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Biokinetic coefficients and efficiency for three activated sludge processes including conventional, extended aeration, and contact stabilization were determined in pilot-scale for six months in Isfahan south municipal wastewater treatment plant. These systems were operated under two different MLSS concentrations in aeration tank. For each MLSS, five periods were considered in base of flow rate and sludge retention time (SRT). The samples from the influent wastewater, reactor and effluent were collected periodically and experimented by the Standard Methods. The data were analyzed by the Excel and SPSS softwares. The investigation showed that the yield coefficient (Y), decay coefficient (kd), maximum specific growth rate and saturation constant (Ks) for conventional activated sludge process were in the range of 0.48-0.8 mgVSS/mg sCOD, 0.0189-0.026 1/day, 0.95-0.98 1/day and 52-71 mg sCOD/L, extended aeration: 0.6174-1.2512 mgVSS/mg sCOD0. 0198-0.0309، 1/day 1.96-3.17، 1/day and 311.7-508 mg sCOD/L and for contact stabilization: 0.6322-0.713 mgVSS/mg sCOD1 0.0172-0.0387،/d0. 23-1 0.42/d and13. 8-50.8 mg sCOD/L, respectively. In the conventional and contact stabilization processes values of the coefficients were within the range of those reported in the literature. However, in the extended aeration process, values of Ks and Y in MLSS of 5000mg/L were out of ranges. The sensitivity analysis of changes in the biokinetic coefficients showed direct relationship of kd and Ks with the concentration of the effluent substrate. Whereas μmax is inversely related to concentration of the effluent substrate. In addition, regardless of …