Director R&D at PhibroChem
An anaerobic digestor (AD) is a microbiological process whereby a consortium of bacteria in the absence of oxygen sequentially biodegrade organic material into a biogas rich in methane and carbon dioxide, and reduce the Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) of the incoming process water. The majority of fuel ethanol plants in North America have an AD which is used to recycle process water within the plant. Fuel ethanol plants at other locations use AD’s prior to discharging process water from the plant.
Numerous species of bacteria (more than 100) have been isolated from anaerobic digestors. Some of the bacteria include Enterobacteriasiae sp , Clostridium sp , Methanobacterium sp, Methanococcus sp, Methanosaeta sp, Methanospirilium sp, Methanosarcina sp, and Methanobacter sp. These bacteria are not part of the traditional Lactic Acid Bacteria (LAB) that are typically isolated from fuel ethanol plants but can nonetheless seriously inhibit yeast viability and fermentation performance. Worse, many AD bacteria have cell wall compositions that are not strictly Gram +ve/-ve and so traditional antimicrobial treatments vary in effectiveness.
To maximize ethanol yield at a fuel ethanol plant, bacterial contamination at any location within the plant is normally brought under control via process optimizations and/or proper dosing of antimicrobial products. Typical bacterial contamination levels that begins to inhibit yeast within a fermentor is 1×105 CFU/ml.
However, to function properly, AD’s require a high level of viable bacteria. In most cases total viable bacteria concentrations isolated from functional anaerobic digestors exceed 1×109 CFU/ml and can reach 1×1010 CFU/ml.
In addition to directly inhibiting yeast (via numbers), AD bacteria also metabolically produce inhibitory organic compounds (fatty acids: SCFA, MCFA, LCFA) and other compounds that are not destroyed in plant unit operations and can stall yeast fermentations – some at concentrations as low as 0.1 %w/v. These organic compounds are not typically detected by HPLC systems located at fuel ethanol plants.
This seminar will initially outline AD microbiology and highlight the types of bacteria and organic chemistries. The seminar will then rapidly accelerate with ethanol plant examples where the AD unit operation was compromised – leading to sluggish yeast fermentations and loss in ethanol yield. Controlled experimental lab results will be discussed that indicates the partial toxicity of methanator discharge water on optimal yeast fermentations. Strategies to mitigate AD process upsets at the plant will be outlined that will aid in recovering ethanol yield and yeast vitality.
About Dr. Bayrock:
Dr. Dennis Bayrock completed his PhD from the University of Saskatchewan. He specializes in multistage continuous fermentations and in microbial contamination in fuel alcohol fermentations. In total, Dr. Bayrock’s career in practical research and international experience has spanned over 18 years in the fuel ethanol industry.
Dr. Bayrock is an Adjunct Professor at the University of Minnesota where he conducts research and provides teaching in fuel ethanol fermentations. In addition to his faculty position, Dennis continues his responsibilities in research and development for Phibro at PhibroChem’s Ethanol Performance group lab in St. Paul, MN.
During his career he has visited numerous fuel and potable alcohol plants in Canada, US, Jamaica, Puerto Rico, Brazil, and Thailand and carried out plant microbiological audits, optimized addition of antibiotics on a plant-by-plant basis, and optimized plant fermentation conditions. Dr. Bayrock has demonstrated his expertise in identifying, cataloging, and screening various microbial contaminants from the fuel and potable alcohol industries. He has made numerous emergency “house calls” to plants suffering from contamination and has successfully diagnosed, treated, and restored plants to normal operation. As an internationally invited lecturer, Dr. Bayrock has been well received at conferences in the fuel and potable alcohol industries, and has presented numerous seminars on general plant operations and on multiple topics relating to yeast and bacteria.