I have been simply continuing to use the same selenium toner, but topping it up with freshly mixed toner at the same dilution when it drops too far below the top of the bottle. I am not sure if it builds up other chemicals that might cause a problem for my prints, but I do a thorough soak in hypo clearing agent and a long wash after toning.(posted 8601 days ago)For my fix I purchased a small silver recovery unit for about $35. Once I have removed the silver, I simply pour the remaining liquid down the drain.
Most developers oxidize rapidly and are not much of a threat to the environment--I neutralize them with the acid stop and pour them down the drain as well.
I have a septic system, and have been doing this for 15 years with no apparent deleterious effects.
Other solutions, such as gold toner, copper toner, and anything else with heavy metals should probably be disposed of as hazardous chemicals.