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This publication gives you a good flow chart for disposal of amateur photo development or dark room chemicals: http://www.kodak.com/global/en/corp/environment/kes/pubs/pdfs/j300.pdf.
However, I would recommend supplementing this by contacting your local waste management service. Certain municipalities have more sensitive household waste water management systems than others (primarily based on where the effluent drains to). You may be instructed to dispose of this waste at a household hazardous waste management drop off location in your hometown. Check for local schedules, many municipalities offer free drop off dates for small volumes of waste (the amounts generally cover household use for hazardous waste such as painting, oil change, and similar chemical wastes, so dark room waste should easily meet these limits).