The effects of a drop-type toilet bowl cleaner(TBC) on the activity of microorganisms of sewage treatment facilities were tested by various experiments such as biochemical methane potential(BMP) measurement, oxygen uptake rate(OUR) measurement, a semicontinuous anaerobic digestion and an activated sludge treatment experiments. The results of the BMP test showed that the methane production rate gradually decreased as the TBC concentration increased, showing little gas production above 5,000㎎/L. In the anaerobic digestion experiment, the gas production and methane content decreased when the TBC concentration in the sludge fed increased to 6,667㎎/L. The OUR test failed to show a threshold TBC concentration which influences the activity of activated sludge. However, the CSTR activated sludge study showed that the activity of the activated sludge can be affected at TBC concentrations above 5,000㎎/L.