ponytug
Super Member
From what I've read about desalization plants is that you end up with a huge amount of brine, that is too salty to just put back, locally in the ocean. It would kill every thing along the environmentally sensitive California coast line. You would have to have huge evaporation ponds and clear out the salt once and a while. Land isn't cheap along the Californa coast. They could probably sell the salt, yet, they couldn't sell it for any sort of profit, as mining and refining salt from Utah is much, much cheaper.
While in some locations, it does make sense to evaporate the brine to dryness, that isn't true in most places. Drying is often complicating, often requiring large areas for ponds or higher costs for things like evaporators and crystallizers, but it can be done.
If your desalination plant is in, say Phoenix, then, yes, evaporating to dryness may be an option, but even then you need either a nearby state wanting salt for roads, a chemical plant, or a landfill that could accept it (most can't).
The reverse osmosis concentrate, or brine, is generally about twice the concentration of the incoming seawater. The limitation is primarily one of energy; while you can concentrate ocean water further, the energy costs to do so increase dramatically. The usual brine disposal strategies are either a discharge pipe with many holes to disperse the concentrated brine rapidly over a long and large area, or, if the local subsurface topography permits, to a deep depth where the environmental impact is small due to low densities of animals, and the dilution tends to be quick. Sometimes a deep brine aquifer is available for disposal, but not commonly. There is one being proposed for Jordan that would use the altitude drop from the Red Sea to the Dead Sea to supply some of the energy for reverse osmosis to generate fresh water for Jordan and Israel. I think the current price tag is on the order of $10B.
Any time that you are doing reverse osmosis, the end fate of the concentrate tends to be an important, and often limiting, design point.
There are technologies on the horizon that might, and I stress might, further process reverse osmosis concentrate into more valuable commodities, but a great deal will depend on the source water. E.g. San Joaquin drain water reverse osmosis concentrate is likely to be high in selenium, fertilizers, and agricultural chemicals. Not super useful in its concentrated form, unless you could sort out, say the fertilizers, from the selenium, pesticide, and herbicide mixtures. Reverse osmosis concentrate from urban waste water tends to be enriched in things like pharmaceuticals, hair conditioner, etc.
Reverse osmosis isn't trivial, but it will likely be an increasingly important of water supplies in many parts of the world as a combination larger populations and more variable water supplies lead to water shortfalls.
All the best,
Peter