#### 3.2. Experimental apparatus and technique

The excess activated sludge concentrated at 1.0 wt% by decantation was flocculated with PACl by stirring at 300 rpm for 1 min. The particle size distributions before and after flocculation were measured by a laser diffraction particle size analyzer (SALD-2200, Shimadzu Corp.). The optimum dosage of PACl was determined from the comparative evaluation of the mean specific surface area size $ d_{s} $ .

Fig. 2(a) illustrates a schematic view of the experimental apparatus used in low-pressure filtration and water permeation through the filter cake formed. A specially designed deliquoring cell with an effective medium area of $ 9.6 \, cm^2 $ was utilized in this research. The cell essentially consists of a stainless-steel cylinder and stainless-steel top and bottom plates. A filter paper (No. 4A, Advantec Toyo Corp.) with a retention size of $ 1.0 \, \mum $ was used as a filter medium. Low-pressure filtration was conducted in the dead-end mode by using excess activated sludge of 80 g flocculated with PACl under a constant pressure condition of 98 kPa adjusted automatically by a computer-driven electronic pressure regulator by applying compressed nitrogen gas.

Once filtration is completed, the amount of water which is equivalent to the volume of liquid contained in the filter cake formed in low-pressure filtration is permeated through the filter cake at the same constant pressure that one applied in low-pressure filtration, in order to re-disperse flocs in the cake. The filtrate in low-pressure filtration and the permeate in water permeation

a

b

Fig. 2. Schematic view of experimental apparatuses: (a) apparatus for low-pressure filtration and water permeation through filter cake and (b) apparatus for ultrahigh-pressure expression.

through the filter cake were collected in a reservoir placed on an electronic balance connected to a personal computer to collect and record mass vs. time data. The weights were converted to volumes using density correlations. The amount of PACl contained in the permeate obtained during low-pressure filtration and water permeation was evaluated on the basis of the measurement of the electrical conductivity using the electric conductivity meter (DS-52, Horiba, Ltd.). The difference of the size distributions of flocs in the cake between before and after water permeation was measured by the laser diffraction particle size analyzer. The photomicrograph of flocs in the cake was taken by Digital HF Microscope (VH-8000, KEYENCE Corp.). The sample for the floc analysis after cake formation was prepared by adding the filtrate (water in the case of the cake after water permeation) to the cake and then by stirring at 100 rpm for 2 min. It should be noted that the stirring speed was set to a lower level than that in flocculation in order to avoid floc breakage.

The experimental apparatus used in ultrahigh-pressure expression is schematically shown in Fig. 2(b). Once water permeation was terminated, the top plate was removed from the deliquoring cell and alternatively the movable piston equipped with a filter paper of No. 4A as a filter medium was inserted in the cell cylinder. The liquid was squeezed out of the cake through the top and bottom filter media by application of a mechanical load through a piston by using a material testing machine (SC-20H (MNS-01), Tokyo Testing Machine Inc.). The cake was consolidated at constant pressures $ p_{c} $ ranging from 98 kPa to 15 MPa. It was suggested that the effect of the side wall friction in the deliquoring cell is negligible if the ratio of the cake thickness to cell cylinder did not exceed 0.6 (Grace, 1953; Tiller and Lu, 1972). In this study, the ratio of the cake thickness to cell cylinder is maintained at relatively low values ranging from 0.010 to 0.39 roughly in order to justify the assumption that the side wall friction is negligible. The time evolution of the cake thickness during constant-pressure consolidation of the cake was measured by a dial gauge fitted on the cell cylinder since the displacement of the cell cylinder was equal to the amount of change in the cake thickness. It should be also noted that the amount of change in the cake thickness is equal to the volume of liquid squeezed out of the cake per unit cross-sectional area. The equilibrium state was considered to be established when the reading of the dial gauge with the minimum scale value of 0.01 mm remained unchanged for 5 h. The residual moisture content in the compressed cake was measured using an infrared-ray moisture meter (FD-720, Kett Electric Lab.) at the end of consolidation in order to rate the degree of deliquoring on the basis of the cake moisture content including liquid inside the microorganism cells.

For comparison, a set of experiments where the flocculants were not employed were also carried out. Additionally, expression experiments were conducted at the same pressure as the one employed in water permeation in place of water permeation experiments.

### 4. Results and discussion

#### 4.1. Flocculation

PACl was added to excess activated sludge at various values of the ratio $ R_{f} $ of added aluminum to the solid mass in sludge, and the particle size distribution was measured by the laser diffraction particle size analyzer to evaluate the flocculation performance of excess activated sludge. For laser diffraction measurements the sludge flocculated with PACl was diluted by adding the aqueous solution with the same PACl concentration in order to avoid the floc breakage. The mean specific surface area size $ d_{s} $ of particles