Why is sewage mixed with air?

Sanitary sewage is treated in plants that mix the raw sewage with bacteria to consume the waste materials. The nature of these bacteria is that they need oxygen to live and grow (they are aerobic bacteria). The plant equipment mixes air into the sewage to increase the transfer of oxygen to the water at much higher rates than quiescent conditions. A an added advantage this mixes the bacteria and sewage so that the bacterial population is much higher and the treatment much faster. The "full" bacteria are separated from the clean water which is discharged to surface watercourses.