Stability of sludge flocs under shear conditions: roles of extracellular polymeric substances (EPS).
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.
Sheng
Guo-Ping
GP
School of Chemistry, University of Science & Technology of China, Hefei, Anhui, 230026 China.
Yu
Han-Qing
HQ
Li
Xiao-Yan
XY
D001422:Bacterial Adhesion; D001673:Biodegradation, Environmental; D001704:Biopolymers / Q000032:analysis / Q000737:chemistry*; D019149:Bioreactors / Q000382:microbiology*; D004492:Edetic Acid / Q000737:chemistry; D005427:Flocculation; D006863:Hydrogen-Ion Concentration; D011134:Polysaccharides / Q000032:analysis / Q000737:chemistry; D011506:Proteins / Q000032:analysis / Q000737:chemistry; D012212:Rheology; D012722:Sewage / Q000737:chemistry* / Q000382:microbiology*; D013816:Thermodynamics
D001704:Biopolymers; D011134:Polysaccharides; D011506:Proteins; D012722:Sewage; D004492:Edetic Acid