Simultaneous partial nitrification, anammox and denitrification (SNAD) process was developed in a sequential batch reactor (SBR) and the influence of hydraulic retention time (HRT) on the SNAD process was investigated. Around 96% NH(4)(+)-N removal and 87% COD removal were observed at 9 d HRT. Marginal decreases in the removal efficiencies were observed when the HRT was reduced to 3 d or the loading rate was increased by three times. On the other hand, a drastic decrease in NH(4)(+)-N and COD removals were observed when the DO, pH and temperature were dropped shockingly. The response of the SNAD system towards the shock in substrate loading and operating conditions was evaluated by sensitivity index. Finally, the extent of total nitrogen (TN) removal by partial nitrification with anammox and denitrification was modeled using stoichiometric relationship. Modeling results indicated a TN removal of 85-87% by anammox with partial nitrification and 7-9% by denitrification. (C) 2010 Elsevier Ltd. All rights reserved.