Screening and Grit Removal Essentials: An Overview of Preliminary Treatment Screens in Wastewater

Screening is a critical first step in the wastewater treatment process that serves to protect the rest of the system from large debris. At wastewater treatment plants, water first passes through screens to remove objects such as rags, paper, plastics, and metals. This is essential not only to prevent physical damage to downstream equipment but also to avoid clogging of piping and other appurtenances that are integral to the treatment process.

Different types of screens are used to address various levels of screening. The granularity of the screens varies, with coarser screens typically used first, followed by finer screens for smaller particles. The specific choice and arrangement of screens depend on the characteristics of the wastewater and the treatment goals of a facility.

Understanding the function and importance of bar screens, for instance, highlights the meticulous design considerations that go into screening systems. These screens play a key role in coarse screening and are typically the first line of defense in a wastewater treatment plant’s preprocessing stage.

Key Takeaways

• Screening is crucial for removing debris and protecting downstream wastewater treatment processes.
• Variety of screens are applied for different particle sizes and treatment needs.
• Bar screens are especially important for coarse screening in wastewater treatment plants.

Overview of Screening and Grit Removal in Wastewater Treatment

Screening is a critical first step in the wastewater treatment process, ensuring that large particles are removed before they can interfere with subsequent treatments or damage equipment.

Purpose of Grit Removal and Screening in Wastewater

Screening is designed to protect the infrastructure of a wastewater treatment plant from large objects which could cause blockages or damage. Objects such as rags, sticks, and other debris are removed at the outset of the screening of water treatment process to prevent potential disruptions.

Screening Process in Water Treatment

The screening process in water treatment involves passing wastewater through screens that can be made of metal or plastic. These screens trap large solids while allowing the liquid to pass through. The efficiency of screening is vital for the protection of pumps, pipes, and other mechanical components down the line.

Classification of Screens in Wastewater Treatment

Screens used in a wastewater treatment plant are classified based on their size, construction, and method of cleaning. They can be categorized as either:

• Coarse Screens: These have larger openings and are used first to remove big solids.
• Fine Screens: With smaller openings, fine screens further refine the screening process, capturing smaller particles.

Each type of screen plays a role in ensuring that the wastewater is satisfactorily treated and that the treatment systems operate efficiently.

Types of Screens Used in Wastewater Treatment

Screening is a fundamental step in wastewater treatment used to remove large solids from the influent. The effectiveness of subsequent treatment processes relies on the efficiency of the screening stage, which utilizes various types of screens designed for different particle sizes and load capacities.

Coarse Screens for Solids

Coarse screens are the first line of defense, designed to capture large debris and coarse solids such as rags, sticks, and plastics. Bar screens are a common form of coarse screens in wastewater treatment, where mechanical action is utilized to clean the collected debris. Coarse screens are essential in protecting downstream processes and equipment from potential damage.

Coarse screens are used as the initial screening step to remove large debris, rocks, gravel, and grit from the wastewater flow. Common types of coarse screens include bar screens, trash racks, and band screens.

Bar screens consist of parallel bars spaced 1 to 3 inches apart that remove large debris as wastewater flows through the bars. Trash racks are similar to bar screens but have larger bar spacing. Band screens are endless belts made of stainless steel or plastic mesh that rotate on two drums. As wastewater flows into the channel, debris is caught on the moving band screen and removed from the flow.

All coarse screens require raking or brushing to periodically remove trapped debris into a collection container for disposal. Grit and rocks also accumulate at the channel bottom and need to be periodically removed by hydraulically operated equipment.

Fine Screens

Fine screens follow coarse screens and have smaller openings, usually from 1.5 to 6 millimeters, to capture finer materials. Types of fine screens include perforated plate screens and band screens. Fine screening plays a crucial role in improving the performance of secondary treatment processes by further reducing the amount of solids.

After initial coarse screening, finer screens are used to remove additional suspended solids and organic matter from the wastewater. Common types of mechanical fine screens include basket strainers, drum screens, disk filters, micro screens, and static inclined screens. Self-cleaning screens reduce operation and maintenance needs compared to manually cleaned screens.

Basket strainers consist of woven wire mesh that traps smaller debris on the inside of the basket. The openings typically range in size from 3 to 10 mm. As debris accumulates, head loss increases and the basket needs to be lifted out and cleaned.

Fine Screen Design Considerations

Proper design is necessary to ensure fine screens efficiently remove solids without frequent blinding or clogging. Key factors that need to be considered include:

– Screen opening size selection based on desired degree of treatment
– Screen loading rates based on peak flows
– Number of units needed to ensure adequate redundancy
– Flow channel design for uniform velocity distribution across screen surface
– adequate screening area to prevent excessive head loss
– Provisions for bypassing flow during maintenance and repair
– Selection of durable corrosion-resistant screening media
– Adequate access for inspection, maintenance, and repair

Micro Screens

Micro screens are used to capture very fine particles and typically have openings less than 1.5 millimeters. They include drum screens, disc screens, and parabolic screens. These screens are often employed when the wastewater is subjected to higher levels of treatment and finer filtration, such as in advanced wastewater treatment facilities where discharge standards are strict.

Drum screens consist of cylindrical drums or disks made of perforated metal or plastic panels with openings ranging in size from 0.5 to 6 mm. As the drum rotates partially submerged in the water, debris becomes trapped on the screen surface. Spray water nozzles clean the panels.

Disc filters contain multiple fabric filter discs mounted on a horizontal shaft. Water flows from the outside of the discs to the inside. The discs rotate, and a backwash spray bar cleans the filter surface. Disk openings range in size from 20 to 150 microns.

Bar Screens in Detail

Bar screens play a crucial role in the initial stage of the wastewater treatment process, tasked with removing large solids from the wastewater stream.

Mechanical Bar Screens

Mechanical bar screens are automated systems designed for the continuous removal of large solids from wastewater. They are essential components in modern wastewater treatment plants, where the volume and flow conditions necessitate automated equipment. Mechanical bar screens operate by using a motor-driven mechanism to move a rake through the bars, collecting debris and transporting it upwards for disposal. These systems can vary in bar spacing, typically ranging from 1 to 3 inches, to accommodate different waste loads.

The efficiency of mechanical bar screens in wastewater treatment hinges on several factors, including:

• Design: The inclination and spacing of the bars are optimized for specific wastewater characteristics.
• Materials: Often made of stainless steel for longevity and resistance to corrosion.
• Automation levels: Higher automation reduces manual labor and can adapt to changing conditions with sensors and programmable controllers.

Mechanical bar screens are evaluated on their ability to reduce manual maintenance and improve the consistency of screenings removal.

Here is an overview of mechanically cleaned bar screens

Chain Driven Rake Screens

These have an endless chain system that drives one or more rakes over the bars to clean the screen. The HUBER TrashMax uses a heavy-duty chain driven dual rake system.

Reciprocating Rake Screens

These screens have a single or multi-rake system that reciprocates up and down the bars powered by a beam and crankshaft mechanism. Provides positive rake control.

Catenary Screens

Catenary screens have a flexible chain system with cleaners that conform to the bars to maintain good contact through rotation. Allows for simple installation.

Continuous Belt Screens

These feature mesh conveyor belts with attached bars that ride on rollers. Belt continuously indexes to transport screenings out of channel. Provides high capacity screening. JWIs Flex-Rake is an example.

Other Types:

• Stationary screens with fixed rakes/brushes
• Traveling multi-strand belt screens
• Rotating drum screens
• Climber screens

Manual Bar Screens

Manual bar screens are the simpler counterparts to mechanical screens and are often found in smaller or less technologically advanced wastewater treatment facilities. The manual operation involves personnel physically raking the collected debris off the bars at regular intervals, which is labor-intensive and less efficient.

Key aspects include:

• Labor requirements: Higher reliance on manual labor for operation and maintenance.
• Cost: Generally lower initial investment compared to mechanical systems, but potential for higher long-term labor costs.
• Flexibility: Suitable for small or seasonal wastewater treatment plants with variable flow rates and debris loads.

Although less sophisticated, manual bar screens are a vital part of ensuring that subsequent treatment processes are protected from large solids. Their simplicity can be an advantage in terms of lower mechanical complexity and ease of repair.

Screening is the first unit operation used in wastewater treatment plants. It removes large objects such as rags, paper, plastics, and metals to protect downstream equipment from damage and clogging. Screens are categorized as either coarse screens (bar screens, trash racks) or fine screens.

Further Treatment

After screening, the treated wastewater undergoes additional treatment processes, such as sedimentation and biological methods, to further improve water quality prior to discharge or reuse. The removed screenings are washed, compacted, and disposed of properly in landfills.

In summary, screening is a crucial first treatment step that protects downstream equipment through progressive removal of solids and debris. Coarse screening with bar screens and band screens removes large debris, while fine screening using basket strainers, microscreens, drum screens, and static inclined screens removes suspended matter. Proper design and maintenance of screening systems helps ensure reliable operation and treatment performance.

Frequently Asked Questions