Wastewater Collection and Preliminary Treatment - Wastewater Operators - page 238

This requires observation of mixing and aeration and sampling of fixed
suspended solids. Actual grit removal is controlled by the rate of aera-
tion. If the rate is too high, all of the solids remain in suspension. If
the rate is too low, both grit and organics will settle out. The operator
observes the same kinds of conditions as those listed for the gravity-/
velocity-controlled system but must also pay close attention to the air
distribution system to ensure proper operation.
5.3.3.3 Centrifugal Force
The cyclone degritter uses a rapid spinning motion (centrifugal
force) to separate the heavy inorganic solids or grit from the light organic
solids. This unit process is normally used on primary sludge rather than
the entire wastewater flow. The critical control factor for the process is
the inlet pressure. If the pressure exceeds the recommendations of the
manufacturer, the unit will flood, and grit will carry through with the
flow. Grit is separated from flow, washed, and discharged directly to a
storage container.
Grit removal performance is determined by calculating the percent
removal for inorganic (fixed) suspended solids. The operator observes the
same kinds of conditions listed for the gravity/velocity-controlled and
aerated grit removal systems, with the exception of the air distribution
system. Typical problems associated with grit removal include mechan-
ical malfunctions and rotten egg odor in the grit chamber (hydrogen
sulfide formation), which can lead to metal and concrete corrosion prob-
lems. Low recovery rate of grit is another typical problem. Bottom scour,
overaeration, or not enough detention time normally causes this. When
these problems occur, the operator must make the required adjustments
or repairs.
5.3.3.4 Preaeration
In the preaeration process (diffused or mechanical), we aerate
wastewater to achieve and maintain an aerobic state (to freshen septic
wastes), to strip off hydrogen sulfide (to reduce odors and corrosion), to
agitate solids (to release trapped gases and improve solids separation
and settling), and to reduce BOD 5 . All of this can be accomplished by
aerating the wastewater for 10 to 30 minutes. To reduce BOD 5 , preaera-
tion must be conducted for 45 to 60 minutes.
5.3.3.4.1 operational Considerations
In preaeration grit removal systems, the operator is concerned with
maintaining proper operation and must be alert to any possible mechan-
ical problems. In addition, the operator monitors dissolved oxygen levels
and the impact of preaeration on influent.