A back-water trap unit comprises a housing having an outer perimetric wall sealed to a base to define an enclosure, and a removable housing cover comprising inlet apertures. The liquid cell includes a liquid cell housing with an internal cavity to contain a fluid, a plurality of conductive feedthroughs traversing the liquid cell housing between the internal cavity and a dry side of the liquid cell, a cantilevered probe coupled to the liquid cell housing, and a piezoelectric drive element disposed on the cantilevered probe. Tubular socket members receive attachment rods of the associated carrier.