Autotrophic nitrogen removal from ammonium-rich wastewaters by partial nitrification-anaerobic ammonia oxidation (PN-ANAMMOX) offers advantages such as energy and resources saving compared with conventional treatment. In view of substrate fluctuations in real applications, performance stability of PN-ANAMMOX was investigated in a new lab-scale internal-loop airlift bio-particle (ILAB) reactor. The response of steady-state reactor to substrate concentration shocks was evaluated at 439.49 mg N/L (shock A) and 489.49 mg N/L (shock B) for 0.5 hydraulic retention time (HRT) and 1.0 HRT respectively. Higher substrate concentrations such as 613.09 mg N/L (shock C) and 629.00 mg N/L (shock D) were also tested for 0.5 HRT and 1.0 HRT respectively. The intensity and duration of shocks (A-D) proportionately affected the disturbance, inertial and recovery phase of the reactor. However, the reactor showed resilience to substrate concentration shocks by tolerating higher pH and maintaining its primary functions in shocks. The inhibition of bio-reaction triggered by substrate concentration shocks was totally reversible and performance indicators returned to steady-state level soon after the end of shocks. The reactor demonstrated full recovery from all shocks with respective recovery time of 13 h, 18 h, 19 h and 23 h and it was capable of recovering 100% even from most disturbing shock D (629.00 mg N/L; 1.0 HRT). The effluent ammonium was sensitive parameter which shot up to 20.5 times the steady-state value and it could be used to monitor the reactor performance. The study results revealed that PN-ANAMMOX was robust and it was not prone to temporary adverse effects of substrate concentration shocks. (C) 2014 Elsevier B.V. All rights reserved.