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According to the Space.com article Astronaut Says It 'Smells Great' Inside the International Space Station (Video):

"We want to increase the level of recycling wastes beyond what we do on the station now. Our ISS water system can recycle about 93 percent of the wastewater back to clean water," Molly Anderson, a principal technologist at NASA, says in the video. NASA scientists plan to fly a demonstration technology to the station soon that should be able to recover most of the other 7 percent, which is referred to as "brine."

Question: Besides water, is it mostly salts and urea, or other organics as well? Is it vented to space or "returned" to earth by spacecraft incineration?

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"Brine" is what is left after the ISS Urine Processor Assembly processes the urine that is input into it, sending product water to the Water Processor Assembly which makes it potable.

(See the schematic in my answer to How close is the ISS to a closed system, in terms of carbon, hydrogen, and oxygen? for an overall ISS water system diagram)

I have seen various numbers quoted for the percent of water recovered, from 70% to 85%. 93% as quoted in the space.com article seems...optimistic, but perhaps things have improved since I left JSC.

As far as composition, the brine is simply super-concentrated urine.

Here's a schematic of the device.

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The original design Recycle Filter Tank Assembly (RFTA) tanks which hold the brine, are returned to the ground for disposal.

The RFTA is or will soon be replaced by a redesigned Advanced RFTA (ARFTA).

In order to support long term ISS operations with reduced up mass capacity, the RFTA is being modified in order to allow on orbit servicing of the system. Instead of returning concentrated brine contained in the RFTA to the ground (or disposing as trash), a new tank is being developed that is serviceable on orbit. The tank would collect the concentrated brine solution in the same fashion as the current tank, but would be removable such that it can drained into an existing disposable storage tank such as the tanks contained in the Progress module or Autonomous Transfer Vehicle (ATV).

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  • $\begingroup$ The "reduced up mass capacity" comes from simply not having to resupply new, disposable tanks as frequently, or does it also involve better water recovery and less fresh water shipments as well? $\endgroup$
    – uhoh
    Commented Apr 22, 2018 at 13:47
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    $\begingroup$ I read that as a euphemism for "not having the shuttle". $\endgroup$
    – Organic Marble
    Commented Apr 22, 2018 at 14:28

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