I used to have a bulletin on just this subject. If i recall correctly they showed the math to figure out concentrations and quantities, but ended by saying call your ag. dept.& they will give you to a local person who will help you, but Kodak said even if allowed you should water ornamentals, not crops- just to be safe. I have some old bulletins somewhere in a box. I'll look for them. Steel wool will remove any silver if you're worried about it, just run it into a plastic trash can as a holding tank and water stuff the next day. Put the drain a hand's width from the bottom, and when the steel wool is gone drain and dry the sludge. When you have an ounce or more send it to a refiner, and get money for film.
This is what I had in mind. Creating another tank to release the water into filled with steel wool, and out the other side.