Screening, Grit Removal, and Preliminary Treatment

Overview of Preliminary Treatment

The first stage of any water intake or wastewater treatment plant is called preliminary treatment or the headworks. Its purpose is to remove large objects, rags, grit, and sand before they can damage pumps, clog pipes, or interfere with downstream processes. Operators must understand each unit at the headworks, keep them running reliably, and calculate the quantities of material removed for proper disposal.

Bar Screens

A bar screen is a series of parallel steel bars or rods placed across an incoming channel to capture large solids such as rags, sticks, plastic, and debris. Bars are spaced anywhere from about 1 inch to 3 inches apart depending on the design and the facility's downstream equipment.

Manually cleaned bar screens rely on an operator using a rake to pull accumulated screenings off the bars on a set schedule. These are common at smaller plants or as a backup unit.

Mechanically cleaned bar screens use an automatic raking mechanism—either a front-rake or back-rake design—that travels up the face of the screen on a timed cycle or when a differential pressure (headloss) sensor triggers the rake. The rake deposits screenings into a collection hopper or a compactor/washer unit.

Key parts an operator monitors:

• Rake teeth or combs – subject to wear and breakage; inspect regularly
• Drive chain or cable – must be tensioned correctly and lubricated per the manufacturer's schedule
• Shear pins or torque limiters – designed to fail and protect the drive motor if the rake jams; keep spare pins on hand
• Overflow or bypass alarm – triggered when headloss across the screen exceeds a set point, indicating a blinded screen or failed rake; the alarm setpoint is established during startup and recorded in the operating permit or O&M manual

Fine screens (openings smaller than about 6 mm) are sometimes installed in place of or after bar screens to capture smaller particles. They use similar mechanical principles.

Safety at the Bar Screen

Screenings contain pathogens. Always wear nitrile or rubber gloves, safety glasses or a face shield, and rubber boots when handling screenings. Screenings hoppers can generate hydrogen sulfide (H₂S) gas—a colorless, extremely toxic gas that smells like rotten eggs at low concentrations but deadens your sense of smell quickly. Never enter a screenings enclosure without first testing the air with a calibrated multi-gas monitor and following confined space entry procedures if required. Use lockout/tagout (LOTO) on the screen drive before any maintenance or unjamming work.

Grit Chambers

Grit refers to heavy, dense inorganic particles: sand, gravel, coffee grounds, eggshells, and similar materials. Grit must be removed early because it is abrasive and will rapidly wear pump impellers, pipe walls, and aeration diffusers.

Grit chambers slow or spin the flow so that grit settles while lighter organic solids remain suspended and move on for biological treatment.

Types of Grit Chambers

Horizontal-flow (velocity-controlled) grit chambers are rectangular channels designed to maintain a flow velocity of approximately 1 foot per second. At that velocity, grit settles but organic matter stays in suspension.

Aerated grit chambers use diffused air along one side of a long tank to create a spiral roll of flow. The roll keeps organics in suspension while grit settles to a hopper at the bottom.

Vortex grit chambers use either an induced vortex (paddle) or a hydraulic vortex to create a centrifugal motion. Grit is flung to the outside wall and drops to a central or peripheral hopper. Vortex units are compact and widely used in modern plants.

Grit Classifier

Settled grit is pumped as a slurry to a grit classifier (also called a grit washer/classifier). The classifier uses an inclined screw conveyor inside a tank; wash water rinses organic material back out while clean, dewatered grit discharges at the top for disposal. Operators check the classifier's water level, screw wear, and discharge chute during each inspection round.

Calculating Screenings and Grit Quantities

Operators must track the volume or weight of screenings and grit removed for disposal records and regulatory reporting. Common units are cubic feet per million gallons (ft³/MG) for screenings and cubic feet or pounds per day for grit.

Basic approach:

• Screenings volume = (screenings generation rate, ft³/MG) × (plant flow, MGD) × (days)
• Grit volume = (grit generation rate, ft³/MG) × (plant flow, MGD) × (days)

Typical generation rates vary by facility and collection system; use the values given in your plant's operating data or the problem statement—never assume a number.

Headworks Inspection Checklist

During each operating shift, the operator walks the headworks and documents:

1. Bar spacing visually clear of debris (or screenings removed on schedule)

2. Screen drive chain tension and lubrication status

3. Rake operation verified (automatic cycling or manual activation test)

4. Overflow/bypass alarm lamp functional

5. Grit classifier water level within normal range

6. Grit discharge chute clear

7. Odor control system (chemical scrubber or biofilter) operating—check chemical feed levels or fan status

8. Any unusual noise, vibration, or odor noted with time and corrective action taken

All findings are logged on the equipment log form with date, time, and operator initials. A written note on any deficiency—what was observed, what action was taken, and who was notified—is required for regulatory compliance and good plant management.