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Extracellular polymeric substances (EPS) - their role in flocculation, settling rates, dewatering, and non-filamentous bulking

4/17/2017

 
i have been doing a lot or work on floc formation and how EPS fluctuates in wastewater systems. Since floc and biofilms are both microbial aggregates of living and dead microbial cells held in a matrix of extra cellular polymeric substances (EPS), I though a general guide to EPS would be useful to a lot of people faced with floc settling problems including non-filamentous or viscous bulking.

To microbes, the aggregate (colony) serves four distinct functions:
  1. Helps retain moisture needed for survival in soils
  2. Buildup of nutrients - here we are referring to organic compounds (food), nitrogen, phosphorus, and trace metals/vitamins
  3. Accumulate enzymes - extracellular enzymes are conserved by being held near the producing cell and making insoluble organics suitable for transport across the cell wall
  4. Physical barrier - this protects against toxic substances, pH swings, and other environmental factors

The EPS is composed of multiple compounds including (from highest to lowest percentage):
  1. Polysaccharides
  2. Proteins
  3. Glycoproteins
  4. Nucleic acids
  5. Phospholipids
  6. Humic acids
The percentage and types of each are determined by:
  1. Organisms present 
  2. organic and other sources of energy
  3. Nutrient concentrations
  4. Environmental conditions
A key takeaway is - EPS suitability for "good" wastewater unit performance is due to all the inputs and the microbial ecology inside the system. EPS is not a uniform substance across all WWTP and bulking is a symptom of deficiency in one or more of the inputs.

Due to the cells and the EPS having a net anionic charge, divalent cations (Ca++. Mg++) are very important to forming dense, stable aggregates. In most wastewater having 14 - 40 mg/L Ca++, and 8 - 24 mg/L Mg++ is sufficient. However, recent research has found that keeping a Divalent to Monovalent cation ratio greater than 0.5 is more important for good floc formation. 

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    Author

    Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater  facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.

    View my profile on LinkedIn

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