To reduce assembling man-hours by providing a communication groove between a plurality of discharge grooves to form a base, and placing a cover on the base to form a reverse wash pipe.

Between a plurality of discharge grooves 4 a communication groove 3 for communicating the grooves 4 with each other is provided to form a base 1 made of resin such as polyvinil chloride resin. A cover 2 is formed from such resin as polyvinil chloride resin. On the base 1 the cover 2 is placed and connected thereto with an adhesive or the like to form a reverese wash pipe 5. The pipe 5 is positioned in a sewage disposal tank. Microbes are activated by separating aged microbes film from a contact member through discharge of air bubbles to the inside of the tank from each groove 4. Accordingly, assembling man-hours at production can be reduced and an easy production is also attained.