During the startup of a full-scale anoxic/aerobic (A/O) biological treatment plant for acrylonitrile wastewater, the removal efficiencies of NH3-N and total Kjeldahl nitrogen (TKN) were 1.29 and 0.83% on day 30, respectively. The nitrification process was almost totally inhibited, which was mainly caused by the inhibitory effects of toxic compounds. To eliminate the inhibition, cultivating the bacteria that degrade toxic compounds with patience was applied into the second startup of the biological treatment plant. After 75 days of startup, the inhibitory effects of the toxic compounds on nitrification were eliminated. The treatment plant has been operated stably for more than 3 years. During the last 100 days, the influent concentrations of chemical oxygen demand (COD), NH3-N, TKN and total cyanide (TCN) were 831–2,164, 188–516, 306–542 and 1.17–9.57 mg L−1 respectively, and the effluent concentrations were 257 ± 30.9, 3.30 ± 1.10, 31.6 ± 4.49 and 0.40 ± 0.10 mg L−1 (n = 100), respectively. Four strains of cyanide-degrading bacteria which were able to grow with cyanide as the sole carbon and nitrogen source were isolated from the full-scale biological treatment plant. They were short and rod-shaped under scanning electron microscopy (SEM) and were identified as Brevundimonas sp., Rhizobium sp., Dietzianatronolimnaea and Microbacterium sp., respectively.

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