Just wondering why you don’t see tall distillation columns or evaporators at water treatment plants to reclaim clean water from goop, like how one might make distilled water (only on a smaller scale)?
I WAG this method is just not economical to simply generate water as the only good byproduct whereas the distillation process of petroleum yields many fine byproducts? Why, even the goop of a refinery’s distillation column has value as tar, etc!
It’s simple economics.
It takes a fantastic amount of energy to boil water (around a KWH/gallon). At an electricity cost of 10¢/KWH, it would cost millions of dollars a day to process even a small cities’ wastewater.
Running an evap is very expensive.
[minor highjack]
As far as I know the ships of the US fleet all have distillation plants that provide fresh water for all the usual purposes. I know for a fact that the most efficient ones will be dumping water overboard while underway.
Of course the energy used is a very small amount of that produced by the ships’ power plants.
I have often wondered, when a ship is decommissioned and sold/donated to a municipality or organization, why they couldn’t hook up the water purification system to the municipal water supply for an added bonus?
Aircraft carriers, for example, support 5000 personnel for extended periods. Coastal desert cities like San Diego (or smaller west coast towns) or those in the ME or Africa could possibly benefit from both tourism and the water. [/minor highjack]
Where would the bonus be? As you point out the energy used is a small amount of that being produced by the ships’ plant, and it’s economic to use that to make water because you have no other options (besides bringing all the water you’ll use), you have lots of waste heat anyway (if that’s being used at all), and you can’t sell the surplus energy because you’re not connected to the grid.
When you’re no longer using the ship as a ship, it will no longer be economic to run the plant, or if it was, you’d build plants of the same design in the cities you mention, without great, big, expensive ships around them.
Do ships distill or operate reverse osmosis membranes? This is much cheaper, but still energy intensive and subject to fouling. Also, most of the intake steam is released back into the sea, just slightly more concentrated, which does you little good for processing waste water.
It was just an idle thought, but occuring at various times in my Naval career and just came back up when I read the thread.
The ships are already built, providing infrastructure and the needed support.
Granted, even though the ships are normally just transferred to cities and other entities, for the cost of transportation, they are usuallly stripped of all the technology and potentially dangerous materials.
However, with certain precautions taken, couldn’t the ship be welded to a dock in BF, Cote Ivoire (for example)? Where the USS Humongous (CVN-###) and it’s nuclear plant could be used to a) hook into the grid, b) provide nearly unlimited potable water, c) potential tourism, d) employment opportunities e) immense good will for the US, etc.?
I’ll wager they are using waste heat from the on-board power plants - either the hot exhaust from big diesel engines/turbines, or the waste heat from the steam turbines on nuclear-powered vessels. That heat will ultimately need to be rejected to the environment anyway, so if you use it to boil water before rejecting it, it’s basically free.
This is not the case at a municipal water treatment facility. You would need to supply heat for distillation, and if you’re not using waste heat from a power plant next door, then you’re actually paying for it - and as noted upthread, distillation requires a lot of heat, so it would be expensive.
Why do water treatment plants need to distill water? Is municipal water usually distilled?
One idea I’ve always had (sort of related) is that obsolete nuclear naval ships, specifically ex-Soviet subs and whatnot, could be repurposed into mobile power generators. The operator can reach an agreement with a coastal city in developing Asia/Africa/India, settle on a price, sail the ship to the city and hook up the ship’s reactor, use your original Russian crew, and you won’t have a problem with proliferation or even with collections, because if anything goes wrong on shore, i.e. political instability, or the customer misses a payment, you raise anchor and leave. Much safer than investing in an actual building in said developing country shithole.
Ships use low pressure evaps to make drinking and boiler water. Low pressure steam is normally used, it is not waste heat steam but extraction steam. That is steam that has run part way through the main engine but still has energy that can be removed from it. The exhaust from the main turbines is at 29" of vacumn and around 70 to 90 degrees depending on sea water temperature.
Ships buy as much water as they can when in port because it is cheaper.
There’s another issue, too… where do you dump the waste?
Distillation doesn’t magically disappear the salt and other dissolved chemicals in water. It remains behind. You have to flush it out of the system and put it… somewhere. And there’s no way in Hell the EPA is just gonna let you dump high-concentration polluted brine back into the environment, and even if it did, it would be a bad idea. (Artificial salt flats generally are.) About the only place it would be practical would be seaside, using huge volumes of sea water to flush (and you’re still increasing the salinity of the local water).
Admittedly, current filteration-oriented water treatment probably needs to be backflushed periodically, and I don’t have a good mental picture of what they do with that. But distillation would make the waste products more concentrated and harder to sanely deal with.
It is essentially the same as dealing with sewage.
I am very familiar with one such fresh water treatment system, and passingly so with many sewage treatment systems.
Backflush water is collected and solids concentrated(de-watered) with settling ponds and/or textile filters. The water from sewage treated with chlorine and returned to the environment, from a freshwater system it is recycled through the treatment system.
The de-watered sludge is still pretty wet…this is sort of the same process that your body uses, but not as good, so the sludge is typically not as firm as you’d like your poop to be. It is typically spread on concrete pads to dry, then collected and disposed in landfills if the landfills charge by the pound. If the landfills charge by volume, then it is dumped wet.