Fig. 3. Relation between mean specific surface area size and ratio of added aluminum to solid mass in sludge.

is plotted against the additive ratio $ R_{f} $ of PACl in Fig. 3. Whilst $ d_{s} $ initially increases with increasing $ R_{f} $ , it decreases by the excessive addition of the flocculant. It is clear that the optimum dosage of PACl where the value of $ d_{s} $ reaches a maximum is obtained at $ R_{f} $ of ca. 0.15. Thus, all experiments were carried out with this optimum dosage of PACl. In addition, it should be noted that inorganic flocculants such as PACl form relatively fragile flocs compared with organic flocculants.

#### 4.2. Low-pressure filtration

Typical data of low-pressure filtration of flocculated sludge conducted under the constant pressure condition of 98 kPa are plotted in Fig. 4 as the form of the filtrate volume $ \nu $ per unit medium area against the filtration time $ \theta $ . For comparison, the result for non-flocculated sludge is also included in the figure. It is obvious that the addition of PACl brings about a remarkable increase in the filtration rate because of the formation of highly permeable filter cake comprised of large flocs (iritani et al., 2011). For instance, the filtration time is shortened by 93% to obtain the filtrate volume $ \nu $ of 6 cm by the addition of flocculants. This indicates that flocculation is extremely effective for the improvement of filterability of activated sludge in low-pressure filtration.

#### 4.3. Water permeation through filter cake

Fig. 5 shows the variation with time $ \theta $ of the moisture content R of the cake on the mass basis obtained from various operations to demonstrate the significance of water permeation through the filter cake. It should be noted that the moisture content of the filter cake decreases pronouncedly in a relatively short time by permeating water through the flocculated filter cake. This implies that the filter cake is efficiently compressed due to the re-dispersion of flocs in the filter cake because the flocculants in the cake are washed away by water permeation through the cake. Such reversible flocculation leads to the significant reduction in the moisture content of the cake. It is expected that the deliquoring of flocculated cake is accelerated in the course of floc breakage occurring during water permeation because the highly resistant skin layer is poorly formed next to the filter medium. In contrast, when the expression operation of the flocculated filter cake is performed in place of water permeation through the filter cake, there is a little reduction in the moisture content of the cake under the same pressure condition (98 kPa) as that employed in

Fig. 4. Relation between filtrate volume per unit medium area and filtration time in low-pressure filtration.

Fig. 5. Decrease in cake moisture content by water permeation through filter cake.

filtration, as shown in the figure. It takes long time to reduce the moisture content of the cake in the course of expression when the flocculants are not employed in low-pressure filtration.

Additionally, it should be stressed that ultrahigh-pressure expression of the cake was impossible except for the cake prepared by permeating water through the flocculated filter cake because a part of the cake leaked out of the narrowest clearance between the piston and cell cylinder. This is probably because the moisture content of the cake is significantly higher near the surface of the filter cake formed in filtration. If elastic diaphragm type of expression equipment which can withstand the ultrahigh-pressure is available, a comparison of ultrahigh pressure expression behaviors between permeated and non-permeated cakes may provide additional insights into the expression mechanisms.

The time variation of the amount of PACl contained in the permeate obtained during filtration and water permeation was measured in order to examine the role of water permeation through the filter cake. In Fig. 6, the aluminum concentration $ C_{a} $ is plotted against the permeate volume $ \nu $ per unit cross-sectional area during filtration and water permeation periods. The aluminum concentration $ C_{a} $ is kept at almost constant value $ (1.5 \times 10^{-6} \, \mathrm{kg/m^{3}}) $ throughout the course of filtration. This constant value nearly corresponds to the additive ratio $ R_{f} $ of aluminum (0.15). Once filtration is followed by water permeation, the aluminum concentration markedly decreases with the progress of water permeation, indicating that a large part of PACl contained in the cake is washed away.