My local sewerage company told me that anything under 5 litres, "in total" per week is considered to be a non issue.

They did ask just what it was and when I replied B&W, C41, E6, RA4 etc, they didn't bat an eyelid.

The only thing she said was that if I was doing Cibachrome (her words) I should mix all the baths together to neutralise, then flush down the toilet.