Hyphens, En Dashes, Em Dashes

Q. In a technical proposal, would you say “400-ton-per-day scrubber” or “400-tons-per-day scrubber”? Thanks a bunch!

A. The first construction is the more usual one. (Btw, what is a 400-ton-per-day scrubber, exactly? And where can we get one?)

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]