However, an unsuspected instability or sudden failure of this biological treatment process, particularly NH4+-N removal, can occur for unknown reasons in full-scale coke wastewater treatment plants. A major cause may be that the microbial PD168393 in the sludge is susceptible to toxic or refractory pollutants (Kim et al., 2009). For instance, the presence of toxic chemicals such as phenol, p-cresol, cyanide (CN−), and thiocyanate (SCN−) in the industrial effluents from mining, electroplating, coke, or steel-producing process can inhibit microbial activity, particularly nitrifying- and denitrifying-microorganisms in activated sludge. This can lead to a notable deterioration in the efficacy of NH4+-N removal ( Kim et al., 2011b, Kim et al., 2008b and Zhu et al., 2009). Besides, elevated levels of NH4+-N may occur via biological oxidation of carbon–nitrogen compounds, such as SCN−, which are common in coke wastewater ( Lim et al., 2008). Because of these problems, a typical sludge-based process for treating coke effluent often fails to satisfy the strict requirements regarding the level of NH4+-N in discharge ( Kim et al., 2008b, Kim et al., 2007 and Zhu et al., 2009). Therefore, the complete remediation of residual nitrogen is needed after an activated sludge process for better treatment of coke wastewater ( Zhu et al., 2009).