I save all my used fixer to go to my county hazardous waste disposal site. Is it safe to add used plain hypo fixer with used rapid fixer? Or do I need to keep them in separate containers? In Alan Greene's book, discussing development of calotype paper negatives, he makes a point of saying to keep used sodium thiosulfate fixer exhaust separate from the other silver nitrate exhausts which consist of used gallic acid developer and the first rinse after development. He says if they are mixed it can produce sulfur dioxide, but not what causes that reaction. I know ammonium sulfate rapid fixer is acidic, but not if that could be what causes sulfur dioxide to be formed. Advice appreciated!