In the sidestream (also referred to as centrate), process wastewater is biologically pretreated before it is introduced into the mainstream of the wastewater treatment plant. The process water is the liquid phase produced while dewatering the anaerobically stabilized sludge.
Typical for the sidestream are high nitrogen concentrations (typically 600 - 1500 mg N/l) along with a low content of organic ingredients (BOD5 100 - 200 mg/l) and high temperatures (> 25˚C).
The centrate can be advantageously treated in relatively small reactor volumes (often using existing, unused basins). The nitrogen load of the entire system is thus reduced by 10 - 20%. In addition, load spikes during dewatering due to introduction of centrate to the plant are avoided.
When deammonification (EssDe®) is used, centrate treatment is particularly resource-friendly, consuming only 40% of the aeration energy as compared to conventional nitrification. In addition, no carbon is needed because the deammonification accomplishes the nitrogen elimination by autotrophic means.
As an alternative to deammonification, the stripping process has occasionally been used in the past. However, this requires significantly greater resources and has practically not experienced further development since the introduction of deammonification.
The separate treatment of process water not only relieves the total plant by 10 - 20% of the incoming nitrogen load, it also improves the C:N ratio and thus enables a more efficient denitrification in the mainstream.

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