven by natural gas, which provides electricity for the sewage disposal site, or gasification to produce fuel for an electricity-producing gas turbine. Disposal routes will depend, among other things, on location, quantity and nature of the sludge. Some alternatives are more sustainable than others. Some in combination may be more (or less) sustainable. Particularly on a regional basis, a sewage disposal authority will have a number of viable options available to it now or in the future. Taken with other factors such as legislation and political concerns, the need to model complex strategies in aiming for a sustainable solution becomes apparent.

Overview of the case study scenario

Our aim was to demonstrate how the IME allows exploration of sludge disposal alternatives. The test scenario is based on disposal of 200,000 tonnes per annum (tpa) of raw sewage, generated on a regional basis. A number of different disposal options are available and used in combination as appropriate since no single option is able to handle the total load.

Sustainability of the disposal options is important. If profit was the only consideration, the sludge would be gasified as this process creates income from generated electricity. However this may not be environmentally or socially sustainable. The optimum disposal option is one that is economically viable but also socially and environmentally acceptable.

A further complication is that the amount of sewage being produced annually is increasing. To allow for this, an increase to 350,000 tpa was also modelled. In such circumstances, important strategic questions arise:

Will the original disposal strategy be able to deal with the increase in sewage or will changes need to be made?
If changes need to be made, is it better to modify the gasifier plant by increasing its size, or build a new gasification plant?

The IME was used to create a meta model of this problem, using simple models to represent the different options including transport and location. A simple sustainability model was also used to rate the sustainability of each disposal option. An overall sustainability index could then be derived for each situation that was modelled.

Fig 2. Sewage disposal option

Example results

With 200,000 tpa of sludge, the best disposal combination was profitable (£1.6M pa electricity and materials), with a high sustainability score (76%)

When the amount of sludge was increased to 350,000 tpa the model showed disposal combinations would make a loss and have low sustainability scores.

If the model was adapted to allow a second gasifier, a satisfactory solution could be generated (£2.8M profit, 49% sustainability score).

However, the most important conclusion to draw from this work is that the IME presents a flexible and powerful environment for such studies, linking as it does disparate external models of any level of detail into a system meta-model which can be explored and also act as a knowledge repository.