In the United States, sewage is required to be purified to high standandards before it can be released back into the environment. Furthermore, no matter how much it's purified, it may not be used as drinking water... however, nothing prevents purified wastewater from being used as a power source to generate drinking water from seawater.
How would this work? Simple:
Build a desalination plant directly next to a wastewater treatment plant, and send the purified wastewater to the desalination plant.
Use a pump to pressurize seawater, and send it through a reverse osmosis device.
The freshwater produced is of course drinkable, as it came from a natural source (the sea).
Then, pass that high pressure brine that's left over through a forward osmosis device, with wastewater being pumped in as the source of "fresh" water. This increases the volume of the saltwater, without decreasing it's pressure.
Pass the now dilute saltwater through a turbine, to generate energy to drive the pump that pressurizes the seawater, and to drive the pump which pressurizes the wastewater.
If the plant creates a smaller volume of freshwater, than the volume of wastewater that it consumes, it should be possible for the system to generate more power than it requires.-- goldbb, Jul 22 2009 Osmotic Power http://en.wikipedia.../wiki/Osmotic_power [goldbb, Jul 22 2009] Not a perpetual energy device. It is powered by the osmotic potential difference between the relatively hypotonic (dilute solutes) waste water and the relatively hypertonic seawater.-- spidermother, Jul 23 2009 This idea to use wastewater to treat brine is a good idea - brine is a toxic effluent but could be diluted to seawater salinity using treated wastewater.
I do not understand how the volume of a quantity of fluid can be changed without changing the pressure it is under. At constant pressure the brine will not take up any free water, because that is why it is at that pressure in the first place - to squeeze the free water out. Lowering the pressure will of course permit ingress of free water. I am not sure there is any energy to be gained except for tiny enthalpy type amounts.-- bungston, Jul 23 2009 The treated wastewater would of course need to be pressurized to force it into the brine. This will of course require *some* energy, but not a lot.
More importantly, as the treated wastewater moves through the membrane into the brine, it's pressure will increase due to the osmotic potential difference.-- goldbb, Jul 23 2009 random, halfbakery