Research Area:
Resourceefficiency of Settlement Structures

Cost evaluation of the adaptation of central sewerage systems in case of shrinking population


Declining populations and an expansion in settlement areas have led to a rise in the cost of pipeline networks for household supply and waste removal. This is particularly true for waste-water removal, which in Germany is primarily managed using public sewage systems. For settlements of low density, decentralised sewage system can help reduce costs in comparison to a centralised system. However, can decentralisation also lower costs in developed settlements undergoing shrinkage which are already connected to the waste-water network? In order to answer this question, the current paper calculates the costs of transforming a centralised sewage network to a more decentrally organised structure. For these calculations, a procedure has been developed to analyse the effect on costs when technical waste-water removal systems undergo stepwise decentralisation.

By making use of geo-analysis software tools and incorporating functional aspects of waste-water technology it has been possible to construct a model which can illustrate the supply and demand of infrastructure in developed settlements according to the infrastructural capacity. This is used to draw up potential solutions for adaptation in the form of decentralisation pathways, for which the costs of transformation can be balanced against the potential reduction in costs. Included in this approach are all relevant cost-related variables in the technical system of waste-water removal which arise as a result of decentralisation. Two evaluation parameters are investigated: the maximum potential for cost savings gives information on the size of the potential costs reductions, while the maximum degree of decentralisation indicates the extent to which decentralisation can be achieved while keeping costs unchanged.

Cost reductions can be realised by the transformation of areas with highly cost-intensive pipeline systems, which lie at the ends of networks and for which major renovation work is necessary. This constellation of factors is, however, rarely met in the area of investigation, and thus the projected cost savings by decentralisation are correspondingly low. For a population reduction of 20 %, savings of only one percent against the total costs of the central system can be expected. Up to 13 % of the area serviced by the pipeline network can be disconnected and decentralised without any rise in total costs for waste-water removal. In particular, recently constructed pipelines in areas with a low density of connections, for which sunk costs will remain into the medium term, counteract the realisation of savings by means of decentralisation. This statement is generally true for the eastern half of Germany. In the west of Germany the situation is different: Here the waste-water networks are older while the population levels are much more stable.

The decentralisation of waste-water infrastructure in a centrally designed system can thus only make a partial contribution to offsetting the rise in costs when population levels fall. The relevant authorities must react at an early stage. Suitable adaption strategies must be drawn up for the long term, and must be coordinated with plans for settlement development as well as other planning requirements. The tool developed here can help to support the process of strategic planning. By estimating the cost impact of alternative plans of transformation, as well as linking this information to geodata, it is possible to draw up detailed development strategies for the transformation of networks, as well as their expansion. Over the long term this will help to break down rigid development pathways. A regional perspective when finding solutions can help to illuminate interdependencies and thus avoid the lower efficiency of uncoordinated action. The developed procedure can also be usefully employed to analyse costs associated with the transformation of other pipeline networks and related infrastructure.


Dr.-Ing. Georg Schiller
Tel. +49 (0)351 46 79 259