2.1 Screening, Grinding, Bar Screens & Grit-Removal Equipment

2.1 Screening, Grinding, Bar Screens & Grit-Removal Equipment

2025 WPI alignment: This section teaches preliminary treatment equipment: screening and grinding, bar screens, and vortex or gravity grit removal in the official Equipment Evaluation, Maintenance, and/or Operation content area.

Why this job task matters

Headworks equipment protects pumps, channels, clarifiers, and biological units by intercepting rags and debris and separating dense inorganic grit before it causes clogging or abrasion.

Core operating concepts

Operating and maintenance workflow

1. Inspect upstream and downstream levels, rake condition, screenings discharge, guards, and bypass-channel status at the start of the round.
2. Confirm mechanical screens cycle by timer or differential level and that sprays, conveyors, compactors, and bins operate without jams.
3. Observe grit-chamber flow pattern, air or vortex equipment, pumps, classifiers, and grit quality; excessive organics indicate poor separation.
4. Trend headloss, run time, grit volume, motor current, vibration, and repeated overloads rather than treating each alarm as isolated.
5. Isolate and lock out rakes, grinders, compactors, conveyors, and grit pumps before clearing a jam or entering guarded areas.
6. Record bypasses and equipment outages and coordinate load protection for downstream pumps and basins.

Diagnostic evidence

Calculation, control, or records connection

For open-channel equipment, connect flow to area and velocity: flow = area × velocity with compatible units. A calculated velocity that is too low explains organic settling; one that is too high explains grit carryover. Screen differential head is not calculated from motor current—it is the water-level difference across the screen. Track the actual upstream and downstream elevations so a failed level sensor is not mistaken for a blocked rack.

Worked operator scenario

After a storm, the upstream screen channel rises, the mechanical rake shows no movement, and grit appears downstream. The defensible response is to protect the headworks from overflow, place available parallel equipment in service, verify power and overload status, and isolate the failed mechanism before inspection. Opening a bypass without authorization would transfer the debris hazard to pumps and clarifiers and may create a reportable event.

Common exam traps

• A grinder changes particle size; it is not a substitute for grit separation.
• More grit removal is not automatically better if organics are being captured and putrefy in the bin.
• A high motor current alone does not prove screen blinding; verify water levels and physical condition.
• Never reach into a screen, grinder, conveyor, or classifier until hazardous energy is controlled.

Field-to-exam checklist

• Screens remove debris; grit units separate dense inorganic particles.
• Differential head and physical inspection reveal screen blinding.
• Grit quality, not only quantity, indicates whether hydraulic separation is correct.
• Headworks failures must be controlled without transferring unsafe debris loads downstream.

Sizing evidence for screens and grit chambers

Headworks decisions become clearer once the equipment is described by opening size, velocity, and particle density rather than by brand.

• Opening size sets what is captured. Coarse screens — trash racks and bar racks — use clear openings of roughly a quarter inch (6 mm) and larger, while fine screens use smaller openings, often perforated plate, wedgewire, or drum designs. Narrower openings capture more rag material but generate more screenings volume, more headloss, and more washing, compacting, and hauling duty.
• Cleaning method sets the reserve area. A manually cleaned rack blinds between cleanings, so it is given generous submerged area and a conservative headloss allowance. A mechanically cleaned unit is cycled by timer, by differential level, or by both, so its reserve depends on the cleaning trigger working.
• Channel velocity sets what settles. Approach velocity must stay high enough to keep grit moving toward the grit chamber instead of depositing in front of the rack, yet low enough that soft debris is not extruded through the bars. Velocity-controlled horizontal-flow grit chambers are conventionally designed around 1 ft/s (0.3 m/s) across the operating flow range.
• Density is the separating property. Mineral grit has a specific gravity near 2.65, while the organic solids the plant wants to keep have a specific gravity close to 1.0. A modest velocity error therefore changes the split dramatically, which is why organic-rich grit is a hydraulics finding rather than a housekeeping complaint.

Worked channel check. A rectangular grit channel is 3 ft wide with 2 ft of flow depth and carries 4.0 MGD. Convert flow: 4.0 MGD x 1.547 = 6.19 cfs. Cross-sectional area = 3 x 2 = 6 ft². Velocity = 6.19 / 6 = 1.03 ft/s, which sits right at the classic design value. If one of two parallel channels were removed at the same flow, velocity would roughly double and grit would carry forward.