distribution enables the volume median diameter of the flocs to be determined. The median diameter of the initial flocs in the raw untreated sludge is 220 $ \mu $ m (Fig. 2). As shown in Fig. 1(a), the activated sludge is considerably heterogeneous because it consists of a large number of different microorganisms. Immediately after ultrasonication with the load power P of 150 W and the net ultrasonic exposure time t of 75 s, the flocs become much smaller than those in the untreated sludge due to floc disintegration (Fig. 1(a) and (b)), and the median diameter of flocs falls to 105 $ \mu $ m (Fig. 2). Although the size distribution of the disrupted flocs in the ultrasonicated sludge remains almost unchanged by the following rapid mixing at 150 rpm for 3 min (Figs. 1(c) and 2), the flocs in the ultrasonicated sludge become significantly large after slow mixing at 50 rpm for 20 min following the rapid mixing (Fig. 1(d)) and the median diameter of flocs increased to a high value of 380 $ \mu $ m (Fig. 2). The intracellular materials, such as proteins, and nucleic acids, released during the breakup of the bacterial cells act as an excellent flocculant. Due to the presence of positive charges, these compounds bind to the anionic sites of the bacteria and the polysaccharides, resulting in reflocculation [25]. However, the slow mixing of more prolonged periods of 30 min following the rapid mixing disintegrates the flocs in the ultrasonicated sludge (Fig. 1(e)), and the median diameter decreases to almost the same size (220 $ \mu $ m) as that of untreated flocs (Fig. 2). This suggests that the flocs enlarged by ultrasonication are likely to be disrupted. It is, therefore, considered that the floc size is determined by a balance of both the growth and erosion of flocs resulting from the shearing action [17,25].

Fig. 3 shows the typical photomicrographs of floc structure in the sludge obtained by the combined pretreatment of ultrasonication and salt addition. The sludge ultrasonicated during 75 s with the power of 150 W was adjusted to the sodium chloride concentration $ C_{s} $ of 0.1 M due to salt addition immediately after ultrasonication. Fig. 4 illustrates the corresponding cumulative size distributions by volume of the flocs. Flocculation effect becomes more conspicuous when sodium chloride is added to the ultrasonicated sludge. Reflocculation of flocs disintegrated by ultrasonication occurs due to the action of sodium chloride once a rapid mixing is conducted, and the floc size increases to the same size as that of the untreated sludge (Figs. 3(a) and 4). It should be noted that the rapid mixing was carried out to mix the salt well with the sludge. The subsequent slow agitation for 20 min provokes the median size to further increase to 450 $ \mu $ m (Figs. 3(b) and 4). Although the slow mixing time is prolonged until 30 min, the agitation does not cause significant disruption of the flocs grown (Figs. 3(c) and 4). Hence, it is inferred that salt addition to ultrasonicated sludge brings about not only the enlargement of the floc size but also the increase in the floc strength. The results obtained by the pretreatment of salt addition alone are also included in Figs. 3(d) and 4. Of particular interest is that salt addition to the ultrasonicated sludge greatly facilitates flocculation of activated sludge whereas the pretreatment of salt addition alone has little effect on flocculation in the sodium chloride concentration of 0.1 M (Fig. 3(b) and (d)). Consequently, it would be expected that the floc coarsening leads to the improvement of solid-liquid separation efficiency of activated sludge. In order to maintain the mixing condition constant, the rapid mixing at 150 rpm for 3 min followed by the slow mixing at 50 rpm for 20 min was adapted as the mixing condition for floc growth in all our subsequent experiments.

#### 3.2. Batch sedimentation curve

In the batch sedimentation tests, the overall settling behaviors are examined by describing the sedimentation curve represented as the form of the height H of the interfacial plane between the sludge and the supernatant normalized by the initial height $ H_{0} $ against the sedimentation time $ \theta $ . Fig. 5 illustrates the batch sedimentation curve of the sludge pretreated under various conditions. Although the particles less than about $ 10 \mu m $ are mostly left in the supernatant without settling out, a distinct sedimentation interface of larger particles is observed. The sedimentation curve is based on the time variation of the height H of this interface. It is obvious that any sedimentation curve is roughly divided into two periods: the hindered settling period in which the interface between the sludge and the supernatant descends at a constant velocity depending on the sludge concentration and the subsequent sludge compression period in which the subsidence velocity of interface declines gradually. The sedimentation behaviors under various pretreatment conditions are compared from the viewpoint of the sedimentation velocity $ u_{0} $ during the hindered settling period and the sludge volume (SV), which is taken as a measure of thickening degree during the sludge compression period. In the untreated sludge, the sedimentation velocity $ u_{0} $ is 0.24 mm/min and the sludge volume $ SV_{300} $ defined as the ratio of the interface height $ H_{300} $ after 5 h (300 min) to the initial height $ H_{0} $ is 0.56, as shown in Fig. 5(a). In the salt-added sludge with the sodium chloride concentration $ C_{5} $ of 0.1 M, the sedimentation velocity increased to 2.1 times that of the untreated sludge and $ SV_{300} $ reduced to 0.49. When the sludge is ultrasonicated during 75 s with the power P of 150 W, the sedimentation velocity increases to 3.2 times that of the untreated sludge and $ SV_{300} $ reduces to 0.42.

The figure clearly indicates that the sedimentation efficiency is surprisingly improved by adding the salt to the ultrasonicated sludge. The combined treatment increases the sedimentation to really 15.4 times that of the untreated sludge. The value of $ SV_{300} $ reduces to 0.31, and it takes only 27.4 min to attain the value of $ SV_{300} $ after 300 min obtained for the untreated sludge. It is demonstrated that an extraordinary improvement of the settling efficiency of excess activated sludge is brought about by ultrasonically-assisted flocculation based on the synergy effect of ultrasonication and salt addition. It is suggested that the fragmentation of particles caused by ultrasonication leads to the creation of more nuclei contributing to the floc development, enhancing the interaction between the particulates and/or the organic solutes and the salt [22]. Thus, cations increased by the salt addition reduce the number of negative charges at the surface of particle fragments due to charge neutralization, resulting in a marked flocculation effect.

Even though the sodium chloride concentration $ C_{s} $ in the sludge increases to 0.5 M, the sedimentation rate of the sludge treated by salt addition alone is comparable with that of the sludge with the concentration of 0.1 M, as shown in Fig. 5(b). However, the sludge volume markedly decreases with the progress of sedimentation, and $ SV_{300} $ decreases to 0.43 of a similar level to that of the sludge ultrasonicated during 75 s with the power P of 150 W. The salt addition to the ultrasonicated sludge increases the sedimentation rate to 25.2 times that of the untreated sludge and 10.3 times that of the sludge treated by the salt addition alone, and decreases $ SV_{300} $ to 0.27. Therefore, it takes only 6.2 min to attain the value of $ SV_{300} $ after 300 min obtained for the untreated sludge, and thus the sedimentation time is strikingly reduced in ca. one-fifty. This clearly indicates that the combined treatment of ultrasonication and salt addition is quite effective even under conditions of high salt concentrations. However, it should be noted that it is preferable to minimize the amount of salts added.

Fig. 5(c) and (d) illustrates the batch sedimentation curves of the sludge pretreated under different ultrasonication conditions from the condition $ (P=150\ W $ and $ t=75\ s) $ tested in Fig. 5(a). Although the flocculation effect becomes more remarkable with increasing power P and time t, it is considered desirable to minimize the ultrasonication exposure from an energy expenditure viewpoint.