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Microbiology of anaerobic digestion
Introduction
All microorganisms just like any other animals require energy for survival and they usually acquire it either through aerobic or anaerobic digestion. However, anaerobic digestion is the most common form of digestion utilized by many microorganisms. It is a biochemical reaction which is complex and carried out in several steps in a variety of microorganisms which usually require very little or no oxygen for survival (Thomas-Hope, 1998). During the process of anaerobic digestion biogas mainly consisting of carbon dioxide and methane is produced and its amount greatly varies with the quantity of fed organic waste to the digester used as well as influence of temperature to the rates gas production and decomposition. However, since these anaerobic organisms survive in environments with less or even no oxygen the energy they release from their food is relatively low compared to the one released by aerobic microorganisms.
There are however three main classes of anaerobic microorganisms which include; obligate anaerobes, facultative anaerobes and aerotolerant. The obligate anaerobes are the organisms which will definitely die in case they are exposed to oxygen, thus they are in other words referred to as strict anaerobes. This is because they are deficient of catalase and superoxide dismutase enzymes responsible for the conversion of deadly superoxide which is formed in the cells of the microorganisms in the presence of oxygen. Examples of these strict anaerobes includes; clostridium and bacteriodes which are spore-forming and non-spore forming respectively (Thomas-Hope, 1998). The facultative anaerobes are capable of surviving in the absence or presence of oxygen. Some of these microorganisms such as the staphylococcus and Yeast are capable of growing in absence or presence of oxygen. The aerotolerant anaerobes survive in the oxygen presence but they end up not using it as their terminal acceptor of electron, hence they are fermentative anaerobes.
Moreover, the process of anaerobic digestion in microorganisms usually takes place in four main steps which include; hydrolysis, acidogenesis or fermentation, acetogenesis and methanogenesis. Hydrolysis involves the decomposition of organic matters which are complex into soluble organic molecules which are simple whereby water is used to aid the splitting of the chemical bonds which exist between the substances. Acidogenesis or fermentation is however the processes through which carbohydrates are subjected to chemical decomposition by yeasts, enzymes, molds, or bacteria in the oxygen absence. The acetogenic bacteria however utilize the acetogenesis process to convert the products of fermentation into hydrogen, acetate and carbon dioxide. This leads to the formation of methane by methanogenic bacteria through the process of methanogenesis which combines acetate and hydrogen.
During the process of anaerobic digestion both acetogenic and methanogenic bacteria usually grow in a close association in stage four of the process. This is because the acetogens’ conversion of fermentation products is only thermodynamically when the concentration of hydrogen is actually kept at sufficiently low levels (Williams, 2005). However, the anaerobic process usually take place under very stringent anaerobic conditions which requires specific conditions for the process which are totally different from the ones required in aerobic treatment. The microbiological anaerobic digestion process can be summarized in the figure below:
Anaerobic digestion pathway
Anaerobic decomposition
This is a biological process which takes place in anaerobes whereby decomposition of organic matter takes place in absence of oxygen. However, anaerobic decomposition usually takes place in two processes. First, the facultative bacteria which form acid use food sources such as organic matter to produce gases such as hydrogen sulfide and carbon dioxide, organic (volatile)acids, stable solids as well as more facultative organisms. Second, the methane forming anaerobes use the formed volatile acids as their food source thus forming stable solids, methane gas as well as more methane forming anaerobes. The produced methane gas in this process is then usable as a fuel (Gerardi, 2003). The methane forming anaerobes usually work at a slower rate compared to the acid forming, therefore there is need to keep the pH constantly basic in order to optimize the methane formation process. Thus, sodium bicarbonate has to be constantly fed to the reaction for the purpose of keeping it basic.
Anaerobic digestion as the favoured technique for the treatment of biodegradable municipal wastes
Municipal waste is mostly generated by commercial activities, households as well as many other sources whereby the activities involved resembles those of commercial enterprises and households. However, anaerobic digestion has been the favored way of treating the municipal solid waste organic fraction. This technique was developed approximately more than twenty years ago whereby it has been rapidly developing over the recent past since mid-nineties. The anaerobic digestion implementation for the purpose of treating the organic fraction or biowaste in the municipal solid waste can be even regarded as the major development which has been achieved in the waste treatment field over the last two decades (Thomas-Hope, 1998). Nowadays, this process of anaerobic digestion of municipal solid waste has captured a very significant portion of the European and United States market whereby the municipal solid waste’s organic fraction undergoes biological treatment (Biodegradable waste, 4).
A greater percentage of municipal solid waste is biodegradable, that is, it is typically originating from animal or plant sources which are able to be broken down into simple products by other living organisms and more preferably the microorganisms. Most of the biodegradable municipal solid waste involves; food waste, green waste, paper waste, human waste and sewage. However, the biodegradable waste if properly managed can be converted into products which are valuable through the anaerobic digestion process hence it is a technique which is mostly favored for the treatment of municipal solid waste (Wang, Tay, Ivanov and Hung, 2010). This is because the process uses microorganisms which are naturally occurring to facilitate the breaking down of the biodegradable waste. Thus, since it is part of the overall integrated waste system, this process of anaerobic digestion contributes to the reduction of the landfill emission into the atmosphere. Another reason why anaerobic digestion is the favored way of treating the municipal solid waste is because it converts the biodegradable municipal waste into other useful products which can be harnessed for commercialization such as organic acids and biogas (Gerardi, 2003).
The issue of biodegradable waste treatment is also a very important topic of concern due to its association with global warming. This is because when this biodegradable waste is disposed of in the usual landfills, it ends up breaking down in anaerobic conditions which are not controlled (Williams, 2005). Thus, this uncontrolled breakdown of biodegradable municipal waste leads to production of landfill gases which are not harnessed hence escaping into the atmosphere. These landfill gases includes methane a greenhouse which is a more potentthan carbon dioxide hence it becomes necessary to favor the anaerobic method of treating municipal solid waste since the products of the process are harnessed for other beneficial uses as well as reducing their effects to the environment. Therefore the landfill gas which is uncontained it escapes into the atmosphere and may greatly contribute to the global warming effects because it contains of methane gas which is actually 20 times more as a potent greenhouse gas compared to carbon dioxide which is another greenhouse gas. Also in addition to the uncontained landfill gas contribution to global warming, the other organic compounds which are volatile present in the landfill gas leads to the formation of a situation referred to as photochemical smog.
Application of anaerobic digestion in Biogas production
Commercial production of biogas can be done utilizing the anaerobic digesters which involves the exploitation of the anaerobic digestion process (Biodegradable waste, 4). The anaerobic digesters are fed with biodegradable waste which includes animal waste, food waste and sewage sludge. However, an air-tight container is used in this process which transforms the biomass waste into biogas which can be used for electricity, heating, and so on (Gerardi, 2003). Moreover, there are two main processes which are used such as thermophilic and mesophilic digestion depending on temperature conditions required. However, the biogas consists of a mixture of several gases in varied compositions which are produced due to the anaerobic digestion of agricultural, domestic and municipal solid waste. Methane forms the greater portion whereas other gases such as carbon dioxide, hydrogen and nitrogen occur in relatively low amounts. Many people living in rural areas around the globe are nowadays using the biogas as source of energy which is cheap, also a pollution control measure as well as improvement of sanitation and health conditions.
The anaerobic digestion process is thus applied in the biogas production whereby animal waste is mostly utilized as well as other biodegradable wastes. The process of biogas production using the anaerobic process involves three main steps such as hydrolysis, acid formation and methane formation. The hydrolysis is the initial step in biogas production whereby it involves the conversion of organic polymers into simple monomers which are then converted to simpler compounds which includes hydrogen, carbon dioxide and ammonia. These simple compounds are finally converted to methane and carbon dioxide through the process of methanogenesis carried out by anaerobic bacteria (Wang, Tay, Ivanov and Hung, 2010).
Moreover, for commercial production of biogas animal wastes as well as other biodegradable wastes are used as the prime source. Hence the biogas production major operations involves mixing of the animal waste together with water in a ratio of 1:1, so as to maintain the inorganic particles present in the animal waste at a level of 10%. This is then followed by a constant feeding rate of the animal waste or other biodegradable waste whose quantity is determined by the digester size (Williams, 2005). However, the biogas production by the anaerobic digester can be drastically increased when the animal and other biodegradable wastes are mixed with other inputs. This is mainly because these inputs are very useful in the multiplication and growth of bacteria which are very crucial in helping anaerobic digestion. For optimum production of biogas using the anaerobic digestion process involves maintaining the required temperature ranges as well as maintaining the pH constant.
Application of the anaerobic digestion process to pathogen destruction
Pathogens present in the municipal solid waste and wastewaters usually pose potential risks to the environment and public health due to their application on open land surfaces such as agricultural lands. This is mainly because most of the sludge produced as a result of municipal wastewater treatment is usually disposed in the landfills while a greater percentage of it is used on land surface. Hence in order to reduce the risk chances associated with these wastes there is need for proper treatment of them before they are applied to the land due to the significant pathogen reduction through treatment.
The processes utilized to reduce the pathogens in wastes plays a critical role in the elimination of pathogens from the wastes.These processes used to significantly reduce pathogens in wastes usually achieve a reduction in pathogens which is almost equivalent to an anaerobic digestion which has been properly conducted. This is evident from the fact that the wastewaters sludge which has been anaerobically digested has been used for so many years as soil conditioner or fertilizer without any harmful effect been recorded. However, in anaerobic digesters some of pathogens such as Salmonella ssp. and E. coli can’t be destructed under the mesophilic conditions but they are rapidly destructed under the thermophilic conditions (Williams, 2005). Thus, mesophilic anaerobic digestion leads to approximately 2-log indicator organisms’ reduction whereas lime stabilization leads to a reduction which is more than 2-long provided the Ph has been maintained at low levels (Wang, Tay, Ivanov and Hung, 2010).
The reduction of the pathogenic microorganisms in the anaerobic digestion process has been determined whereby a lot of studies have already been carried out in the recent past on the single-stage conventional anaerobic digester (Gerardi, 2003). Some of the anaerobic microorganisms utilized in the process of anaerobic digestion are pathogenic, that is, they can cause harmful effects to animals and human beings hence their destruction in the process of anaerobic digestion becomes essential. This is because most of the end products of anaerobic digestion such as the animal waste slurry is finally used in the agricultural lands and if proper destruction of the pathogenic microorganisms was not thoroughly carried out it would lead to harmful effects to the animals or human beings who comes into contact with it because it is infectious.
Application of anaerobic digestion to treatment of different waste types
Anaerobic digestion has been extensively used in the process of treatment of different types of wastes. The overall aim this waste treatment is actually to reduce the potential risks associated with the wastes as well as using them to produce other useful products in the process of their treatment (Wang, Tay, Ivanov and Hung, 2010). However, there are very many types of wastes which are constantly treated to reduce the harmful effects which these wastes might have to the environment and living things as well as harnessing the beneficial products which are produced. Moreover, the most commonly treated wastes include; agricultural waste, industrial waste and wastewaters.
Agricultural waste treatment; In agriculture crop and animal wastes are commonly used as the anaerobic digesters feedstock (Williams, 2005). Thus, this provides a better way the waste treatment to produce useful products such as the biogas used for cheap source of energy. This type of waste treatment is very common in rural areas where the agricultural wastes are commonly available.
Industrial waste treatment; The organic wastes which are commonly generated through industrial processes, specifically the food processing industries wastes can be treated through the anaerobic digestion by acting as anaerobic digesters feedstock. This is mainly because they make very excellent feedstock in methane production. Sometimes the food wastes are mixed with animal wastes to improve generation of methane in a process referred to as codigestion. Different digesters are used depending on the feedstock moisture content. The biogas generated in industrial waste treatment is mostly used for heating as well as other energy production.
Wastewater treatment; The sewage sludge is usually broken by use of anaerobic digesters for the purpose of eliminating pathogens in wastewaters (Williams, 2005). Often, the biogas which is captured from the anaerobic digesters is typically used to heat the facilities which are nearby. However, some municipalities are nowadays diverting food wastes from the landfills to wastewater treatment plants for the purpose of relieving waste burden which is generally placed on many local landfills thus allowing energy production.
Sterilization and its uses
In order to reduce the menace of pathogenic microorganisms different sterilization procedures have already been devised to facilitate the elimination or killing of these microbes. This is mainly because the transmissible infective agents are generally present in many parts including on bench surfaces, fluids, medications as well as many other compounds. Hence sterilization has been developed to facilitate total destruction or disabling of the infectious microorganisms which otherwise would have led to harmful effects (Block, 2001). The process of sterilization has for so many years been used in the food, medicine and surgery sectors to sterilize the instruments and materials used. For instance, the process of sterilization has been extensively used over a long period of time to preserve foods for longer periods of time as well as preventing food poisoning. The main processes used for food sterilization includes; canning, irradiation and pasteurization. These methods achieve a greater percentage of sterilization thus extending the time for which these foods can be preserved for future use or distribution over long distances (Reinchert and Young, 1997).
Moreover, medications and surgical instruments entering the patients body parts which are already aseptic including bloodstream and the skin must be sterilized first into very high levels of sterility. This is because some of these instruments are invasive such as hypodermic needles, scalpels, artificial pacemakers and surgical instruments hence they should be thoroughly sterilized to prevent infections to the patient.
Methods of sterilization