There are a host of non-toxic things that have been used for processing film! Some use only a plant extract like coffee for a developing agent, while many add some form of Ascorbate, or vitamin-c. Ascorbate is a described developing agent, and the general idea is to complement it with another developing agent that will work with it "Superadditively," in a similar manner to combining Metol with Hydroquinone, the idea being that the combination works better than either developing agent works alone.
The general principles of developer composition apply: you need at least a developing agent or two (the plant extract or ascorbate + coffee or wine or whatnot) plus a base, which is almost always sodium carbonate (available at grocery stores as "Washing Soda" or "Soda Crystals"). Other easy to come by bases are Borax, used in, for example, D-76 (it has a lower pH and thus creates slower but often finer-grained developers), or Baking Soda, aka sodium bicarbonate, which makes a good buffer with sodium carbonate.
You can use your imagination to find other potential developing agents. One likely source is things rich in phenolic compounds (their being dark, staining, or bitter are a give away for this) or aromatic things (hence the herbs). Plant extracts with these characteristics are likely candidates because compounds that work as developers frequently have the sort of carbon-ring structure referred to as "aromatic" (such structures often have strong odors) or "phenolic," so if you have an aromatic soup some of the things in it might work as developing agents.
One place to start is to have a look at http://www.caffenol.org/, which has lots of recipes and records of things people have done (beet juice, anyone?). Use your imagination!
The Handmade Film Institute came up with a recipe to more or less act like D-19, in particular to produce a high maximum density with a realively short development time. We tested this in a cave on the beach in Jamaica in the spring of 2010, and got excellent results:
All of this, though, is for negative processing, and the developer is only the first step. To complete a negative process, you need to fix. And here, alas, we have not found or heard of a decently functioning alternative to the standard fixing agent, the various versions of Thiosulfate. Thiosulfate IS a naturally occuring substance -- one finds it in very small concentrations in many hot springs -- and it is not inherently toxic, as such things go; it is produced by heating Sulfite, another naturally occuring substance. But making it work as a fixer requires using it at a much higher concentration than can reasonably produced by nature. In other words, we're not aware of an effective alternative to using the standard commercial stuff. This is not the end of the story, and we would love to hear about an alternative that works! But meanwhile, how about....reversal?
The idea that reversal processing could be less toxic than negative processing probably sounds like a bizarre typo to anyone who has used the standard black and white R-9 (dichromate) bleach, or even its "less toxic" (ha!) alternative, permanganate. The hexavalent chromium in R-9 was actually the villain (causing cancer and death) in the 2000 film, Erin Brockovich. However, Ricardo Leite has apparently created a bleach out of Hydrogen Peroxide and lime juice. Though its toxicity relative to standard fixer might be arguable, it promises to be easy to make out of commonly available non-chemical materials, which is always a goal, and to smell good, too! So in on April 17, 2016, in a workshop at with Sam Houlihan in Minneapolis, we set out to come up with a working, reproducible formula, and were very peased at the results: it works! Here are the results: