The photo chemicals we use are mainly water. The developers are mildly alkaline; the fixers are mildly acid as is the stop-bath. All can be comingled without fear that some adverse chemical reaction will cause harm. Additionally, all are quite benign. Whereas spent fixer contains silver in solution, the federal government labels it toxic. The toxicity is due to the test method. Some forms of silver are toxic. Photo effluent, especially comingled fluids contain silver sulfide. This is the result of complexes formed by the fixer which is sodium or ammonium thiosulfate. Silver sulfide is inert. You can comingle to your heart’s content. The comingled stuff has reduced toxicity.
Back to the silver: Ionic silver is toxic however some home remedies have you drink the stuff. After consumption, internal organs and the skin take on a bluish pallor. Silver compounds have been used in water purification, for ointments to heal burns; a slug of silver is commonly placed in household water filters to kill bacteria that often grow as the filter captures organic matter. Farmer’s Reducer is based on a compound of cyanide. The cyanide is tightly compounded with iron thus it has been used for nearly 100 years as a photo bleach solution. Ektachrome and Kodachrome bleaches were switched to EDTA to avoid this stigma.
If you live in a metropolitan location, one with a modern sewer system, you need not fear disposal of spent photo chemicals down the drain. Now giant photofinishing labs (I was technical manager of 7) must pre-treat and haul away some. The pre-treat is bubbling air. This reduces oxygen demand to reduce the load on a municipal sewer system. Oxygen demand and chlorine demand are the main nemeses. The sewer system must chlorinate to kill disease causing bacteria. They must oxygenate to reduce oxygen demand. This because, all sewerage will be released to a creek, river or lake; it must not harm the environment. The stuff of fixer is the same stuff that removes chlorine from tropical fish aquariums.
OK, the bottom line, your hobby darkroom is just a drop in the bucket compared to the trillions of gallons presented to your sewer system. Your impact will be just a thimble full compared to the Pacific Ocean. Haul your comingled effluent to a disposal site if it makes you feel noble. Only gigantic photo labs with have any significant impact. My 7 labs did 20,000 rolls a day.
I had to become skilled on this subject. I was a registered environmental inspector State of California, for photo effluents.