Abstract: Threshold concentration(S_(min)),as the lowest pollutant(substrate) concentration that may be achieved by biodegradation,is significant in wastewater biological treatment.Major factors affecting S_(min) were studied in this paper.Lowering the substrate concentration in the feed resulted in a lower S_(min).Continuous culturing produced microorganisms capable of achieving a lower S_(min) relative to that from batch culturing.The effect of supplemental carbon on S_(min) was dependent on the sludge acclimation;it enhanced degradation rate of non-acclimated system but reduced biodegradation rate of well acclimated system.