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Grit Removal Overview
Purpose
Grit is removed at the headworks to reduce its accumulation in downstream piping and tanks, and to minimize damage to sludge pumps.
Description
Grit is removed from the liquid flow stream in two concrete chambers, each equipped with a rotating paddle mechanism, an air scouring mechanism, and a baffle plate for di­recting incoming wastewater flow downward. The chambers consist of an upper and lower compartment separated by a plate with an opening in the center. The hydraulics through the upper compartment move the grit toward the center opening. The rotating paddle mechanism serves to re-suspend organic material that has settled with the grit before it drops into the lower compartment.

Grit accumulates in the lower compartment in each chamber and is pumped out by one of two 250 gpm recessed impeller grit chamber pumps. Two 75 scfm rotary lobe blowers provide air for the scouring mechanism. The air scouring mechanism serves to fluidize the grit for pumping.

The grit is conveyed by a grit chamber pump to one of two cyclone separators. The cyclone separator returns lighter material to the channels upstream of the grit cham­bers and discharges the heavier grit into a grit classifier.

Each grit chamber is designed to handle the design year maximum month hydraulic flow of 5.5 MGD. Under normal operation, only one grit cham­ber is in service. When a storm event is foreseen that will result in hy­draulic flows approaching the 10.6 MGD maximum day flow, then the second grit cham­ber should be placed in service.
Operation
Each grit chamber paddle mechanism has a local control panel adjacent to it with a Hand/Off/Auto (HOA) switch and status indicator lights for On and high torque. On the HCP, each grit chamber mechanism has an On status indicator light and alarm annunciation for high torque.

The start of an automatic grit removal cycle for the grit chambers is based on time or totalized plant flow, whichever occurs first. The timer set point (presently 30 minutes) and plant flow set point (presently 90,000 gallons) are selected by the operator through the PCMS. An automatic cycle will also be started after a plant power failure.

The automatic grit removal cycle for each grit chamber is as follows:

1. The grit chamber blower starts to fluidize the grit. This will run for a set period of time selectable by the operator through the PCMS.

2. After a set period of time (about 60 seconds, selectable by the operator through the PCMS) a signal is sent from the PLC to start the grit pump. One minute after the grit pump is called to run, the PLC will also send a signal to start the grit classifier. The grit pump will run for a set period of time (5 minutes). The grit classifier will run for a set period of time (6 minutes).

3. The grit removal cycle can operate for only one grit chamber at a time, be­cause the single grit classifier is not expected to have sufficient capacity for grit from both chambers simultaneously.

4. If the PLC detects the grit chamber blowers HOA switches are not in the auto position, the rest of the grit removal cycle will still operate.

5. If the PLC detects that the grit pump or grit classifier HOA switches are not in the AUTO position, the grit removal cycle will not operate.

6. If at any time during a grit removal cycle the PLC detects equipment that (1) has its HOA switch in the AUTO position, and (2) has an Off status when it should be an On status, the PLC will halt the grit removal cycle and output a GRIT SYSTEM FAILURE alarm to the HCP annunciator. Toggling of hand switch HS‑5071 in the screenings loading area to the Off position will reset the alarm.

In the manual mode, the grit chamber blower, grit chamber pumps, and grit classifier will all be operated from the HCP. Grit piping will allow for operating Grit Chamber Pump No. 1 with Grit Chamber No. 2, or for operating Grit Chamber Pump No. 2 with Grit Chamber No. 1. In this situation, the grit removal cycle must be operated manually.
Equipment
The grit collection system includes the following equipment: 
  • Grit Chamber
  • Grit Chamber Blower
  • Grit Pumps
  • Grit Hydrocyclones
  • Grit Classifier 
Grit Chamber
The two grit chambers are concrete tanks, each equipped with a rotating paddle mechanism, an air scouring mechanism, and a baffle plate for di­recting incoming wastewater flow downward. The chambers consist of an upper and lower compartment separated by a plate with an opening in the center. The hydraulics through the upper compartment move the grit toward the center opening. The rotating paddle mechanism serves to re-suspend organic material that has settled with the grit before it drops into the lower compartment.
 
  
Grit Chamber Blower6
 
Two 75 cfm rotary lobe blowers provide agitation air for the scouring mechanism in the grit chamber. The blowers operate in a lead lag strategy.
The air scouring mechanism serves to fluidize the grit for pumping.
 
Grit Pump 6
 
Two recessed impeller pumps are utilized for grit removal. These pumps pull grit from the two grit chambers and pump it to the hydrocyclones.

Each pump has a 3-inch suction and discharge line with the discharge arranged vertically up. The suction and discharge piping is increased to 4-inch to reduce the flow velocity and minimize pipe damage from the gritty material in the pipes.

Each pump is equipped with inlet and discharge isolation plug valves and a check valve.
Grit Hydrocyclones6
The grit pumps discharge a grit slurry to the hydrocyclones, where the material rotates and causes the heavy grit to move to the outside and down to the apex where it is discharged to the classifier. The lighter liquid is forced up to the top and out of the hydrocyclone where it flows to the channels upstream of the grit cham­bers.

The hydrocyclone has a 4-inch inlet flange with a 3.5-inch vortex finder that requires 7 psi pressure drop to operate correctly. The inlet head liner and apex liner are replaceable.
Grit  Classifier6
The grit classifier receives the  heavy grit discharge from the grit hydrocyclones and discharges the dewatered grit into a dumpster for disposal.
 
The grit classifier has a 1 HP parallel helical gear motor which couples directly to the 12-inch diameter screw conveyor. The screw conveyor is supplied with abrasive resistant wearing shoes. These shoes protect the auger. They can be replaced as needed to increase the life of the equipment. The system is made of T-304 stainless steel.

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Last Updated: 9/17/2013 7:06:21 AM
Version 4.0.1