The role of two types of carrier in a continuous activated sludge (AS) reactor-powdered zeolite (AS+Z) and powdered activated carbon (AS+PAC)-for nitrification was evaluated under high organic loading, with addition of zinc and phenol. When the organics concentration of the influent was increased from 240 mg/l of COD to 360 mg/l, increasing the mass ratio of COD to total nitrogen (C/N ratio) to 5 decreased the nitrification efficiencies of AS+Z and AS+PAC from 62.5 and 64.2% to about 5%. However, considerable recovery of nitrification efficiency was observed in AS+Z during much shorter periods than AS+PAC. When 10 mg/l of zinc was fed into AS+Z and AS+PAC, the nitrification efficiencies severely deteriorated to about 2.9 and 3.1%. The suppressed nitrification efficiency in AS+Z slowly recovered to reach 43.23 mg/l NO3--N concentration (61.8% of nitrification efficiency) in days after stopping zinc addition. In contrast, AS+PAC remained at 1.54 mg/l (2.2% of nitrification efficiency). The retrieval rates of nitrification efficiency in both reactors were accelerated when sludge retention times (SRT) of AS+Z and AS+ PAC were increased. The influence of phenol on nitrification was also evaluated. When 400 mg/l of phenol was added, the effluent concentrations of NO3--N in AS+Z and AS+PAC sharply decreased from 69.24 and 51.24 mg/l to 1.89 and 1.51 mg/l, respectively, in only 14 days. After lowering the influent concentration of phenol to 60 mg/l, nitrification efficiencies were gradually recovered. Adsorption capacities for both carriers were also estimated under the same conditions in continuous bioreactors. The adsorption capacity of zeolite and activated carbon (AC) was important in improving nitrification in both reactors. Enhanced nitrification efficiency in AS+Z was accomplished by the attached growth of nitrifier on-the-surface of carriers because zeolite has a superior ammonium adsorption capacity. (C) 2003 Elsevier Science Ltd. All rights reserved.