Water quality aspects of optimal operation of rural water distribution systems for supply of irrigation and drinking water

This paper considers the control of quality in rural water supply systems by means of dilution and treatment plants. Diverse quality sources can be used to supply consumers with differing quality requirements, such as irrigation of agricultural crops with differing water quality tolerance levels, domestic requirements at drinking water standards, and industrial and municipal uses. Models for optimal operations of such systems are difficult to develop due to the complexity of formulating dilution conditions at junctions as functions, for which the derivative can be calculated (because the flow directions at incident pipes change as the solution procedure progresses); strong scaling effects due to the treatment plants; and the presence of several water quality parameters. The model presented here solves the operation problem for a single period under steady state conditions. The formulation is simplified by representing the hydraulics by a conveyance cost in each pipe with bounds on the maximum allowed discharges (the authors have also developed a model that includes the hydraulics explicitly). The model is therefore suited both to canal and pipe systems for supply of irrigation and drinking water. Improvement of water quality by treatment plants located at the supply system is also considered here. This improvement has been achieved by lowering the concentration of conservative substances (those whose concentration remain time invariant) in the water only such as in the reverse osmosis process. The applicability of the model to practical situations is demonstrated by a case study of a regional rural water supply system with 39 pipes and 37 nodes (of which I I are source nodes with different qualities, 10 are agricultural consumers with different quality requirements, and 4 are domestic consumers requiring high quality drinking water). The model determines the optimal discharges in all pipes, values of quality parameters at consumer nodes, and removal ratios of the treatment plants. Three cases were studied: (1) a single water quality parameter (salinity) without treatment plants; (2) three quality parameters (salinity, sulphur, and magnesium) without treatment plants; and (3) three quality parameters and eight treatment plants, each located at a different source node. Water quality requirements are represented by constraints on the maximum allowable concentrations at consumer nodes, and, for the case of irrigation, a crop yield function that relates yield loss to salinity. The value of the objective function for Case 3 was 10% lower than for the other cases due to savings in yield loss at the agricultural consumers that were obtained by improvement of the quality of the irrigation water. Copyright (C) 2004 John Wiley Sons, Ltd.