Economic Guidelines for Public Utilities Planning

This paper uses the wastewater collection utility to demonstrate the need and possibility of developing a normative mathematical relationship between the cost of installing such a utility on the one hand and the area being served and population density on the other. On the basis of an analysis of optimal network designs which were obtained by the use of a nonlinear programming algorithm, diseconomies of scale exist with increasing network size. This implies that a number of smaller and simpler networks may be constructed at a greater economy than a large enveloping system and treatment plant size. The combination of these factors—the network costs and the treatment costs—indicates that a minimum per capita cost solution exists. The size of the system at which a minimum cost is obtained is highly sensitive to the density of the population in the area being served but is relatively insensitive to the size of the service area.