Knowledge, or a test

What the rule permits

A person must use knowledge of the waste to determine whether it meets any listing description, and acceptable knowledge may include waste origin, composition, the process producing the waste, feedstock, and other reliable and relevant information.

Which fits spent fixer well

The process is well understood, the chemistry is published, and the reason silver accumulates is the mechanism of the product. That is exactly the kind of knowledge the paragraph describes.

So most generators treat it as characteristic

Rather than testing each batch to establish what the process guarantees. The record then says the determination was made on knowledge of the process, and explains why.

Testing is still the stronger position

Where you want to show a waste is below a level rather than above it. Arguing that spent fixer is under 5.0 mg/L on knowledge alone is the harder direction, because the process points the other way.

And a representative sample is the standard if you do test

The characteristic is framed around the extract from a representative sample of the waste. A sample drawn from the top of a settled container is not representative, and the result will not support the conclusion you wanted.

Record which route you took

Knowledge or test, with the reasoning or the result, dated, per waste stream. That document is what answers the question years later, and without it the determination is an assertion.

Re-determine when the process changes

A new film, a different chemistry, a change in throughput or a change in how long fixer is used before replacement all change the waste. The rule anticipates that with its requirement to determine again when properties may have changed.

The two routes point in opposite directions here

Knowledge supports the conclusion that spent fixer is characteristic, because the process puts silver in it deliberately. Arguing the other way needs a test, and that asymmetry decides which route is sensible.

References