To answer the OP's question first: No, normal black-and-white chemicals will not damage a porcelain toilet bowl, and you can certainly use it for disposing of chemicals. However, for film developing it will likely be more convenient to just dump the chemicals down the sink with a long water rinse following to make sure the chemicals are diluted and will not hurt pipes (even though there is little chance of that anyway). I used to have a darkroom with toilet in it (a converted bathroom) and found the toilet bowel handy for dumping larger volumes of printing chemicals (pour stop and developer trays into the toilet bowl, then flush a few times).
The question of what you should be dumping to start with from an environmental point of view is another question, but one that you should consider. Do download and read the Kodak publication linked to above. It has been my guide for years. My interpretation of it is this: Developer and stop are fine to dump down plumbing connected to public sewers. (I believe the caveat in the Kodak J-300 publication about not dumping unneutralized indicator stop bath was for the concentrated product with a pH of 1.0, which is quite low. Working solutions are much less acidic, even less acidic than vinegar you put on your salad, and won't hurt your pipes and can be safely dumped down the drain without neutralizing first. When stop bath has been used almost to capacity, it is, for all intents and purposes, neutral already. However, mixing it with spent developer ensures neutralization of both stop and developer and is certainly not a bad idea.) Wetting agents and wash-aids are safe to dump into the sewer; they are detergents and sodium sulfite (with maybe a little metabisulfite) and are harmless. Even potassium ferricyanide bleach breaks down to harmless chemicals, and is usually used in such small quantities, that disposing of it down the sink is safe. Fixer and selenium toner need special treatment (see below).
FWIW, here is my method of disposing of darkroom chemicals: I have a couple of working areas on two different continents. For the areas hooked up to public sewer and water-treatment plants I simply dump developers and stop baths down the drain. I have one darkroom hooked up to a septic system. For this I also dump developer and stop down the drain. Of course, I dilute well and try to minimize the amounts of chemicals going into the system by efficient use. My research some years ago indicated that common developing agents, stop baths and even pyrogallol were broken down into harmless compounds by the septic system. Unfortunately, I don't have my sources here and cannot document this, so don't take my word for it as an authority!
Fixer is collected in large plastic jugs and simply taken to a local photofinishing business, which is happy to reclaim the silver from my fixer in exchange for disposing of it. If there is a photofinisher in your area, you can likely make a similar arrangement. This is a lot simpler for me than reclaiming the silver myself. Sure, steel wool, etc. will remove the silver from the fix, but then it has to be dealt with and disposed of too. And my volume is not large enough to justify a professional silver-recovery system. Much easier just to let someone else do the work for the profit they get from the tiny amount of silver I dispose of every year. If there is a hazmat collection site that is more convenient for you, then simply use that.
Selenium toner is toxic and should not be dumped into a septic system or into the sewer. Selenium is a heavy metal and local water-treatment plants do not effectively remove it from effluent (same with silver in the fix, and why it should not be dumped down the drain). Heavy metals collect in the sludge and can reach dangerous levels. I replenish and reuse my selenium toner, never discarding it. I've been doing this for years with good results, and have posted my techniques here and elsewhere (do a search on my posts and you will likely find one of my diatribes on the subject...). If you must dispose of selenium toner, use it to exhaustion (i.e., until toning times are extremely long), then toss a few scrap prints into the toner and let them sit overnight. The resulting solution will contain very little selenium. This should probably then go to the hazmat collection, but small amounts of very exhausted selenium toner can probably be legally and safely discarded into the sewer system in most areas (do check).
That covers it for the chemicals I use. In your case, if all you want to do is develop film at home, you can dump everything but the fixer down the sink. Get your fixer disposed of by a photofinisher or someone with a silver-recovery system for maximum environmental safety. If you are looking for the lowest impact, you can also try to use environmentally-friendly developers (such as Kodak's Xtol or other vitamin-C-based formulas), very dilute standard developers, as well as other more environmentally-friendly alternatives. A bit of research here will turn up lots of info. Of course, using your chemistry efficiently, to capacity, and purchasing and mixing in amounts that prevent waste will reduce your impact as well.
Photochemistry is not all that dangerous, but informed and responsible use is still a really good idea (especially if you need to convince your parents that you know what you are about...). Read up, then have fun.
Best,
Doremus Scudder
www.DoremusScudder.com
