The Collaborative International Dictionary of English v.0.48:

aerator \aerator\, Aerator \A"["e]r*a`tor\, n.
   1. That which supplies with air or gas; specif.:
      (a) An apparatus used for charging mineral waters with gas
          and in making soda water.
      (b) A fumigator used to bleach grain, destroying fungi and
          insects.
          [Webster 1913 Suppl.]

   2. an apparatus for exposing something to the air (as
      sewage).
      [WordNet 1.5] Aerenchym

WordNet (r) 3.0 (2006):

aerator
    n 1: an apparatus for exposing something to the air (as sewage)