You'd need to check but here in the UK ferricyanide, and thiocyanatres are not treated as Cyanide for effluent disposal. They test for free cyanide, I was on secondment to a water treatment plant back in 1972 when a local company discharged a quanity of Cyanide which killed two treatment beds.

If you're worried about disposal of ferricyanide bleach just dilute well before flushing away.

Ian