ca. 40 J/g. There is no distinct difference in the supernatant quality between salt added (0.1 M) and non-added sludge samples. Therefore, it is considered that ultrasonication particularly contributes to high concentration of non-flocculated and non-settled fine particles, and soluble organic matter in the supernatant, arising from the floc breakage $ [22,29] $ . Although the combined pretreatment is extremely effective in sludge thickening, the quality of the supernatant evaluated by turbidity and TOC was considerably deteriorated when ultrasonication was incorporated in the pretreatment. However, it would be expected that the ultrasonic pretreatment enhances biodegradability of contaminated supernatant prior to anaerobic digestion or recycling in aeration tank.

#### 3.5. Relation between sedimentation velocity and floc size

It is well known that the sedimentation velocity of particles strongly depends on its size. In Fig. 11, the sedimentation velocity $ u_{0} $ during the hindered settling period is logarithmically plotted against the median diameter $ d_{50} $ of flocs in the sediment. It should be noted that the particles less than about 10 $ \mu $ m are mostly left in the supernatant without settling out. Consequently, the sedimentation curve and the resulting sedimentation velocity are little influenced by such small particles. Therefore, such particles are ignored when the relation between the sedimentation velocity and the median diameter of flocs is evaluated. Plots show that the sedimentation velocity increases with the increase in the median diameter of flocs. Plots can be approximated by the straight line, as shown in the solid line in the figure, although there is some variation in the data. The value of slope is found to be ca. 1.66 (the correlation coefficient [30] r=0.91). According to the Stokes law applicable to settling of very dilute suspension, the sedimentation velocity increases directly with the square of the particle diameter [31]. In the case of flocs, the floc density decreases with the increase in the floc size [32], and hence the sedimentation velocity $ u_{0} $ is represented as

$$ u_{0}=\frac{d_{f}^{D-1}g}{18\mu} $$

where D is the fractal dimension for a self-similar structure, g is the acceleration of gravity, and $ \mu $ is the viscosity of the supernatant. The fractal dimension D in Eq. (3) is determined from the slope of the

Fig. 11. Relation between sedimentation velocity and median diameter of flocs in sediment.

regression line in Fig. 11 and found to be ca. 2.66. This value is plausible because a lot of computer simulations show that the fractal dimension D may range from less than 1.7 to 3.0, depending on the condition of floc formation $ [33,34] $ .

### 4. Conclusions

The effect of the pretreatments of ultrasonication and addition of sodium chloride was examined by conducting batch sedimentation experiments and the measurements of the floc size for excess activated sludge. The sedimentation velocity in the initial period of settling was increased due to the increase in the floc size with the increase in the specific ultrasonic energy dissipated into a liquid through the increase in the load power or the sonication time. The increase in the sedimentation rate was surprisingly noticeable when the sodium chloride was added to the ultrasonicated sludge. The sludge volume decreased significantly by the combined pretreatment of ultrasonication and salt addition.

## Acknowledgements

This work has been partially supported by Grants-in-Aid for Scientific Research from The Ministry of Education, Culture, Sports, Science and Technology, Japan and from The Ministry of the Environment, Japan. The authors wish to acknowledge with sincere gratitude the financial support leading to the publication of this article. The authors also wish to express their sincere appreciation to the Ueda Sewage Treatment Works for their generous contribution of the activated sludge used in this research.

## References

[1] C.C. Wu, C. Huang, D.J. Lee, Effects of polymer dosage on aluminum sludge dewatering characteristics and physical properties, Colloids Surf. A: Physicochem. Eng. Aspects 122 (1997) 89-96.

[2] V. Agridiotis, C.F. Forster, C. Carliell-Marquet, Addition of Al and Fe salts during treatment of paper mill effluents to improve activated sludge settlement characteristics, Bioresour. Technol. 98 (2007) 2926–2934.

[3] X. Yin, P. Han, X. Lu, Y. Wang, A review on the dewaterability of bio-sludge and ultrasound pretreatment, Ultrason. Sonochem. 11 (2004) 337-348.

[4] J. Laurent, M. Casellas, M.N. Pons, C. Dagot, Flocs surface functionality assessment of sonicated activated sludge in relation with physico-chemical properties, Ultrason. Sonochem. 16 (2009) 488-494.

[5] A.R. Mohammadi, N. Mehrdadi, G.N. Bidhendi, A. Torabian, Excess sludge reduction using ultrasonic waves in biological wastewater treatment, Desalination 275 (2011) 67–73.

[6] J. Kopp, J. Müller, N. Dichtl, J. Schwedes, Anaerobic digestion and dewatering characteristics of mechanically disintegrated excess sludge, Water Sci. Technol. 36 (1997) 129–136.

[7] Y.Y. Li, T. Noike, Upgrading of anaerobic digestion of waste activated sludge by thermal pretreatment, Water Sci. Technol. 26 (1992) 857-866.

[8] S.T. Yan, L.B. Chu, X.H. Xing, A.F. Yu, X.L. Sun, B. Jurcik, Analysis of the mechanism of sludge ozonation by a combination of biological and chemical approaches, Water Res. 43 (2009) 195–203.

[9] Y. Chen, S. Jiang, H. Yuan, Q. Zhou, G. Gu, Hydrolysis and acidification of waste activated sludge at different pHs, Water Res. 41 (2007) 683–689.

[10] R.V. Rajan, J.G. Lin, B.T. Ray, Low-level chemical pretreatment for enhanced sludge solubilization, Res. J. Water Pollut. Control Fed. 61 (1989) 1678–1683.

[11] B. Park, J.H. Ahn, J. Kim, S. Hwang, Use of microwave pretreatment for enhanced anaerobiosis of secondary sludge, Water Sci. Technol. 50 (2004) 17–23.

[12] Y.C. Chiu, C.N. Chang, J.G. Lin, S.J. Huang, Alkaline and ultrasonic pretreatment of sludge before anaerobic digestion, Water Sci. Technol. 36 (1997) 155–162.

[13] R.O. King, C.F. Forster. Effects of sonication on activated sludge. Enzyme Microb. Technol. 12 (1990) 109–115.

[14] R. Dewil, J. Baeyens, R. Goutvrind, The use of ultrasonics in the treatment of waste activated sludge, Chin. J. Chem. Eng. 14 (2006) 105-113.

[15] R. Dewil, J. Bacyens, R. Goutvrind, Ultrasonic treatment of waste activated sludge, Environ. Prog. 25 (2006) 121-128.

[16] F. Wang, M. Ji, S. Lu, Influence of ultrasonic disintegration on the dewaterability of waste activated sludge, Environ. Prog. 25 (2006) 257–260.

[17] K. Kakii, H. Kato, K. Ohara, Y. Kawai, M. Yoshida, M. Kuriyama, T. Shirakashi, Reflocculation of sewage activated sludge disrupted by ultrasonication. J. Biosci. Bioeng. 72 (1994) 371–377.

[18] X. Feng, H. Ley, J. Deng, Q. Yu, H. Li, Physical and chemical characteristics of waste activated sludge treated ultrasonically, Chem. Eng. Process. 48 (2009) 187–194.