We use an Osmosis system to clean the water when laying water on rinks.
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The water is pumped under pressure through cartridges packed with hollow tubes designed to pass water, but reject most other things (salt, urine, oils, etc).
So you get pure water oozing through the walls of the tubes that you can use to fill your rink.
Efficiency of the process isn’t generally all that great. Most typically you end up discarding more contaminated water than how much pure water you get.
You have two pools of water, one with salt and one without salt. You now remove the barrier between them, allowing them to mix. Now, both pools are equally salty and much less salty than the original pool.