Appropriate control of waste activated sludge (WAS) is required to solve the annual increment of WAS volume. The low bacterial activity at low temperatures poses a difficulty in reducing or utilizing WAS in cold weather regions and/or during the winter season. This study reveals a practical method to enhance sludge reduction at a low temperature using isolated strains of Pseudomonas and Aeromonas species. The effect of inoculating each strain into WAS was examined at different temperatures (4 degrees C, 10 degrees C, 12 degrees C, 15 degrees C, 20 degrees C, and 30 degrees C) and under an aerobic condition. Sludge reduction showed 2-to 8-fold improvement at the temperature range from 4 degrees C to 15 degrees C. Both strains are psychrophilic and can produce protease and lipase for sludge degradation even at low temperatures. Thus, biological WAS treatment at a psychrophilic temperature can be enhanced by inoculating these promising strains. (C) 2014 Elsevier Ltd. All rights reserved.