Screen rakes are critical mechanical devices used in water and wastewater treatment facilities to remove solid waste and debris from incoming water flows. These specialized machines combine screening technology with automated cleaning mechanisms to ensure continuous and efficient operation of treatment processes.

Device Composition

Screen rakes consist of several integrated components working together to effectively remove solid materials from water flows. Understanding these components helps facility operators properly maintain and troubleshoot these critical systems.

Rake: a tooth-shaped or bucket-shaped structure made of metal, installed on a mechanical arm or chain, which can move up and down along the surface of the screen. These rake elements are specifically designed to match the profile and spacing of the screen bars they clean. The teeth or buckets are typically manufactured from durable materials such as stainless steel (commonly grades 304 or 316) to withstand the corrosive environment of wastewater and the mechanical stresses of operation. The specific design of rake teeth varies significantly based on the application, with some featuring simple tine arrangements while others employ more complex geometries optimized for particular types of debris. The spacing between rake teeth must precisely match the bar spacing of the screen to ensure effective cleaning without causing damage to the screen structure.

The screen itself forms a critical component of the overall system and typically consists of evenly spaced parallel bars mounted in a frame. These bars are positioned perpendicular to the flow direction and are angled from vertical (usually between 60 to 85 degrees) to facilitate debris removal. Bar spacing varies based on application requirements, with coarse screens featuring gaps of 15-100mm and fine screens having much narrower openings of 3-15mm. The bars themselves are often teardrop-shaped in cross-section rather than rectangular, which helps minimize head loss while improving screening efficiency.

Drive system: The rake is driven to and fro by a motor, chain, gear, or hydraulic device to achieve automatic slag removal. This system provides the mechanical force required to move the rake through its cleaning cycle. Electric motors with appropriate power ratings (typically ranging from 0.75kW to 7.5kW depending on system size) provide the primary energy source. These motors connect to the rake mechanism through various power transmission systems. Chain-driven systems use heavy-duty industrial chains similar to those found in conveyor applications but with special corrosion-resistant properties. Gear-driven systems employ reduction gearboxes to convert the high-speed rotation of electric motors to the slower, more powerful movement needed for rake operation. Hydraulic systems utilize hydraulic cylinders and pumps to generate powerful linear motion, particularly valuable in applications requiring exceptional force to remove heavy debris.

Workflow

Screen rakes operate through a systematic process that ensures effective debris removal while maintaining continuous water flow. Understanding this workflow helps facility operators optimize system performance and troubleshoot issues when they arise.

When wastewater flows through the screen, solid impurities are intercepted by the bars and accumulated on the surface of the screen. As water carrying suspended solids approaches the screen, it passes between the bars while particles larger than the bar spacing become trapped. This accumulation process is not uniform across the screen surface; areas with higher flow velocity or turbulence often experience more rapid buildup. The nature of the captured material varies widely depending on the source of the water being treated. Municipal wastewater typically contains textiles, hygiene products, plastic items, and organic matter, while industrial applications might encounter specific process byproducts, raw materials, or manufacturing residues.

The interception process creates a phenomenon known as "blinding," where the accumulated material itself becomes a finer filter that captures progressively smaller particles. This can improve removal efficiency but accelerates the restriction of flow through the screen. The rate of accumulation depends on several factors, including flow rate, solid content of the incoming water, and screen bar spacing. During heavy rainfall events or industrial discharge peaks, accumulation can occur much more rapidly than during normal operation.

When the screen slag accumulates to a certain extent (triggered by time setting or differential pressure sensor), the drive system starts, and the rake moves down along the inclined surface of the screen and inserts into the bottom of the screen slag. Modern systems typically use one or both of the two triggering mechanisms. Timer-based operation initiates cleaning cycles at predetermined intervals, suitable for situations with predictable debris loads. Differential pressure monitoring provides a more responsive approach, initiating cleaning only when necessary based on actual screen clogging conditions. The differential pressure trigger point is typically set between 50-200mm water column difference, depending on the specific application requirements and screen design.

The initial downward movement of the screen rake is carefully controlled to ensure proper positioning at the bottom of the screen. This positioning is critical as it determines how effectively the rake will engage with the accumulated debris during the upward cleaning stroke. In chain-driven systems, the rake assembly follows a continuous path guided by sprockets. In hydraulic arm systems, the movement is more direct, with hydraulic cylinders extending to position the rake at the screen bottom before initiating the upward cleaning motion.

When the rake moves upward, it scrapes the screen slag from the gap between the bars, lifts it to the top discharge port, falls into the collection tank or conveyor belt, and then sends it to subsequent treatment (such as compaction, dehydration). During this upward movement, the rake teeth or buckets physically engage with the accumulated debris, lifting it along the face of the screen. The inclined orientation of the screen facilitates this movement, allowing gravity to assist in keeping the debris in contact with the rake as it ascends. The upward movement typically occurs at speeds between 3-10 meters per minute, with the optimal speed depending on the nature and quantity of the debris being removed.

At the top of its travel path, the screen rake reaches a discharge point positioned above the maximum water level. Here, the collected debris naturally falls away from the rake due to gravity, aided in some designs by mechanical scrapers or water jets that ensure complete discharge. The discharged material then enters a collection system such as a chute, hopper, or conveyor. Many installations incorporate screw compactors or dewatering presses that reduce the volume and water content of the collected screenings, minimizing disposal costs and improving handling characteristics.

After discharge, the screen rake returns to its standby position, ready to begin another cleaning cycle when triggered. The entire cleaning cycle typically takes between 30 seconds and several minutes, depending on the size of the installation and the specific design of the mechanism. During normal operation, these cycles might occur anywhere from once per hour to once every 8 hours, though during storm events or unusual conditions, continuous operation might be necessary.

Screen Rake Manufacturers

Screen rake systems find extensive use across various water treatment scenarios, with applications tailored to specific industry needs and water quality challenges.

The coarse or fine screen system of large and medium-sized sewage treatment plants and industrial wastewater treatment stations is especially suitable for scenes with high flow and high impurity load (such as municipal sewage, food processing wastewater, papermaking wastewater, etc.). In municipal wastewater treatment plants, screen rakes typically operate at the headworks as part of the preliminary treatment process. These installations handle flow rates ranging from hundreds to millions of gallons per day, removing tons of screenings annually that would otherwise damage pumps, clog pipes, or interfere with biological treatment processes. Coarse screens (typically 20-50mm spacing) provide initial protection against large debris, while fine screens (3-10mm spacing) capture smaller materials that could accumulate in process tanks or interfere with treatment efficiency.

Tianjin Kairun provides comprehensive after-sales support, including technical assistance, maintenance guidance, and prompt response to any issues. If you are choosing your screen rake manufacturers, welcome to contact us at catherine@kairunpump.com. Their engineering team specializes in customizing screen designs for specific application requirements, ensuring optimal performance in various municipal and industrial settings. With manufacturing facilities equipped with modern fabrication and testing equipment, they maintain strict quality control throughout the production process.

References

1. Water Environment Federation. (2023). Design of Municipal Wastewater Treatment Plants: WEF Manual of Practice No. 8.

2. Metcalf & Eddy, Inc. (2024). Wastewater Engineering: Treatment and Resource Recovery, 6th Edition.

3. American Water Works Association. (2023). Water Treatment Plant Design, Fifth Edition.

4. European Committee for Standardization. (2022). EN 12255-3: Wastewater Treatment Plants – Part 3: Preliminary Treatment.

5. US Environmental Protection Agency. (2024). Wastewater Technology Fact Sheet: Screening and Grit Removal.