The answer lies in the Venn Diagram which describes the intersection of the following three phrases:
- "What people don't know won't hurt them."
- "It's better to ask forgiveness than to beg permission."
- "Ignorance is bliss."
Most lay people really are ignorant about the chemicals used in photography. As far as they are concerned, "All chemicals are bad."
They have no idea what the chemicals are, what they are used for or just how common many of them are:
Hydroquinone/Metol = An ingredient is skin whitening creams to remove age spots. (Not used very much anymore.)
Ascorbic acid/Sodium Ascorbate = Vitamin C.
Acetic acid = vinegar.
Sodium thiosulfate = An agent sometimes used as an antidote for cyanide poisoning. Also used in topical medications to treat certain forms of skin rash. (Pityriasis versacolor.) (Not common uses.)
These chemicals are fairly common and are all things that can be ingested or spread on the skin.
I wouldn't put concentrated stop bath on my salad nor would I eat powdered XTOL as a vitamin supplement but my point is that the chemistry we use to develop film is not nuclear waste. Properly handled and responsibly disposed of, the chemicals that the average home photographer who does not use any exotic chemicals or alternative processes should pose virtually no problem if disposed down the municipal sewer in reasonably small quantities.
If you are an industrial user or a business, things are different. You'll probably have to get a permit. But, the average guy who develops a couple-few rolls of film per week will have no problem.
One caveat is fixer. I would not dump used fixer down the drain. It contains dissolved silver which is an environmental pollutant. Secondarily, you can recover that silver and cash it in for money. The average home photographer won't get rich off the silver but, over the span of a year or two, it will likely be enough money to buy a dozen rolls of film and the chemistry to process it.
Here's what I've been doing: I recover the silver from the spent fixer then I store what's left in old 5-gallon buckets with tight fitting lids. I peel off the label from the original container and stick it on the sealed buckets. Our town has a "Household Haz-Mat Amnesty Day" where, one day a year, you can collect up all the old household chemicals, paint thinner, cleaning agents, used oil, etc., and turn them in to the municipal recycling center for free disposal. With that label on the bucket, I have not had any questions asked.
Past that, I would not go around asking questions. Most people, ignorant as they are about photography, have no idea what photographic chemistry is, what the potential hazards are, nor do they understand just how safe it is, given reasonable precautions. Out of that ignorance, most people will just make up their own answers with no regard to truth or logic. The most likely answer you hear will be something like, "Chemicals? You can't pour CHEMICALS down the drain!!"
You're probably better off not asking. Just go about your business in the responsible manner you already know how and let sleeping dogs lie.