PURPOSE: To increase the amount of dissolved oxygen by means of a unit which causes turblent flow to water to increase the contact surface for aeration, so that the amount of sewage components is decreased by connecting such uints, and the amount of water to be treated is increased by series connection of the units.

CONSTITUTION: A single unit consists of a hemisphere 1 with an internal barrier plate 10 having passage 11 at the bottom. Sewage water passed as shown by the route 4 is subject to aeration at the zone 2 and stayed in the zone 12 for a while. Then, the sewage water overflows to the outer wall 3 to permit microbes to act to decompose sewage components. The water is collected to the bottom wall and flows into the next hemisphere unit. The single units are connected in series to permit more chance for decomposition and increased treatment efficiency. To treat much water, the units are connected in parallel. At the bottom of the last unit, a tank is installed to receive the water, and collected water is returned by a storage tank for treatment circulation.