Removing biological solids and insoluble compounds requires microbes to form floc or biofilms. Many desirable wastewater microbes prefer to exist in a biofilm matrix. Extracellular Polymer Substances (EPS) are the glue in biofilm and floc. Consisting of polysaccharides, proteins, DNA, and humic substances, EPS can be either attached to the cell (capsular) or free in solution (non-capsular). In typical floc, EPS constitutes 50 - 90% of total organic matter.
The Zooglea family of microbes was one of the first wastewater organisms associated with floc formation. While not the only producer of floc forming EPS, the Zooglea family are very common in most wastewater plants and can be the cause of non-filamentous bulking, difficult to dewater biological solids, and difficult to settle floc.
Why biofilm and floc forms:
For keeping biological solids under control, you need EPS to be present in quantities that allow for binding biological solids. It is also better to have capsular EPS - attached to the cells - over free, unattached EPS. In free solution, the EPS acts much like agar or gelatin. Increasing viscosity, entrapping water, and creating the gelatinous matrix we associate with non-filamentous bulking. So what triggers the excess non-capsular EPS production?
The Zooglea family of microbes was one of the first wastewater organisms associated with floc formation. While not the only producer of floc forming EPS, the Zooglea family are very common in most wastewater plants and can be the cause of non-filamentous bulking, difficult to dewater biological solids, and difficult to settle floc.
Why biofilm and floc forms:
- More consistent environmental conditions - protection from toxic compounds, pH swings, and predatory protozoa
- Allows syntrophic organisms to function efficiently, or one microbes' waste is another microbe's food source
- Helps to bring insoluble compounds close to microbes where extracellular enzymes can function
- Promotes concentration of nutrients - nitrogen, phosphorus, and micronutrients
For keeping biological solids under control, you need EPS to be present in quantities that allow for binding biological solids. It is also better to have capsular EPS - attached to the cells - over free, unattached EPS. In free solution, the EPS acts much like agar or gelatin. Increasing viscosity, entrapping water, and creating the gelatinous matrix we associate with non-filamentous bulking. So what triggers the excess non-capsular EPS production?
- Low nutrients - usually nitrogen and phosphorous - when microbes face nutrient related stress, they attempt to concentrate/and capture vital nutrients in their EPS matrix.
- High levels of soluble organics - microbes use the EPS to accumulate soluble organics which can lead to excess EPS over extended periods of high organic loading
- Insoluble compounds such as grease, oil, and long chain fatty acids - while usually associated with Nocardia form growth, grease can also result in non-capsular EPS production
- Environmental stress - extreme temperatures, pH, and inhibitory influent organics can stress microbes which resort to producing more EPS for protection.
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