A mathematical model for assessing the water supply alternatives with the reuse option is described, and its potential to yield results useful for economic analysis and policy making is explored. The results of the model for Salt Lake County, Utah, used as a case study area are shown. The relative marginal values reflecting quality and accessibility of various sources can aid in planning future water resource development. The supply curves derived from the model are useful for pricing policies. The relative 1ocational advantages of water and waste water treatment plants enter importantly into capacity expansion decisions.The least cost solution indicates the necessity of a facility to recycle waste water for municipal use under increased demand conditions.
INTRODUCTIONAs regions across the country continue to experience growth in population and economic activity, there is also a corresponding increase in the demand for water for various purposes. As a result, communities frequently find that they are rapidly approaching the supply capacity of their present water systems. Traditionally, their response to increases in demand for water has been to develop new sources by reaching out further in distance for additional high-quality water. However, with the steady improvement of technology, various water reuse alternatives have been proposed and are, in fact, being implemented in scores of individual situations. These include recycling of industrial process water, greater reuse of effluents by agriculture, renovation for municipal reuse, and desalting sea and brackish water. Models relating these options to specific water supply planning have been developed previously. Dracup [1966] proposed a water allocation model that includes recycling aspects. Maximization of a quadratic profit function to allocate water for Tucson, Arizona, was considered by Clausen [1970]. An integrated approach to regional planning of water quality and allocation through water reuse was presented by Bishop and Hendricks [1971].The effects of these reuse options have the economic implications illustrated in Figure 1. The usual approach to expanding water supplies, further developing primary (surface water and groundwater) sources, is depicted by supply curve So. As the safe yield of primary supplies is approached, it becomes more costly to deliver additional units of water. Thus at some point, say, E, water reuse technologies will provide an increase in supply at a lower cost than that of primary sources.
At that point there is a shift in the supply curve from So to S•. At lower levels of demand indicated by curve Do, reuse options need not be implemented. As the demand for water increases, Do shifts upward and may eventually lie to the right of point E as represented by the demand curve D•, where implementationof reuse options could represent a welfare gain to society. This can be illustrated as follows: Point B, the intersection of demand and supply curves D• and So, yields the equilibrium Q• and P• for traditional development of primary supp...