Anammox process is emerging as a promising biotechnology to remove ammonium (NH4+) from waste-water. Co-existing organic matters such as toluene could pose potential inhibitions on this process. In this study, the mechanisms of toluene-nitrite (NO2-) inhibition synergy of anammox activity (AA) was investigated in batch culture. In results, the M (mmol N-2-N L-liquid(-1) h(-1)) in reference assays achieved values of 0.21 +/- 0.0078. The toluene (0.1-3.0 mM) demonstrated different inhibitory effects on the AA and caused 50% inhibition (IC50) at 0.79 mM. The pre-exposure to toluene and NO2- or NO2- alone in the absence of NH4+ demonstrated a much severer inhibition, compared to toluene alone. The IC50 of the AA due to pre-exposure to NO2- lacking NH4+ was of 6.25 mM NO2-. These results indicated that toluene by itself did not severely inhibit the anammox process but did so in the presence of NO2-. (C) 2013 Elsevier Ltd. All rights reserved.