Page 1 of 16 Anaerobic Digestion: General Processes and Applications By Jon Curry Introduction Energy and water are necessities for people. Finding more efficient and environmentally sound ways of producing energy and treating water is of vital importance for society and civilization. As a process of waste water treatment, anaerobic digestion reduces the emissions of landfill gasses into the atmosphere. It is now widely used as a source of renewable energy due to its high yield of biogas (mostly methane and carbon dioxide). This article explains the processes involved with anaerobic digestion, its applications, and its place in human history. Anaerobic digestion (AD) is the biochemical process by which microorganisms break down biodegradable material, in the absence of oxygen, and produce biogas. There are four main steps that occur in anaerobic digestion: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The process is much like composting, where microorganisms, like bacteria and fungi, break down the biodegradable material; however, composting occurs in the presence of oxygen. There are also many different methods for carrying out the AD process for many different purposes. Depending on what the desired purpose is and what method the operator intends to use, conditions surrounding the process can change significantly. All things considered, the basic process, products, and by products remain the same. Page 2 of 16 Historical Perspective It is believed that biogas was used to heat bath water in Assyria during 10th century BC and Persia during the 16th century. In the 17th century, a Flemish chemist determined that flammable gas could evolve from decaying organic matter. An Italian physicist concluded that there was a direct correlation between the amount of decaying organic matter and the amount of flammable gas produced in the year 1776. In 1808, a British chemist found methane to be present in the gases produced during the AD of cattle manure. The first digestion plant was built in Bombay, India in 1859. Soon, biogas was used to fuel street lamps in Exceter, England in 1895 by using AD sewage treatment center. Scientists identified anaerobic bacteria and the conditions that promote methane production in the 1930s. In the years of and following WWII, Europe used AD quite extensively because energy supplies were reduced. It has maintained use of AD facilities to date. Applications Six to eight million family-sized, low-technology digesters are used to provide biogas for cooking and lighting fuels with varying degrees of success. Many industries use AD as a pretreatment step that effectively lowers sludge disposal costs, controls odor, and reduces costs of final treatment at a municipal wastewater treatment facility. Processes like AD and composting offer the only biological means of recycling matter and nutrients from the organic portion of municipal solid wastes (MSW). Composting, being an energy-consuming process, requires 50-75 kWh of electricity per ton of MSW input. AD is an energy-producing process, with around 75-150 kWh of electricity produced per ton of MSW input. Page 3 of 16 The Process Feedstock A feedstock of some sort is required to be broken down and eventually turned into gas and its effluent by product. Common feeds include bio waste, manure, fats, oils, greases, organic household waste, municipal solid wastes, sewage treatment works sludges, dairy farm piggery waste slurries, dairy products, and other food processing wastes. Digesters can generally accept any biodegradable material but when optimal biogas production is the aim, putresciblility—the ability of microorganisms to decompose organic matter—is the main factor. Another factor is moisture content. The wetter the material the more volume and area it takes up relative to the levels of gas that are produced. Also, wetter content is more suitable to be handled with standard pumps instead of energy intensive concrete pumps and physical means of movement. The carbon/nitrogen ratio is another factor to consider; this is the ratio that is required for microbes to grow. Excess nitrogen can lead to ammonia inhibition of digestion. Contamination of feed is a key consideration also. Plastic, glass, and metal are common contaminants that, if left unfiltered, can block digesters and will prevent them from functioning efficiently. After contaminants have been removed, feed is then shredded, minced, and mechanically or hydraulically pulped to increase surface area. With more surface area, microbes can digest more material at once; resulting in faster digestion. Page 4 of 16 At Stahlbush Island Farms Biogas Plant, silage (a common biofuel feed) is put into a hopper and mechanically broke down by augers as part of the initial breakdown process. Page 5 of 16 The feed is then further broken down by a hammermill which crushes and grinds the material into even smaller particulates. Hydrolysis After the mechanical and hydraulic breakdown of the feed, the particulates (like cellulose) and colloidal (like proteins) wastes are broken down even more so in a process called hydrolysis. Page 6 of 16 In hydrolysis, a certain molecule is split into two parts by the addition of a molecule of water. This is the solubilization of what would otherwise be insoluble material. Polymers are broken down into monomers. Complex organic molecules like carbohydrates, fats, and proteins are then broken down into their simple constituent sugars, fatty acids, and amino acids. This process makes the solids more readily biodegradable. This reduces the hydraulic retention time in the digester, meaning lower capital costs. Additionally, greater biodegradability improves removal of volatile suspended solids, which increases biogas yield and decreases the quantity of sludge for final disposal. Acidogenesis The simple monomers are then converted into volatile fatty acids (VFAs) in a process called acidogenesis. The simple sugars, fatty acids, and amino acids are broken down into carbonic acids, alcohols, hydrogen, ammonia, and carbon dioxide. This is done by acidogenic (fermentative) bacteria. Acetogenesis In acetogenesis, VFAs are converted into acetic acid, carbon dioxide, and hydrogen by acetogens. Acetogens grow in a symbiotic relationship with methane-forming bacteria. Acetate serves as a substrate for methane-forming bacteria. When ethanol is converted to acetate, carbon dioxide is used and acetate and hydrogen are produced. The equation is: CH3CH2OH CO2 CH3COOH 2H2 When acetate-forming bacteria produce acetate, hydrogen is also produced. If the hydrogen accumulates and significant hydrogen pressure occurs, it will result in termination of activity of Page 7 of 16 acetate-forming bacteria and acetate production will cease. However, methane-forming bacteria use hydrogen in the production of methane and as illustrated by the equation below, significant hydrogen pressure does not occur: CO2 4H2 CH4 2H2O Acetogens survive only at very low concentration of hydrogen in the environment. They can only survive if their metabolic waste—hydrogen—is continuously removed. This is why their symbiotic relationship with hydrogen-utilizing bacteria like methanogens is so vital. Because acetogenic bacteria reproduce slowly, generation time for these organisms is usually greater than 3 days. Methanogenesis In methanogenesis, methanogens convert acetate, hydrogen, and methyl substrates into methane. There are many different methane-forming bacteria with many different shapes, sizes, and growth patterns. There are bacteria in found as rods, curved rods, spirals, and cocci. They can be grouped as irregular clusters of cells, chains of cells or filaments, and sarcina or cuboid arrangements. Motile and nonmotile bacteria as well as spore-forming and non-spore forming bacteria exist. These methane-forming bacteria thrive in heat. They are found in habitats rich in degradable organic compounds. In these habitats, oxygen is rapidly removed through microbial activity. Many occur as symbionts in animal digestive tracts. Methane-forming bacteria also have high sulfur content. Page 8 of 16 In AD, there are three main types of methanogens: hydrogenotrophic, acetotrophic, and methylotrophic. Hydrogenotrophics were previously discussed with their function as a symbiotic relationship with acetogens. Acetotrophic methanogens “split” acetate into methane and carbon dioxide. The acetotrophic methanogens reproduce more slowly than the hydrogenotrophic methanogens and are adversely affected by the accumulation of hydrogen. Therefore, the maintenance of a low partial hydrogen pressure in an anaerobic digester is favorable for the activity of not only acetate-forming bacteria but also acetotrophic methanogens. Under a relatively high hydrogen partial pressure, acetate and methane production are reduced. Approximately 70% of the methane produced in an anaerobic digester is by both this acetotrophic and the hydrogenotrophic methanogens. The other 30% of methane may be produced by what are called methylotrophic methanogens. These methanogens grow on substrates containing methyl groups. They produce methane directly from methyl groups and not from carbon dioxide. An equation that represents this is: 3CH 3 OH 6H 3CH 4 3H 2 O 4CH 3 3 N 6H 2 O 9CH 4 3CO 2 4NH 3 Reproduction of methane-forming bacteria is slow and is carried out by fission, budding, constriction, and fragmentation. Slow growth rates can be attributed to the relatively small amount of energy obtained from the use of their limited substrates. Generation times can range from a few days to several weeks. If solid retention times are short or early withdrawal of digester sludge occurs, the population size decreases because bacteria is removed from the digester faster than they can reproduce. Page 9 of 16 Stahlbush Island Farms' 2 Digesters and single Post digester. Page 10 of 16 The Torch is a device at Stahlbush Island Farms that is activated when there is an excess of gas for some reason. Gas production may bottleneck before it reaches the turbine engine; consequently automating the flare to burn. Products A production of about 50-75% methane is typical for AD. Carbon Dioxide accounts for approximately 25-50% while the remaining 0-15% is composed of nitrogen, hydrogen sulfide, hydrogen, and oxygen. Wastewater from the original content and produced by the microbial Page 11 of 16 organisms is the other major product. This water will typically have elevated level of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). These cannot be put directly back into watercourses so they are moved onto the next appropriate stages for wastewater treatment. The final major product is digestate. This is composed of whatever non-digestible material the microbes could not feed on and any dead bacterial remains. Lignin is a common nondigestible element that must be subjected to aerobic digestion. Ammonia, which may also be found in the digestate, is a phytotoxin that inhibits plant growth so it cannot be used as a soil improving material until exposed to aerobic digestion as well. Page 12 of 16 The Squeeze Auger, which is part of Stahlbush Island Farms De-Cantering (De-watering) system, provides a means by which solids from the digestate can be obtained and therefore used as a soil enhancer in the crop fields. Page 13 of 16 Conclusion Types of Digester Systems Anaerobic digesters have been used as a means to treat wastewater for some time now. This process, however, is becoming utilized much more for the purpose of biogas production. For both purposes, there are many different types of digesters. High-strength digesters deal with insoluble wastes such as colloidal and particulate organics. Digester retention times of at least 10-20 days are typical for high-strength wastes. High-rate digesters are used for the treatment of soluble wastewaters. Because high-rate digesters do not require hydrolysis and solubilization of wastes, retention times can be less than 8 hours. In suspended growth systems, the bacteria are suspended in the digester through intermittent or continuous mixing action. The mixing action distributes the bacteria or biomass throughout the digester. Because these digesters do not incorporate a means for retaining and concentrating the biomass, the solid retention time (SRT) and hydraulic retention time (HRT) are the same. Anaerobic fixed-film (sludge blankets) systems provide a dormant environment for the growth of a clumped mass of bacteria. Because bacterial growth requires relatively long periods of time to develop, the media used in fixed-film systems hold the bacteria in the digester for relatively long periods and provide for long SRTs and short HRTs. Psychrophilic sludge digestion and methane production occur at a relatively low temperature range (5-20 C ). Due to this low temperature, anaerobic bacteria operates at a less than optimal Page 14 of 16 level and digestion of sludge is limited to small-scale operations such as Imhof tanks, septic tanks, and sludge lagoons. Mesophilic sludge digestion occurs at a moderate temperature range (30-35 C ). Mesophilic digestion of sludge is common for municipal and industrial wastewater treatment. It has two advantages: more mesohpiles exist in nature and it is less expensive to maintain mesophilic temperature over thermophilic temperature. Thermophilic sludge digestion and methane production occur at a high temperature (50-60 C ). This is more often used at industrial wastewater treatment plants. Because of the high temperature of these digesters, sludge digestion and methane production is rapid. Also significant destruction of pathogens is achieved. A typical single-stage digester consists of only one tank or reactor. Digester operations consist of sludge addition and withdraw, mixing, heating, gas collecting, and supernating (concentrated digestate). These operations are possible because of the stratification of the digester content. Stratification results in the following layers from top to bottom: gas, scum, supernatant, active digester sludge, and digested sludge and grit. Both acid forming and methane forming bacteria exist in the same digester. Two-stage digester systems consist of at least two separate tanks or reactors. This systems has improved efficiency and stability over a single-stage system. It is capable of performing acid production in the first tank and methane production in the second. Also, some allow for utilization of both mesophilic and thermophilic digestion. Page 15 of 16 Long Term Viability The world depends on fossil fuels, a limited resource, but it is becoming more readily apparent that that dependence is not only a hindrance, but harmful as well. We are now pursuing other means by which we can support our energy needs. Among these many alternative energy sources is biofuel. Not only is the yield of energy from biomass relatively excellent, but this natural process also produces by products that are exceptionally applicable. It also doesn’t exhaust the environment or any of the elements involved in the process. Surely, there is a promising future for the process of anaerobic digestion and its place in the production of biogas as an alternative, renewable energy. Page 16 of 16 References Gerardi, Michael H. The Microbiology of Anaerobic Digesters. Hoboken, New Jersey: Wiley Interscience, 2003. Print. Lippert, Jeffrey B. Bioplant Manager. Employed by Stahlbush Island Farms. Corvallis, OR. jeffl@stahlbush.com Lyberatos, Gerasimos, and I.V. Skiadas. "Modelling of Anaerobic Digestion-A Review." Global Nest 1.212 June (1999): 63-76. Web. 1 Feb. 2011. <http://www.gnest.org/journal/Vol1_No2/Lyberatos.pdf>. "A Short History of Anaerobic Digestion." PennState. N.p., n.d. Web. 6 Mar. 2011. <http://www.biogas.psu.edu/pdfs/ShortHistoryAD.pdf>. Anaerobic Digestion The AD Community: An Independant Web Site. N.p., 2011. Web. 23 Feb. 2011. <http://www.anaerobic-digestion.com/html/anaerobic_digestion_faqs.html>. Crabtree, Dennis. "Anaerobic Digestion Systems Tackle Growing Clean Water and Renewable Energy Challenges." biostar systems. Biostar, n.d. Web. 22 Feb. 2011. <http://www.biostarsystems.com/whitepaper/BioStar_Whitepaper_093009_.pdf>.
