For example: Should it drop TDS(Total Dissolved Solids) to 0? That makes the water taste "flat" and remove any fluoride and any minerals that people may want. Also makes it easier for the water to absorb co2 if sitting outside for a while.
Should it remove bacteria? That involves a different filtering method and can really reduce flow rate.
Should it just improve the taste of the water? What is involved in that? Thats also subjective. Typically it involves reducing chlorine and thats it.
Or should it just generally remove all known harmful contaminants and leave the healthy minerals/fluoride? After all, the government put the fluoride in the water for a reason(at least in the US). You may agree with their reasoning and want to keep the flouride.
Here is a test: https://youtu.be/ja0ioX6GSz0?t=535
In this test, the Berkey seems to remove contaminants but does not reduce the TDS to 0.
FYI for anyone else reading: The main winner in this test (Zerowater) dropped TDS to 0 as advertised but does not remove bacteria which some of the other filters do. The water also looks clearer from the Berkey and Aquatru compared to the Zerowater so it would be interesting to see what else is floating in there that is not picked up by a TDS meter.
"Forever chemicals can’t be removed from drinking water but they can be reduced in a fairly affordable way."
It can be done by having a material in the filter medium that absorbs some of the chemical thereby reducing whats left in the water. I think the material will need to perform an ion exchange function:
From Wikipedis: https://en.wikipedia.org/wiki/Ion_exchange
"Ion exchange is a reversible interchange of one kind of ion present in an insoluble solid with another of like charge present in a solution surrounding the solid with the reaction being used especially for softening or making water demineralised, the purification of chemicals and separation of substances"
Here is what Zerowater says: they don't claim to remove just significantly reduce: https://zerowater.com/blogs/filtration/what-are-forever-chem...
Berkey is more ambiguous, they claim to "Remove or Reduce PFCs/PFAS (PFOA and PFOS)". Which one is it?
https://www.berkeyfilters.com/pages/do-berkey-water-filters-...
I guess what they are implying is that in low concentrations they may be able to remove completely?
I don't know if Berkey uses the resin beads but in the video I posted he cuts open the Zerowater filter and it definitely seems to have both activated carbon and the ion exchange resin beads. I think Berkey has two different filters the water goes through and he only cut open one in the video.
EDIT: I want to also go back to my original point, this is why there is no one method to judge a water filter's performance. You have to judge it on what you are looking to reduce. TDS is something that can be electrically measured. It is possible that not all chemicals will be detected by a TDS meter so it is not a singular solution.
[1]: https://en.wikipedia.org/wiki/Total_dissolved_solids#Measure...
EDIT 2: The Zerowater article references a study from Duke University: https://nicholas.duke.edu/news/not-all-home-drinking-water-f...
In the study it says that filters that are reverse osmosis or two stage reduced PFAS by 94% and that Activated carbon filters reduced by 74% but inconsistently and sometimes removed nothing. This makes me believe that the activated carbon is just absorbing some of the PFAS maybe through a some physical method but it is not holding onto it and just releasing it back into the water in subsequent filter attempts (this is just a random guess because the report also states that activated carbon sometimes added PFAS, it must have come from previous filter removals).
The Reverse osmosis filters in the video included the Aquatru brand and the Zerowater is "five stage" which includes first activated carbon and then ion exchange (plus some other stuff that I forget) so I guess they are technically a "two stage" filter?? But what type of filtering does two stage imply? The basic version of Brita/Pur is just activated carbon confirmed by tearing down the filters.