Wastewater Screen Design For Membrane Bioreactor Protection

A membrane bioreactor, or MBR, is a wastewater treatment process that combines a microfiltration or ultrafiltration process with a suspended growth biological reactor. Membrane systems require influent to be treated with ultra-fine screens with openings that are usually 2mm or less. This protection is important to remove the inorganic trash, fibers, and hair present in the wastewater stream before they get into the tank. Once in the membrane tank, they have no place to go.

High-quality screening performance matters. Just a small debris amount of bypass can lead to a major problem for your downstream processes. Every 1mg per liter of debris amounts to 8.5 pounds of material per million gallons per day.

If the membranes bundles accumulate too many solids, it prevents the rewetting of the membrane’s fiber bundles. In turn, this causes the membranes to dry out, which renders them fouled. More importantly, it reduces the system’s capacity for filtration.

Two stage screening is a best practice to protect a process that includes a membrane bioreactor. Two stage screening includes a coarse screening stage and a fine screening stage. Grit removal is typically placed in between screening stages. A current trend includes retrofitting the MBR process into a traditional wastewater treatment plant.  In this case, if the plant has a primary clarifier, a single fine screening stage is typically used placed after the primary clarifier to screen undesirable material from the treatment process. 

The benefits of drum screens in an MBR environment

Inclined or horizontal drum screens are commonly used to protect a smaller (<7 MGD) MBR treatment process.  Drum screens have a minimum of seals, typically just an inlet seal and outlet seal so the capture ratio for this screen is high. However, drums screens have their drawbacks:

  • Drum screens require a larger footprint to operate. Larger equipment can reach 10 ft diameter drums rated for screening to 10 MGD each.  Redundancy is typically included in most MBR screening designs.
  • Wide channels allow for grit settling in front of the screens during a low-flow period. If a peak-flow event follows, accumulated grit is picked up and deposited inside the drum.
  • Not the best option for retrofits, since wide channels are required.
  • Drum screens can be more difficult and costly to maintain and operate given larger equipment sizes.
  • Channel design is limited and operational flexibility to handle varied flows is limited.
  • More space is required for solids handling equipment due to the wide channel requirements needed with drum screening equipment.

The benefits of center-flow band screens for MBR systems

The center-flow band screen has a lot of advantages, including its efficiency at screenings capture. Unlike drum screens, they are good for smaller widths and deeper channels, which allows them to handle a wide range of flows.

The advantage of center-flow band screens for large installations:

  • Can be used in narrow and deeper channels to provide best hydraulics.
  • Their performance requires fewer channels
  • Small footprint. This makes them a good choice for retrofits.

The center-flow band screen offers up to two-times the grid area than a through-flow band screen design. This configuration delivers lower headloss at a higher capacity.  And because of their ability to operate in deeper channels, center-flow band screens offer much greater operational flexibility.

Like it’s drum counterpart, the center-flow band screen has its drawbacks, but not nearly as many:

  • A center-flow screen design must be made specifically for MBR application including low tolerances between connecting elements. The market has many center flow screen designs that do not have the precision design needed to limit debris leakage through seals.
  • The installation base for center flow designs is smaller than drum screening designs used for MBR processes.

Hydro-Dyne supports your MBR operations

Hydro-Dyne is the leader is building screens that meet the needs of plant operations throughout the United States and around the world. We can do the same for your facility’s flow and channel dimensions.

Our work is distinguished by having some of the highest SCRs (screening capture rates) in the industry. From our Clearwater, Fla., headquarters, we manufacture components that exceed the industry standards for performance and reliability. We’ve led over 2,000 installations of our equipment worldwide.

Hydro-Dyne’s Great White Center Flow Screen has a formal approval from Kubota to be used as a pre-screen to protect their MBR Systems. The Great White is the only center flow screen to have this distinction; Suez and Evoqua have also given the Great White formal approval.

To speak with a Hydro-Dyne expert about which screening system is right for your wastewater treatment facility, call us at 813-818-0777 or contact us online.

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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.