NOVELTY - A sewage in situ purification system comprises a drainage ditch which is separated into treatment ditch and diverging ditch (7) in parallel by longitudinal separating wall. A diverging trough (1) for controlling flow rate flowed into treatment ditch and leading sewage that flows in to the diverging ditch, is arranged at the front end of treatment ditch and diverging ditch. The treatment ditch is provided with a sand settling tank (2) at its front end, a depositing tank at its rear end (6), a contact oxidation tank and an artificial marsh tank (5) between sand settling tank and depositing tank.    USE - System for sewage in situ purification.    ADVANTAGE - The system not only can make an effective use of ditch space to treat and purify sewage in situ for realizing the continuous treatment and purification, but also can make the ditch become a harmonious landscape with surroundings. The system can also oxygenize the water and reduce sewage treatment cost by utilizing the height differences between every part of ditch. It produces no secondary pollution or has no influence to the flood discharge of ditch, and can effectively intercept and remove high-concentration pollutant caused by scouring effect of rain in the early period.    DETAILED DESCRIPTION - A sewage in situ purification system comprises a drainage ditch which is separated into treatment ditch and diverging ditch (7) in parallel by longitudinal separating wall along the water flow (15). A diverging trough (1) for controlling the flow rate flowed into treatment ditch and leading the sewage that flows in to the diverging ditch, is arranged at the front end of treatment ditch and diverging ditch. The treatment ditch is provided with a sand settling tank (2) at its front end, a depositing tank at its rear end (6), a contact oxidation tank and an artificial marsh tank (5) between the sand settling tank and the depositing tank. The sand settling tank, the contact oxidation tank, the depositing tank and the artificial marsh tank are separated by transverse separating wall.    DESCRIPTION OF DRAWING(S) - The drawing is schematic view of sewage in situ purification system.    Diverging trough (1)    Sand settling tank (2)    Artificial marsh tank (5)    Rear end (6)    Diverging ditch (7)    Water flow (15)