It doesn't - waste material is stored in special tanks and removed and dosposed of when the plane lands. Water from the washbasin is released into the air, it simply becomes water vapour and disperses harmlessly. All the urban myths about giant icebergs formed from toilet waste killing innocent hosehold pets are just that - urban myths.
blue ice is not a myth, liquid waste used to leak from old DC3 s and freeze on the fusilage, then whilst descending out of sub zero temperatures it would then fall off in lumps. people have been killed from it, like in the michael caine and ian holme film Blue Ice