The discharge of nitrogen is increasingly restricted. The autotrophic nitrogen removal process, which combines partial nitritation and anaerobic ammonium oxidation (Anammox), is recommended for high ammonium loaded streams with no or low organic carbon content because of the lower operational costs. However, the cultivation and start-up period can be long and time-consuming due to the slow growth rate of Anammox bacteria. The aim of this research was to evaluate the feasibility of Anammox cultivation in an anaerobic continuous stirred tank reactor (CSTR). Monitoring of nitrogen conversion showed Anammox activity with an increasing nitrogen removal rate up to 700 mg N L-1 day(-1) in a reactor inoculated with OLAND (Oxygen-Limited Autotrophic Nitrification/Denitrification) seed sludge. A second lab scale reactor, a modified CSTR which was operated as an anaerobic upflow fixed bed reactor (UFBR), was inoculated with nitrifying and denitrifying sludge from a manure treatment plant. The fixed bed consisted of polyurethane sponge which served as a carrier material for the slowly growing biomass. The UFBR showed removal according to the Anammox metabolism within 3 months and a nitrogen removal rate up to 250 mg N L-1 day(-1) was achieved. Primary research on the treatment on pig manure wastewater indicates that organic load had an inhibitory effect on the Anammox activity. The results also indicate that Anammox bacteria can stay dormant or hidden for longer periods, but will reactivate under the suitable conditions. (c) 2012 Elsevier B.V. All rights reserved.