1.2 Bar Screens, Trash Racks & Mechanically Cleaned Screens

1.2 Bar Screens, Trash Racks & Mechanically Cleaned Screens

Exam Focus: Preliminary screening is the very first physical unit process encountered by incoming raw wastewater. The Class I exam emphasizes screen classifications, clear bar opening dimensions, channel approach velocity boundaries (1.25 to 2.5 ft/s), differential headloss control setpoints (3 to 6 inches), screenings handling protocols, and operational troubleshooting.

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1. Engineering Purpose of Preliminary Screening

The primary objective of screening at a wastewater treatment facility headworks is the physical removal of coarse solids and fibrous debris before wastewater enters pumps, piping, and downstream unit processes. Municipal wastewater carries an immense volume of non-biodegradable and fibrous contaminants—rags, synthetic non-woven wet wipes, tree branches, plastics, hygiene products, beverage containers, and rocks.

If these objects bypass preliminary screening, they cause severe operational consequences:

• Pump Binding and Clogging: Rags wrap around open and enclosed centrifugal pump impellers, stalling electric drive motors and tripping thermal overload relays.
• Check Valve Jamming: Solids lodge under flapper check valves or swing checks on raw sewage and sludge transfer lines, preventing full closure and causing backflow or pump cavitation.
• Piping Stoppages: Stringy fibrous debris agglomerates in pipe ells, reducers, and flow meters, causing line blockages.
• Aeration System Fouling: Rags drape across dissolved oxygen probes, foul submerged level sensors, and blanket fine-bubble membrane diffusers, creating high blower backpressure and poor oxygen transfer.
• Clarifier Mechanism Failure: Tree limbs, structural steel, or rocks wedge beneath traveling sludge bridge scrapers or clarifier bottom flight squeegees, snapping drive chains, shearing pins, or warping collector arms.

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2. Classification of Screening Devices & Clear Openings

Screens are classified according to the clear spacing (open distance) between adjacent parallel steel bars, rods, or perforations.

   SCREEN OPENING SPECTRUM
   [Trash Racks]          [Coarse / Medium]          [Fine Screens]         [Micro-Screens / MBR]
   50 mm - 150 mm         12 mm - 50 mm              6 mm - 12 mm           1.5 mm - 6 mm
   (2" to 6")             (0.5" to 2")               (0.25" to 0.5")        (0.06" to 0.25")
   Protect Lift Pumps     Protect Clarifiers/Pipes   Protect Diffusers      Protect Membranes

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3. Manually Cleaned Versus Mechanically Cleaned Bar Screens

Manually Cleaned Bar Screens

• Construction & Incline: Bars are set at an incline of 30° to 45° from the horizontal (or 45° to 60° from vertical). The flatter angle allows an operator to rake debris upward by hand using a special long-handled rake whose tines match the bar spacing.
• Drainage Plate: Debris is pulled upward onto a perforated drain pan at the top of the rack, where excess wastewater drains back into the channel before the solids are shoveled into disposal bins.
• Application: Typically limited to very small wastewater treatment plants (<0.5 to 1.0 MGD), package plants, or as an emergency bypass channel around a mechanically cleaned screen.
• Disadvantages: Labor-intensive; presents ergonomic strain; exposes operators to biohazards and pathogenic aerosols; prone to blinding during unstaffed nighttime storm events.

Mechanically Cleaned Bar Screens

• Construction & Incline: Bars are set steeper, typically 60° to 85° from the horizontal (near vertical), to minimize the headworks footprint and optimize mechanical rake efficiency.
• Rake Configurations:
  • Front-Clean, Front-Return: Rake teeth enter the bars from the upstream (front) side at the bottom and travel upward, pushing debris to the discharge chute. Avoids wrapping debris around bottom sprockets.
  • Back-Clean, Front-Return: Rake teeth penetrate the bars from behind and pull debris upward. Prevents solids from jamming the rake carriage at the channel floor.
  • Catenary Screens: Rakes are suspended from continuous chains without bottom submerged sprockets or bearings, drastically reducing underwater maintenance.
  • Continuous Multi-Rake Screens: Multiple rakes mounted on continuous drive chains provide high cleaning frequency during storm surges.

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4. Automation and Control Mechanisms for Mechanical Screens

Mechanically cleaned screens rely on three primary control modes to balance equipment wear against hydraulic headloss:

Differential Headloss Control (Primary Mode)

Differential headloss is the physical difference in water surface elevation between the upstream and downstream sides of the screen rack ($\Delta h = h_{\text{upstream}} - h_{\text{downstream}}$). Water levels are monitored continuously using ultrasonic level transducers, differential pressure bubbler tubes, or float switches.

• Operating Trigger: When debris accumulates on the screen, the open area decreases, restricting flow. The upstream water level rises while the downstream level drops. When the differential headloss reaches 3 to 6 inches (0.25 to 0.50 ft / 75 to 150 mm), the PLC automatically energizes the drive motor to initiate a cleaning cycle.
• Stop Setting: The rake runs until the differential drops below a preset low limit (typically 1 to 2 inches) and the rake reaches its park position at the top of the channel.
• Consequences of Excessive Headloss (>6 inches):
  • Causes upstream wastewater to back up into collection sewers, inducing surcharging and basement backups.
  • Generates severe structural hydrostatic pressure that can bend screen bars or buckle support frames.
  • Creates a sudden hydraulic surge downstream when the rake cleans the screen, upsetting grit basins and primary clarifiers.

Time-Clock Control (Secondary / Backup Mode)

A programmable interval timer initiates a rake cleaning cycle at regular elapsed intervals (e.g., every 15 to 30 minutes) regardless of headloss.

• Why Timer Mode is Crucial: During low-flow nighttime periods, debris accumulation may not generate a 3-inch differential headloss for several hours. Without periodic cleaning, organic solids trapped on the screen begin anaerobic decomposition, producing foul hydrogen sulfide (H2S) odors and fly breeding. Furthermore, grease and scum dewater and cement the debris to the bars, making removal extremely difficult.

Emergency High-Level Override

A mechanical float switch or high-level electrode mounted in the upstream channel above the normal high-water mark overrides all timers and differential controls. If the upstream water level reaches this critical threshold during a torrential storm or flash surge, the screen operates continuously to prevent channel overtopping and catastrophic facility flooding.

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5. Screen Channel Hydraulic Principles & Approach Velocity

Maintaining the correct wastewater flow velocity through the screen channel is critical to overall headworks performance.

$\text{Velocity } (V) = \frac{\text{Flow Rate } (Q)}{\text{Cross-Sectional Area } (A)}$

Strict Velocity Boundaries

To maintain an approach velocity between 1.25 and 2.5 ft/s across wide diurnal flow variations (such as low night flows versus peak morning flows), facilities utilize multiple parallel screen channels with automated motorized influent sluice gates that open or close channels in response to total influent flow.

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6. Handling, Dewatering & Disposal of Screenings

Screenings Characteristics and Quantities

Screenings consist of highly objectionable, putrescible material saturated with fecal matter, pathogens, and grease. The volume of screenings collected typically ranges from 0.5 to 5.0 cubic feet per million gallons (ft³/MG) of wastewater treated, peaking sharply during the first flush of heavy rain events.

Screenings Processing Train

1. Discharge & Sluice: The mechanical rake ejects solids into a discharge chute, screw conveyor, or water sluice trough.
2. Screenings Washer-Compactor: Modern facilities direct raw screenings through an enclosed wash-water spray cycle. High-pressure wash water strips organic and fecal solids off the debris, returning the wash water containing biodegradable organics to the wastewater stream.
3. Dewatering & Compaction: A heavy-duty screw press squeezes the washed screenings against an adjustable discharge cone. This process:
   • Reduces screenings volume by 50% to 70%.
   • Increases cake solids from 15% to approximately 45% to 50% dry solids.
   • Expresses foul water back into the plant liquid flow.
4. Final Landfill Disposal: Dewatered screenings drop into a sealed roll-off dumpster or continuous bagging system for haulage to an approved municipal sanitary landfill.

Critical Exam Rule on Disposal: Screenings must NEVER be returned to the wastewater stream or dumped into open pits. They are classified as municipal solid waste and vector attractants and must be buried in a licensed sanitary landfill or incinerated in compliance with state solid waste rules.

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7. Operational Troubleshooting & Diagnostic Procedures