TM 5-814-3/AFM 88-11, Volume III
10-1. General considerations.
Preliminary treatment of wastewater includes screening, grinding, grit removal, flotation, equilization, and
flocculation. Screens, grinders and grit removal are provided for the protection of other equipment in the
treatment plant. Air flotation and flocculation aid in the removal of suspended solids and oil in the primary
clarifier and reduce the biological loading on secondary treatment processes. Prechlorination or pre-aeration
may be required to prevent odor problems and to eliminate septic conditions where wastewater has
abnormally long runs to the plant. Equalization structures are used to dampen diurnal flow variations and to
equalize flows to treatment facilities.
10-2. Bar screens.
a. Description and function. The primary function of coarse screening is protection of downstream
facilities rather than effective removal of solids from the plant influent. All screens used in sewage treatment
plants or in pumping stations may be divided into the following classifications:
(1) Trash racks, which have a clear opening between bars of 1 to 4 inches and are usually cleaned
by hand, by means of a hoist or; possibly, by a power-operated rake.
(2) Standard, mechanically cleaned bar screen with clear openings from to 1 inches (fig 10-1).