Disposal cost of excess sludge becomes one of major expenses in sewage treatment plants since ocean dumping was prohibited. In this study sludge reduction agent which contains macromolecular carbohydrates was used to assess reduction of excess sludge. The agent was injected into bioreactor for 7 months in order to reduce excess sludge production generated from sewage treatment plants, and then there was found about 41% reduction effect according to the measurement of generation amount of excess sludge production. And also, an analysis was conducted to investigate influence of macromolecular carbohydrates injection on water quality of effluent in sewage treatment plants (BOD, COD, SS, T-N, T-P). As a result, it was found that there was no side effect at all, and treatment efficiencies of total nitrogen and total phosphorus became a little higher. Reduction mechanism of excess sludge in a bioreactor is that microorganism enters into endogenous respiration phase if any nutrient supply is blocked off by macromolecular carbohydrates surrounded the surface of activated microorganisms with thin film. Microorganism in this phase uses energy released from the substances which are stored in cells and are decomposed by catabolism and thus, the amount of excess sludge production becomes reduced. DGGE analysis was conducted to identify that macromolecular carbohydrates, sludge reducing agent, effect on the microorganism community within the sewage treatment plants. It is expected bands from DGGE observed, the DNA sequence was analyzed and compared with known base sequence. All bands are consistent with unculturable microorganism which would be easily found in sewage treatment facilities and categorized as aerobic bacteria. From this result, it is judged that in case the injection of macromolecular carbohydrates does not cause new microorganism species, but reduce the sludge production implying increment of microorganism activity.

Key words : Macromolecular carbohydrates, Biological sewage treatment, Excess sludge reduction, Endogenous respiration, DGGE