To improve the activity of aerobic bacteria and microorganism and to improve digestion efficiency by using an existing digestion tank and digesting operation as it is by adding an extracted liq. of a Common Soap-Wort having the effect on the activation of the aerobic bacteria and the microorganism to sludge.

About 2% saponin is incorporated in the extracted liq. extracted from the Common Soap-Wort with alcohol. The saponin exhibits a reaction accelerating function and a stress relaxation function of the microorganism and also brings an improving effect of fat-splitting function and oxygen dissolving efficiency by its physiological activity. Further, sufficient effect is generated by addition of order of 2% to waste water at an aerobic treatment. The oxygen dissolving is eliminated at an anaerobic treatment and an oil content in the sludge is low, but there is a fear that the activity of the bacteria is lost conversely when its concn. it too high since the saponin has antifungal function. Then the digestion is markedly improved when the digestion is executed by using the extracted liq. of the Common Soap-Wort most much in crude protein ratio.