Activated sludge consists essentially of bacteria and unicellular organisms. Protozoa feed on bacteria. Unicellular organisms give valuable information on the state of the biological system solely because of their presence or absence in a biological system.
Crucial for the formation of sludge floc but also of biological granules is the ability of the microorganisms to secrete biological polymers, so-called extracellular polymeric substances (EPS). Without this mechanism, an aeration plant could not operate because the sludge could not be recycled and enriched.
Depending on the substrate and load situation, each treatment plant forms a very specific biological system with a specific structure of species. The sludge retention time, i.e. the average retention time of the sludge in the system is a crucial characteristic for the characterization of the biology (nitrifying, stabilizing etc.). The higher the retention of sludge, the more limited the food supply for the biomass. This can also establish species that are very specialized and that would not survive at high substrate concentration.
In urban wastewater, nitrogen and phosphorus are significantly over-represented compared to organic carbon. Therefore, organic carbon is usually the limiting factor in biomass production.


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