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

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.