How to Get the Right Screen for Your Wastewater Treatment Plant

What are Screen Capture Rates?

In wastewater plant operations, a screen’s capture rate is defined by the amount of screenings captured and removed by the screen, divided by the total amount in the influent stream.

Screening capture rates vary by screen type and grid opening size. This is why selecting the right wastewater screen type and grid opening is important to your overall plant performance. The screen is one of the first and best ways to ensure that your plant can effectively remove influent debris.

General Benefits of Screening

As we noted, screens improve your plant’s performance by removing debris. Some other benefits of screening:

• • Protects downstream processes and increases life of equipment
  • Saves money by reducing maintenance costs
  • Reduces organic material in wastewater sludge that goes to your offsite collection facilities

How much a screen collects is based on the amount and size of debris in the influent and the screen design. Some screen designs have a higher SCR than others.

Screens with a lower SCR can lead to:

• Underperforming headworks
• Shorter equipment lifespan
• Higher maintenance costs, due in part to the failure to protect downstream equipment

   

As process equipment becomes more sophisticated and more sensitive to foreign material, it will influence the type and opening size of the wastewater screening equipment needed to function properly.

Selecting the Right Screen is Important

Screens are designed to improve the performance of your plant by removing debris from influent. The amount of screenings collected can vary by a factor of 70 based on individual plant influent characteristics. From this, we can see that traditional screen sizing methods may not account for fluctuations in the total screenings.

Every plant has unique needs because of the factors that affect the influent wastewater. Some of these factors include:

• Population characteristics – including size and density, as well as the presence of hospitality and entertainment businesses like restaurants
• Collection systems – including pump stations and storage basins
• Flow variations – weather conditions and water-use restrictions
• Headworks – is it pumped or gravity-fed, and what is the length and slope of the channel?

   

Consider that not all screens of a certain grid size capture and remove debris equally well. For example, UKWIR testing shows that a center-flow design with a 6mm grid has an average SCR of 80%, while at the low end a step screen has an average SCR of 35%.

Even screens of the same type may have a narrow or wide range of performance. Again, UKWIR data shows that band screens have a SCR performance varying from 78% to 84%, while drum screens have shown a wide range of SCR performance from 32% to 66%.

But this doesn’t mean that choosing a screen with a high SCR is right for your facility. As we showed in last month’s article, a plant’s hydraulic profile is a major factor in determining which screen design is right.

Testing to Determine the Right Screen

 As industry standards become stricter, testing your plant’s influent for the right grid type and opening size becomes important so that you have a good idea of the performance you can expect. You’ll want to optimize your screen selection by choosing the correct open area for the expected blinding from your influent.

Undersizing a screen can result in high grid velocities leading to blow-through or stapling of screenings. This reduces screenings capture and degrades the ability of the equipment to unload. Oversizing a screen increases capital costs and footprint, as well as the electrical and wash water requirements of the system.

A screen that is not properly sized for an application could:

• Suffer excessive wear from overuse
• Fail due to structural deficiencies under hydraulic forces
• Flood headworks structures and upstream force mains

   

Find Your Right Screen with Hydro-Dyne

Every wastewater treatment plant is unique, so selecting the right screen is important.  Hydro-Dyne will help you:

• • Conduct on-site testing of your plant’s influent characteristics
  • Identify the best type of screen, grid type and grid opening
  • Optimize your screen sizing to maximize protection and extend the life of your downstream processes, while reducing the amount of equipment maintenance and chance of equipment failure

     

See how we helped the Albuquerque Wastewater Treatment Plant find the screening systems that would be most effective for their unique facility.

To determine the screen that’s best for your plant, contact Hydro-Dyne and ask us about our Hammerhead on-site screening sizing process.