Why Wastewater Screen Sizing Matters

How Wastewater Screen Size Affects Operations

To keep your entire downstream processes running effectively, you must screen influent as reliably and efficiently as possible. When you have the right size screen, you will be able to support:

• The hydraulic conditions of the plant. This ensures you can process the peak flows coming into your plant.
• The structural integrity and reliability of the headworks process. This ensures the equipment will hold up and operate during maximum water level differentials.
• The lowest overall installation and operating cost. The optimized screen size results in the smallest screen possible, reducing the size and cost of your headworks and overall operating costs.
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Let’s look at how to determine the right screen size for your wastewater treatment facility.

Hydraulic Impacts on Screen Design

Some screen designs perform better than others when placed in certain environments. Finding the right screen requires expert assessment of various factors. Screening capture rates will depend on the type of screen, size of the openings, the manufacturer’s design and manufacturing quality, and the nature of the influent.

Other factors affecting screen size that you need to include:

• Peak flow. Consider the peak flow and other flow ranges, as these will affect how efficiently you can process the influent with a screen of a certain size.
• Opening size and type. Consider the opening size and grid type needed to maximize removal of inorganic debris to minimize negative impacts on downstream processes.
• Channel dimensions. Newer channels offer the flexibility to consider multiple design options across wide rages of flows, while existing channels present more design constraints (e.g. width, depth, water levels, headloss, footprint).
• Minimum water levels at peak flow. To support your hydraulic modeling, you’ll need the channel water levels downstream of the screen during peak flow.
• Channel and grid velocities. Controlling velocities during hydraulic modelling helps to minimize settling of grit in the channel and minimizes undesirable effects of high grid velocities on screen performance.
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Some Design Options to Counter Hydraulic Sizing Constraints

• Reduce the flow. Design around an appropriate peak flow. Some design conditions call out expected peak flows in 25 years or peak flows associated with 100-year storm events. Consider other headworks design options (multiple channels, overflow weirs, etc.) to reduce peak design flow.
• Increase screen opening sizes to reduce blinding. Larger openings mean more area for flow and lower blinding percentage. Different screen types can offer the same opening size, but an overall larger open area for flow and higher overall hydraulic capacity.
• Increase the exposed surface area of the screen. Using more of the screen surface area leads to overall higher screen hydraulic capacity. You can increase the area in a couple of ways:

• Increasing the water levels. Increasing downstream water levels increases the wetted surface area of the screen and allows more flow to pass through the entire screen.
• Running the equipment faster. This doesn’t make the influent pass over more surface area, but it does quickly clean off the area it is passing through, effectively giving you more screen area to capture solids.

In both scenarios, we’re trying to increase the hydraulic capacity of the screen. However, you must balance running the equipment faster with long term wear and tear of screen components.

Find Your Right Screen with Hydro-Dyne

As we’ve seen, screens are critical to treating wastewater effectively. Wastewater screen sizing is affected by:

• Hydraulic and structural capacity
• The flow levels and influent entering the plant
• Your ability to control the velocities of the influent
• Screens of different sizes and types and how they perform under a variety of conditions

Consider all of these together and how they impact one another so you can optimize your plant’s performance.

Every wastewater treatment plant is unique, so selecting the right screen is important. To determine the screen that’s best for your plant, contact Hydro-Dyne and discuss options for your specific application.  Also ask us about our on-site screen sizing process. It usually takes just a day and a half, and you’ll be on your way to knowing exactly what you need to have a high-performing screening system.