In Trier, photo chemistry is listed as "Problemabfall", problematic waste, like old paint and a lot of other stuff. You can take it to a special collection truck that stops at certain places at scheduled times, or ask for collection at your house. The service is included in the annual rubbish collection fee. You should check if a similar service is available where you live.
My extremely diluted Rodinal goes usually down the drain but I collect all more concentrated waste. It's also important to keep them separated and labelled (or in original bottle), so the recycling people will know what to do with it. 5 litre canisters from distilled water are extremely helpful in this regard.