Water Treatment Screening: Principles and Applications

Screening is a fundamental preliminary treatment step in both surface water and wastewater facilities. Its primary objective is to protect downstream equipment and processes by removing larger suspended solids that could otherwise cause obstructions, damage machinery, or impede the efficiency of subsequent treatment stages.

Purpose of Screening

The core functions of effective screening include:

• Equipment Protection: Safeguarding pumps, valves, and other mechanical units from damage or clogging by large debris.
• Process Optimization: Preventing accumulation of coarse materials in clarifiers, aeration tanks, and other treatment units, which can reduce their effective volume and operational efficiency.
• Solids Separation: Facilitating the easy removal of gross solids carried by raw water, simplifying the overall treatment process.

Classification of Screening

Screening is typically categorized based on the clear spacing between the screen bars, which dictates the size of solids removed:

• Fine Screening: Bar spacing less than 10 mm (0.39 in). Often used for municipal wastewater before fine treatment or for industrial effluents.
• Medium Screening: Bar spacing between 10 mm (0.39 in) and 40 mm (1.57 in). Commonly used in many general applications.
• Coarse Screening: Bar spacing greater than 40 mm (1.57 in). Primarily serves as a preliminary protective screen, often preceding finer screens.

Typical Bar Spacing Guidelines

• Surface Waters: 20 to 40 mm (0.79 to 1.57 in), typically upstream of a strainer.
• Municipal Wastewater (Raw): 15 to 30 mm (0.59 to 1.18 in). Fine screening is necessary if a straining or lamellae settling process follows.
• Municipal Wastewater (Sludge): 10 mm (0.39 in) or less, if screening is required.
• Industrial Effluents (e.g., Agri-food): Often requires fine bar screening, sometimes preceded by medium screening and then straining.

Hydraulic Sizing and Clogging Management

Proper hydraulic design is critical to ensure efficient screening without excessive head loss or complete clogging.

Crossing Velocity

The velocity of water passing through the clear area of the screen (i.e., between the bars) is a key parameter:

• Normal Crossing Velocity: Maintained between 0.6 and 1.0 m/s (2.0 and 3.3 ft/s) to ensure solids attach to the screen without causing excessive head loss or being carried through.
• Maximum Crossing Velocity: Can reach 1.2 to 1.4 m/s (3.9 to 4.6 ft/s) during peak flow conditions.

These velocities apply to the unclogged portion of the bar screen.

Degree of Clogging

The extent to which a screen can be clogged before cleaning is necessary varies by application and screen type:

• Automatic Bar Screens:
  • Surface Water: Up to 10% clogging.
  • Wastewater (High Solids): Up to 30% clogging.
• Manually Cleaned Bar Screens: Designed with a larger immersed area to reduce the frequency of manual cleaning, implying a higher tolerance for clogging before head loss becomes critical.

Automation and Control

The trend in screening is towards increased automation, even in smaller facilities, to reduce manual labor and improve reliability. Automation is essential in situations with high solids loading or sporadic influxes of large debris (e.g., plant matter) that can rapidly clog screens.

Control Mechanisms for Automatic Cleaning Systems

Automatic bar screen cleaning systems typically operate intermittently and can be controlled in several ways:

1. Cyclic Control: Based on adjustable frequencies (e.g., 1 minute to 1 hour intervals) and operation durations (e.g., 1 to 15 minutes per cycle).
2. Differential Head Loss Indicator: Activates cleaning when the water level difference (head loss) across the screen reaches a predefined threshold, indicating significant clogging.
3. Combined System: Utilizes both cyclic and differential head loss control for enhanced responsiveness and efficiency.
4. Pump Start-up Linkage: When located downstream of a pumping station, the cleaning mechanism can be linked to pump operations, often with a built-in timer (1 to 30 minutes) to ensure cleaning cycles align with pumping activity.

Protection Features

Automatic bar screens are equipped with torque limiters to prevent damage from overloading or jamming. Reciprocating cleaning bar screens (both curved and straight) often include a device to automatically stop the rake outside the screen area, preventing jamming upon restart.

Types of Screens

Various screen designs cater to different flow rates, solids characteristics, and operational requirements.

Manually Cleaned Screens

• Static Screen: For smaller flows, these screens (e.g., inclined bar type or parabolic static sieve bend) require manual cleaning. Parabolic static sieve bends can remove solids down to 0.5 mm (0.02 in).

Automatically Cleaned Screens

• Inclined Bar Screen: Sewage flows through the inclined screen, which is periodically raked by a mechanized comb system. Actuation is typically by level switches or a time clock. A doctor blade at the top of the comb travel removes collected screenings into a container.
• Curved Bar Screen: Uses hydraulics for kinematic operation. Similar to inclined screens, it's periodically raked by a mechanized comb, with screenings removed by a doctor blade.
• Radial Bar Screen: Similar features and operation to the curved bar screen but often simpler for small installations and offers two cleaning cycles per revolution, enhancing screening capacity.
• Step Screen: Popular for their ability to remove smaller sized solids compared to traditional bar screens. These utilize moving steps to convey screenings upward.
• Brush Type Inclined Screen: These are inclined screens featuring a rotary brush belt cleansing system, offering an alternative to step screens.

Overview of Upstream Mechanical Bar Screens

The following table outlines common types of upstream mechanical bar screens and their typical operational parameters:

Note: For 'Rack and Pinion Bar Screen', the raw data listed 0.65 and 1.3 for both Bar Thickness and Disposal Height. For other fine screens, Bar Thickness was not specified.

Screenings Handling

The handling of collected screenings has become increasingly important, especially with the rise in problematic debris like sharps (e.g., hypodermic syringes). Modern wastewater treatment plants often incorporate mechanical screenings handling equipment, including:

• Dewatering Processes: To reduce the volume and weight of screenings.
• Shaftless Screen Conveyors: For efficient transport of sticky or fibrous materials.
• Bagging Units: For clean and contained storage and disposal.

These technologies enhance worker safety, reduce odor, and minimize disposal costs.

AquaChain Engineering Tip

When designing a screening system, always ensure easy access for maintenance and cleaning of both the screen and the channel upstream. Unexpected blockages, even in automated systems, can halt operations, and quick manual intervention is often critical. Consider installing an emergency bypass channel with a coarser screen to maintain flow during severe blockages or maintenance.

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Self-cleaning screeners are part of various advanced filtration technologies that enhance water treatment processes.

Frequently Asked Questions

Q1: Why is screening considered the "first treatment station" in water and wastewater treatment? A1: Screening is the initial physical process designed to remove large, visible solids like rags, plastics, and debris, protecting downstream pumps, pipes, and treatment units from damage, clogging, and inefficient operation.

Q2: What is the significance of "crossing velocity" in screen design, and what happens if it's too high or too low? A2: Crossing velocity is the speed of water passing through the screen's clear openings. If too low, solids may settle upstream; if too high, solids might be forced through the screen or dislodged prematurely, reducing removal efficiency and potentially damaging downstream equipment.

Q3: How does automation in screening systems benefit a water treatment facility? A3: Automation reduces manual labor, ensures consistent cleaning, adapts to varying solids loads (e.g., via differential head loss control), prevents catastrophic clogging events, and improves overall reliability and efficiency of the preliminary treatment stage.