Biomass to Biogas—Anaerobic Digestion
by Michael Biarnes
Anaerobic digestion is a multistep biological and chemical process that is beneficial in not only waste management but also energy creation. There are four fundamental steps of anaerobic digestion that include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Throughout this entire process, large organic polymers that make up Biomass are broken down into smaller molecules by chemicals and microorganisms. Upon completion of the anaerobic digestion process, the Biomass is converted into Biogas, namely carbon dioxide and methane, as well as digestate and wastewater.
Source: Anaerobic Flow Diagram
Anaerobic Digester—Fundamental Steps
In general, hydrolysis is a chemical reaction in which the breakdown of water occurs to form H+ cations and OH- anions. Hydrolysis is often used to break down larger polymers, often in the presence of an acidic catalyst. In anaerobic digestion, hydrolysis is the essential first step, as Biomass is normally comprised of very large organic polymers, which are otherwise unusable. Through hydrolysis, these large polymers, namely proteins, fats and carbohydrates, are broken down into smaller molecules such as amino acids, fatty acids, and simple sugars. While some of the products of hydrolysis, including hydrogen and acetate, may be used by methanogens later in the anaerobic digestion process, the majority of the molecules, which are still relatively large, must be further broken down in the process of acidogenesis so that they may be used to create methane.
Acidogenesis is the next step of anaerobic digestion in which acidogenic microorganisms further break down the Biomass products after hydrolysis. These fermentative bacteria produce an acidic environment in the digestive tank while creating ammonia, H2, CO2, H2S, shorter volatile fatty acids, carbonic acids, alcohols, as well as trace amounts of other byproducts. While acidogenic bacteria further breaks down the organic matter, it is still too large and unusable for the ultimate goal of methane production, so the biomass must next undergo the process of acetogenesis.
In general, acetogenesis is the creation of acetate, a derivative of acetic acid, from carbon and energy sources by acetogens. These microorganisms catabolize many of the products created in acidogenesis into acetic acid, CO2 and H2. Acetogens break down the Biomass to a point to which Methanogens can utilize much of the remaining material to create Methane as a Biofuel.
Methanogenesis constitutes the final stage of anaerobic digestion in which methanogens create methane from the final products of acetogenesis as well as from some of the intermediate products from hydrolysis and acidogenesis. There are two general pathways involving the use of acetic acid and carbon dioxide, the two main products of the first three steps of anaerobic digestion, to create methane in methanogenesis:
CO2 + 4 H2 → CH4 + 2H2O
CH3COOH → CH4 + CO2
While CO2 can be converted into methane and water through the reaction, the main mechanism to create methane in methanogenesis is the path involving acetic acid. This path creates methane and CO2, the two main products of anaerobic digestion.
Temperature is a key component to the efficiency of anaerobic digesters. As in other chemical and biological processes, the more energy that is put into a reaction, the faster the reaction runs, until a point of degradation occurs. Being that organisms are responsible for the digestion process, it is vital to ensure that the entire process is kept within a certain temperature range to maximize reaction speed and organism livelihood. That being said, the range depends on the species of organism used. The two most widely used types of microorganisms are mesophiles, which undergo mesophilic digestion, and thermophiles, which undergo thermophilic digestion.
Mesophiles are most efficient at moderate temperature ranges between 25-40°C while thermophiles run most efficiently at higher ranges between 45-80°C. The words mesophile and thermophile are simply terms used to classify broad groups of organisms and just describe the temperature of their livable environment. Ideally, one should know the exact microorganism used and know the range that is most suitable for that specific species. As in other chemical processes, if the temperature is monitored during the digestion process and kept in the upper portion of the specific range given, the rate of reaction will be at its highest value producing more gas in the same amount of time. While higher temperatures produce higher gas yields, they are also more difficult and expensive to maintain making temperature a vital aspect to measure throughout the digestion process.
CO2 and Methane Measurement
Biogas production rate is also a key component to maximizing anaerobic digestion efficiency. It is vital to constantly measure Biogas emissions (methane, carbon dioxide, trace amounts of siloxanes, hydrogen sulfide, ammonia, hydrocarbons, and water) as the production levels are good indications of production abnormalities, Biomass quantities, and microorganism well being. Being that in anaerobic digestion, biogas is produced with a normal distribution; the gasses emitted should indicate how much Biomass is left to be broken down. If a batch system is used, measuring gas emissions will provide a timetable for complete biomass digestion and will indicate when new Biomass must be added. If a continuous system is used, measuring Biogas production will indicate how efficiently the whole process is running and allow for perpetual emissions by maintaining enough biomass to keep the microorganisms at the peak of their digesting capability. Not only can emissions be a good indicator of digestible material and waste presence, but they can also be an indication of problems that may be occurring such as pH variations or temperature fluctuations that may be limiting the digestion process.
pH is the measure of H+ ions in a solution, otherwise known as a method of determining whether a solution is an acid or a base. The pH scale ranges from 0-14, with 7 being neutral, less than 7 being acidic, and greater than 7 indicating a basic solution. In anaerobic digestion, it is crucial to measure the pH throughout the entire process to ensure the health of the methanogens. As with all living beings, methanogens require a particular environment so that it may live and prosper. The microorganisms that are responsible for the creation of methane require an environment between the pH ranges of 6.5-8. Being that in the process of acidogenesis, acidogenic bacteria produce acid, which thus lowers the pH of the digestion tank, it is crucial to constantly measure the pH throughout the entire process to ensure the continued wellbeing of the methanogens and thus, methane production.