What is a Bar Screen? Headworks Equipment Guide

Introduction

Headworks is the first treatment step at a wastewater treatment plant — the point where raw sewage enters the facility and large solids are removed before any biological or chemical treatment can occur. Headworks equipment protects all downstream processes, pumps, and equipment from damage, clogging, and interference.

The bar screen is the most fundamental piece of headworks equipment. Without it, rags, wipes, sticks, and other large debris would clog pump impellers, wrap around mixer shafts, blind filter media, and generally wreak havoc on everything downstream.

What is a Bar Screen?

A bar screen is exactly what it sounds like: a series of parallel bars or rods through which wastewater flows while solids above a certain size are retained and removed. The gaps between bars (called the "clear opening") determine what is removed — typically from 1/4 inch (6mm) to 2 inches (50mm).

Manually Cleaned Bar Screens

Simple stationary screens with raking done manually by operators. Used in small plants (< 0.1 MGD) or as backup screens. Low cost, no moving parts — but require frequent manual attention to prevent blinding.

Mechanically Cleaned Bar Screens

The standard at modern facilities. Motorized cleaning mechanisms (rakes, rotating bars, tines) continuously or intermittently remove accumulated solids from the screen face and discharge them to a screenings conveyor.

Types of mechanically cleaned bar screens:

• Front-cleaned (catenary) screens: Rake travels up the face of the screen, pushes solids to the top
• Back-cleaned screens: Rake travels behind the screen bars
• Step screens: Multiple stairstepped stages, each finer than the last
• Perforated plate screens: Solid plate with holes, combined with rotating wipers

Fine Screens

As non-flushable wipes have proliferated (personal care wipes, baby wipes, disposable cleaning cloths), fine screening has become critical protection for downstream equipment.

Fine screens typically have clear openings of 1-3mm and are placed downstream of coarse bar screens. They are effective at removing "rags" — the tangled masses of non-woven wipes, hair, and other fibrous material that clog pump impellers, wrap around shaft seals, and bridge MBBR effluent screens.

Types of fine screens:

• Rotary drum screens: Rotating cylindrical screen submerged in the channel; solids collect on the inside surface and are washed to a discharge chute
• Step screens (fine): Multiple stairstepped panels at 1-3mm spacing
• Band screens: Continuous belt of perforated panels; used in stormwater and CSO applications

Grit Removal

Grit — sand, gravel, eggshells, coffee grounds, and other dense inorganic solids — is removed at headworks to protect pumps, equipment, and biological treatment processes.

Grit settles rapidly (specific gravity ~2.65) and must be separated from organic solids (which should remain in the flow for biological treatment). Grit removal systems create conditions where grit settles while organics are kept in suspension.

Types of grit removal systems:

Vortex Grit Systems

A tangential inflow creates a controlled vortex (cyclonic) motion in a circular tank. Dense grit settles to the center by centrifugal/gravitational forces while organics are carried upward in the flow.

Manufacturers: Smith & Loveless (PISTA Grit)

Aerated Grit Chambers

Diffused air injected along one side of a rectangular channel creates a spiral roll, separating grit from organics. Grit settles in a hopper while organics remain suspended.

Detritor (Stirred) Systems

Slow-speed propeller agitator keeps organics suspended while grit settles to a central collection hopper. Common in UK practice; some US installations.

Hydrocyclone/Grit Classifier

Cyclone separator concentrates and dewaters grit slurry pumped from the collection hopper. Grit classifiers (wash/degritting screws) remove organics from the grit slurry and produce a clean, dewatered grit product.

Screenings Handling

Removed screenings (material from bar screens and fine screens) must be:

1. Dewatered — pressed or centrifuged to reduce free moisture
2. Washed — to reduce organic content and odor
3. Compacted — for volume reduction
4. Disposed of — typically to a sanitary landfill

Integrated screenings washers/compactors are common — the screenings exit the bar screen into a combined washer/compactor that continuously processes and dewaters them into a hopper.

Manufacturers of screenings handling equipment:

• HUBER Technology — RakeMax bar screens, ROTAMAT fine/drum screens, WAP wash press.
• Headworks International — integrated screenings management
• JWC Environmental — Muffin Monster and Auger Monster shredders/grinders
• Franklin Miller — Taskmaster grinders (inline alternatives to screening)

Comminution (Grinders/Shredders)

An alternative or supplement to screening: comminutors/grinders cut solids into small pieces rather than removing them from the flow. Less common today because the cut-up solids must still be removed downstream.

Inline grinders (JWC Environmental Muffin Monster, Franklin Miller Taskmaster) are used to protect specific equipment (pumps, centrifuges) rather than as the primary headworks treatment step.

Odor Control at Headworks

Headworks is the highest odor zone at a wastewater plant. Hydrogen sulfide from anaerobic conditions in the sewer system is released when wastewater cascades over screens and into the treatment plant.

Modern enclosed headworks buildings capture and treat vent air through odor control systems:

• Biofilters (wood chip or volcanic rock media, biological sulfide oxidation)
• Chemical scrubbers (caustic, bleach, or multi-stage)
• Carbon adsorbers (activated carbon)
• Regenerative thermal oxidizers (RTO) for very high H2S loads

Key Headworks Equipment Manufacturers

• WTP Equipment — front-cleaned reciprocating bar screens, deep-well screens, and conveyor screens
• Amwell — heavy-duty front-cleaned bar screens (to 7 ft wide, depths to 30 ft) and screenings handling
• Grande — CSO/SSO overflow screening (ACU-SCREEN, TRU-RAKE)
• DryCake — Twister fine screens.
• Headworks International — full headworks systems, grit, screens
• Parkson Corporation — fine screens, grit, grease removal
• JWC Environmental — Muffin Monster grinders, fine screens
• Huber Technology — rotating screens, grit, screenings handling
• Smith & Loveless — PISTA Grit vortex systems, packaged headworks
• Duperon Corporation — FlexRake bar screens
• Franklin Miller — Taskmaster inline grinders
• BioAir Solutions — headworks odor control