Which activated sludge process does not achieve complete nitrification?

The contact stabilization process is a type of activated sludge treatment that focuses on the rapid removal of organic matter and is designed primarily for the efficient removal of pollutants rather than fostering conditions conducive to complete nitrification.

During this process, influent wastewater is mixed with returned activated sludge in a contact chamber where it experiences a high level of oxygen and mixing, but this does not provide sufficient time for the microbes to convert ammonia to nitrate and then to nitrogen gas fully. The design of the contact stabilization process involves a relatively short aeration phase followed by a settling phase, which limits the extent of nitrification that can occur.

In contrast, processes such as extended aeration, sequential batch reactors (SBR), and oxidation ditches provide longer aeration times along with stage management that enhances conditions for the growth of nitrifying bacteria. These systems can effectively achieve complete nitrification, converting ammonia into nitrates, which is not the case with the contact stabilization method.