sewage-treatment (sewage system) process in which sludge, the accumulated, bacteria-rich deposits of settling tanks and basins, is seeded into incoming waste water and the mixture agitated for several hours in the presence of an ample air supply. Suspended solids and many organic solids are absorbed or adsorbed by the sludge, while organic matter is oxidized by the microorganisms. The amounts of air and sludge used can be varied to control the level of treatment obtained. The sludge is then separated out in a settling tank.