Q&A with

    Gerhard Forstner


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      To help you understand more about PONDUS, Gerhard Forstner answers engineers' questions typlically asked during an evaluation process. Download a full copy of PONDUS Q&A: A Cost-Effective and Simple Solution for Sludge Hydrolysis.

      Q&A with Gerhard Forstner, President of CNP

      PONDUS is a thermal hydrolysis process (THP) that uses low-grade heat (140 °F to 160 °F) and sodium hydroxide for sludge hydrolysis. PONDUS does not use pressurized steam as in conventional thermal hydrolysis systems. It's a cost-effective hydrolysis process to install and operate. The PONDUS design uses standard industrial control equipment; the compact size allows it to fit within a small footprint, allowing for indoor installation with protection from the elements, make it ideal for plants of all sizes and locations with harsh climates.

      Operating and maintaining PONDUS requires little to no day-to-day plant supervision, with 99% reported uptime. Pump maintenance, heat exchanger cleaning, and sodium hydrolyzed refilling are routine, straightforward maintenance tasks that plant technicians can easily perform.

      To understand PONDUS better, Gerhard Forstner, President of CNP, answers common engineers' questions.


      When was the PONDUS technology developed? When as was the the frist PONDUS system installed?

      PONDUS is a thermal hydrolysis technology in the marketplace for over 20 years. The first PONDUS system was commissioned in 2005 for Gifhorn, Germany. The plant’s objective for installing PONDUS was to increase biogas production, which it achieved with an increase that averages between 25 to 30%. Another advantage of the PONDUS system is low maintenance and operation costs.

      Visit our page on PONDUS Installations for up-to-date information.

      Compared to a leading thermal hydrolysis technology, PONDUS delivers 80% of the performance with 20% of the cost.

      PONDUS Thermal Hydrolysis technology | Kläranlage Gifhorn  |  Gifhorn, Germany

      Do you prefer 24/7 operation? Is operating for 8 hours per day possible or problematic?

      It is possible to operate at 8 hours per day or 12 hours a day. CNP prefers 24/7 operation because a steady uninterrupted sludge feeding schedule also helps stabilize the anaerobic digester's operation. The simplicity and automation of the PONDUS system makes 24/7 operation safe and straightforward. One installation operates on 8hr/day, but the PONDUS system is typically sized and designed for 24/7 operation.

      PONDUS is designed as a fully automatic system to run 24/7 year-round with 99% uptime. Download our brochure, Reduce Sludge Volume, for more details.


      What skills are required for operator and maintenance staff to operate and service PONDUS?

      The PONDUS system is designed for low maintenance and low operational costs. Because of this, most treatment plants will already have the staff needed to operate the system. No special skills or tools are necessary, nor are any outside resources required for maintenance. No steam is used, and no pressure vessels are needed for a conventional PONDUS system. Typical maintenance includes replacement of stators in the PC pumps, and a once-a-year high-pressure wash of the heat exchanger. The PONDUS reactor has no moving parts.

      Are there any special parts or components used in the PONDUS system?

      No, all PONDUS components are sourced in the USA. There are no proprietary parts on the system, such as controls or mechanical components, and no special tools are required for maintenance. None of the PONDUS parts have a long lead time and can be replaced by the operator, eliminating system downtime for our customers.

      How long does it take to start-up a PONDUS system, and how long does it take until a plant sees its impact?

      A start-up typically takes less than one week. After the system is started and the hydrolyzed sludge is fed into the anaerobic digester, an increase in gas production is observed within a couple of hours. After one SRT in the digester, dewatering improvements (higher cake solids and lower polymer dosage) are also observed.

      PONDUS was taken out of service for one week at a U.S. installation. Within 24-hours of restarting PONDUS, the plant's biogas production increased by 23%.

      Does PONDUS treatment improve digestion rates? Has this been proven?

      Yes, PONDUS improves the anaerobic digester’s operation and allows the digester to operate with a higher solids content in a shorter hydraulic retention time. Plants have reduced the SRT from 25-30 days down to 16-18 days. This increases anaerobic digestion capacity by up to 50%. Increasing the volatile solids (VS) loading rate depends on the current loading rate and your state’s standard (i.e., PONDUS can’t lower the volatile solids loading below the given state standard if applicable).


      What are the primary benefits for installing a PONDUS system

      The passing of the Inflation Reduction Act in late 2022 is a great incentive for facilities to install PONDUS as plants can take advantage of the new Investment Tax Credit (ITC). By installing PONDUS, a plant will have:

      • 25 to 35% more biogas production
      • 15 to 20% less polymer consumption at dewatering
      • 20 to 30% less disposal costs
      • Up to 80% less viscosity of thickened WAS
      • Increase digester capacity

      At what temperature does PONDUS operate?

      PONDUS operates from 140 °F to 160 °F. Heat is supplied by hot water from the heat exchanger. PONDUS uses the heat sources of the hot water system, such as the biogas boiler, the combined heat and power system (CHP), and other sludge thermal treatment processes. Lower temperatures also mean lower energy consumption and no need for high-pressure vessels to prevent boiling. This keeps the initial investment costs of the system lower and results in lower long-term operating costs.

      Does PONDUS need a cooling heat exchanger?

      The classic PONDUS system only hydrolyzes thickened waste activated sludge (TWAS). Primary sludge (PS) is mixed with the 150 °F TWAS to cool the PS+TWAS mix to mesophilic temperatures of around 100 °F. In most cases, a heat exchanger to cool the hydrolyzed sludge is not needed. If the blended sludge stream does not achieve the desired mesophilic temperatures because of an imbalance of the PS and TWAS flow, a small cooling heat exchanger can be added to the system.

      Excerpt from a TPO Mag interview with Gerhard Forstner on how the PONDUS thermal hydrolysis system works:

      "We first thicken the waste activated sludge to 6 to 10% solids. We then dose that with about 1,500 parts per million of 50% caustic soda. Next, we mix one part of that fresh sludge with two parts of hydrolyzed sludge recycled from the PONDUS reactor. That reduces the viscosity by 80 to 90%. This material passes through a heat exchanger and enters the reactor."

      The following questions were asked by engineers during the recent PONDUS Digester Intensification discussion held on June 13, 2023.

      If you were thickening both the waste activated sludge (WAS) and the primary sludge (PS), have you looked at the benefit of hydrolyzing both? Is there any benefit, or do you recommend hydrolyzing just the WAS?

      The only time you want to hydrolyze primary sludge (PS) is when your goal is to achieve Class A biosolids with a thermal hydrolysis process (THP) and mesophilic anaerobic digestion (MAD). More than 90% of the benefit from the THP comes from TWAS hydrolyzation as thickened waste activated sludge (TWAS) which is much harder to digest than primary. Hydrolyzing PS would mean twice the size of the THP system, more than double the caustic soda amount, and a cooling heat exchanger to cool down the hydrolyzed PS+WAS from 160 °F to 100 °F. In summary, it would require almost twice the capital cost to hydrolyze PS+WAS together and twice the OPEX with little benefit. A better way would be to thicken the primary sludge to about 6% and blend it with the Lysed TWAS to achieve maximum benefit in terms of increased digester capacity and improved energy balance for the solid’s treatment train.


      Can you comment on comparing CAMBI’s thermal hydrolysis system (THP) with PONDUS for medium-sized wastewater treatment plants?

      Sure, I think there are four main points to mention:

      • PONDUS uses significantly less polymer. For the PONDUS THP, we thicken to 6% with little to no polymer (0-2 lbs./DT) vs. pre-dewatering at CAMBI, which thickens to 12-15% dry solids (DS) and requires 12-15 lbs./DT of polymer.
      • PONDUS is easier and less costly to operate and maintain. Our installation in Kenosha, a 28 MGD facility, hired one person to run the THK sludge thickener, PONDUS, and dryer. Every wastewater treatment plant has the staff to operate and maintain PONDUS THP, which consists of PC pumps, a heat exchanger, a stainless-steel reactor, caustic soda tanks, and simple instruments such as IDMs and temperature gauges. No pressure vessels, and no boilers. The system easily fits indoors due to its compact footprint, which is a much smaller footprint than Cambi’s THP.
      • Simple start-up and run time. Recently a PONDUS installation at Erlangen was started up in 24 hours. Uptime in Kenosha is >95%.
      • Lower side-stream loading rates for PONDUS THP, especially for ammonia. No discoloration of centrate with PONDUS THP.


      Regarding Bio-P, if facilities have issues with struvite coming out of their digesters, then the thermal hydrolysis process will probably worsen the problem. Would you recommend a phosphorous removal process, like P-recovery or sequestration, for such applications?

      Absolutely! If a plant operates with biological phosphorus, then phosphorus removal should be part of the discussion with or without thermal hydrolysis. THP will increase the loading rates of both phosphorus and nitrogen, making P removal even more vital. We would encourage our clients to see this as an opportunity to recover phosphorus paired with THP. Since the loading rates are higher with THP, there might even be an economic case to recover the phosphorous in fertilizer form and obtain an ROI for that process. If you do it before the digester, you can use a technology like CalPrex. Post anaerobic digestion, we recommend MagPrex for struvite sequestration or recovery. Please contact us for more information and we would be happy to answer any questions regarding P-sequestration or recovery.

      For more details on how a PONDUS system works, please see Reduce Sludge Volume: PONDUS Thermal Hydrolysis Process (THP).


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