What's wrong with sortation? Wouldn't 'you' say 'at the sewage filtration plant' rather than 'at the sewage filtering plant' or 'at the distribution centre' rather than 'distributing centre'? English is odd though eh?
Edited to add: Just googled it and others have had this discussion and Royal Mail used to use the term - and it's an old term (doesn't appear to be a recent Americanism or some such) - https://forum.wordreference.com/threads/sortation.2014092/
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Or, better still perhaps, "Being sorted"... ...which removes the need for "facility" as well.