The Aerators switch off and the effluent becomes still. 

Having no oxygen supply, the microorganisms use carbon in the organic matter as a food source, converting nitrates to nitrogen gas which is released to the atmosphere (“Denitrification”).

During this phase the solids separate and settle from the liquid giving clarification to the upper part of the tank.

Alum is added to help remove phosphorus. Phosphorus salts sink to the bottom of the tank along with other solids.