1. INTRODUCTION Treatment of industrial waste has recently been concerned as a social problem. Waste soil, yielded at underground construction fields, is one of the typical industrial wastes. Figure 1 shows flow and treatment of waste soil in Tokai district, Japan. The waste soil is separated into sand available to material for construction, and sludge hard to deal with because it composes of mainly clay and water. The sludge has often been thrown away in the mountain area after being de-watered. Because land price is souring and residents are becoming nervous about environmental problems, however, it is difficult to find a place where the sludge can be left. Therefore, it is important to develop a new treatment of the sludge instead of throwing it away. In this study, porous ceramics named here "bio-module" was made from the sludge by adding sand or powder rubber in order to recycle the waste soil. Then physical characteristics of the modules were tested for shrinkage rate, porosity, permeability, and unconfined compression strength. 2. EXPERIMENTAL Material The material was waste soil yielded at underground construction fields in Nagoya, Japan. Only that portion passing a 74 micrometer-mesh sieve was used. The material contained bentonite, kaolinite, and cement, of which the ratio were 1:5:6.5 by dry weight, respectively. According to an elute test, toxic objects such as mercury, hexad-chrome, PCB and arsenic were not detected in the material. Sand, yielded at Experimental farm of Mie Univ., and powder rubber ground from an old tire were mixed with the material. Five kinds of test piece were prepared according to compounding ratio shown in Table 1. Method The material, previously conditioned to the required initial compounding ratio, was packed into an aclylite tube 5 cm long and with a 2 cm inside diameter. Three pieces were prepared for each experiment. The packed tubes were dried at 45 ¡î for 12 hours; so that because the test pieces shrank slightly, they were easy to pull out. The pieces were then baked with an electrical oven at 500 ¡î for 3 hours and at a fixed temperature from 1090 to 1150 ¡î for 6 hours. After the temperature returned to about 100 ¡î, the baked pieces were taken out of the oven and tested for physical properties such as shrinkage rate, porosity, permeability, and unconfined compression strength at room temperature. 3. RESULTS AND DISCUSSION Color of baked ceramics The color of baked ceramics changed from light yellow to dark brown as the fixed temperature was increased. The pieces of A, D and E had dark luster at 1150 ¡î. This is attributed to the melting of some component in the materials. Shrinkage rate The diameter of a test piece was compared before and after baking. Shrinkage rate, SR, was here defined as a ratio of diameters before and after baking; namely, SR=(Db-Da)/Db (1) where Db and Da are the diameters of a piece before and after baking, respectively. Figure 2 shows the shrinkage rate (SR) versus baked temperature. A mean of three pieces is shown in Fig. 2. The SR increased with temperature for each piece. This result means that the skeleton structure of the piece changed with the temperature because of sintering. Mixing sand (B, C) or powder rubber (D, E) with the waste soil decreased the shrinkage rate. This indicates that adding sand or rubber with the sludge has the effect of maintaing the original skeleton shape. Porosity Porosity is an important physical property as a microbial habitat in environmental water and soil. The porosity of a piece can be determined by the measurement of water absorption. After drying at 105 ¡î, test pieces were sunk into water and vacuumed for half an hour. Since air was exchanged for water in the test piece, porosity, ¦Õ, was calculated from the change in weight before and after sinking; namely, ¦Õ=(Ma-Mb)/¦ÑV (2) Permeability Permeability is an important physical property related to nutrients flow and exchange for microorganism that recycle chemical elements. The permeability, K, was measured with an apparatus shown in Fig. 4 and calculated from equation (3). K=2.30aL log(h1/h2)/A(t2-t1) (3) where L and A are the length and the sectional area of the sample, respectively. a is the sectional area of the thinner stand tube. h1 and h2 are the water levels at the times of t1 and t2, respectively. where ¦Ñ is the density of water, 1 g/cm3. V is the volume of the piece. Mb and Ma are the weights, g, of the piece before and after sinking, respectively. Figure 3 gives the porosity versus temperature. Each piece had the porosity of 40-55 %, which will be enough as a habitat for microorganism. While the porosities of A, B and C decreased with increasing temperature, those of D and E were independent of the temperature. This result can be interpreted that while the components of the waste soil and sand had remained over 1090 ¡î the rubber had already melt below the temperature. Figure 5 shows the permeability versus temperature. The pieces of B and C, which contained sand, had higher permeability than the others for each temperature. This result indicates that sand plays a role to keep a high permeability. Also, the permeability of C was twice as high as B, which is proportional to the quantity of additional sand. Unconfined compression strength Strength is an important property in a material. Longitudinal strength of the piece was measured by unconfined compression test. The test was conducted at the strain velocity of 2 mm/min. Figure 6 shows unconfined compression strength (UCS) versus temperature. UCS increased with temperature for each piece. This result can be explained in terms of sintering. n addition, the permeability of B and C increased at the temperature below 1110 ¡î with increasing temperature, and became highest at the temperature, then decreased over 1110 ¡î. This can be interpreted that the component of sand started to melt and clogged the pores in the piece at 1110 ¡î. This result suggests that there is an optimum temperature for permeability. The pieces of A and B at 1115 ¡î had the strength of 100 kgf/cm2 as same as a first-class brick based on JIS (Japanese Industrial Standard). The strength of the pieces of B and C, which contained sand, were larger than the piece of A, but adding powder rubber(D, E) decreased the strength. This suggests that the strength of the piece is dependent on the kind of additional materials. Practical aspect The bio-module should sustain an enormous population of microorganism that recycle chemical elements. The experiment suggests that the baked porous piece is promising as a material with respect to the physical characteristics such as porosity, permeability and strength. However, we need to experiment the bio-module from biological aspects. One of the aspects is to investigate the adherent property of microorganisms on the module which should be the best habitat for them. Another aspect is to find best microorganisms who are useful to purify eutrophic environmental water. If we find and use good microorganisms, we will be able to increase the efficiency of purifying the eutrophic water. From such a practical viewpoint, we are studying the optimum condition for the bio-module, and testing to purify an eutrophic moat. 5. CONCLUSION It was found from the experiment that (1) Adding sand or powder rubber with sludge reduced the shrinkage rate, and increased porosity and permeability of the baked piece; (2) Strength of the piece is dependent on added materials and baked temperature. We have a plan to use the bio-module for purifying eutrophic environmental water. In order to apply the bio-module to practical aspect, we need to find the best condition for a compounding ratio and a baked temperature while considering the nature of microorganism. Acknowledgements The authors would like to thank Murakami-Giken Co., for financial support of this work. They also gratefully acknowledge the help made by P. Stephen and T. Shibata to refine the manuscript. M. Mizoguchi, (81)592-31-9574, mizo@bio.mie-u.ac.jp