Sewage biological reaction tank

The utility model discloses a sewage biological reaction tank, which includes an activated sludge aerobic zone and a pretreatment zone, wherein a first anoxic/anaerobic tank and a second anoxic/anaerobic tank are arranged inside the pretreatment zone, and are communicated through a penetration hole in the bottom of a first partition, so as to form an anoxic/anaerobic treatment zone; an MBBR treatment tank is arranged on each side of the anoxic/anaerobic treatment zone, and only the second anoxic/anaerobic tank is communicated with the activated sludge aerobic zone through a through hole in the bottom of the corresponding second partition; internal return pipelines are arranged on the second partition corresponding to the two MBBR treatment tanks. Under the premise of unchanging the volume of the original biological tank, the sewage biological reaction tank improves the sewage treatment efficiency as original muddy water mixed liquid not sufficiently nitrified and arranged in a CAST/CASS primary reaction zone is lifted and enters the two additionally arranged MBBR treatment tanks through an internal reflux submersible sewage pump.