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Grit Removal System Overview
Purpose
 The purpose of grit removal is to provide protection of downstream equipment from abrasive materials and to prevent the accumulation of excessive amounts of grit in downstream processes.
Theory of Operation
Screened wastewater enters the grit basin upper chamber. A set of rotating paddles in the upper basin maintains the proper velocity to keep organic material in suspension at all flow rates. The grit basin is capable of removing the following at the peak flow rate of 16 MGD:
    95% of grit greater than 50 mesh in size
    85% of grit greater than 70 but less than 50 mesh in size 
    65% of grit greater than 100 but less than 70 mesh in size

The grit moving across the bottom of the upper chamber is hydraulically scoured by the rotating paddles to keep organic material in suspension. The grit slurry is pumped from the bottom of the grit chamber through a grit concentrator (cyclone) which separates the lighter organics and excess water from the grit.  The grit from the concentrator is discharged to the grit classifier which dewaters the grit and conveys the grit to a dumpster for disposal.
Equipment
The Grit Removal system includes the following equipment: 
  • Grit vortex drives
  • Grit pumps
  • Grit pump flushing control valves
  • Grit control panel
  • Grit cyclones
  • Grit classifier
  • Influent sampler
  • Influent flow meter
Description
The two concrete grit basins are divided into an upper vortex chamber and a lower grit slurry hopper. The vortex action in the upper chamber keeps the organics in suspension while allowing the grit particles to settle to the bottom of the chamber.  A rotating paddle in the upper chamber provides a minimum velocity in the upper chamber which works in conjunction with the vortex flow pattern to keep the organic material in suspension.  The heavier grit particles settle in the lower portion of the grit chamber.  The grit slurry in the grit chamber is removed by the grit pumps, one for each chamber.  The pumps are controlled to run intermitently by the PLC in the grit control panel.  The suction line for each pump runs fom the pump to the center of the grit chamber.  Flush valves on the suction and discharge lines for each pump open to allow nonpotable water to flush the lines before and after each pumping cycle.

The discharge for each grit pump is directed to individual grit concentrators (cyclones) located directly above the grit classifier.  The grit concentrators separate the organic material from the heavier grit particles and concentrate the grit slurry by removing much of the wastewater from the grit.  The overflow from the grit concentrator is directed to the filtrate pump station and the underflow discharges to the grit classifier.  The grit classifier dewaters the grit with the overflow directed to the filtrate pump station and the dewatered grit conveyed to a dumpster.  Grit piping is painted dark brown.

The effluent from the grit chamber flows through a long throated flume for measurement of the plant influent flow.  An automatic sampler in the headworks building collects composite samples of the influent wastewater and is flow paced by the flowmeter.
 Grit Vortex Drives6
 
 A rotating paddle in the upper vortex portion of the grit basin provides a minimum velocity at all flow rates to keep the organic materials in suspension while allowing the grit particles to settle to the bottom section of the grit chamber.  The paddle is driven by the motor and gear drive unit at the top of the grit chamber.
Grit Pumps6
   Grit is removed from each grit chamber by the two grit pumps.  The torque flow pumps have specially hardened impellers to resist wear from pumping the grit slurry from the grit chambers to the grit concentrators.  The grit pumps are controlled by the PLC in the grit control panel to run intermittently.  Seal water to the pumps is controlled by solenoid valves that open five seconds before the pump starts to keep the pump seals clean.
Grit Pump Flushing Control Valves 6
Each grit pump has a flushing control valve on the suction and discharge lines to the pump.  The two control valves apply nonpotable water to the lines to flush the grit piping before and after the grit pump operates.  The flushing action ensures that the grit piping remains open for efficient pump operation.
Grit Control Panel6
  The grit control panel, 21GT-CP01, is the local control station for the grit drives, grit pumps, and grit classifier.  Manual operation of the grit removal equipment is accomplished from this control panel.
Grit Concentrators (Cyclones)6
Each grit pump discharges grit slurry to separate concentrators located at the grit classifier.  The grit concentrator creates a vortex movement, much like that of the grit chamber, to separate the lighter organic material from the heavier grit particles.  Approximately 6% of the grit slurry flow from the grit pumps is discharged to the grit classifier with the remaining 94% is returned to the headworks through the filtrate pump station.
Grit Classifier6
A single grit classifier receives the concentrated grit solution form the discharge the two grit concentrators.  The grit classifier consists of a tank for dewatering the grit and a conveyor to transport the dewatered grit to a dumpster.  The dewatering tank contains a flow dissipation zone where the underflow from the grit concentrators is introduced which allows the concentrated grit slurry to enter the classifier without disturbing the already settled grit.  An outlet weir trough in the grit classifier collects the overflow which is conveyed through drain piping to the filtrate pump station.
Influent Sampler 6
An automatic sampler collects flow composite samples of the influent wastewater.  The sampler is flow paced from the influent flow meter.  The sampler contains a refrigeration unit to preserve the sample over the 24 hour sampling period.
Influent Flow Meter 6
A long throated open channel flow meter provides measurement of the influent wastewater flow.  The level of the wastewater is monitored as it travels through the flume and a flow transmitter converts the level to flow.  The flow meter has an operating range of 0 - 20 MGD.
 

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Last Updated: 6/13/2013 10:50:50 AM
Version 3.0.1