Lab. reactor consisting of 2 connected helical rotors with a diam. of 340 mm and a length 2 x 1400 mm was placed in a flowing stream of mech. cleaned domestic sewage (flow rate of 0.65 dm(3)/min). The rotors were made of tubes with a diam. of 32 mm wound with a helix stroke 2800 mm clockwise as well as anticlockwise around a shaft of diam. 40 mm. Rotors floated with approx. 50% of immersion in the wastewater and were mech. rotated to the right or left with speeds 1.0, 1.95 or 2.95 rpm. Periodically, 1 L of saline was injected into a reactor contg. 200 L of sewage, the salt concn. was measured at the outlet as a function of time and then the Morrill dispersion, short circuiting, ay. retention time and median rate of half-time residence were detd. The indexes were typical for reactors with complete mixing for rotor speeds above 1 rpm.