How are grease and fats typically removed from sewage?

Grease and fats are typically removed from sewage through the process of skimming in sedimentation tanks. This method is effective because fats and grease are less dense than water, allowing them to float on the surface of the liquid. During the sedimentation process, the liquid settles, and the floating material can then be easily skimmed off. This helps in reducing the amount of fats and grease that enter the subsequent treatment processes, preventing issues such as clogging and reduced efficiency in later stages of treatment.

The other methods listed do not effectively target the removal of grease and fats. Pumping from the sump in the grit chamber primarily removes heavier inorganic materials rather than lighter greases. Decanting in the digestion tank is more focused on separating solids from liquid after digestion has occurred, and backwashing fine screens is intended to clear out debris from the screening process, not specifically grease. Therefore, skimming in sedimentation tanks is the most suitable method for effectively removing fats and grease from sewage.