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A nitrification study was conducted in continuous flow stirred tank reactors using high strength nitrogenous wastewater (concentrated stream from a urea plant mixed with pharmaceutical wastewater). The reactors were operated at different solids retention times (SRT = 10–62.5 d) and hydraulic retention times (HRT = 1.5–2.1 d). Pharmaceutical wastewater was used as an organic carbon source to maintain a COD/TKN ratio of 1. The nitrification achieved at different SRTs varied from 87 to 99%. The nitrogen balance data show that ammonia assimilation and denitrification accounted for 4–53% of the total nitrogen removed. The yield coefficients and decay coefficients were Y_b = 0.5 (COD basis), k_db = 0.07 d⁻¹ (COD basis) for heterotrophs and Y_n = 0.15 (TKN basis), k_dn = 0.06 d⁻¹ (TKN basis) for nitrifiers respectively.