Ozonation is used for the tertiary treatment, particularly for removal of colour, of the effluent from a night soil treatment plant in Japan. The practical system for effectively controlling the ozone treatment was investigated. The optimum ozonation was accomplished by monitoring the colour. This treatment plant worked in response to a sudden variation in the quality of raw water, i. e., nitrite ion concentration, COD, etc, and the quality of the ozonated water was kept at the levels of COD of 20 ~ 30 mg/2, and colour of 30 units. The water treated by this ozonation plant was found to be free from coliform bocilli, and to exhibit neither so strong acute toxicity to fishes nor mutagenicity.