Ecological effects of development chemicals

I am completely new to this developng game following the gift of an enlarger from some friends. I have set up a darkroom but am concerned about the effect of throwing out the used chemicals. I understand that I can keep the fixer and use it again but the developed goes off and should be disposed of. I never know what to do with it and have been pouring it down the sink with plenty of water. What environmental pollution am I causing by doing this?

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