a serious shortage of the remaining landfill capacity, and more severe environmental regulations. Deliquoring by mechanical expression of residual sludge arising from wastewater treatment has become increasingly important owing to the relatively low energy consumption compared to thermal drying which follows in the process sequences. It is desirable to remove the maximum feasible amount of liquid by mechanical expression prior to drying and incineration. There has been considerable research on the expression mechanism of sludge in recent decades (Kawasaki et al., 1990b, 1996; Kang et al., 1990a, 1990b; Yoshida, 1993; Chang and Lee, 1998; Chu et al., 1998; La Heij et al., 1996a, 1996b; Lee and Wang, 2000; Mihoubi et al., 2003; Christensen and Keiding, 2007; Huang et al., 2010; Grimi et al., 2010; Petryk and Vorobiev, 2013). Among them, Kawasaki et al. (1990b, 1996) analyzed the expression characteristics of freeze/thaw conditioned excess activated sludge, and examined the effect of bound water, which cannot be easily removed by mechanical expression, on the expression performance. Kang et al. (1990a, 1990b) examined the role of extracellular polymer in expression of activated sludge. Chang and Lee (1998) modified the conventional Terzaghi-Voigt model (Shirato et al., 1974) by considering the ternary consolidation appeared in the final phase of expression in order to accurately evaluate the expression characteristics of sludge (Chu et al., 1998). However, there is not much information on expression characteristics, especially cake moisture content, of excess activated sludge under ultrahigh-pressure expression conditions more than 10 MPa (Bergins, 2004; Venter et al., 2007).

Unfortunately, the deliquoring rate and moisture content of the compressed cake produced by mechanical expression of biological sludge are currently unsatisfactory. The relative effect of expression pressure on decreasing moisture content of the compressed cake and decreasing permeability is fundamental to develop a promising deliquoring method. Although it is to be expected that the increase in the expression pressure leads to the decrease in the moisture content of the compressed cake, the deliquoring rate is unsatisfactory because of the formation of highly compactible filter cake (Tiller and Yeh, 1987; Tiller et al., 1999). In contrast, whilst the deliquoring rate is significantly improved with the advent of high-performance polymer flocculants, the moisture content of the compressed cake is insufficient due to the highly aggregated structure of flocs in the cake. In any case, it is difficult to sufficiently reduce the sludge volume at high rate.

In the present article, a deliquoring method which combined reversible flocculation with ultrahigh-pressure expression is developed as an innovative technique to overcome the defects of the conventional mechanical deliquoring methods. The effectiveness of the method is examined from the viewpoint of the achievement of high-rate and high-level deliquoring using excess activated sludge discharged from a municipal waste water treatment plant, and the kinetics of ultrahigh-pressure expression is revealed on the basis of the multi-stage creep model combined with the modified Terzaghi model.

### 2. Deliquoring process developed

The outline of the deliquoring process developed in this research is schematically shown in Fig. 1. Inorganic flocculants are initially added to sludge to form large flocs, which enhance the filtration rate in the subsequent low-pressure filtration. After cake formation is completed in the filtration process, water permeation through the filter cake is performed to re-disperse flocs in the cake on the basis of the principle of reversible flocculation achieved by washing out the flocculants. As a result, the cake structure undergoes the substantial change, and thus the denser cake is produced due to the floc breakage in the cake. Finally, the moisture content of the compressed cake is dramatically reduced by expression under action of ultrahigh pressure. Low-pressure filtration combined with flocculation leads to high-rate deliquoring of sludge, and ultrahigh-pressure expression combined with water permeation through the filter cake results in the high degree of deliquoring of the cake.

### 3. Experimental

#### 3.1. Materials

The excess activated sludge employed was collected from the aeration tank of the Ueda Sewage Treatment Works (Nagoya City, Japan), and concentrated at 1.0 wt% by decantation for 20 h in the refrigerator kept at 278 K. True density of solids in the sludge measured by the pycnometer was $ 1.45 \times 10^{3} $ kg/m $ ^{3} $ . Polyaluminum chloride (PACl, 250A, Taki Chem.) was employed as inorganic flocculants. Water used in water permeation through the filter cake was ultrapure, deionized water (minimum resistivity: 18 MΩ cm) prepared by purifying tap water through ultrapure water systems equipped with both Elix-UV20 and Milli-Q Advantage for laboratory use (Millipore Corp.).

Fig. 1. Schematic view of deliquoring process developed.