(1) Round or circular tanks (see Figure 2-7). The tanks are usually made
of concrete, but can be made of metal. These tanks usually have mechanical
conveyors. The sweeping or skimming device on the surface moves floating solids and
scum to a holding tank or well for further treatment or disposal. The skimmer must be
adjusted to a level position so that just enough emersion is allowed to remove all
floating solids without removing too much water. The surface baffles (deflector vanes)
prevent the discharge of floating solids from going over the weirs. The skimming device
is attached to the same shaft that drives the sludge collection mechanism. These tanks
usually have rotating scraping mechanisms which continuously force the sludge to a
sump near the center of the bottom of the tank. As wastewater enters through a center
pipe, the solids settle out and the sludge collecting device mechanism (scrappers and
plows) removes sludge from the slopping perimeter to the sloping floor of the tank.
Figure 2-7. Circular primary settling tank with powered sludge scraping and scum
(2) Rectangular tanks (see Figures 2-8, 2-9, and 2-10). Except for the
dimensions and location of the unit, these tanks serve the same purpose and have
about the same component parts as the round tanks. However, instead of circular
motion removal, mechanical conveyor or rail motion is used. Floating solids are
skimmed into a scum trough by the discharge end. A baffle prevents the floating solids
from going over the weirs. Sludge is normally collected in a hopper at the bottom of the
tank. For most rectangular clarifiers, surface skimming and sludge collecting are
accomplished with chain-and-flight type devices or scrappers for larger installations.
Two or more endless chains are installed crosswise from the clarifier.
(3) Imhoff tank. The Imhoff tank is a type of combination primary settling
tank and sludge digester used at some Army installations. It is a two-story structure