Figure 2-6. Parshall flume.
b. Detention Time and Period. The detention time is the time required to fill
the tank with wastewater at a given flow or the time required for wastewater to pass
through a tank. The detention period is the time required to displace the contents of a
tank or unit at a given rate of discharge. The normal detention time for average daily
flow should be 2.5 hours. Under average conditions, most of the settling in primary
sedimentation tanks occurs within one hour. The quantity and rate of sedimentation
after 2 hours is almost negligible. A 2-hour primary clarification period will usually effect
removal of 50 to 60 percent of the suspended matter and 20 to 40 percent of the total
BOD. A detention period of 2 to 3 hours in an Imhoff tank will reduce nonfiltrable
residue by 45 to 60 percent. With the addition of coagulants (aluminum sulfate, ferrous
sulfate, ferric chloride, and ferric sulfate, plus lime for pH adjustment) which are
commonly used in water treatment processes, as much as 75 to 85 percent of the
suspended matter may be removed. This process will also remove 50 to 70 percent of
the total BOD.
c. Primary Clarification Tanks. Different shaped clarification tanks or basins
are frequently employed for holding wastewater until primary settling can take place.
Three designs are common -- round, rectangular, or square. The round and
rectangular sedimentation tanks are used more frequently than the square one.