PURPOSE:To make it possible to measure the water content of dehydrated cake instantly with an infrared-ray moisture meter by decreasing the errors of the water content of the dehydrated cake of the sludge generated from sewage, human waste or an industrial waste-water treating facility. CONSTITUTION:An infrared-ray water moisture meter 2 and a color-difference meter 4 are arranged in the vicinity of the output port of dehydrated cake of a belt-press type dehydrating machine 1. The data of a calibration curve corresponding to the mixing ratio with respect to the mixed sludge of digestive sludge and raw sludge are stored beforehand. The color of the dehydrated cake S, which is discharged from a second filter cloth 12 at the outlet port of the dehydrated cake is measured with the color-difference meter 4. The absorbance of the dehydrated cake S is measured with the infrared-ray moisture meter 2. The calibration curve of the infrared-ray moisture meter 2 is set based on the value of the lightness L of the color of the dehydrated cake S measured with the color-difference meter 4 and the data of the calibrating curve. The absorbance of the dehydrated cake S, which is measured with the infrared-ray moisture meter 3, is converted into the water content with the set calibration curve.
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