simplified disposal. The value of the reclaimed water offsets the cost of RO, and dilute wastewater concentration leads to economies in any further required liquid waste treatment.

Unrestricted use of reclaimed wastewater for drinking water, however, requires careful examination. While practically a complete barrier to viruses, bacteria, and other toxic entities that must be kept out of a potable supply, RO membranes could pose serious problems should any defect develop in their separation mechanism. Given the purity and clarity of RO-treated wastewaters, however, it might be advantageous to use RO and then subject the product to well-established disinfection procedures.

You should remember that RO uses a semi-permeable membrane. As such, the membrane is permeable to only very light molecules like water. Under atmospheric condirtions the fresh water flows into the solution which is called osmotic flow. But for purification purposes, this is no use, and hence we employ the reverse of osmotic flow. For this to happen, we need to apply external pressure in excess of osmotic pressure. The osmotic pressure is given by:

Of course, you should be familiar with this equation (the Ideal Gas Law), where 'n' is the molar concentration of solute, R is the universal gas law constant, and T is absolute temperature in °K. The permeate flow can be calculated from:

In this expression, Am is the membrane permeability coefficient.

It is useful to compare the merits of the various processes for seawater desalination. Although the comparison will be primarily qualitative, it should be helpful in providing a deeper insight into the strengths and weaknesses of process. Foremost among the aspects of comparison is the energy consumption of each process you consider. With the known process specification, it is theoretically possible to calculate the minimum work or energy needed for separation of pure water from brine. For the real process, however, the actual work required is likely to be many times the theoretically possible minimum. This is because the bulk of the work is required to keep the process going at a finite rate rather than to achieve the separation.

The minimum work needed is equal to the difference in free energy between the incoming feed (i.e. seawater or brackish water) and outgoing streams (i.e. product water and discharge brine). For the normal seawater (3.45 per cent salt) at a temperature of 25° C, for usual recoveries the minimum work has been calculated as equal to about 0.86 kWh/m"3. Table 5 makes the desired comparison.

Table 5. Energy requirements of four industrial desalination processes. (Source. International Atomic Energy Agency 1992.)
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