Figure 2-5. Shapes of weirs.
b. Parshall Flume. The Parshall flume is one of the most common means of
metering wastewater flow in open channels. This flume has an open and constricted
channel (see Figure 2-6) in which the difference in wastewater elevation above and
below the constriction can be accurately translated into rate of flow by means of tables.
The Parshall flume is preferred over the rate of flow by means of tables. The Parshall
flume is preferred over the weir in that it is self-cleaning, can handle a wide variation in
flow, and is useful when available the head is limited.
PRIMARY CLARIFICATION OF SEDIMENTATION
a. Process. Sediment is material carried in a fluid by reason of motion of the
fluid. The purpose of primary clarification is to separate floating and setttleable solids
from the wastewater. The objective is to clarify or settle out the sediment when the
motion of the fluid lessens or stops. When fresh domestic wastewater stands quiescent
or flows very slowly, a considerable portion of the suspended organic matter and the
solid mineral matter remaining after grit removal settles rapidly. Floating solids may be
skimmed off the top. The accumulation of nonfiltrable residues, which is called sludge,
settles to the bottom of the tank or basin and is removed from the bottom of the tank.
This sludge is still quite fluid, containing 95 percent or more of water and 4 to 5 percent
(40,000 to 50,000 ppm) of solids. Most of the residue is highly putrescible organic
materials. The sludge must be removed frequently to prevent septic conditions in the
basin and to maintain the designed basin volume. Removal of the settleable organic
sediment markedly reduces the biochemical oxygen demand of the remaining waste
stream. Treatment disposal of sludge is discussed in Section II.