Stability of sludge flocs under shear conditions: roles of extracellular polymeric substances (EPS)

Biotechnol Bioeng. 2006 Apr 20;93(6):1095-102. doi: 10.1002/bit.20819.


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 floc stability. The sludge flocs 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 flocs, 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 flocs and 15 mg/g SS for the anaerobic flocs, 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 flocs and 2.66:1 for the anaerobic flocs, also appeared to be an important factor governing the microbial floc 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 flocs. The total content of the EPS, however, did not show a direct correlation with the floc stability. A hypothesis about biological flocs 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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Adhesion
  • Biodegradation, Environmental
  • Biopolymers / analysis
  • Biopolymers / chemistry*
  • Bioreactors / microbiology*
  • Edetic Acid / chemistry
  • Flocculation
  • Hydrogen-Ion Concentration
  • Polysaccharides / analysis
  • Polysaccharides / chemistry
  • Proteins / analysis
  • Proteins / chemistry
  • Rheology
  • Sewage / chemistry*
  • Sewage / microbiology*
  • Thermodynamics


  • Biopolymers
  • Polysaccharides
  • Proteins
  • Sewage
  • Edetic Acid