medium · GMAT Verbal
In reverse-osmosis desalination, energy is spent pushing seawater against osmotic pressure to force pure water through a membrane. Thermodynamics sets a minimum energy per liter that no membrane can reduce, because it is fixed by the salt concentration itself. Real plants already operate close to that minimum. Newer membranes mainly let water pass with less resistance, raising throughput.
Which of these statements, if true, most strongly support which of the following?
- In time, reverse osmosis will turn out to be cheaper than every other available method of producing fresh water.
- Improving membranes alone cannot bring the energy needed per liter of desalinated water below the thermodynamic minimum.
- With a sufficiently advanced membrane, the energy required to desalinate a liter of seawater could be eliminated altogether.
- Desalinating water that has a lower salt concentration would actually require more energy per liter than desalinating seawater.
- Future improvements to desalination membranes will have no effect whatever on the operating costs that these plants incur.
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