Page 2 - 3. LESSON NOTE
P. 2
Sewage water has to be treated before releasing it into the rivers and seas. For this
sewage is treated in a wastewater treatment plant. The treatment is done in three
steps- Primary, Secondary and Tertiary.
Primary treatment: - primary treatment of sewage happens in three stages
Screening: the sewage is screened by passing the wastewater through
screens of vertical bars that help to remove all large objects such as cans,
rags, sticks and plastic packets that are carried in the sewage stream. The
solids are collected and later disposed into a landfill
Grit removal: grit removal is done after screening. The screened wastewater
is passed through a sand or grit chamber where the speed of the incoming
wastewater is adjusted to allow the settling of sand, grit, stones etc. these
particles are removed as they might damage pumps.
Sedimentation: After grit removal by passing through large tanks, commonly
called sedimentation tanks. The solid waste called sludge settles down while
lighter impurities (such as oil and grease) called scum float on the water
surface. The tanks gently pump out the sludge from below while the scum on
top gets skimmed away.
2
