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Pre-Treatment for THP/TCHP – Effective Sludge Treatment

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Thermal and Thermochemical Hydrolysis (THP/TCHP) systems offer a variety of benefits—reduced polymer usage, improved digestion, and enhanced dewatering performance, to name a few. But to fully unlock these advantages, one critical step is often overlooked: effective pre-dewatering through sludge thickening.

In the second session of our Sludge Treatment Webcast Series, Centrisys CEO Michael Kopper and CNP President Gerhard Forstner explored how upstream sludge thickening directly enhances the efficiency and stability of hydrolysis and biosolids treatment systems.

How sludge is prepared before it enters the reactor affects everything from polymer usage and pump reliability to energy efficiency and downstream process consistency.

The Link Between Sludge Thickening and Process Stability

A thickening step is needed before sludge enters a hydrolysis reactor. The method and degree of thickening influence how efficiently the sludge can be heated, pumped, and ultimately digested.

In traditional high-pressure THP systems, sludge typically needs to be thickened to approximately 17% total solids. Achieving this level often requires significant polymer dosing and presents mechanical challenges that many facilities are not equipped to handle.

For facilities using or considering Centrisys/CNP’s Thermochemical Hydrolysis Process (TCHP), the pre-treatment requirements are less demanding. TCHP is optimized for sludge in the range of 6–10% total solids and operates under atmospheric pressure using moderate heat and alkaline pH adjustment. This allows for a simpler and more manageable thickening strategy—without compromising performance.

Choosing the Right Sludge Thickening Equipment

Not all sludge thickening technologies perform the same, and the differences become even more important when preparing sludge for hydrolysis. In the webcast, our experts examined several commonly used thickening methods and what to consider for each including but not limited to the following equipment:

  • Gravity Belt Thickeners (GBTs): Familiar to many operators and well-suited to certain conditions, but their performance can fluctuate when sludge characteristics or flow rates vary.

  • Rotary Drum Thickeners: Known for reliable performance and moderate throughput. However, they require attentive polymer dosing and regular maintenance to maintain consistency.

  • Modified Decanter Centrifuges: When facilities modify existing centrifuge equipment, operators typically face increased power consumption, space constraints, higher polymer costs, and less effective dewatering results. 

  • THK Sludge Thickening Centrifuges: Compact, high-capacity design with patented control system that can run polymer-free under normal conditions, reducing operating costs and delivering fast ROI.

Choosing the right thickening solution depends on your facility’s specific constraints—such as space availability, sludge type, and processing goals. But when aligned with the overall system requirements, well-matched equipment can boost process reliability and set the stage for smoother, more efficient downstream operations.

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The Polymer Factor

Polymer usage is a major operational cost in both sludge thickening and dewatering. If polymers aren’t mixed properly or sludge isn't uniformly conditioned, downstream hydrolysis and digestion performance will suffer. Inconsistent thickening leads to uneven feed characteristics, which in turn stress pumps, slow digestion, and reduce biogas yield.

One of the main advantages of the TCHP system application in many plants is due to the fact that this process doesn’t require extreme solids concentrations prior to treatment. This means that the polymer dosage during pre-thickening is significantly reduced. In most installations, facilities have cut polymer usage by several folds compared to conventional high-pressure THP systems.

Lower and more consistent polymer demands allow operators to fine-tune chemical dosing and optimize equipment settings, instead of constantly reacting to fluctuations. The result will be smoother performance and better control of operating costs.

Spending too much on polymer? Input your plant data into our polymer savings calculator to see how you can improve polymer savings.

Operational Flexibility and System Design

Sludge isn’t always predictable. Feed composition can shift due to collection schedules, seasonal weather changes, or even community behavior. Systems that lack flexibility often become bottlenecks.

This opens the door to smarter system design. With fewer pressure-related constraints, facilities can avoid overbuilding their thickening systems. In many cases, they can use existing thickening infrastructure with only minor upgrades, lowering the upfront cost of implementing hydrolysis while reducing the learning curve for operators.

Lessons from the Field

During the webcast, we shared two real-world examples where sludge thickening strategies directly improved performance:

  • Case Study 1: One facility used gravity belt thickeners+centrifuge to achieve a 6.5% solids target before feeding into a TCHP reactor. With consistent feed characteristics, they avoided straining their polymer system or pumps—resulting in smoother digestion and increased gas yield.
  • Case Study 2: The same facility employed a THK+ centrifuge dewatering system to manage highly variable sludge input. The system’s adaptability allowed them to maintain performance during peak flows and seasonal changes. Reduced polymer usage and simplified pumping contributed to a quicker return on investment due to high polymer efficiency of the THKs.

Sludge thickening is a core component of overall process performance. Getting it right means fewer problems downstream and a system that’s easier to run, optimize and sustain.

Final Thoughts

Sludge thickening should never be treated as an afterthought when planning for thermal hydrolysis. Whether you’re using traditional THP or exploring the advantages of TCHP, upstream preparation has a significant influence on the efficiency, reliability, and cost of the full biosolids treatment process.

Centrisys/CNP’s experience in designing and optimizing end-to-end solutions has shown that the best results start with informed decisions about thickening. Facilities that plan for variability, align equipment with their unique needs, and manage inputs carefully are more likely to realize the full benefits of hydrolysis—without the setbacks caused by overlooked pre-treatment factors.

FAQS

Below, we answer the questions asked during the webinar about the Centrisys' THK Series Sewage Sludge Thickener, including the ROI of the equipment, installation process, and more.

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Does switching from a centrifuge to a THK require major infrastructure changes?

No, switching from a centrifuge to a THK does not usually require major infrastructure changes. If you already have a centrifuge in place, the THK has similar or lighter requirements for space and power. In fact, your existing setup will often be more than enough. In most cases, it’s simply a matter of installing the THK and connecting it—no big modifications to power, footprint, or buildings are needed.

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From a carbon footprint perspective, how does power use in a THK compare to the footprint of polymer production?

Power use can be offset or even produced with zero carbon footprint (for example, from renewable energy or waste heat recovery). Polymer, however, is made from hydrocarbons, and its carbon footprint cannot be reduced to zero. That means even if a THK uses a bit more electricity than other thickening options, shifting operating costs from polymer to power generally lowers the overall carbon footprint—especially as more utilities move to renewable or carbon-free electricity sources.

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With THK, does the polymer dosage increase linearly with the targeted percent solids?

Yes, more or less. When thickening waste-activated sludge with a THK, polymer demand does rise fairly linearly as you push for higher solids. For example, you can often reach ~6% TS with zero polymer (assuming a good SVI, ~150 or below). To reach ~10% TS, you typically need only ~2–3 pounds of polymer per dry ton. The exact curve depends on sludge characteristics (like SVI and filamentous content), but overall, the increase in dosage is relatively linear.

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Does THK have a higher risk of plugging compared to a centrifuge?

No. In practice, THK does not have a higher risk of plugging. The system works well with a grinder installed upstream—which most wastewater plants already have—to handle screenings and rags. This setup eliminates the need for fine sludge screening, keeps all solids in the process flow (so there’s no separate waste stream to manage), and avoids plugging issues.

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Why can’t I just use my centrifuge as a thickener instead of a THK and get the same results?

A standard centrifuge can be used for thickening, but it isn’t as efficient. To match a THK, the centrifuge would need to be much larger in G-volume, which means higher capital and operating costs. It also relies on a conveyor to move solids out, which consumes more power. By contrast, the THK is designed specifically for thickening—it uses a large clarification volume and discharges solids at the smallest possible diameter. That design lets it handle the same flows with about half the power consumption of a dewatering centrifuge.

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Can both a THK and a centrifuge be integrated with the same controls?

Yes. THKs and centrifuges use similar control panels and can be integrated into the same system without issue. Any differences in control logic would be minor.

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If our plant already uses a centrifuge, is it worth switching to a THK?

It depends on your plant’s current polymer and power use. THKs typically save significant polymer—often running from 0 lbs/dry ton up to ~3 lbs/dry ton—and use about half the power of a dewatering centrifuge. Those two factors drive the ROI. We recommend calculating payback case by case: if you share your flows, % solids targets, polymer usage, and energy costs, we can run an ROI analysis to show how quickly a THK would pay for itself. In many evaluations, the polymer savings alone make the switch worthwhile.

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Based on its design, how does a THK achieve a higher concentration of sludge than a centrifuge?

A dewatering centrifuge uses a conical section that fills with solids during operation, which limits its effective thickening volume. By contrast, the THK is designed as a cylindrical machine with a much larger clarification volume. It uses a hydraulic disc to separate the clarified liquid from the solids, and an air injection system at the discharge point to lift and release the thickened sludge. This design avoids wasting the cone area, maximizes hydraulic capacity, and makes it possible to reach up to ~10% total solids with lower power and less polymer.

Watch the full webinar here to dive deeper into these insights and see how your facility can benefit from a smarter approach to sludge thickening.

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