A novel aerobic animal waste treatment system (Tertiary Oxygen Activated Sludge Treatment – TOAST trademark) was operated on an pig farm in southern Minnesota, USA. The fully operational system was evaluated for odour reduction impact by comparing the treated barn with an adjacent barn operated in anaerobic mode. There was a reduction in odour in the treatment barn compared with the odour in the control barn. For most tests the odour units in the control barn were double those in the treated barn. The odour units measured on the untreated manure were twice those on the treated manure. Hydrogen sulfide emissions were greater from the untreated manure. The method of evaluating odour used an olfactometer with sensory panels to judge air samples.