Partial nitritation using inhibition of free ammonia and free nitric acid is an effective technique for the treatment of high concentrations of ammonium in wastewaters. This technique was applied to the digester liquor of swine wastewater and the stability of its longterm operation was investigated. Partial nitritation was successfully maintained at a nitrogen loading rate (NLR) of 1.0 kg N m(-3) d(-1) for 120 days without acclimatization of nitrite oxidizing bacteria (NOB) to the inhibitory compounds (free ammonia and free nitric acid). The conversion efficiencies of NH4-N to NO2-N and to NO3-N were determined to be around 58% and < 5%,), respectively. After the establishment of partial nitritation, the influence of swine wastewater on the Anammox reaction was examined using continuous flow treatment experiments. Consistent nitrogen removal was achieved for 70 days at a nitrogen removal rate (NRR) of 0.22 kg N m(-3) d(-1) and the color of Anammox bacteria changed from red to greyish black. The NO2-N consumption and the NO3-N production increased concurrently and the Anammox reaction ratio was estimated to be 1:1.67:0.53, which is different from that reported previously (1:1.32:0.26). (c) 2007 Elsevier Ltd. All rights reserved.