Anaerobic Digestion
Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is widely used to treat wastewater sludges and organic wastes because it provides volume and mass reduction of the input material. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertilizer.
The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Methanogens finally are able to convert these products to methane and carbon dioxide.
Previously, the technical expertise required to maintain anaerobic digesters coupled with high capital costs and lower process efficiencies had limited the level of its industrial application as a waste treatment technology. Anaerobic digestion facilities have, however, been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply, as they are less capital intensive than large power plants.
Recent, patented developments in the anaerobic digestion process, namely the Advanced Anaerobic Digester System (AADS), have resulted in both sequential batch reactors (SBRs) and continuous process plug flow designs that are highly automated and consequently much lower in annual operating costs. These systems are also readily scalable from 50 Animal Unit installations supporting a 25kW engine-generator to Biogas generators supportive of 20 Megawatts or more of equivalent electrical generation, steam production, or furnace fuel. The application of programmable logic controllers (PLCs), coupled with oversight by a local, reliable and commercially available digital control system (DCS) software application assures high-performance reliability and results. Heuristic algorithms, self-tuning capability, and the ability to perform Data Mining from the collective AADS database will ensure continually improving operations and promote scientific research into anaerobic bacteria populations and their reactions to changing stimuli.
The Advanced Anaerobic Digester System (AADS) requires the presence of few full-shift personnel who will typically have high school or trade school level education in addition to specific training in AADS operations. Monitoring of feedstock receiving and post-process Biosolids and Bioliquids handling will be the responsibilities most often addressed. All process parameters and system equipment conditions are constantly monitored in real time by the AADS control system. Non-nominal conditions or events will be subsequently annunciated or alarmed both locally and to the remote AADS data center.
Careful control of the digestion temperature, pH, and loading rates is crucial to obtaining efficient breakdown of the material, and disturbances to a digester can lead to process failure. Ensuring that the quality of input materials to the digesters is maintained and that the process effectively monitored is essential for ensuring that a digester's performance is reliable. The advent of the AADS process makes these tasks much easier for the owner-operator.
The new AADS process mechanically and electronically duplicates the digestive processes of a mature Holstein dairy cow, one of Earth's most prolific Methane producers. Effective contaminant removal at the system's front end assures that the multiple-source feedstocks are suitable for Brew preparation. The prepared Brew, prior to injection, and the contents of all SBR or plug flow digester cells are maintained at a constant 101 degrees F. This assures that the process, like a dairy cow, remains healthy at all times. Blending-in or blending-out of multiple feedstocks, based on their changing availability, is directed by menus and recipes within the associated DCS system. This eliminates operator error and substantially increases the variety of feedstocks that can be processed. I.e.; Natural disasters often produce large amounts of ruined crops and municipal solid wastes that can be readily processed by AADS digesters - in addition to their standard diet of animal wastes, agribusiness sludge, and meat processing wastes, Ethanol plant thin stillage, Switchgrass, prairie grasses or other resources. More information on this process can be obtained at www.biogasusa.com
Another matter very successfully addressed by the AADS system design is Biogas storage - without added special equipment. A double-cover system is employed on the in-ground plug flow digester cells. This accomplishes a number of goals. The outer cover, pressurized with low-oxygen exhaust from an engine-generator or boiler, etc., provides a durable and intrinsically safe, warm gas shell over the flexible, inner Biogas collection cover and digester cell itself. The constantly pressurized outer cover is essentially taught and tornado proof. Snow, sleet and ice is promptly melted off of the outer cover due to its warmth. Consequently, the inner digester cover is never directly exposed to wind or sunlight, which extends its useful life considerably. The flexible inner cover is allowed to rise and fall based on the rates of Biogas production and Biogas fuel demand. This provides up to 48 hours of Biogas storage per cell should Biogas fuel demand cease during the client(s) weekend or maintenance outages. This design substantially reduces having to waste valuable Biogas to the system's safety torch. It also promotes engineering the AADS system to support Peaking Power requirements for local utilities and alternative or associated Biogas users.
History
Gas street lamp
Scientific interest in the gasses produced by the natural decomposition of organic matter was first reported in the sixteenth century by Robert Boyle and Stephen Hale, who noted that flammable gas was released by disturbing the sediment of streams and lakes. In 1808, Sir Humphry Davy determined that methane was present in the gasses produced by cattle manure. The first anaerobic digester was built by a leper colony in Bombay, India in 1859. In 1895 the technology was developed in Exeter, England, where a septic tank was used to generate gas for street lighting. Also in England, in 1904, the first dual purpose tank for both sedimentation and sludge treatment was installed in Hampton. In 1907, in Germany, a patent was issued for the Imhoff tank, an early form of digester.
Through scientific research anaerobic digestion gained academic recognition in the 1930s. This research led to the discovery of anaerobic bacteria, the microorganisms that facilitate the process. Further research was carried out to investigate the conditions under which methanogenic bacteria were able to grow and reproduce. This work was developed during World War II where in both Germany and France there was an increase in the application of anaerobic digestion for the treatment of manure.
Applications
Anaerobic digestion is particularly suited to wet organic material and is commonly used for effluent and sewage treatment. Anaerobic digestion is a simple process that can greatly reduce the amount of organic matter which might otherwise be destined to be landfilled or burnt in an incinerator.
Almost any organic material can be processed with anaerobic digestion. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage and animal waste. The exception to this is woody wastes that are largely unaffected by digestion as most anaerobes are unable to degrade lignin. The exception being xylophalgeous anaerobes (lignin consumers), as used in the process for organic breakdown of cellulosic material by a cellulosic ethanol start-up company in the U.S. Anaerobic digesters can also be fed with specially grown energy crops such as silage for dedicated biogas production. In Germany and continental Europe these facilities are referred to as biogas plants. A co-digestion or co-fermentation plant is typically an agricultural anaerobic digester that accepts two or more input materials for simultaneous digestion.
In developing countries simple home and farm-based anaerobic digestion systems offer the potential for cheap, low-cost energy for cooking and lighting. Anaerobic digestion facilities have been recognized by the United Nations Development Programme as one of the most useful decentralized sources of energy supply. From 1975, China and India have both had large government-backed schemes for adaptation of small biogas plants for use in the household for cooking and lighting. Presently, projects for anaerobic digestion in the developing world can gain financial support through the United Nations Clean Development Mechanism if they are able to show they provide reduced carbon emissions.
Pressure from environmentally-related legislation on solid waste disposal methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful by-products. Anaerobic digestion may either be used to process the source separated fraction of municipal waste, or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. These facilities are called mechanical biological treatment plants.
Utilizing anaerobic digestion technologies can help to reduce the emission of greenhouse gasses in a number of key ways:
- Replacement of fossil fuels
- Reducing methane emission from landfills
- Displacing industrially-produced chemical fertilizers
- Reducing vehicle movements
- Reducing electrical grid transportation losses
Methane and power produced in anaerobic digestion facilities can be utilized to replace energy derived from fossil fuels, and hence reduce emissions of greenhouse gasses. This is due to the fact that the carbon in biodegradable material is part of a carbon cycle. The carbon released into the atmosphere from the combustion of biogas has been removed by plants in order for them to grow in the recent past. This can have occurred within the last decade, but more typically within the last growing season. If the plants are re-grown, taking the carbon out of the atmosphere once more, the system will be carbon neutral. This contrasts to carbon in fossil fuels that has been sequestered in the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in the atmosphere.
If the putrescible waste processed in anaerobic digesters was disposed of in a landfill, it would break down naturally and often anaerobically. In this case the gas will eventually escape into the atmosphere. As methane is about twenty times more potent as a greenhouse gas as carbon dioxide this has significant negative environmental effects.
Digestate liquor can be used as a fertilizer supplying vital nutrients to soils. The solid, fibrous component of digestate can be used as a soil conditioner. The liquor can be used as a substitute for chemical fertilizers which require large amounts of energy to produce. The use of manufactured fertilizers is therefore more carbon intensive than the use of anaerobic digestate fertilizer. This solid digestate can be used to boost the organic content of soils. There are some countries, such as in Spain where there are many organically depleted soils, and here the markets for the digestate can be just as important as the biogas.
In countries that collect household waste, the utilization of local anaerobic digestion facilities can help to reduce the amount of waste that requires transportation to centralized landfill sites or incineration facilities. This reduced burden on transportation has and will reduce carbon emissions from the collection vehicles. If localized anaerobic digestion facilities are embedded within an electrical distribution network, they can help reduce the electrical losses that are associated with transporting electricity over a national grid.
The process
Main article: Anaerobic respiration
There are a number of bacteria that are involved in the process of anaerobic digestion including acetic acid-forming bacteria (acetogens) and methane-forming archaea (methanogens). These organisms feed upon the initial feedstock, which undergoes a number of different processes converting it to intermediate molecules including sugars, hydrogen & acetic acid before finally being converted to biogas.
Different species of bacteria are able to survive at different temperature ranges. Ones living optimally at temperatures between 35-40°C are called mesophiles or mesophilic bacteria. Some of the bacteria can survive at the hotter and more hostile conditions of 55-60°C, these are called thermophiles or thermophilic bacteria. Methanogens come from the primitive group of archaea. This family includes species that can grow in the hostile conditions of hydrothermal vents. These species are more resistant to heat and can therefore operate at thermophilic temperatures, a property that is unique to bacterial families.
As with aerobic systems the bacteria in anaerobic systems the growing and reproducing microorganisms within them require a source of elemental oxygen to survive.
In an anaerobic system there is an absence of gaseous oxygen. In an anaerobic digester, gaseous oxygen is prevented from entering the system through physical containment in sealed tanks. Anaerobes access oxygen from sources other than the surrounding air. The oxygen source for these microorganisms can be the organic material itself or alternatively may be supplied by inorganic oxides from within the input material. When the oxygen source in an anaerobic system is derived from the organic material itself, then the 'intermediate' end products are primarily alcohols, aldehydes, and organic acids plus carbon dioxide. In the presence of specialized methanogens, the intermediates are converted to the 'final' end products of methane, carbon dioxide with trace levels of hydrogen sulfide. In an anaerobic system the majority of the chemical energy contained within the starting material is released by methanogenic bacteria as methane.
Populations of anaerobic microorganisms typically take a significant period of time to establish themselves to be fully effective. It is therefore common practice to introduce anaerobic microorganisms from materials with existing populations. This process is called 'seeding' the digesters and typically takes place with the addition of sewage sludge or cattle slurry.
Stages
The key process stages of anaerobic digestion
There are four key biological and chemical stages of anaerobic digestion:
In most cases biomass is made up of large organic polymers. In order for the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available by other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of these high molecular weight polymeric components is the necessary first step in anaerobic digestion. Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.
Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilized by methanogens.
The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products. The process of acidogenesis is similar to the way that milk sours.
The third stage anaerobic digestion is acetogenesis. Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.
The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here methanogens utilize the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water. It is these components that makes up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8. The remaining, non-digestible material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.
A simplified generic chemical equation for the overall processes outlined above is as follows:
C6H12O6 → 3CO2 + 3CH4