Sodium Thiosulfate is relatively harmless as are Sodium Sulfite and Ammonium Thiosulfate. These are used in swimming pools to adjust chlorine level. Ammonium salts are fertilizers. Alum is quite harmless. It is used to pickle vegetables. The pH of the solution is sometimes a problem as it will either raise or lower your soil pH.
The real problem is related to the Silver salts in the water. Silver is often banned from effluents, or at least controlled.
Any photographic solution, dumped as-is is harmful to plants. It must be diluted by quite a bit to be harmless. Concentrated solutions are always bad for plants.