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This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics of the conventional MSF process. The research project involved an optimization study where the water cost per unit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.

1. In the first two parts of this study [1,2], the feasibility of the hybridization of RO and MSF technologies to improve the MSF economics were investigated through an optimization study based on minimum water cost. In this study, nine different water only plants, having the same desalting capacity, were considered for comparison. Besides a brine-recycle MSF plant, a two-stage RO plant and seven other hybrid RO/MSF plant configurations were studied. 2. Comparison revealed that two-stage RO has the most favorable economics and performance characteristics from the standpoint of water cost, specific capital cost and recovery fraction. The study concluded that in designing new desalination plants, hybridization of the RO and MSF processes would result in better economics and operation characteristics than those corresponding to conventional MSF plants. 3. The MSF process proved to have high specific capital costs, and this is the main reason for the high cost of water produced by this process. For the MSF process to compete, it is suggested that the process design may be revised for possible reduction in its capital investment. 4. We investigated the sensitivity of the minimum water cost to: steam cost, membrane cost, top brine temperature, cost of chemicals and cost of electrical energy. Results showed that RO technology has the most favorable economics regardless of the cost element varied and the range of variation. A sensitivity study showed that RO technology is the most sensitive to variations in the cost of electricity, while the MSF process is the least sensitive. Also, membrane cost variation will affect RO water cost more significantly than is the case with hybrid RO/MSF plants con-sidered in this study. The MSF process is the most sensitive technology to variation in steam cost. Chemical cost variation has the least effect on water cost from the studied plants.