A batch test procedure, based on manometric measurements, was used to study the Anammox process, in particular the inhibition due to nitrite and the effects of hydroxylamine and hydrazine, indicated as possible intermediates of the process. The maximum nitrite removal rate (MNRR) was measured. The method showed good reliability with a standard error of 4.5 +/- 3.3% (n: 41). All the tests were carried out on samples taken from a pilot plant with Anammox suspended biomass. The tests were used also to monitor the reactor activity. By testing different spiked additions of nitrite (10-75 mg NO(2)(-)-N L(-1)), a short-term inhibition, with more than 25% MNRR decrease, was found at concentrations higher than 60 mg NO(2)(-)-N L(-1). Repeated additions of nitrite higher than 30 mg NO(2)(-)-N L(-1) caused losses of activity. After a complete loss of activity, spiked additions of hydroxylamine (30 mg N L(-1) in total) determined a 20% permanent recovery. Low amounts of the intermediates (1-3 mg N L(-1)) applied on partially inhibited samples and uninhibited samples produced temporary increases in activity up to 50% and 30%, respectively. (C) 2009 Elsevier Ltd. All rights reserved.