Shucks! I guess I'd better give up on my idea of keeping a small tray of fixer next to my computer, just for the nostalgia of that good darkroom smell.
My father was a keen amateur photographer from the age of 15 when he got his first camera. I was raised in a household where there was a permanent darkroom. I believe that the fumes from those chemicals could eventually prove harmful to anyone spending several hours every day in such an environment. My father lived to the ripe old age of 96.
The original question was in relation to setting up a small scale, domestic darkroom, not a large scale, commercial enterprise. Small quantities of waste developer and fixer should not be a problem tipped into a small pit in the garden.
If you were really fussy, you could line the pit with builders' plastic and fill with sawdust. The sawdust would allow the chemicals to slowly evaporate and the plastic would prevent any chemicals from seeping into the soil. Then once every 5 years or so, you could remove the plastic, leaving the dried chemicals and sawdust compost, and try growing tomatos. You might need a bit of lime .