Before any dilution
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What the rule says
The hazardous waste determination for each solid waste must be made at the point of waste generation, before any dilution, mixing, or other alteration of the waste occurs.
So the first idea anybody has does not work
Pour the spent fixer into a larger volume of rinse water, measure the mixture, and report a figure under 5.0 mg/L. That is dilution before the determination and the clause is written to prevent exactly it.
Mixing is named separately
Combining spent fixer with developer, stop bath or wash water is mixing, and it happens naturally if everything goes into one carboy. Keeping the fixer separate is both the practical and the regulatory course.
And the obligation can arise again later
The rule also requires a determination at any time in the course of management that the waste has, or may have, changed its properties as a result of exposure to the environment or other factors. It is not a one-time exercise.
Which argues for separate containers at the tray
One container for spent fixer and others for the rest, filled at the point of generation. That is the arrangement the rule effectively describes, and it is also the one that makes silver recovery possible.
Recovery is the reason to keep it separate anyway
Silver in spent fixer is worth recovering, and a recovery service wants it unmixed. The regulatory requirement and the commercial interest point the same way, which is unusual and convenient.
The sequence has three steps, in order
Is the waste excluded from regulation. If not, is it listed. If not, does it exhibit a characteristic. Going straight to the third question is the common shortcut and it skips a step the rule puts first.