To reduce the rising time of a sewage treatment apparatus and system by promptly making a sponge carrier supplied into an aeration tank compatible with sewage, and absorbing water into the carrier to make the carrier floatable in the aeration tank; to make the treatment capacity of the sponge carrier high even at the rise time by fully exhibiting its function immediately after its supply; to maintain the treatment capacity at a high level even when replacing the carrier.

In a treatment system for treating organic sewage using the activated sludge process, a sponge carrier impregnated with a hydrophilicity improving agent, microorganisms, and nutritive ingredients is used when another sponge carrier for supporting microorganisms is newly supplied to the aeration tank.