But what a cool café, and how fun to find it so fortuitously! "Pump house" seems to be a euphemism for "flushing station":
The Flushing Station was built in 1888 to pump fresh water into the Milwaukee River from Lake Michigan, to flush out pollution. The 1,700-sq. ft. cafe takes up two-thirds of the building, with the still-functioning flushing pump filling the remaining third.
Is there a better place than Alterra to drink chocolate and ogle nineteenth century machinery?