People dump all kinds of waste into the sewer everyday: salty (food scrap, urine), organic (bodily fluids, shower water), toxic (bleach, household cleaner), etc. And the swer system just takes it and handles it. What makes treating sea water so difficult when a typical city sewer has far more complicated and varying chemicals to deal with?
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Well though it isn’t usually done because it is expensive most of the stuff contaminating sewage is still often more economical to get out than plain old salt. So treatment for direct potable reuse (DPR) is still generally cheaper then desalination of seawater.
Big solids you can just let settle out. Small solids you toss in a flocculant, they turn into big solids and they settle out. Complex chemicals you can trap with stuff like activated carbon filters, or break up into non harmful elements with ozone treatment or similar since while they are small they still aren’t like individual atoms like salt is.
After that you are left with stuff like membrane filtration to remove the small amount of really tiny stuff like salts left in. This last step is what makes desalination so expensive. You have a lot of salt that needs pulled out and those membrane filters are consumables. Either that or it’s very very energy and space intensive distillation. It’s just one step, but with how much you depend on that one step it’s more costly than DPR. And with wastewater recycling often you can cut out a good deal if you have large natural reservoirs to help filter things.
To ELI 5 it even more, DPR is sortof like cleaning up a bucket of mud, rocks, sand, and a spoonful of glitter from your carpet. It sucks but it’s doable, you can get most of it with a shovel, and a wet vac. Desalination is like if that entire dang bucket was glitter.
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