Effective debris removal screening is an essential step in preliminary treatment as clean-water plants, and bar screens are the devices of choice for many.

Automated screens free operators from the miserable job of manual cleaning. A key issue is how to balance optimum debris capture and hydraulic capacity to handle high-flow events.

Duperon last January introduced two next-generation versions of its FlexRake bar screen. The FlexRake IQ is designed to enhance debris removal by automatically increasing capacity and speed to accommodate rising flows. The FlexRake IQ2 adds capability to widen the bar openings automatically during peak events, providing greater resiliency under challenging flow conditions.

Both incorporate primary features of the original FlexRake that are designed to enable simple operation, reliable performance and low maintenance. Bryce Funchion, mechanical engineer and product owner, and Mark Turpin, company president, talked about the new versions in an interview with Treatment Plant Operator.

What qualities have made the original FlexRake popular in the marketplace?

Turpin: The FlexRake has been around for about 30 years, and a couple thousand have been installed. In designing the product, our founder Terry Duperon focused on minimizing complexity for operators. His basic philosophy is that two parts are one too many. He tries to eliminate as many wear parts and moving parts as possible. In that way you end up with a machine that’s very simple to operate and maintain and has a very low life-cycle cost.

Was the machine designed with any particular operating challenges in mind?

Turpin: There is a propensity for bar screens to get jammed by large debris that tries to enter near the headworks channel floor. When that happens, the channel has to be dewatered, and an operator has to enter that confined space and clear the jam. To combat that, Terry developed a link formed into a chain that eliminated the lower sprocket and shaft, so there is nothing down there to maintain. Now if a large object meets the screen at the channel floor, the link system reaches out over it with a scraper, which engages the object and brings it up the screen. So we can bring very large debris out of the channel without creating a jam.

What market need were you looking to meet with the new-generation screens?

Turpin: In looking at what operators are dealing with, we’ve found there is huge variability in flows. That can be because of I&I issues. It can be because a developer created a new subdivision and now plant flows are significantly higher. Perhaps they have a college in town and the flows are much higher when the students are on campus. Or maybe that college has a football stadium and there are massive flows on game days.

How does that affect the screening process and the choice of screening equipment?

Turpin: The dilemma it creates is the screens have to be designed with an opening wide enough to address those peak flows. That sub-optimizes the operation of the plant, because 90% of the time when those high flows are not occurring, they could be screening at a much finer opening.

How did you address that issue with the new screens?

Turpin: We asked: What if engineers didn’t have to modify their design to accommodate changing flow conditions? What if the screen itself adapted to them? We created the new design to provide a larger safety factor that assures system reliability during normal flows and worst-case events. We also recognized that there is a large hydraulic load on the machine in high-flow events, but that early in those events there can be a very significant load of solids and grit. So we developed a machine that responds to those conditions.

How exactly do the two models accommodate those conditions?

Turpin: In the FlexRake IQ we redesigned the scraper and some other components to handle grit more effectively. In addition, during a high-flow event, if the machine engages with a large piece of debris, it will do what the original FlexRake does. And the link will re-engage more rapidly, so the screen remains clean during those extreme events. The IQ2 is an additional feature. If the hydraulic head continues to build ahead of the screen, then the screen will automatically change to a larger opening.

How is the change to the larger screen openings accomplished?

Turpin: After the first flush comes in and the debris is removed from the channel, the bar screen changes from a 1/4-inch opening to a 3/4-inch opening. This occurs without any operator interaction. When the high-flow event is over, the speed and the screen opening return to normal. This also occurs automatically.

What triggers the change in the screen opening?

Funchion: There are ultrasonic transducers that detect the water levels upstream and downstream of the unit. From there an algorithm calculates the headloss across the screen. We set parameters to change the bar opening at a certain head differential.

Are these screens designed mostly for plants with highly variable flows?

Funchion: They are good technologies for any treatment plant. There are probably some plants that have very stable flows, but the vast majority have high-flow events to manage. And in case a plant should encounter an extreme event or some other form of variability in the future, the FlexRake IQ2 especially gives them a belt-and-suspenders approach, so they have flexibility to allow more flow at any given time.

What was done to prove out this product?

Funchion: We did an alpha test at the wastewater treatment plant in Saginaw, Michigan, where we’re located. We studied it at high speed for six months and simulated roughly a five-year life cycle. It didn’t show any wear, so we know the service life is going to be much longer than that. We also installed a beta unit in Saginaw Township. They purchased the unit, and it has been running since June 2020.

What are customers saying about these technologies so far?

Turpin: When we show it to people, they say it just makes sense. People understand the logic of what we’re doing and the advantages to it. The feedback has been favorable.