i never said or suggested i was a
chemist
John, I am certain PE's comment was not addressed to you. PE has made similar comments about what the experts have to say regarding EDTA and thiosulfate toxicity in several threads before, for example in
this post from 2005. His point, I believe, is that it is not always clear what is toxic. I might offer another example: selenium (Se) and arsenic (As) antagonism. On their own, both are highly toxic, but one can safely ingest stoichiometric quantities of both and they will be co-excreted in the bile duct. There's a number of trials ongoing in Bangladesh administering careful doses of Se, including dietary Se from enriched lentils to counteract As poisoning from contaminated well water. It really works!
Everyone should of course follow local laws.
My point is simply that "toxicity" and "environmental effects" are often neither rigorously defined nor well-understood. Thankfully, I don't have to be the arbiter of what these things mean. Regulations quantify toxicity and environmental impacts by stating how various things should be disposed of on different scales. Clearly it's bad to aquatic systems to dump in significant amounts of silver. But, if your local area doesn't have regulations forbidding the dumping of small quantities of darkroom chemicals down the drain, you probably needn't to be too concerned. Any water treatment plant worth its salt monitors concentrations of metal ions in the discharged water.
Not to be vulgar, but consider the environmental impact of the large quantity of waste material disposed every day: sewerage. Even after sewerage is treated, the organic material has a "biochemical oxygen demand, BOD" (This is to say that it can be oxidized, thereby depleting oxygen in the surface waters that the sewerage is discharged into.) This can have significant harmful effects for fluvial ecosystems, and the BOD of the effluent of any sewerage treatment plant needs to be carefully monitored.
There's a lot of really nasty chemicals out there, but most photographic solutions are not terribly toxic or environmentally hazardous. If one takes precautions and complies with local regulations, normal photographic processing solutions (including E-6) can be used safely without significant detrimental effect to the user or the environment.
Better safe than sorry is generally a good approach. FWIW, I follow local regulations, and furthermore, I don't dump any solutions containing metals (bleaches, fix). I do dump the small number of litres of developer and stop bath I use annually. I practice good lab technique, wear appropriate PPE, and try to avoid toxic and environmentally hazardous chemicals. Personally, I am much more concerned about my environmental impact caused by landfilled waste (which is not very regulated where I live) such as plastic packaging than the photographic chemical solutions I dispose of down the drain (including some of the E-6 process solutions.) But I haven't conducted studies on the effects of waste plastic in the environment, neither do I make the regulations for disposing waste plastic. So, I do what I reasonably can to minimize my impact, and for what trash I do produce, I use the trash can rather than littering. I don't feel compelled that it is my personal responsibility to reform my city's practice of landfilling domestic waste.
I will mention one more thing for those who are concerned with disposing metals: most fogging baths for E-6 contain tin (II) chloride. Of course, you can use fogging by light instead.
