Fresh, usable water is the problem. Where plentiful, we waste it freely. Where it is short, severe human conflict is apt to prevail.
Industrial societies use water for drinking, washing, irrigation, waste disposal, and as a heat sink for power stations. Consequently, pollution exacerbates our problems. For example, the Ganges and the Yellow rivers are both nearly tapped out and significantly polluted.
Big water problems differ a great deal and are complex – with social, technical, economic, and political aspects. Saudi Arabia has severe dryness. Bangladesh is soaked, but short of unpolluted water. Fixing any of these problems presumes fixing more than water technology, although every technical advance helps.
If we reuse water, we usually have to use equipment and energy – plus ingenuity – to process it in some way. If we can’t treat water ourselves we must reduce use until natural processes can treat it for us.
China is the poster child for water issues. With a huge population and substantial industrialization, it has it all. The World’s Water (sort of a biennial bible in water) devotes whole chapters to China.
Water shortages are related to decreasing return on energy, but the impact of Compression could felt sooner with water than with energy.