Design and economics of RO seawater desalination

Large-scale seawater desalination is an attractive and viable alternative for the production of potable water in the absence of natural fresh water resources. Over the past two decades, the reverse osmosis (RO) process has allowed a tremendous reduction in the cost of potable water from seawater desalination. Indeed, cost studies conducted by various researchers have indicated that it is possible to obtain product water cost of about US$ 0.80/m(3). This paper introduces a comprehensive, but tractable, means of modelling large-scale seawater RO desalination plants that can accommodate various flow configurations. The two most important considerations in RO plant modelling are the permeator performance characteristics and the permeator replacement scheme. In particular, the flux degradation of the permeator with operational age has to be accounted for. The RO model introduced here was used for extensive plant cost analysis. The cost studies indicate that the permeator cost makes up a large percentage (similar to 37%) of the total plant capital cost. This means that large-sized permeators, which offer smaller cost per unit membrane area, should be utilized in order to appreciate economies of scale. In terms of operating cost, the annual permeator replacement cost also forms the major portion. Interestingly, with the use of hydraulic energy recovery systems, the product water cost is relatively insensitive to energy price fluctuations. Also, cost comparisons conducted for a brine-stage RO plant configuration indicated that it is more costly than single-stage plants, especially for high concentration seawater.