Fig. 1. Photomicrograph of floc structure in ultrasonicated sludge (P = 150 W, t = 75 s): (a) untreated sludge, (b) sludge immediately after ultrasonication, (c) ultrasonicated sludge after rapid mixing at 150 rpm for 3 min, (d) ultrasonicated sludge after slow mixing at 50 rpm for 20 min, and (e) ultrasonicated sludge after slow mixing at 50 rpm for 30 min.

the initial height $ H_{0} $ of 12.0 cm) was gradually poured into a graduated setting cylinder, the sedimentation experiment starts and the sedimentation height H of the interface plane between the top of settling bed suspension and the supernatant liquid was recorded by taking the photographs with the lapse of the specified sedimentation time 0 for 24 h. The liquid temperature was almost kept constant (20°C). The experiments were conducted more than twice in order to ensure the reproducibility of the results. The sedimentation velocity $ u_{0} $ was determined from the slope of the linear relationship between the sedimentation height H vs. the sedimentation time 0 during the initial stage of sedimentation. The sludge volume SV defined as the ratio of the interface height H to the initial height $ H_{0} $ was also determined for the specified sedimentation times.

After the sedimentation experiment is completed ( $ \theta = 24 $ h), the supernatant containing fine particles and macromolecules was pipetted up to the surface of sedimented sludge. To evaluate the quality of the supernatant, the turbidity and total organic carbon

Fig. 2. Cumulative size distribution by volume of flocs in ultrasonicated sludge $(P=150\mathrm{W}, r=75\mathrm{s})$.

(TOC) in the supernatant were measured by the turbidimeter (ODYSSEY DR2500 spectrophotometer, Hach Co., USA) and TOC analyser (TOC-5050A, Shimadzu, Corp., Japan), respectively. The viscosity of the supernatant measured by using a capillary viscometer was ca. 1 mPa s, which was the same value as that of water. The sediment was diluted with the filtrate obtained by filtering the supernatant of the untreated sludge through a syringe filter with a 0.45 μm, pore diameter (cellulose acetate) to remove any suspended solid particles, and then the photomicrographs of flocs were taken (x 40) with the digital photomicroscope (BA210EINT, Shimadzu Rika Corp., Japan). The floc size distribution by volume and the median diameter were determined from the ferret diameters of at least 200 floc samples based on the photomicrographs. Alternatively, the floc size distribution was measured using a laser diffraction particle size analyzer (SALD-2200, Shimadzu Corp., Japan).

For comparison, the experiments were also carried out for the untreated sludge and the sludges conditioned by ultrasonication alone and salt addition alone, in addition to the sludge conditioned both by ultrasonication and by salt addition.

### 3. Results and discussion

#### 3.1. Microscopic observation and floc size measurement

In Fig. 1, the typical photomicrographs at 40× magnification of floc structure in the ultrasonicated sludge followed by different mixing operations are compared with the photomicrograph of floc structure in the untreated sludge. The corresponding cumulative size distributions by volume of flocs are illustrated in Fig. 2, where $ d_{f} $ is the floc diameter. The contrast between the floc size distributions shown in Fig. 2 and the photomicrographs shown in Fig. 1 reveals a better understanding of the change of floc structure induced by ultrasonication. The size distributions are based on the ferret diameters measured from the photomicrographs of flocs since the measurements obtained using a laser diffraction particle size analyzer underestimates the real floc size probably as a result of floc breakage caused by the introduction of the sample into the measuring chamber. It should be noted that the particles less than 10 μm are ignored in the measurements because such small particles are mostly present in the supernatant after settling and have little influence on the sedimentation velocity. The cumulative size