Forming Floc or Slime---- it all depends on your EPS (Extacellular Polymeric Substances)

Extracellular polymeric substances (EPS) is literally the glue that hold bacteria in a biofilm or floc. This glue also allows the floc to act like a sponge - adsorbing organic and inorganic insoluble compounds and forming the matrix that we call biofilm or floc (depending upon your system).

​What exactly is the EPS composition? Well it has been found to be:

• ​Polysaccharides
• Proteins
• DNA
  
• Lipids

​The exact ratio and composition of the materials determines the nature of the EPS and how strong a floc is formed. To fully understand the EPS and its role in floc formation, we need to define the types of EPS that we see in wastewater:

• Crude - seen in early growth phases, is not developed and often in free solution
• Capsular - attached tightly to the bacterial cell which forms the eventual floc
  
• Loosely bound - the EPS is weak, not forming tight bonds with bacterial cells. This is a function of charge density and EPS molecular weight.
  
• Tightly bound - EPS forms tight bonds with cells. In this case charges are a match between bacterial cells and EPS forming a dense floc.
  
• Slime - EPS has significant quantities of entrained water, grease, or fatty acids. This is the non-filamentous bulking condition.
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​What factors change the EPS formation?

• Dissolved Oxygen
  
• pH
  
• Temperature
  
• Oil & Grease - encapsulates and reduces floc density
  
• Toxic shocks - floc structure breaks down under severe shocks
  
• Nutrients - N, P - chronic low N & P can favor filaments, slime and loosely bound EPS
  
• Micro-nutrients - Fe, Ca, Mg, etc - sever lack of micronutrients can create loosely bound EPS