Extracellular Polymeric Substances - the "glue" binding floc & biofilms

India Ink is used to evaluate EPS levels in floc.

As microbial cells develop into floc or biofilm, they are bound tegether by Extracellular Polymeric Substances (EPS). Consisting of a mixture of polysaccharides, proteins, enzymes, and DNA, EPS provides several benefits to the microbial community:

• Protection from the external environment including pH swings, dehydration, predators, and toxic compounds.
• Storage of excess organic compounds or particulate organics for later use by the microbes
• Accumulate and store valuable macronutrients & micronutrients such as phosphate and trace metals.

While EPS is needed for floc formation, certain conditions can promote excess EPS production which can lead to non-filamentous or Zoogleal bulking. This phenomenon is seen with high levels of soluble organic acids near the influent (EPS acts to store food). We also see Zoogleal bulking with low nutrients where EPS is used to store scarce compounds such as phosphate. EPS can also be capsular which is attached to cell walls which is beneficial in most cases. Under stress conditions such as pH shock, inhibitory compounds including heavy metals, EPS can become non-capsular and create a "gel" that protects the individual cells but creates problems with solids separation.