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Fine Screening System Overview
Purpose
The fine screens are designed to remove objects larger than 0.25 inches from the wastewater influent to protect downstream equipment.
Theory of Operation
The perforated plate screen is installed at a 60o angle in the influent channel.  The screening system consists of traveling panels with openings of 0.25" diameter.  Solids are captured on the perforated face of the screen panels as wastewater flows through the screen.  The retained solids restrict the flow through the screen causing an increase in the differential pressure.  When the differential pressure setpoint is reached, the screen is acuated and the screen panels with the retained solids are moved upward with clean panels following reducing the differential pressure through the screen.
 
The screen is self cleaning through the interaction of the screening panel, water spray system and rotating bristle brush.  The screenings are deposited into the hopper of the screenings washer compactor.  The washer compactor removes organics from the screenings and compacts the screenings to reduce their volume.  The cleaned screenings are deposited in a dumpster for disposal at the local landfill.
Equipment
The Fine Screening system includes the following equipment: 
  • Water control gates
  • Fine screens
  • Fine screen control panels
  • Fine screen wash screen press
  • Wash screen press control panel
  • Gas detection system
Description
Influent wastewater from the Beaver Channel Pump Station enters the headworks building influent channel containing three screen channels.  Each channel contains a manually operated water control gate to isolate the associated screen.  Normal operation would involve the use of the two automatic fine screens (control gates open) with the bypass channel containing the manual screen closed.  The fine screens are traveling perforated plate type screens.  Level monitors are installed upstream and downstream of each automatic screen to provide level measurements and associated differential pressure through the screens.  A control panel for each screen controls the operation of the screen based on differential pressure through the screen or on time.
 
Solids collected on the screen restrict flow through the screen and cause an increase in the differential pressure through the screen.  This will activate the screen causing the movement of the screen panels.  Clean panels are moved into position in the channel and allow the influent level to return to normal.  The screenings are removed from the screen panels by a combination of water spray and rotating bristle brush.  The screenings are deposited in the hopper of the wash screen press.
 
The wash screen press removes organics from the screenings using water and the forward motion of the screw.  The screenings are pushed forward to the discharge tube where the screenings are compacted and dewatered.  The wash screen press operation is controlled from the local control panel.
 
The headworks building contains a bypass channel containing a manual screen.  This manual screen can be used as needed when both fine screens are out of service.

The Headworks Building is equipped with a combustible gas detection system consisting of a combustible gas sensor, gas transmitter, and three combination strobe light/horn alarms, one inside the headworks building and the other two located on the building exterior.  The high alarm setpoint is at 10% LEL with the high-high alarm at 25% LEL.  Under either condition, all strobe lights/horns are activated and alarms are generated at the HMI.
Water Control Gates6
 
The water control gates are located in the influent and effluent channels for each screen (including the manual screen) and are used to isolate the screens.  The gates are manually operated and are upward opening.
Fine Screens6

 

The two fine screens are traveling perforated plate screens with 0.25" openings.  The screens are actuated by differential pressure through the screen or based on time.  Screenings are removed from the screens by a combination of water spray and rotating bristle brush.
Fine Screen Control Panels6
  The fine screen control panel provides operational control of the perforated plate screens.  Ultrasonic level transducers upstream and downstream of the screens provide signals to the control panel where the differential pressure across the screen is determined by a PLC for control of the screen operation.  The screens are also controlled by a time setting in the PLC.
Wash Screen Press6
Screenings from the fine screens are deposited in the hopper of wash screen press.  The wash screen press uses water to remove organics from the screening and compacts the screenings as they are transported to the dumpster.  There are two wash screen presses, one for each fine screen.
Wash Screen Press Control Panel6
The wash screen press operation is controlled from the local control panel. The operation of the wash screen compactors are interlocked with the screen operation.
Combustible Gas Detection System 6
The Headworks Building is equipped with a combustible gas detection system consisting of a combustible gas sensor, gas transmitter, and three combination strobe light/horn alarms, one inside the headworks building and the other two located on the building exterior.  The high alarm setpoint is at 10% LEL with the high-high alarm at 25% LEL.  Under either condition, all strobe lights/horns are activated and alarms are generated at the HMI.  The combustible gas sensor is located on the west wall of the Headworks Building, adjacent to the influent channel.
 

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Last Updated: 1/7/2014 1:15:57 PM
Version 3.0.1