Abstract: The present invention relates to a process and a treatment system for biologically removing phosphorus and nitrogen from wastewater. In the anaerobic contact and settlement zone 1, wastewater 7 is first mixed with the returned activated sludge 8 to form a mixed liquor which is then allowed to settle in zone 1, and organic matters present in influent are mostly taken up by the sludge. The supernatant from zone 1 is passed through an aerobic reaction zone 2, furnished with packings, for complete nitrification; the settled sludge from zone 1 is hold in an anaerobic zone 3 for a sufficient period of time for controlling the sludge carbohydrate content to low levels in order to enlarge the sludge capacity for phosphorus removal because the extended anaerobic sludge degradation in zone 3 is very effective in controlling the sludge carbohydrate content to low levels.