Fig. 3. Photomicrograph of floc structure in sludge obtained by combined pretreatment of ultrasonication $ (P=150\mathrm{W}, t=75\mathrm{s}) $ and salt addition $ (C_{s}=0.1\mathrm{M}) $ : (a) sludge after rapid mixing at 150 rpm for 3 min, (b) sludge after slow mixing at 50 rpm for 20 min, (c) sludge after slow mixing at 50 rpm for 30 min, and (d) sludge treated by salt addition alone after slow mixing at 50 rpm for 20 min.

Fig. 4. Cumulative size distribution by volume of flocs in sludge obtained by combined pretreatment of ultrasonication and salt addition.

#### 3.3. Effect of salt addition to ultrasonicated sludge on sedimentation rate and sludge volume

Fig. 6 expresses the effect of the sodium chloride concentration $ C_{s} $ in the ultrasonicated sludge on the sedimentation velocity $ u_{0} $ during the hindered settling period. It must be stressed, once again, that the sedimentation velocity $ u_{0} $ was determined from the slope of the linear relationship between H and 0, as indicated by the dotted lines in Fig. 5. Accordingly, the time interval of calculating $ u_{0} $ is different depending to pretreatment conditions such as ultrasonication and salt addition. In general, as $ u_{0} $ increases, the time interval of hindered settling becomes shortened. The unsonicated sludge is used as a control. The increase in the sedimentation velocity with increasing salt concentration is insignificantly small for the unsonicated sludge since the flocculation effect of the monovalent cation $ (\mathrm{Na}^{+}) $ is unnoticeable. However, the salt addition to the ultrasonicated sludge has a profound effect on the increase in the sedimentation velocity. The sedimentation velocity markedly increases with increasing salt concentration for the ultrasonicated sludge. Considering that the amount of salt added to the sludge should be minimized, it should be noted that $ C_{s} $ of 0.1 M have a substantial effect on settling enhancement in this ultrasonication condition ( $ P = 150 \, W, t = 75 \, s $ ). In Fig. 7, the sludge volume $ SV_{300} $ after 300 min is plotted against the sodium chloride concentration $ C_{s} $ in the sludge. The sludge volume gradually decreases as the salt concentration increases. The sludge volume at the salt addition to the ultrasonicated sludge reduces by more than 10% points than that at the salt addition to the untreated sludge, indicating the noticeable effect of ultrasonication on the reduction in the sludge volume.

#### 3.4. Effect of ultrasonic energy in combined treatment on sedimentation velocity and sludge volume

The effect of ultrasonic energy on the settling behaviors are examined by conducting the batch sedimentation tests using the sludge ultrasonicated under conditions of the load power P ranging from 50 to 200 W and the net ultrasonic exposure time t ranging from 30 to 150 s. In order to examine the influence of ultrasonic irradiation quantitatively, the net ultrasonic power is evaluated by the ultrasonic power dissipated into a liquid not the load power $ [26] $ . The correlation between the net ultrasonic power I and the load power P obtained for the same ultrasonic homogenizer as that used in this study is given as $ [27] $ .

$$ I=0.0171P^{1.47}+15.2\quad(40\leq P\leq200) $$

Thus, the effect of ultrasonication examined in this research is reviewed in the light of the ultrasonic energy dissipated in the sludge in order to comprehensively assess the effects of both the ultrasonic power I and ultrasonication time t. The specific ultrasonic energy E dissipated into a liquid is defined by [25,27,28]

$$ E=I t/M $$

where M is the mass of sludge sample.

Fig. 8 illustrates the plots of the sedimentation velocity $ u_{0} $ during the hindered settling period against the applied specific