Physical-chemical treatment of wastewater focuses primarily on the separation of colloidal particles in the wastewater with the introduction of chemicals to change the physical state of the colloids, turning them into particles or flocs with settling properties. The settled material or primary sludge is sent for sludge treatment.

Fats, oil, grease and suspended solids are removed by means of flotation, and is skimmed off at the top of the tank.