Enhancement of dewaterability of sewage sludge by thermal treatment

The paper presents an experimental study on thermal treatment of sewage sludge. The sewage sludge forms more than 40 percent of industrial waste in Japan, therefore its efficient treatment is urgent problem. The objective of this research is to develop an effective method of sewage sludge treatment. To study the effect of thermal treatment on dewaterability, two types of experiment were executed. The first one is floatation with continuous warming and the second one is only indirect heating. Microbubbles in sewage sludge blanket sometimes prevent settling, however in this study, they were inversely utilized for thickening by flotation. That is, the generation of microbubbles was promoted with heating. Floatation experiments were executed in thermally controlled water bath by setting 500ml messcylinder filled with sewage sludge. Treatment temperature were set at 40, 50 and 60 degrees c, and some factors which influences the floatation process, such as, variation of pH, generated gas volume and gas composition were also measured. In case of 40 degrees c, the gas generated by some biochemical reaction promoted flotation, while in case of 60 degrees c, the gas such as N-2 and CO2 played an important role, which had resolved in water at room temperature and educed due to water temperature rise. At 50 degrees c, the flotation speed was slow compared with above two cases. Using this floatation method, primary sludge of 95 percent of initial moisture content was thickened to 90 percent. Another thermal treatment method is only heating the sludge in a tank. Although most of the heat treatment of this kind were conducted under high pressure, in this study the treatment was done under atmospheric pressure. This is the unique point of our research. After heating the sludge in a water bath, the sludge was dewatered with a dewatering tester. As the best condition for the treatment, 60 degrees c and one hour was selected. A new type compressibility and dewaterability tester was made. This tester has a series of pressure filtration cells. After loading the sludge in these cells, the air pressure was set at previously determined value in each cell. Using this apparatus, the dewatering process was measured. At the final stage where the filtration pressure and the cake resistance balanced, the moisture content of this cake was measured. For the case of primary sewage sludge with thermal treatment, the moisture contents of the final cake were 61-65 percent, while for the case of no treatment sludge, this value was about 72 percent. There was not so much difference in the dewaterability between the floatation thickened sludge and that of only thermal treatment. Following the laboratory experiment, a pilot plant test was also done and the similar results were obtained. Considering that no flocculant is added for dewatering, this thermal treatment method can be concluded as a very useful one.