Anaerobic is a technical word which literally means without air (where "air" usually means oxygen), as opposed to aerobic. In wastewater treatment the absence of oxygen is indicated as anoxic; and anaerobic is used to indicate the absence of any electron acceptor (nitrate, sulfate or oxygen)

Anaerobic may refer to:

• Anaerobic exercise, a form of exercise
• Anaerobic digestion, the breakdown of organic matter by bacteria with the absence of oxygen
• Anaerobic glycolysis, the conversion from sugar to alcohol using yeast - see Fermentation (biochemistry)
• Anaerobic organism, any organism that does not require oxygen for growth
• Anaerobic respiration, oxidation of molecules in the absence of oxygen
• Anaerobic ammonium oxidation, anammox, the microbial process combining ammonium and nitrite.

See also

• Hypoxia (environmental) for discussion on oxygen depletion

Anaerobic Exercise Introduction

Anaerobic means "without air", and refers to the energy exchange in living tissue that is independent of oxygen. Anaerobic exercise is brief, high intensity activity where anaerobic metabolism is taking place in muscles. During extended periods of exercise aerobic metabolism supplies the bulk of the energy and the exercise is termed aerobic exercise.

Examples of anaerobic exercise include weight lifting, sprinting, and jumping; any exercise that consists of short exertion, high-intensity movement, is an anaerobic exercise. Anaerobic exercise is typically used by athletes in non-endurance sports to build power and by body builders to build muscle mass. Muscles that are trained under anaerobic conditions develop biologically differently giving them greater performance in short duration-high intensity activities.

Aerobic exercise, on the other hand, includes lower intensity activities performed for longer periods of time. Activities like walking, running, swimming, and cycling require a great deal of oxygen to generate the energy needed for prolonged exercise.

There are two types of anaerobic energy system, the ATP-CP energy system, which uses creatine phosphate as the main energy source, and the lactic-acid (or anaerobic glycolysis) system that uses glucose (or glycogen) in the absence of oxygen. The latter is an inefficient use of glucose and produces by-products that are thought to be detrimental to muscle function. The lactic-acid system is the dominant energy system during high to maximal intensity exercise over short durations (up to about 1 min), but the lactic acid system can still provide a proportion of the required energy during aerobic exercise, as the body has the capacity to get rid of the anaerobic by-products at a certain rate. The efficiency of by-product removal by muscles can improve through training.

Anaerobic biochemistry

Anaerobics are activities that are carried out 'without oxygen'. This terminology refers to the molecular level of respiration, not the respiration of the organism as a whole (i.e., breathing). During anaerobic exercise, the muscles being exercised have insufficient oxygen to meet the demands of the activity, and thus must also use alternate, non-oxygen-dependent processes to produce energy. The muscle does still receive oxygen during anaerobic exercise; the average drop in blood oxygen content throughout the body is likely minimal.

Anaerobic exercise begins with muscles utilizing stored creatine phosphate to generate the ATP that produces muscle contraction. After several seconds, further ATP energy is made available to muscles by metabolizing muscle glycogen or sugars in the blood to lactate. The following aerobic phase is limited by the ability of the heart and lungs to supply the muscle with oxygen. Aerobic metabolism can utilise carbohydrates or fats, as well as lactic acid from anaerobic metabolism, as fuel, releasing larger amounts of energy.

Anaerobic threshold

The anaerobic threshold (AT) is the exercise intensity at which lactate starts to accumulate in the blood stream. This happens when it is produced faster than it can be removed (metabolized). This point is sometimes referred to as the lactate threshold, or the onset of blood lactate accumulation (OBLA). When exercising below the AT intensity any lactate produced by the muscles is removed by the body without it building up.

The anaerobic threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. distance running, cycling, rowing, swimming and cross country skiing), and can be increased greatly with training.

Fartlek (speed-play) training and interval training take advantage of the body being able to temporarily exceed the anaerobic threshold, and then recover (reduce blood-lactate) while operating at below the threshold, but still doing physical activity. Fartlek and interval training are similar, the main difference being the relative intensities of the exercise, best illustrated in a real-world example: Fartlek training would involve constantly running, for a period time running just above the anaerobic threshold, and then running at just below it, while interval training would be running quite high above the anaerobic threshold, but then slowing to a walk during the rest periods.

Fartlek would be used by people who are constantly moving, with occasional bouts of speed, such as basketballers, while interval training is more suited to sprinters, who exert maximum effort and then can stop exerting completely. With both styles of training, you can exert more effort before fatiguing and burn more calories than exercising at a constant pace (continuous training), but will emphasize training the anaerobic system rather than the aerobic system. Long duration training below the anaerobic threshold is recommended to primarily work the aerobic system.

Accurately measuring the anaerobic threshold involves taking blood samples (normally a pinprick to the finger, earlobe or thumb) during a ramp test where the exercise intensity is progressively increased. Measuring the anaerobic threshold can also be performed non-invasively using gas-exchange methods, which requires a metabolic cart to measure air inspired and expired.

Although the anaerobic threshold is defined as the point when lactic acid starts to accumulate, some testers approximate this by using the point at which lactate reaches a concentration of 4 mM/L (at rest it is around 1 mM/L)....

Anaerobic Digestion

Anaerobic digestion (AD) is the harnessed and contained, naturally occurring process of anaerobic decomposition ^{[1] [2]} . An anaerobic digester is an industrial system that harnesses these natural process to treat waste, produce biogas that can be used to power electricity generators, provide heat and produce soil improving material ^[3] .

Anaerobic digesters have been around for a long time and they are commonly used for sewage treatment or for managing animal waste. Increasing environmental pressures on waste disposal has increased the use of AD as a process for reducing waste volumes and generating useful byproducts. It is a fairly simple process that can greatly reduce the amount of organic matter which might otherwise end up in landfills or waste incinerators .

Almost any organic material can be processed in this manner. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage and animal waste. Anaerobic digesters can also be fed with specially grown energy crops to boost biodegradable content and hence increase biogas production. After sorting or screening to remove inorganic or hazardous materials such as metals and plastics, the material to be processed is often shredded, minced, or hydrocrushed^[4]. to increase the surface area available to microbes in the digesters and hence increase the speed of digestion. The material is then fed into a airtight digester often with extra water added depending on the digestion process and feedstock.

Stages of anaerobic digestion

There are two conventional operational temperature levels:

• Mesophilic which takes place optimally at 37°C or at ambient temperatures between 20°-40°C with mesophile bacteria
• Thermophilic which takes place at elevated temperatures up to 70°C with thermophile bacteria

The residence time in a digester varies with the amount of feed material, type of material and the temperature. In the case of mesophilic digestion, residence time may be between 15 and 30 days. In the case of mesophilic UASB digestion hydraulic residence times (1hour-1day) and solid retention times (<90 days) are separated. In the thermophillic phase the process can be faster, requiring only about two weeks to complete. Thermophilic digestion is more expensive, requires more energy and is less stable than the mesophillic process. Therefore, the mesophillic process is still widely in use.

Many continuous digesters have mechanical or hydraulic devices to mix the contents and to allow excess material to be continuously extracted to maintain a reasonably constant volume.

The digestion of the organic material is done by a range of many different species of naturally occurring bacteria, all doing a different job at a different step in the digestion process. Maintaining suitable conditions in the digester is essential in maintaining a healthy bacterial population.

Four stages of anaerobic digestion have been recognised.

1. The first is hydrolysis, where complex organic molecules are broken down into simple sugars, amino acids, and fatty acids with the addition of hydroxyl groups.
2. The second stage is acidogenesis where a further breakdown into simpler molecules occurs, producing ammonia, carbon dioxide and hydrogen sulfide as byproducts.
3. The third stage is acetogenesis where the simple molecules from acidogenesis are further digested to produce carbon dioxide, hydrogen and mainly acetic acid, although higher-molecular-weight organic acids (e.g., propionic, butyric, valeric) are also produced.
4. The fourth stage is methanogenesis where methane, carbon dioxide and water are produced.

By-products of anaerobic digestion

There are three principal by-products of anaerobic digestion.

• Biogas, a gaseous mixture comprising mostly of methane and carbon dioxide, but also containing a small amount hydrogen and occasionally trace levels of hydrogen sulphide. Biogas can be burned to produce electricity, usually with a reciprocating engine or microturbine. The gas is often used in a cogeneration arrangement, to generate electricity and use waste heat to warm the digesters or to heat buildings. Excess electricity can be sold to electricity suppliers. Electricity produced by anaerobic digesters is considered to be green energy and may attract subsidies such as Renewables Obligation Certificates.

Since the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle biogas does not contribute to increasing atmospheric carbon dioxide concentrations; because of this, it is considered to be an environmentally friendly energy source. The production of biogas is not a steady stream; it is highest during the middle of the reaction. In the early stages of the reaction, little gas is produced because the number of bacteria is still small in size. Toward the end of the reaction, only the hardest to digest materials remain, leading to a decrease in the amount of biogas produced.

• The second by-product (acidogenic digestate) is a stable organic material comprised largely of lignin and chitin, but also of a variety of mineral components in a matrix of dead bacterial cells, some plastic may be present. This resembles domestic compost and can be used as compost or to make low grade building products such as fiberboard.

• The third by-product is a liquid (methanogenic digestate) that is rich in nutrients and can be an excellent fertilizer dependent on the quality of the material being digested. If the digested materials include low levels of toxic heavy metals or synthetic organic materials such as pesticides or PCBs, the effect of digestion is to significantly concentrate such materials in the digester liquor. In such cases further treatment will be required in order to dispose of this liquid properly. In extreme cases, the disposal costs and the environmental risks posed by such materials can offset any environmental gains provided by the use of biogas. This is a significant risk when treating sewage from industrialized catchments.

Nearly all digestion plants have ancillary processes to treat and manage all of the by-products. The gas stream is dried and sometimes sweetened before storage and use. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration. Excess water is also sometimes treated in sequencing batch reactors (SBR) for discharge into sewers or for irrigation.

Digestion can be either wet or dry. Dry digestion refers to mixtures which have a solid content of 30% or greater, whereas wet digestion refers to mixtures of 15% or less.

Reactor types

The two main types of reactors are continuous and batch. Batch is the simplest, with the biomass added to the reactor at the beginning and sealed for the duration of the process. In the continuous process, which is the more common type, organic matter is constantly added to reactor and the end products constantly removed, resulting in a much more constant production of biogas.

Although there will always be a net loss in energy in the whole system (the energy to grow the biomass is more than the output of the reactor), for the processing of waste organic material, anaerobic digestion is the preferable choice because it is environmentally friendly. The biggest impacts on the environment include the energy and materials used to build the plant, transport costs and fuel use in transporting material to site and visual and audible impacts of the site operation. Odor can be a severe problem during emptying cycles. This is a particularly difficult issue for batch reactors.

Considerations

To be economically viable, there must be a market for the end products. Biogas can be sold or used in almost all parts of the world, where it will offset demand on fossil fuel stocks. The digester liquor is suitable for use as a fertilizer, although frequently supplemental nutrients need to be added.

The sludge component, even when dried and available as a soil conditioner, is not easily disposed of. However, it has its uses in non-agricultural areas, such as golf courses, and as cover for landfills. In some localities, the sludge itself is used as a fuel in heating systems, and the residual ash is disposed of in a landfill.

Contribution to prevention of climate change

Production of Renewable Fuel

Processing biodegradable waste using anaerobic digestion helps to reduce global warming. If this waste was landfilled it would break down naturally however the biogas would escape directly into the atmosphere. In this way anaerobic digestion is considered to be a sustainable technology and biogas is considered to be a renewable fuel.

Associated technologies

Mechanical biological treatment

related topics: mechanical biological treatment

New developments in anaerobic digestion have led to systems being integrated with sorting units. Mixed waste streams such as unsorted household waste can undergo a mechanical pretreatment stage. These systems come under the category of mechanical biological treatment. They enable the recovery of the organic fraction of the waste in a form that can be processed in anaerobic digesters.

Inhibition of methanogenesis & production of alcohols

related topics: bioconversion of biomass to mixed alcohol fuels

Anaerobic digestion can be inhibited from reaching the methanogenic stage. The organic acids (i.e., carboxylic acids) from the acidogenic and acetogenic stages of the digestion can be recovered. The acids can then undergo further chemical transformations into useful chemicals or fuels.

Potential in the Hydrogen Economy

related topics: steam methane reforming

As anaerobic digestion is a renewable source of methane it offers the potential to contribute to the hydrogen economy:

Steam methane reforming (SMR) is the most common method of producing commercial bulk hydrogen. It is also the least expensive method. At high temperatures (700 – 1100 °C) and in the presence of a metal-based catalyst, steam reacts with methane to yield carbon monoxide and hydrogen.

CH₄ + H₂O ? CO + 3 H₂

The United States produces nine million tons of hydrogen per year, mostly with steam reforming of natural gas. This process is different from catalytic reforming, an oil refinery process that also produces significant amounts of hydrogen along with high octane rating gasoline.

anaerobic organism

An anaerobic organism or anaerobe is any organism that does not require oxygen for growth.

• Obligate anaerobes will die when exposed to atmospheric levels of oxygen.
• Facultative anaerobes can use oxygen when it is present.
• Aerotolerant organisms can survive in the presence of oxygen, but they are anaerobic because they do not use oxygen as a terminal electron acceptor.

Microaerophiles are organisms that may use oxygen, but only at low concentrations (low micromolar range); their growth is inhibited by normal oxygen concentrations (approximately 200 micromolar). Nanaerobes are organisms that cannot grow in the presence of micromolar concentrations of oxygen, but can grow with and benefit from nanomolar concentrations of oxygen.

Obligate anaerobes may use fermentation or anaerobic respiration. In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen some of them ferment, some use anaerobic respiration. Aerotolerant organisms are strictly fermentative. Microaerophiles carry out aerobic respiration, and some of them can also do anaerobic respiration.

There are many chemical equations for anaerobic fermentative reactions.

Fermentative anaerobic organisms mostly use the lactic acid fermentation pathway:

C₆H₁₂O₆ + 2 ADP + 2 phosphate ? 2 lactic acid + 2 ATP

The energy released in this equation is approximately 150 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose. This is only 5% of the energy per sugar molecule than the typical aerobic reaction generates.

Plants and fungi (e.g., yeasts) generally use alcohol (ethanol) fermentation when oxygen becomes limiting:

C₆H₁₂O₆ + 2 ADP + 2 phosphate ? 2 C₂H₅OH + 2 CO₂ + 2 ATP

The energy released is about 180 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose.

Anaerobic bacteria and archaea use these and many other fermentative pathways, e.g., propionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis or methanogenesis.

Some anaerobic bacteria produce toxins (e.g., tetanus or botulinum toxins) that are highly dangerous to higher organisms, including humans.

Obligate(strict)anaerobes die in presence of oxygen due to the absence of the enzymes superoxide dismutase and catalase which would convert the lethal superoxide formed in their cells due to the presence of oxygen.

Anaerobic respiration refers to the oxidation of molecules in the absence of oxygen to produce energy. These processes require another electron acceptor to replace oxygen. Anaerobic respiration is often used interchangeably with fermentation, especially when the glycolytic pathway exists in the cell. However, certain anaerobic prokaryotes generate all of their ATP using an electron transport system and ATP synthase.

The word & symbol equation for the anaerobic respiration of glucose is:

Glucose ---> Lactic Acid + Energy (ATP)

C₆H₁₂O₆ ---> 2C₃H₆O₃ + 2 ATP

The energy released is about 120kJ per mole Glucose.

When glycolysis is used

Oxygen is not necessary for glycolysis to occur in all organisms.

Obligate Anaerobes

In some organisms called obligate (strict) anaerobes (ex: C. tetani (causes tetanus), C. perfringens (causes gangrene)), the presence of oxygen is lethal. This is because the presence of oxygen is processed by the organisms into the extremely toxic molecules of singlet oxygen (¹O₂), superoxide ion (O₂^-), hydrogen peroxide (H₂O₂), hydroxyl ion (OH^-), and other dangerous byproducts.

Faculative Anaerobes and Obligate Aerobes

Faculative anaerobes (organisms that can survive in either oxygenated or deoxygenated environments and can switch between cellular respiration or fermentation, respectively) and obligate (strict) aerobes (organisms that can survive only with oxygen) have special enzymes (superoxide dimutase and catalase) that can safely handle these products and transform them into harmless water and diatomic oxygen in the following reactions:

1. 2O₂^- + 2H⁺ ---Superoxide Dimutase---> H₂O₂ (hydrogen peroxide) + O₂

The hydrogen peroxide produced is then transferred to a second reaction...

2. 2H₂O₂ ---Catalase---> 2H₂O + O₂

The oxidative powers of the superoxide ion have now been neutralized. Only faculative anaerobes and obligate aerobes possess the two enzymes necessary to reduce the superoxide.

In organisms which use glycolysis, the absence of oxygen prevents pyruvate from being metabolised to CO₂ and water via the citric acid cycle and the electron transport chain (which relies on O₂) does not function. Fermentation does not yield more energy than that already obtained from glycolysis (2 ATPs) but serves to regenerate NAD⁺ so glycolysis can continue. Various end products can also be created, such as lactate or ethanol.

Fermentation in animals is essential to human life.

In lactic acid fermentation, the following reaction occurs:

1. Glycolysis C₆H₁₂O₆ (glucose) + 2 NAD⁺ ---> 2 C₃H₄O₃ (pyruvic acid) + 2 NADH

2. Lactic Acid Creation 2 C₃H₄O₃ (pyruvic acid) + 2 NADH ---> 2 C₃H₆O₃ (lactic acid) + 2 NAD⁺

Net Reaction: C₆H₁₂O₆ (glucose) ---> 2 C₃H₆O₃ (lactic acid)

The lactic acid can then oxidize by losing hydrogen ions (H⁺) and turning into lactase ions. The introduction of ions into the solutions lowers the pH, acidifying the environment. When the pH reaches a critical threshold, enzymes essential to respiration fail to function, cutting off either aerobic respiration or anaerobic respiration.

Fermentation in other organisms

In some plant cells and yeasts, fermentation produces CO₂ and ethanol. The conversion of pyruvate to acetaldehyde generates CO₂ and the conversion of acetaldehyde to ethanol regenerates NAD⁺.

The end product of fermentation in C. perfringens is a gas which causes the condition of gas.

When the Oxygen levels are low, it takes turn to give out a ? helix.

Anaerobic respiration is also defined as a membrane bound biological process coupling the oxidation of electron donating substrates (e.g. sugars and other organic compounds, but also inorganic molecules like hydrogen, sulfide/sulfur, ammonia, metals or metal ions) to the reduction of suitable alternative electron acceptors other than molecular oxygen. During these redox processes, protons are translocated over the membrane from "inside" to "outside", establishing a concentration gradient over the membrane which temporarily stores the energy released in the chemical reactions. This energy is then converted into ATP by the same enzyme used during aerobic respiration, ATP synthase. Possible electron acceptors for anaerobic respiration are nitrate, nitrite, nitrous oxide, oxidised amines and nitro-compounds, fumarate, oxidised metal ions, sulfate, sulfur, sulfoxo-compounds, halogenated organic compounds, selenate, arsenate or carbon dioxide (in acetogenesis and methanogenesis). All these types of anaerobic respiration are restricted to prokaryotic organisms.

Commercial applications of anaerobic respiration

• Anaerobic digestion
• Mechanical biological treatment

See also

See also

• Anaerobic digestion
• Anaerobic decomposition
• Anaerobic digester types
• Anaerobic lagoon

• Biogas powerplant
• List of solid waste treatment technologies
• List of waste water treatment technologies
• Mechanical biological treatment
• Sewage treatment

External links

• European Anaerobic Digestion Network
• Independent Anaerobic Digestion Website
• Methane (Biogas) from Anaerobic Digesters
• Application of anaerobic digesters

References

1. ^ Friends of the Earth FOE (2004 Anaerobic digestion Briefing of the breakdown of organic matter by anaerobic organisms in environments lacking oxygen
2. ^ Anaerobic Digestion Cardiff University Anaerobic Digestion Page
3. ^ Anaerobic Digestion Remade Scotland Anaerobic Digestion Page
4. ^ ArrowBio Reference Finstein, M. S., Zadik, Y., Marshall, A. T. & Brody, D. (2004) The ArrowBio Process for Mixed Municipal Solid Waste – Responses to “Requests for Information”, Proceedings for Biodegradable and Residual Waste Management, Proceedings. (Eds. E. K. Papadimitriou & E. I. Stentiford), Technology and Service Providers Forum, p. 407-413