S::Select ® - the ideal biological activated sludge system

The problem of bulking sludge

The activated sludge process is being utilized in the great majority of biological wastewater treatment plants (>= 90%). The settling properties of the activated sludge are therefore crucial for the design and function of the activated sludge plants.

The problem of bulking sludge surfaced once biological nitrogen removal requirements were instated in the 1980s and SRTs rose correspondingly along with massive enlargement of the plants. Today, approximately 65% of all wastewater treatment plants suffer at least temporarily from a progressing deterioration of sludge settling and bulking sludge.

Bulking sludge is the excessive development of filamentous organisms. The filamentous organisms cause a very voluminous sludge which has correspondingly poor settlement properties. As a consequence, the bioreactors must be dimensioned even larger and the subsequent operational issues are enormous.

The cause of the growth of filamentous organisms is primarily due to the significantly lower nutrient availability as a result of the much larger basin volumes. With their specifically larger surface area, the filamentous organisms have a much better access to the remaining substrate. Therefore, they are able to establish sufficiently faster as compared to the "normal" organisms. The result is bulking sludge.

The requirement for nitrogen removal is essential. At the same time, there is a strong desire for well settling activated sludge. Thus, it seems that there is an unsolvable problem.

The ideal biology

The trade-off is achieved by turning the structure of the activated sludge from floccular to granular sludge. The slower growing bacteria, especially the nitrifying bacteria, settle in these granules. However, they will soon be overgrown by fast growing heterotrophic bacteria. In order for the nitrifying bacteria to permanently maintain good access to the substrate (NH4) and oxygen, the granules are being shaved regularly. This takes place during the passage through the specifically designed hydrocyclone.
The shaved sludge is discharged as waste sludge (WAS). With the operation of the selector, in which a series of cyclones are combined, the granules are washed and the suspended sludge is withdrawn.

Thus, 2 completely different SRTs can be set within a biological system:
  • a short flock SRT that has a very high biological activity (C-removal, denitrification) while showing no trend in the formation of bulking sludge (for example, 3 days)
  • a high SRT of the biomass attached to the granules and as a result provides a high nitrification capacity (for example, 10 days).

The ideal biology is therefore created by combining two SRTs.

The advantages are:
  • high specific biological activity with respect to nitrification and C-removal
  • securely performing nitrification, even at low temperatures
  • high denitrification potential, superior recovery of alkalinity and energy
  • reduced level of stabilization and therefore high gas yield
  • small footprint
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