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Screenings Removal Overview
The purpose of influent screening is to protect downstream equipment from debris that could clog or damage the equipment.
Theory of Operation
Two mechanically cleaned, perforated plate screen removes all material larger than 8 millimeters (5/16 inch) in diameter from the wastewater. Additionally, finer material is removed as a debris mat builds up on the screen plates prior to cleaning.  The influent screens are designed as follows:

Screening Design Criteria
Criteria Units Value
Quantity Installed each 2
Type each Perforated Plate
Hydraulic Capacity, each mgd 15
Channel Width feet 3
Channel Depth feet 5
Perforation Size mm 8 ( ~ 5/16 inch)
The screenings system includes the following equipment:
  • Isolation gates
  • Perforated plate fine screen  
  • Washer/compactor
Wastewater normally flows through one of the automatically cleaned screens. Debris is trapped on the screens and removed and the screened wastewater then flows to the influent pumping station.  The mechanical screens discharge directly to dedicated washer/compactors where organic materials are washed out, screenings are dewatered and discharged to a dumpster for landfill disposal. The washer/compactor drain water drops into the channel downstream of the screen.
Isolation Gates6
Two manually operated 36-inch slide gates provide isolation to each screen from the influent flow. Two additional manually operated 36-inch slide gates provide isolation on the downstream side of each screen. Opening both the inlet and outlet gates places a screen into service.
Perforated Plate Fine Screens6
  Two 36-inch wide perforated plate screens with 8 millimeter (5/16 inch) openings are installed in the screen channels. The perforated plates rotate through the wastewater, which flows through the plates to the influent pumping station.  Each screen is rated for a peak flow of 15 MGD.  Each mechanically cleaned screen has two drives, one for the perforated plates and one for the cleaning brush located at the top of the screen on the downstream side. The perforated plate drive and the brush drive are connected to variable speed drives. After the brush removes the majority of the screenings from the perforated plates, the plates are backwashed with a high pressure water spray. The screenings and spray water drop into a discharge chute that connects to a screenings washer/compactor.
Screenings and spray water are discharged from each mechanical screen into a dedicated washer/compactor. Each washer/compactor has a rated screenings capacity of 10 cu.ft./hour. The screens remove fecal matter and other biodegradable materials that are returned to the wastewater stream and treated in the primary and secondary treatment processes.

Each washer/compactor has a hopper, a spiral discharge screw and two automatic spray valves. One spray washes the screenings and the second spray flushes the washer/compactor drain at the end of the washing and compacting cycle. The washer/compactor spiral screw pushes the washed screenings into a long discharge tube where the screenings are further dewatered before dropping into dedicated screenings dumpsters.

The washer/compactor drain discharges into the 36-inch line to the influent pump station.

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Last Updated: 9/17/2013 9:40:31 AM
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