Synergy effect of ultrasonication and salt addition on settling behaviors of activated sludge

• Ultrasonication pretreatment increased sedimentation velocity of activated sludge.
• Ultrasonication effect was well evaluated by specific ultrasonic energy.
• Ultrasonication effect was dramatically facilitated by NaCl addition.
• Sedimentation velocity was related to median diameter of flocs.
• Sludge volume reduced by combined treatment of ultrasonication and salt addition.

The batch sedimentation tests were conducted for the excess activated sludge treated by ultrasonication and addition of sodium chloride, and the settling behaviors such as the sedimentation velocity, the sludge volume, and the quality of the supernatant were examined for different values of the load power of ultrasonication, the sonication time, and the sodium chloride concentration. The ultrasonication pretreatment significantly increased the sedimentation velocity in the initial stage of sedimentation, and the effect was dramatically facilitated by the addition of sodium chloride even when the effect was unnoticeable in the pretreatment of the salt addition alone. The effect of ultrasonication was increased with increasing load power and sonication time and found to be evaluated by the specific ultrasonic energy dissipated into a liquid. The increase in the sedimentation velocity brought about by the pretreatment was caused by the increase in the floc size, and the sedimentation velocity was related to the median diameter of flocs, based on the Stokes law considering the effect of both the floc size and floc density. Moreover, the combined pretreatment markedly reduced the sludge volume, as compared to the pretreatments of ultrasonication alone and salt addition alone.