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The growing demands on our existing water supplies and the current problems of water shortage emphasize the need for a comprehensive approach to analysis and planning of water reuse. The primary focus, heretofore, has been on the treatment technology for achieving water reuse.

The concept of reuse, however, should be broadened to consider a totally integrated urban and agricultural system. This necessitates a systems analysis where water reuse, together with all other water dispositions, is considered in the context of its contribution to the total water resources pool of a region.

The components of the water resource system are shown in the matrix of Figure 1, including both sources of sources are indicated by row headings, and each origin of water is classified as : (1) primary or base supply, (2) secondary or effluent supply, or (3) supplementary or imported supply. Each row represents a different possible origin of supply. The system of water users is indicated by the column headings in Figure 1. They are grouped into the broad sectors of municipal, industrial, agriculture demands, or other uses. Both the sectors of water use, the columns, and the supply categories, the rows, can be specified to any degree of refinement desired.

In the context of broad system planning, the matrix of water supply sources and demand sector requirements depicts all possible combinations for satisfying the aggregate system demand with the aggregate available supply. Thus, each element in the matrix represents a possible means of satisfying all or part of the demand requirements of a sector with all or part of the water from a given source.

In the past water planning and management has been concerned mainly with the design and optimum operation of storage and distribution systems to regulate water allocation to each use sector in both time and space. This approach is generally adequate when water resource development is at a stage where the primary water supply is in large excess of demand requirements, and the entire demand can be satisfied by the primary supply vectors. However, in many areas the primary supply is no longer sufficient to meet the diversion requirement of all users. Thus, secondary and supplemental sources of water become important, and water demands must be met by recycle-reuse and sequential-reuse from secondary supply vectors or development of supplementary supplies. This means that all combinations in the matrix of Figure 1 need to be considered for comprehensive planning of water utilization. The purpose of this paper is to delineate the manner in which all system permutations can be explored and how the best alternatives can be selected.

Specifically, the objectives are:

1. To formulate a conceptual framework for analyzing water reuse alternatives.

2. To present a model for analyzing alternatives of sequential-reuse and recycle-reuse in an integrated agricultural and urban environment. The function of the model is to determine the optimal allocations of water from each supply category to each use sector at minimum cost, which is the focus of this paper, or maximum net benefits. Quality constraints may necessitate treatment of water before reuse. Therefore, three possible levels of treatment are considered in the analysis: (a) conventional primary-secondary, (b) tertiary, (c) desalting.

3. To illustrate the application of the reuse model by application to a specific metropolitan area.

Some questions to be answered are:

1. Which origins of primary and secondary water supply might best be allocated to which use sectors, considering quantity and quality constraints of minimum costs?

2. What should be the design capacities of waste water treatment facilities and when should they be phased into operation?


Publication made available electronically April 27, 2012.