the water to hold particles in suspension. As the water flows through a sedimentation
basin within a tank, at an extremely slow velocity, suspended material settles toward the
bottom of the basin. The water passing out of the tank is relatively clear. Correct basin
design keeps the water from flowing directly from inlet to outlet. Additional baffles are
sometimes needed to control the flow. Best results are obtained when flow is evenly
distributed, properly baffled, and enters the basin at about 1 foot per minute velocity.
The process normally takes between 3 and 6 hours. Sedimentation basins may be
separate units or they may be combined with flocculators. The latter is shown in
Figure 1-5. Combined flocculator and sedimentation tank.
e. Solids Contact Process. The solids contact process incorporates chemical
mixing, flocculation, and sedimentation in a single tank referred to as an upflow clarifier
(see figure 1-6). In the military version of the up flow clarifier, the process also includes
disinfection (chlorination). A brief description of the process follows: The raw water
enters through the mixing zone where it is mixed with coagulation chemicals and rotated
at high speed by motor driven disks (impellers). The water flows downward through
mixing zone, forming floc that absorbs colors and traps other impurities as it settles.
The water passes through openings at the bottom of the mixing zone into the
clarification zone where a counter rotary motion is imparted to the coagulated water and
the speed of rotation reduced to a gentle agitation. As the coagulated water rises in the
clarification zone, the floc (or slurry) remains in suspension so that the water at the top
of the clarifier (separator zone) is clear and relatively free of floc. The clarified water
flows from the effluent launder into the wet well tank from which it is pumped to fiIters
for final treatment. The level of slurry in the clarifying chamber is maintained by the
continuous withdrawal of slurry through a draw-off port into a sludge concentrator, which
also acts as a small auxiliary clarifier. The solids contact process provides for retention
periods of 1 to 2 hours.