Thirty Years of Spiral Rakes in Clarifiers – What Have We Learned?
The year 2019 marks thirty years since WesTech’s first installation of spiral blade clarifiers at the 23rd Avenue Treatment Plant for the city of Phoenix, Arizona. The then-new clarifier design included many innovations, including (but not limited to) deep, continuous spiral rake blades, an energy dissipating inlet (EDI), and a Stamford baffle below the effluent weir. It was christened the COP™, which stood for Clarifier Optimization Package.
The introduction of a new, optimized design for what was essentially a very mature product in the industry was extremely successful. In the last 30 years, WesTech has installed 1,700 COP clarifiers around the world, and every other clarifier mechanism supplier has developed its own version of a spiral-blade rake mechanism.
The old segmented rake blade design is now universally recognized as obsolete, particularly for larger tank diameters where effective transport of settled solids to the tank center is more difficult. Spiral blade mechanisms have been successfully applied in primary clarifiers, secondary clarifiers following fixed film processes, and secondary clarifiers following activated sludge processes. In all applications, performance has been excellent.
Spiral Blade vs. Suction
In general, it is fair to say that spiral-blade mechanisms offer superior performance when measured against suction mechanisms. Even so, there continue to be a few skeptics who maintain that suction sludge-removal systems are superior to spiral rakes in secondary clarifiers that treat activated sludge. Virtually all of this skepticism derives from a leftover perception that arose from comparisons with the old segmented rake designs, which do not transport settled solids very effectively. Indeed, suction removal systems do outperform old-style segmented scrapers, producing shallower sludge blankets and reduced solids inventory in the clarifier. This in turn results in more capacity to handle high flow events and less chance of denitrification or secondary phosphorus release in nutrient removal facilities.
Multiple studies have shown similarly significant reduction in blanket depths when old-style segmented scrapers were retrofitted with spiral rake blades. One study in adjacent flat-floored clarifiers at Knoxville, Tennessee even directly compared a suction header sludge removal system with a spiral rake mechanism. At high return activated sludge (RAS) rates, the sludge blankets were the same depth, and at lower return rates, the spiral scrapers produced a lower sludge blanket than the suction system.
Given this kind of data, skeptics sometimes fall back on arguments that sound reasonable but are based on supposition. I call this kitchen logic because it’s the kind of logic I might use when arguing with my brother-in-law in the kitchen. For example, some say that scrapers don’t actually move settled solids, but simply resuspend or fluff them up so that they will flow by gravity to the central draw-off point. They base this logic on the fact that settled biosolids are indeed thixotropic and require shear to flow, and because once or twice, they have seen a plume of light solids rising behind a too-fast moving scraper mechanism.
We at WesTech have also seen this phenomenon. I have personally seen small plumes of light solids rising behind virtually every type of clarifier mechanism arm, including old-style segmented rakes, spiral rakes, suction pipe arms, and suction header arms. In every case, the process biosolids were very light, and the clarifier mechanisms were turning at the upper end of recommended design speeds. Even though the plumes resulted in no significant increase in effluent total suspended solids (TSS), a simple reduction in tip speed eliminated the visual problem in every case.
A Simple Test
There is a simple way to determine whether a spiral rake blade actually moves settled solids. I have done it many times, and you can do it in your own treatment plant, or at least in one close by if you don’t have a spiral rake clarifier in your own plant. Take a core sampler and measure the sludge blanket depth just in front of the spiral rake arm and again just behind it. You will find a significant reduction in the depth of the blanket, without observing significant fluffing or resuspension of solids above the sludge interface. Spiral rake blades do move settled biosolids and they do so very effectively.
Spiral Blade Systems Require Less Maintenance
Spiral rake blades require less maintenance than suction sludge removal systems. Suction pipes can easily clog with debris or solids settling in the horizontal runs. Orifices in suction headers can also clog. An operator at a Colorado brewery complained that grocery bags kept blowing into his clarifiers and plugging the withdrawal orifices, requiring him to take the tanks out of service to remove them. Another plant I visited in Illinois often had to pump down its clarifiers in the fall to clean out leaves that had blown in and plugged the orifices.
Also, all suction sludge removal systems include seals between the rotating and stationary parts of the mechanisms. These seals wear out regularly and require taking the clarifier out of service to replace them. According to one operator, this
always happens during the wet weather season, when the plant needs its full capacity.
Two Final Words: Effluent. Quality.
I still haven’t addressed the most important question: How does the effluent quality of an activated sludge secondary clarifier with a spiral rake mechanism compare to that of a suction removal system? After all, this is where the rubber meets the road; producing clear effluent is the main function a secondary clarifier is supposed to perform.
Recently, WesTech reached out to a large cross-section of treatment plants with COP secondary clarifiers and asked them to share their plant effluent data. As expected, the results agreed with early studies: Plant effluent performance was uniformly excellent, with all plants easily meeting their permits and most plants achieving single-digit TSS and biochemical oxygen demand (BOD) numbers.
Cases in Point
As an example of spiral blade mechanisms’ performance, the Medford Regional Water Reclamation Facility (WRF) in Medford, Oregon, upgraded its existing suction-header clarifier mechanisms with WesTech COP clarifiers. The following figures compare the effluent TSS of the WRF’s existing suction clarifiers with that of its new spiral blade clarifiers. (Download the case study about this upgrade.)
Average effluent TSS concentrations dropped from 5.6 milligrams per liter (mg/L), with spikes well over 10 mg/L, to 4.2 mg/L, with spikes below 10 mg/L, even though the plant has experienced higher overflow rates since the upgrade.
WesTech performed flocculated suspended solids (FSS) tests and a dispersed suspended solids (DSS) tests on Medford WRF’s spiral blade clarifiers in July, 2018. In these tests, a sample of mixed liquor was optimally flocculated in a lab stirrer for 30 minutes, then statically settled for 30 minutes. Supernatant was then withdrawn and tested for TSS. The tests showed that the lowest achievable effluent TSS for the Medford biosolids was 2.5 mg/L. The actual effluent TSS for the COP clarifiers was consistently within 1.7 mg/L of this best theoretical value. This is best-in-class performance.
West Jordan, Utah
South Valley WRF in West Jordan, Utah, also retrofitted existing equipment with COP spiral blades. In this case, the WRF was using old-style rake blades. Because the segmented rake blades push solids a short distance with every revolution and spiral blades maintain consistent contact, the facility’s spiral blades moved settled solids four times faster than its old segmented rake blades. (Download the case study about this retrofit.)
The facility is operating under a permit that limits effluent TSS to 35 mg/L per week, and 25 mg/L per month. The following figure demonstrates that the facility’s spiral blade clarifiers keep even the peak TSS well below the 25 mg/L monthly limit.
A third example WRF, Pace Water System, Inc., in Pace, Florida, expanded its current WRF to include two WesTech OxyStream™ oxidation ditches to treat biological phosphorous and two COP clarifiers to remove suspended solids. Here, treated liquor flows over an effluent weir into the COP clarifiers, which then remove suspended solids. The WRF’s COP clarifiers deliver an effluent that ranges from 2.0 mg/L to 2.3 mg/L. (Download the case study about this expansion.)
So, what have we learned about spiral blade clarifier mechanisms in the last thirty years? We’ve learned a lot. For example, we’ve learned that:
- Deep spiral rake blades are significantly more effective at transporting settled solids than old-style segmented shallow rake blades
- As a result, spiral blades are now the acknowledged industry standard
- Spiral blade systems compare favorably to suction sludge removal systems
- Spiral-blade clarifiers require less maintenance than suction sludge removal systems and therefore reduce system downtime
- Effluent quality is uniformly excellent with spiral blade systems
Given the obvious benefits of clarifiers that use spiral blades, I predict that these systems will maintain their status in the industry for years to come.