What determines whether nitrification will occur in the activated sludge process?

Nitrification in the activated sludge process is primarily influenced by several key factors, with one of the most crucial being the mean cell residence time (MCRT). MCRT is the average time that the activated sludge is retained in the treatment system. This retention time is significant because nitrifying bacteria, responsible for the conversion of ammonia to nitrate, require appropriate conditions to thrive and reproduce.

Longer MCRT times allow for more substantial growth and activity of these bacteria within the system, optimizing the nitrification process. If the MCRT is too short, it may lead to an insufficient population of nitrifying bacteria, which can severely limit or even prevent nitrification from occurring.

While factors like oxygen levels, MLSS concentration, and temperature do play a role in the effectiveness of the nitrification process—such as ensuring the presence of enough dissolved oxygen for aerobic bacteria, maintaining an adequate concentration of mixed liquor suspended solids, and being within a suitable thermal range for bacterial activity—it is the MCRT that fundamentally determines the ability of the system to sustain the necessary bacterial populations for nitrification to occur effectively.