In general, used darkroom chemicals, except fixer, may be disposed of in a municipal sewer system if one has a home-based darkroom. Used solutions should never be disposed of in a septic system. Septic systems are designed for biological treatment of standard household waste, and do not properly treat chemical wastes associated with photographic processing.
Photographic processing facilities, such as schools, labs, and professional photographers’ studios, must comply with stricter regulations than home users. Federal guidelines for waste disposal are usually administered by local/state/regional sewer authorities, who have the power to enforce the regulations. Managers of educational, commercial and professional darkrooms should begin by contacting these agencies to obtain guidelines. Please note that some local regulations may be stricter than federal ones; they may require lower levels of chemicals in the effluent. It may be necessary to obtain a permit from the local/state/regional sewer authority to discharge photolab effluents into the sewer line.
Used fixer, because it contains dissolved silver, requires special attention. For home-based darkrooms, there may be specific limits on the amount of dissolved silver allowed in the effluent. Commercial users must process fixer in a silver recovery system or have it taken away by a waste disposal company. Remember, silver is valuable.