A three-phase draft-tube fluidized-bed bioreactor packed with granular activated carbon (GAC) was employed to start up a nitrification process for high-strength ammonium (500 mg/l as N) wastewater. Supplement of sucrose into the ammonium wastewater was expected to enhance the attachment of nitrifier biofilm on GAC medium, and also to promote a high rate of ammonium conversion to predominant nitrite, i.e. nitrification process. In this study, various concentrations of sucrose provided various COD/NH4+-N ratios (0 to 1) to the fluidized bed bioreactor with an extremely high loading of 2.0 kg NH4+-N/m3-day and a limited air supply at the dissolved oxygen level of 3.0 mg/l. The results clearly indicated that the addition of organic carbon would inhibit the bioactivity of ammonium oxidizer (Nitrosomonas sp.) and nitrite oxidizer (Nitrobacter sp.). When the COD/NH4+-N ratio was increased to 0.33, the applied sludge loading to the nitrite oxidizer was greater than the specific substrate utilization rate, so the intermediate product, nitrite was accumulated without further nitratification. While the COD/NH4+-N ratio was increased to 1.0, a significant nitrite accumulation above 95% was achieved with the high ammonium feeding concentration of 500 mg/l NH4+-N. Meantime, the addition of sucrose in the start-up stage promoted heterotrophic growth and biofilm attachment on GAC surface. The filamentous structure of heterotrophs provided a biofilm surface on which the discrete nitrifying bacteria could be easily entrapped. In this fluidized-bed reactor, a large amount of entrapped nitrifier and a high efficiency of oxygen transfer rate had contributed to a high removal loading rate of ammonium with short hydraulic retention time of 6 hours.

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