Best Practices for Wastewater
Screening Design

The Use of Weirs to Optimize Screen Design

Wastewater screening is the crucial first step in the wastewater treatment process, designed to remove large solid materials and debris from the influent wastewater. This process is essential to safeguard downstream treatment units and prevent potential equipment damage, operational inefficiencies, and clogging issues. By effectively screening out unwanted solids, the treatment plant can maintain smooth operations and minimize maintenance requirements. 


Wastewater screening involves the use of specialized screens tailored to specific debris removal needs. Properly sizing and designing the screens, along with implementing appropriate operational mechanisms, ensure optimal solids capture and maintain hydraulic efficiency. A well-executed wastewater screening process lays the foundation for successful treatment downstream, making it a fundamental aspect of any wastewater management strategy.

Types of Wastewater Screens

There are various types of screens available, each designed to address specific requirements. Selecting the appropriate wastewater screen is paramount in achieving the right solution for effective solids removal. With a range of screens available, each designed to address specific requirements, it is essential to carefully match the screen type with the unique characteristics of the influent wastewater.


Commonly used screens include:

  • Bar Screens: These consist of vertical or inclined bars spaced to intercept and retain debris. They are suitable for large and medium-sized debris removal.
  • Fine Screens: Fine screens are designed to capture smaller particles, such as grit, hair, and plastics. They use finer mesh or perforated plates to achieve this.
  • Drum Screens: Drum screens are rotating cylindrical devices with fine mesh or perforated plates that capture and remove solids as wastewater flows through them.
  • Step Screens: Step screens feature a series of fixed or movable steps, creating a barrier that captures and lifts solids out of the wastewater.

Wastewater Screening Design

Correct screen design is crucial to optimize the screening process, as it ensures that the screening system can handle the expected flow rates and effectively capture the desired size range of solids. 


Key design parameters to consider are:

  • Solids Loading: Analyze the concentration and characteristics of solids in the influent to estimate the amount of debris the screen will encounter.
  • Screen Openings: The size and shape of screen openings depend on the type and size of solids expected in the wastewater.
  • Screening Capture Ratio (SCR): SCR is the percentage of solids captured by the screen, and it should be maximized for effective screening.
  • Hydraulic Headloss: Ensure that the screen design minimizes headloss to maintain the desired flow rate.
  • Flow Rate: Calculate the average and peak flow rates to determine the required capacity of the screen.
  • Screen Grid Velocity: The speed at which the wastewater flows through the screen openings.


It is crucial to maintain an appropriate grid velocity to prevent clogging and achieve efficient solids capture. If the grid velocity is too high, smaller particles may not be adequately retained, leading to reduced screening effectiveness. Conversely, a grid velocity that is too low may result in clogging and hinder the flow of wastewater. 


For more Best Practices for Wastewater Screening Design when it comes to screen grid velocity, download our article.

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Weirs in Wastewater Screening

Weirs play a crucial role in wastewater screening, especially in bar screens and step screens, enhancing their efficiency and overall performance. A weir is a device that allows water to flow over its crest while retaining solid particles and debris. 


Integrating weirs into the screening process offers several benefits:


  • Flow Control: Weirs help regulate the flow of wastewater through the screening system, ensuring a consistent and controlled flow rate. By maintaining a stable flow, the screens can operate optimally, maximizing the capture efficiency of solid particles.
  • Equalization: In cases where the influent flow varies significantly, weirs provide equalization by stabilizing the flow rate. This prevents sudden hydraulic surges that could overwhelm the screening equipment, minimizing the risk of screen clogging and system failure.
  • Pre-Screening: Weirs can act as preliminary screens by capturing larger objects and coarse debris before they reach the main screen. This pre-screening step reduces the burden on the primary screen, allowing it to focus on removing smaller particles effectively.
  • Distribution: Weirs facilitate even distribution of the wastewater flow across the width of the screen, ensuring that all parts of the screen receive a relatively uniform amount of wastewater. This uniform distribution enhances the screening process’s efficiency and prevents localized issues such as screen blinding.
  • Hydraulic Profile Management: Weirs assist in managing the hydraulic profile within the screening system. By adjusting the height and length of the weir crest, operators can control the depth of the water on the screen surface. An appropriate hydraulic profile ensures efficient solids capture and prevents solids from bypassing the screen.
  • Baffle Effect: The positioning of weirs within the screening chamber can create a baffle effect, encouraging solids to accumulate near the screen face. This effect increases the likelihood of particles contacting the screen surface, further improving screening efficiency.
  • Solids Accumulation: Weirs can act as collection points for retained solids and debris. The accumulation of solids at the weir’s crest makes it easier to remove them during cleaning and maintenance operations.
  • Safety and Reliability: Properly designed weirs contribute to the overall safety and reliability of the screening system. They minimize the risk of hydraulic disturbances and uneven flow distribution, which can lead to operational challenges and increased maintenance requirements.

Weir Design Considerations

When incorporating weirs into wastewater screening systems, several design considerations should be taken into account:


  • Weir Type: Common weir types with screening are a Sutro weir or an underflow weir
  • Weir Length and Height: Calculate the optimal length and height of the weir crest to achieve the desired hydraulic profile and flow control.
  • Materials and Construction: Select corrosion-resistant and durable materials for constructing the weirs to withstand the harsh wastewater environment.
  • Flow Rate and Headloss: Analyze the flow rate and headloss across the weir to ensure it operates within the desired range and does not negatively impact the overall treatment process.

Weir Installation and Maintenance

Proper installation and regular maintenance are essential to ensure reliable screening performance. Installing a weir downstream after screening equipment is a strategic decision that brings several advantages to the wastewater treatment process. 


Once the influent wastewater passes through the screening equipment and larger solids and debris are removed, the weir acts as a flow control and equalization device, regulating the flow rate and stabilizing hydraulic surges. The weir’s positioning ensures a uniform distribution of wastewater across the downstream treatment units, optimizing their performance and preventing overloading. Additionally, the weir serves as a preliminary screen, capturing any residual coarse particles that might have bypassed the initial screening process. This aids in maintaining the efficiency of the downstream treatment units and reduces the risk of clogging or damage caused by large debris. 


The weir’s presence also facilitates the accumulation of retained solids, making cleaning and maintenance operations more manageable. By carefully designing and installing a weir downstream after screening equipment, wastewater management professionals can enhance treatment efficiency, prolong equipment lifespan, and ensure the smooth operation of the entire treatment system.


Mastering wastewater screening design is fundamental to successful wastewater management. By understanding the various design elements, you can optimize your screening system’s performance and protect downstream treatment units. 


In a compelling case study, Hydro-Dyne’s innovative solution proved to be a game-changer for Tulsa, Oklahoma’s Cherry Creek Lift station. By implementing their advanced technology, the lift station’s performance witnessed a remarkable improvement, primarily due to simplified maintenance requirements, enhanced waste capture capabilities, and robust protection for their Heavy Duty Through Flow Screens. This study sheds light on the critical maintenance challenges that were negatively impacting pump operation and downstream processes. Hydro-Dyne’s alternative solutions, tailored for superior debris removal and ease of maintenance, played a pivotal role in transforming the lift station’s efficiency and reliability. For a more in-depth understanding of the success achieved through this remarkable project, delve into the details of this case study.

Incorporating weirs into wastewater screening systems enhances the efficiency and reliability of the screening process. By managing flow rates, promoting equalization, and acting as preliminary screens, weirs contribute significantly to the overall success of the wastewater management process. 


Implementing weirs will not only improve your wastewater treatment plant’s efficiency but also contribute to a cleaner and more sustainable environment.