Seven Common Wastewater Mechanical Screens

Today’s sophisticated wastewater treatment processes require highly effective operations. It all starts in the headworks where optimal performance is largely determined by effective screenings capture ratio (SCR). There are three major design principles for maximizing SCR through tailored screen design: type, size and grid opening. Let’s talk about screen types and how they support the mission of wastewater treatment plants.

 

Through the decades, we’ve seen a variety of screen types emerge, each in their own way aligning to the performance that plant operators need for their facilities to run effectively. In this overview, we’ll look at the different types of screens for wastewater treatment plants, and the benefits and challenges that each of them present.

Single Rake Mechanical Bar Screen
This design features a stationary bar rack with large openings, greater than 1”, that is anchored within the channel. A rake arm engages the screening materials and lifts it up and out of the channel.

 

Pros:

  • Can lift large objects out of deep channels
  • Very rugged and reliable with heavy duty carbon steel painted frame

Cons:

  • High overhead heights make maintenance points difficult to access
  • Potential to jam

Step Screen
This design includes two vertical stainless-steel plates shaped like steps; one of the plates is stationary, while the other rotates. The plates alternate across the width of the channel opening to form the step screen face, elevating solids one step at a time to the discharge point.

Pros:

  • All stainless-steel housing
  • Small footprint and low profile
  • Highly cost effective

Cons:

  • Does not have a high capture rate
  • Has problems lifting larger solids
  • Does not screen high flows

Through Flow Continuous Belt Screen
This design signaled the birth of fine screens and is known for being both a filtering and removal mechanism. Usually there is a rotating or static brush to knock the screenings off. A spray wash is also commonly used for offloading materials.


Pros:

  • All stainless-steel housing
  • Small footprint and low profile
  • Highly cost effective

Cons:

  • Does not have a high capture rate
  • Has problems lifting larger solids
  • Does not screen high flows

Center Flow Screen

These screening systems became popular in the U.S. in the 2000s. The center flow screen provides a solution to the limitation of the drum screen design by elongating the screen opening, which allows for a higher flow into the equipment. It features an all stainless steel construction.

 

Pros:

  • Works well with wastewater applications
  • High screening capture ratio
  • Offers a submerged screen area in a relatively narrow channel
  • Zero carryover of debris into downstream processes

Cons:

  • The challenge with this screen type is having an effective seal with the panels and the frame
  • One of the most expensive screens

Multi-Rake Bar Screen

The single rake type bar screen has evolved into a design that includes all or mostly stainless-steel construction with multiple rakes.

Pros:

  • Durable
  • Good solution for pump protection or headworks coarse screening
  • Maintenance tends to be accessible from the top channel
  • More positive engagement of rakes into bars than previous versions of bar screens

Cons:

  • High overhead heights make maintenance points difficult to access
  • Potential to jam

Spiral Screw Screen

This design, which originated in Europe, includes a trough with a ¼” perforated hole basket. The screening is cleaned by a rotating screw which transports solids up into the compaction area. The design includes a stainless-steel housing.

Pros:

  • All stainless-steel housing
  • Highly cost effective

Cons:

  • Does not have a high capture rate
  • The screw tends to push material through the perforated basket
  • Can be expensive to maintain, especially with grit in the sewage
  • Only suitable for small plants

Inclined Drum Screen

Developed by screw screen manufacturers for screening, washing, and handling in single unit. Inclined rotating drum screen in the channel replaces the static basket. Spray bars offload solids into a trough containing a rotating screw.

Pros:

  • Efficient screening capture uses perforated drums
  • Zero carryover of debris into downstream processes
  • Very high screening capture ratio

Cons:

  • Restricted in capacity, multiple units needed for larger plants
  • Potential for higher headloss and blinding

 

Finding the right screens for your wastewater operations
We’ve looked at several screening systems that can be matched to municipality wastewater treatment needs. Independent testing will help with the process of selecting the right screen type for your operations, and on-site sampling will help optimize the proper screen size and screen grid opening. If you’d like to discuss your specific needs, connect with Hydro-Dyne’s highly experienced professionals for more information about determining the right screen – type, size, and grid opening — for your wastewater treatment facility.

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