Gravitational thickening is the prevailing method to reduce bio-sludge volume though the process is slow and usually requires addition of polyelectrolyte(s). This paper investigated the potential benefits of sonication on enhancing the bio-sludge gravitational thickening with very low energy dose, thereby called “weak ultrasound”. Results showed that weak sonication significantly changed the bio-sludge settleability and the main mechanism was release of the loosely bound extracellular polymeric substances. The changes were strongly influenced by power density and sonication duration. Low frequency was better but the difference was insignificant. Weak sonication (<680 kJ/kg DS) improved the sludge gravitational thickening while high ultrasonic energy deteriorated the process. Considering both the sludge thickening efficiency and energy consumption, the optimum conditions were 0.15 W/mL, 7 s and 25 kHz. Under such conditions, the energy dose was only 155 kJ/kg DS, much lower than literature reports and the sludge settling time shortened from the original 24 to 12 h. Weak sonication could substitute expensive polyelectrolyte coagulant for bio-sludge thickening. Combination of weak sonication and polyelectrolyte could further reduce the settling time to 6 h. The final water content of the thickened bio-sludge was not changed after sonication or polyelectrolyte addition.