Figure 2-4. Grit chamber.
b. Preaeration Tanks.
(1) Preparation. At some Army installations, air is sometimes forced into
the wastewater in one of several ways before the wastewater undergoes primary
sedimentation (para 2-5). Specially designed preaeration tanks also serve as grit
chambers. Preaeration is done when it is necessary to keep wastewater fresh or to
freshen it up by adding air (oxygen). Much of the oxygen content of the air forced into
the wastewater dissolves in the wastewater. This dissolved oxygen (oxygenation)
keeps the wastewater from becoming stale. The rolling action of the aeration tank
causes the particles to collide many times and thus scour (clean) the grit.
(2) Disadvantage of preaeration. Grease, which usually settles with the
solids during the primary sedimentation process, rises to the top of preaerated
wastewater. Removal of grease by preaeration helps make secondary treatment more
effective, but the extra labor and power necessary for disposal of the large quantity of
grease is not considered justified at Army installations.
c. Prechlorination. Chlorine is sometimes added at the plant inlet for special
purposes. This chlorination is termed prechlorination. A residual is not normally
(1) Low flows. When flows are below rated plant design and detention
periods in primary-settling tanks are excessive, chlorine may be added at the plant inlet
to keep wastewater fresh and prevent odors.