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This answer includes this schematic showing the overview of the water processing system on the ISS, found on page 341 (page 375 of the pdf) of The International Space Station - Operating an Outpost in the New Frontier.

 A schematic showing the overview of the water processing system. Wastewater from the Waste and Hygiene Compartment (WHC) is transferred to the Urine Processor Assembly and the WPA. From the WPA, clean water is placed on the potable bus where it can be transferred to the O2 generator, the crew’s drinking station (Potable Water Dispenser), or the Sabatier, or used for EVA Mobility Units or for flush water in the WHC

The top part of the diagram contains an "Archival Sample".

  1. How often are the samples collected?
  2. If they are continuously collected, what fraction of the water is collected?
  3. Are they returned to earth? If so, how often?
  4. How are they used? Have they served important roles yet? (If so how?)
  5. These samples seem to be potable water collected from the Water Delivery ORU. Wouldn't it make more sense to collect waste water for analysis (as well)?
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There are many papers on NASA NTRS called "Analysis Results for Potable Water for ISS" for various expeditions and time frames. I have not looked at all of them.

  1. How often are the samples collected?

From the papers I skimmed, each Expedition brought back 2 - 4 samples. See tables below for examples.

  1. If they are continuously collected, what fraction of the water is collected?

It seems that they are not continuously collected. Sample collection bags were 1 liter in size. A typical sample returned on shuttle was > 500 mL, on Soyuz < 500 mL.

  1. Are they returned to earth? If so, how often?

They are returned to Earth as shown in the tables below.

  1. How are they used? Have they served important roles yet? (If so how?)

They are analyzed to see if the water is safe for the crew. Sample results:

enter image description here

  1. These samples seem to be potable water collected from the Water Delivery ORU. Wouldn't it make more sense to collect waste water for analysis (as well)?

I am not sure why you think it doesn't "make sense" to ensure that the water is safe for the crew.

Example sample return tables:

enter image description here

enter image description here

References:

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    $\begingroup$ For question 5, I absolutely think it makes sense to analyse the potable water to make sure it's safe, but I thought that analyzing waste water would be more important since that's a way to detect diseases. $\endgroup$ Sep 11 '20 at 18:16
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    $\begingroup$ @Speedphoenix What was left of the wastewater/urine "brine" after it was processed used to be returned to earth in Recycle Filter Tank Assemblies. Probably it got analyzed. But I think now they dump it overboard. I don't know if samples of that are taken or not. Your part 5 just seemed odd. ntrs.nasa.gov/citations/20140002837 $\endgroup$ Sep 11 '20 at 18:43

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