In order to prevent the toxic effect caused by high strength ammonium in a swine wastewater treatment system, a patented upflow multi-layer bioreactor (UMBR) as a pre-anoxic tank was applied to a pilot-scale plant with a treatment capacity of 5 m(3)/d. This plant was operated for 4 months at a high IR ratio in the range of 10-17, in order to alleviate the toxic effects caused by high strength ammonium. A computational fluid dynamic (CFD) analysis was also conducted to design and configure the rotating distributors in the UMBR. At an IR ratio of about 17, the influent NH4+-N (1169 mg N/L) was diluted to less than 80 mg N/L at the head of the UMBR, and then was completely nitrified (about 98.3%) in the aeration tank, without any inhibition caused by high strength ammonium. The nitrate at a concentration of about 58.2 mg N/L recycled from the aeration tank was completely denitrified in UMBR #1, which was operated at an actual hydraulic retention time (HRT) of 3.5 h. (C) 2007 Elsevier Ltd. All rights reserved.