Desalination plants could still be a viable option for water supply if the operating cost could be brought down. To this end I looked at a small plant of about 5 million liters per day.
Although there are various means of desalination, reverse osmosis seems to be the easiest option to implement, but the high operating cost is a drawback. This cost is shared between that of maintaining and replacing the membranes, as well as the cost of running the pumps to create the pressure differential needed to drive the process. My estimates for the power usage for a 5 million liter plant is about R35000 worth of electricity per day. On the membrane side I don’t yet have figures, but that could be the subject of a second round of optimization.
The electricity however, is a problem that I believe has a solution. The pumps can be replaced with a pressure reservoir charged with a wave powered ram pump system. Apart from the saving of almost a million rand in power every month, there is the added benefit of not putting more strain on an already burdened power grid.