Q&A with

    Gerhard Forstner


      Download Your Guide

      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 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 that is safe and simple to install and operate. The PONDUS design uses standard industrial control equipment; the compact size allows it to fit within a small footprint, making it ideal for plants of all sizes. The typical payback period of a PONDUS system is less than 5 years.

      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.


      Is PONDUS a new technology? When was the first PONDUS system installed?

      PONDUS is not a new thermal hydrolysis technology. The first PONDUS system was commissioned in 2004 for the town of Gifhorn, Germany. The plant's objective for installing PONDUS was to increase biogas production. PONDUS has increased biogas production at Gifhorn between 25 to 30%. The system is still going strong with minimal maintenance requirements 19 years later.

      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

      Is PONDUS available for plants in the USA?

      To date, we have 11 full-scale PONDUS systems installed internationally along with two more in design. The Kenosha Water Utility in Kenosha, Wisconsin was the first PONDUS installation in the USA with 24/7 operation since March 2016.

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


      Have there been any issues with digester pH getting too high with a PONDUS system?

      We have not seen the digester pH getting too high due to the PONDUS system. The PONDUS reactors raise the pH to 10-11. With the release of volatile fatty acids (VFAs) in the hydrolysis process, the pH returns to approximately 7 in the PONDUS reactor's outflow.

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

      Related from a TPO Mag interview with Gerhard Forstner on how the PONDUS hydrolysis 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."


      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 helps stabilize the operation of the anaerobic digester as well. The simplicity and automation of the PONDUS system makes 24/7 operation safe and straightforward. We have one installation that operates on 8hr/day, but the PONDUS system is typically sized and designed for 24/7 operation.

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

      The primary components of PONDUS are progressive cavity pumps and a heat exchanger. Most water resource recovery facilities will have operators who can operate the PONDUS system. No special skills or tools are necessary, nor are there are any outside resources required to maintain the system. There's no steam used, and no pressure vessels needed for a conventional PONDUS system. Typical maintenance includes replacement of stators for PC pumps, and a once a year high-pressure wash of the heat exchanger. The PONDUS reactor has no moving parts and is maintenance-free.

      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 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 observed as well.

      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.

      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 downtime of the system for our customers.

      Install 200907 Basis piping 11 overview

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

      Yes, PONDUS does improve the operation of the anaerobic digester and allows the digester to operate with a higher solids content in a shorter hydraulic retention time. We've seen plants reduce the SRT from 25-30 days down to 16-18 days. This increases anaerobic digestion capacity up to 50%. The ability to increase 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


      What other benefits does PONDUS provide a plant?

      With cell lysing, the organic matter in the bacterial biomass is released together with an increased soluble fraction of organic matter, often measured as chemical oxygen demand (COD). Some fraction of soluble COD is VFAs, readily available for methane foaming bacteria in the anaerobic digesters. This is how PONDUS will accelerate the rate of biogas formation. The VFAs generated by the PONDUS system are beneficially used as a carbon source for the biological nutrient removal (BNR) process.

      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 that are part of the hot water system, such as the biogas boiler, the combined heat and power system (CHP), and other sludge thermal treatment processes.

      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 cooling down heat exchanger 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.

      Is it possible to predict the performance of PONDUS with a feasibility test?

      Yes, it is possible to predict some parameters of PONDUS in the lab. Centrisys/CNP can arrange feasibility testing to replicate the hydrolysis process with sludge provided by the plant. This testing confirms the pH, viscosity reduction, COD solubilization, and VFA accumulation by PONDUS. It is also possible to provide a breakdown of VFAs and biological methane potential testing to evaluate a potential biogas improvement. The combination of these values provides a good scenario of what to expect within a full-scale installation.

      Contact us today if you have questions or want to find out more about how a PONDUS system could benefit your plant. By requesting a sample analysis or pilot test, our knowledgeable team can perform either an on-site or off-site sample analysis.