PURPOSE: To remove nits, maggots, etc. of filter bed flies in sludge and to suppress the generation of the filter bed flies, by intermittently interrupting purification of organic waste water, and bringing an alkaline solution into contact with a trickling filter bed at this time.

CONSTITUTION: Purification of organic waste water is intermittently interrupted, and an alkaline solution of pH9W14, preferably 10W12, containing caustic soda, slaked lime, soda ash or the like is brought into contact with a trickling filter bed on this occasion. By this treatment, nits, maggots and chrysalides of filter bed flies in sludge are removed with the exfoliation of the sludge surface, so that the generation of the filter bed flies is suppressed. For instance, the supply of waste water was interrupted, and a caustic soda solution adjusted at pH10 was circulated for one hour through the trickling filter bed. As a result, a part of sludge was peeled off. Operation was then restarted. The generation of the filter bed flies was suppressed over 38 days.