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Processes
Grit Removal
Grit
is a problem in all combined sewer systems. A majority of the equipment
maintenance and operational problems at the treatment plant can be associated
with excess grit being transported through the system. Studies were done
to evaluate various alternative grit collection technologies for use at the
plant.
HDR Engineering, Inc.
recommended that WWMA consider the use of a vortex separator for grit removal at
the plant. The most significant reason for poor grit removal is variable
flow, especially during high flow periods. The vortex system is designed
to maintain a relatively constant design flow throughout the process regardless
of flow variation. This allows the system to be operated at constant and
efficient rates which, in turn, results in a higher level of grit removal.
Studies have shown that grit systems that do not have flow control are 70% less
effective at removing grit and separating organics from incoming wastewater
during peak or highly variable flows. A grit concentrator was recommended
for additional separation of organics that may not have been separated during
the initial vortex process. Separated organics are returned to the head of
the vortex system and allowed to cycle through again without being removed with
the grit. This is extremely helpful in assuring a much higher level of
organics separation from the discharged grit.
Raw sewage flow that
enters the plant is initially pumped up to the vortex grit tank. This
mechanism uses a cyclonic swirling action that separates material such as sand,
small rocks, dirt, and other heavy particles from the raw water waste
flow. The flow then exits the grit tank and is transported to the primary
diversion structure. Grit removal early in the process is very important
because grit wears down the plant equipment and can accumulate in the tanks and
reduce treatment efficiency.
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