Miljøprojekt, 1065/2006 – Afløbssystemets levetid og renovering

Afløbssystemets levetid og renovering

Summary and conclusions

The purpose of this project is to contribute to the process of achieving “the best possible sewer system for the money available”. This task is undertaken by developing a methodology for sewer rehabilitation, taking into account a large number of criteria for technical operation, environmental impacts, economy and service level. It is considered important that such methodology is developed with due regard to sustainability, technological, environmental and economic principles, at the same time allowing consideration of local objectives and needs.

The project presents methods and strategies, which can be applied as decision support tools for renovation and development of the Danish sewer system. The starting point of the tool is the operational problems and environmental impacts resulting from poor operation, state and structure of the sewer system. The tool then addresses the economic consequences and puts them in relation to the local objectives and strategies for rehabilitation.

By means of a technical-objective tool, the problems in a sewer catchment area are identified and quantified. The sewer system is then analyzed with respect to conditions that may result in the need for rehabilitation. Potential problems originating from the network itself are discussed. The tool links problems and adverse effects with their technical origin. Quantifiable performance indicators (PI's) are defined for a broad range of issues. The performance indicators are quantified in terms of PI values that can be used to determine to which degree objectives are met.

Meeting a certain objective for the technical operation of a sewer system is at any time overlayered by the economic consequence hereof. Consequently, a tool for evaluation of the economic perspectives of the technological rehabilitation solution, in terms of a Cost-Effectiveness Analysis (CEA), is presented.

The CEA establishes a sound basis for the technological prioritization of each rehabilitation project or solution method. For each project or solution the “Cost” is measured by the annuity of the net costs, and “Effectiveness” is measured by the change in selected PI's. The ratio between “Cost” and “Effectiveness” indicates how cost-effective an investment is with respect to selected performance indicators. To compare different projects or solutions, the cost-effectiveness ratios are ranked, and the best ratio is favoured. For some alternatives, the performance indicators are not comparable, and the prioritization must be done on a qualitative basis.

The “impartial” and system-depending approach for choosing a rehabilitation strategy is overlayered by objectives and demands originating from local and national interests. Thus, a service level is defined in terms of objectives and demands, which vary in time and space. The technical and economic possibilities for a rehabilitation project are hereby filtered through the objectives and strategies of each municipality, finally resulting in an overall and sustainable solution. In other words, the methodology does not result in a fixed technological/economic solution of a problem, but paves the way for an active interaction with the decision maker.

In agreement with this, the procedure in the rehabilitation process is as follows:

Procedure in the rehabilitation process

Focus of the project is on the establishment of a holistic rehabilitation process, including socio-economic influences. In this context, the capability of the sewer system to adapt to future – and, at the time, unknown – aspects of operation and environmental impacts, is crucial. To meet this objective, a broad spectrum of operational and environmental possibilities and economic consequences of sewer rehabilitation is addressed. Furthermore, the final prioritization, with starting point in the desired service level and flexibility of the system, is dealt with.

Overall, the project has the purpose of establishing a basis for rehabilitation and development of a sewer system, which a municipality in practice can operate. An example is given of application of the developed methods, with this purpose in mind.

Emphasis is on the systematization and awareness of elements and decision processes, which are part of the overall sewer rehabilitation process. To further illustrate the underlying ideas, examples are given that illustrate the use and importance of elements of the rehabilitation process.

The report provides the background for and the procedures with which technological, environmental, economical and service-related aspects can be included in the sewer rehabilitation process. Hereby, operation of the sewer within its service life time is optimized. As part of this procedure, proposals are given on how to include e.g. the choice of performance indicators (PI's) for the characterization of the sewer, as well as details regarding economic aspects. It is in this respect important to emphasize that the procedure is not rigid, but open for interpretation and choices according to the needs and objectives of individual users.

The concept of “lifetime” is central for any construction or rehabilitation work of the sewer system. Briefly stated, the requirement for this task is expressed through the technological lifetime, which, basically, is determined by the actual demand for a certain level of service. A long physical lifetime of a sewer system is therefore only meaningful when the system is based on flexibility. Hereby it becomes possible later to add technological lifetime, and, thus, make use of the residual physical lifetime.

It would be advisable to apply the method described in this report in this context.