A storage device for use with at least one batch furnace for batch treatment of wafers supported in a wafer boat is disclosed. The storage device comprises a cassette storage carousel for storing a plurality of wafer cassettes on rotatable platform stages. A carousel housing bounds a mini-environment chamber in which the platform stages are accommodated. A gas recirculation circuit of the storage device subsequently comprises a gas inlet channel, a gas inlet filter, the mini-environment chamber, a plurality of gas outlet openings in a bottom wall of the carousel housing, a plenum housing bounding a plenum chamber, a plenum chamber outlet, a gas circulation pump connecting the plenum chamber outlet to an inlet end of the gas inlet duct.