Abstract: A wastewater treatment process 100 comprises passing wastewater through an anaerobic reaction zone 12 so that organic matter in the wastewater is subjected to anaerobic biological degradation. Wastewater from the anaerobic reaction zone is passed through an aerobic reaction zone 18 in which organic matter in the wastewater is subjected to aerobic biochemical breakdown. A wastewater side stream 30 is withdrawn downstream of the anaerobic reaction zone and upstream of the aerobic reaction zone, and passed through a facultative oxidation zone 32, 33, 34 wherein organic matter therein is subjected to aerobic biochemical breakdown. At least a portion of the side stream is passed directly to the aerobic reaction zone. Optionally, a portion of the side stream can be returned to the anaerobic reaction zone.