The roles of extracellular polymer substances (EPS) in the shear stability of aerobic and anaerobic flocs were investigated. Both pH and EDTA concentration had a significant effect on the floe stability. The sludge floes became much weaker as the solution pH increase to above 10. Addition of 1 mM EDTA or more could cause considerable cell erosion and deflocculation of the anaerobic floes, whereas more than 3 mM EDTA was needed to show its adverse effect on the stability of aerobic flocs. A fraction of the EPS, around 10 mg/g SS for the aerobic floes and 15 mg/g SS for the anaerobic floes, could be extracted by fluid shear when the dispersed mass concentration approached the equilibrium. This suggests that most of the dispersed particles were glued by a small amount of readily-extractable EPS fraction. In addition to the abundance of this EPS fraction, its proteins/carbohydrates ratio, about 0.22:1 for the aerobic floes and 2.66:1 for the anaerobic floes, also appeared to be an important factor governing the microbial floe stability. A lower content of the readily-extractable EPS fraction and a lower ratio of proteins/carbohydrates were responsible for the greater stability of microbial floes. The total content of the EPS, however, did not show a direct correlation with the floe stability. A hypothesis about biological floes with two distinct structural regions was proposed. The outer part contained dispersible cells loosely entangled by the readily-extractable EPS fraction. This part was layered and would become completely dispersed at an infinite shear intensity. On the other hand, the inner part contains biomass in a stable structure tightly glued by EPS, which could not be dispersed by shear except under unfavorable conditions. (c) 2006 Wiley Periodicals, Inc.