The term anaerobic digestion refers to a series of steps that involves the usage of microorganisms to break down any material that is considered to be biodegradable in a situation that does not require the presence of oxygen gas. Its application is usually found in both domestic and industrial processes where it is employed in the management of waste products so as to release some useful energy (Antonio et al., 2011). It uses the process of fermentation that is also used in the production of certain drinks and food products. Either, silages are a product of the anaerobic processes. It is considered one of the oldest technologies in the history of mankind and over the years it has been used in the management of waste products. Critics argue that amounts of waste products that are released to the environment have steadily increased thereby necessitating the need to effectively manage them. This has been a general area of concern to both health officials and environmental crusaders who believe the problem of waste management is not only an environmental concern but also a threat to the health of individuals. This paper looks at the anaerobic digestion process, its application in the management of waste products and the various challenges faced in its usage (Lawrence & Joo-Hwa, 2000).
Historically, there has been a widespread interest on the area of gas manufacturing. Robert Boyle and his counterpart Stephen Hale were the first people to produce gas through the process of decomposing organic matter. Studying the sediments of lakes and streams, they realized that inflammable gases were being produced. Their work was furthered by the work of Humphrey Davy who found out that methane gas could be produced from the decomposition of manure. In 1859, the first decomposer was built in India and later developed in England where septic tanks were employed in the production of natural gas that was alighting gas. Later on, further scientific research resulted in anaerobic digestion being included within the academic system where it led to the revelation of the presence of anaerobic bacteria in organic materials. Conditions under which the methanogenic bacteria grew to multiply were also revealed (Lawrence & Joo, 2010).
The anaerobic digestion process has various stages that involve different bacteria. It begins with a process that involves the hydrolysis of the bacteria in all inputs materials so that it can break down all organic insoluble polymers. These polymers include elements such as carbohydrates and they are transformed into a state that the acidogenic bacteria can act on them. The acidogenic bacteria then break these polymers, already transformed into amino acids and sugar, into smaller elements that include ammonia, organic acids and hydrogen gas. The resulting elements are then acted upon by the acetogenic bacteria that further break them down into acetic acids, carbon dioxide, hydrogen and ammonia. The final stage involves the conversion of these products into carbon dioxide and methane gas. The whole process involves the active participation of the methanogenic archaea especially in waste management.
It has over the years acted as a source of a biogas energy that is renewable consisting of carbon dioxide, methane and small amounts of others gases otherwise considered contaminants. The produced biogas is usually used as a direct source of fuel in homes. It can be also used in heat engines or it can be refined industrially and upgraded into natural gas of higher quality such as biomethane. Its usage has been found to be advantageous as it reduces the reliance on fossil fuels. Either, the by-products in the form of digestate can be also used as farm fertilizers thus avoiding wastages and environmental pollutions. With the increase in agitation for the use of environmentally friendly sources of energy that are also cost effective, anaerobic digestion has been vouched for by the United Nations as a resourceful and environmentally friendly source of energy as opposed to other sources that involved the usage of huge power plants that are otherwise costly. Countries such as Denmark, Germany and the United Kingdom have employed its extensive use in production, management of waste products and as a source of energy.
The Anaerobic Process
Studies show that a variety of microorganisms are involved in the process of aerobic digestion. These bacteria include those that help in the formation of acetic acid such as acetogen bacteria and those that lead to the formation of methane gas that are known as archaea or methanogens. They act on the initial stocks that undergo various processes to convert them into molecules that are in an intermediate state such as acetic acids, sugar, hydrogen that are then converted into biogas. The bacteria involved in this process are capable of living at different temperatures and are thus named in accordance to the temperature range they are capable of living at. The bacteria that are capable of living at temperatures between 35 to 40°C are generally known as mesophilic bacteria while those capable of living conditions of temperatures between 55 and 60°C are referred to as thermophilic bacteria. There are also methanogen bacteria that are capable of living at very high temperatures (Mata, 2003).
In aerobic systems, the multiplication and growth of the microorganism require the presence of oxygen, while in the case of anaerobic systems the oxygen condition is not required for the process to be successful. Oxygen is contained so that it does enter the anaerobic system. Sealed tanks are majorly used to prevent oxygen from entering the system. The oxygen can either be generated from outside the system or within the system itself. The oxygen generated within the system comes from various organic materials; as a result, the end products are aldehydes, organic acids, carbon dioxides and alcohols. When methanogen bacteria are introduced in the system, these products are then converted into the desired final products that are methane gas, hydrogen sulfide traces and carbon dioxide. Since anaerobic microorganisms take a considerable amount of time to multiply and establish, this process is started by introducing materials that already have anaerobic bacteria microorganisms through the addition of cattle slurry or even sewage sludge. This process is referred to as seeding.
The main stages in the anaerobic digestion are hydrolysis, acidogenesis, acetogenesis and methanogenesis. The hydrolysis process is the first step where the large materials are broken down into smaller parts that can be easily acted upon by the bacteria. This process ensures that the organic materials are broken down into fatty acids, amino acids and simple sugars. The methanogens can directly act on the produced hydrogen and acetates but the fatty acids have to be further catabolised so that the methanogens can act on them. The second stage of acidogenesis involves actions by the acidogenic bacteria that further breaks down the remaining components. This process is similar to the process of milk fermentation and its end products are ammonia, fatty acids, hydrogen sulfide, carbon dioxide and other by-products (Arun, 2011).
In the acetogenesis stage, the acetogens act further on the products that result in acetic acids, hydrogen and carbon dioxide. The final stage is that of methanogenesis where the methanogens convert the acetogens into their final product which is methane, water and carbon dioxide. They are the major components of the biogas products that are the end product of the anaerobic digestion. The methanogenesis stage is very sensitive to both higher and lower pHs and the overall anaerobic process can be represented using a chemical equation as below.
C6H12O6 → 3CO2 + 3CH4
Issues to be considered in the anaerobic digestion system
The first thing to take into consideration when initiating the anaerobic digestive system is the materials that will be used in the process. Research findings show that any organic material can be anaerobically processed but when it comes to situations of biogas production, the digestibility of these materials must be taken into consideration as those that are easily digestible tend to produce more gas. Materials such as waste papers, animal wastes, food remnants, sewage and grass are easily digestible thus can be used as a source of anaerobic digestion. Materials such as wood should be avoided since their digestibility is very low.
The anaerobic system usually employs the usage of co digesters where two or more materials are used as feedstock materials. An example is where a farm manure and grass are used as primary and secondary feedstock respectively. The anaerobic digesters that use crops as their feedstock materials produce high levels of biogas products. There are also those that only depend on slurry, which is considered cheaper and only uses a small amount of material (Mata, 2003).
Moisture content is the second element to be considered in any anaerobic digestion system. Feedstock materials such as food and wastes from the yard are recommended to be digested in chambers that are tunnels in nature. These chambers do not discharge waste waters which is an advantage in conditions where the discharge of waste water is not required. When the waste water is produced, it will occupy larger volumes than the intended products which are the gases. This is majorly affected by the levels of the moisture contents in the feedstock materials as it determines the type of digester to use. So as to increase the solid contents of the feedstock materials of bulky materials such as the compost materials, a high-solids anaerobic digester is used.
Carbon nitrogen ratios of feedstock materials should be also taken into consideration. There should be between 20 to 30 elements of carbon for every single element of nitrogen as this gives the requisite ratio considered balance and an imbalance where there is an excess of nitrogen that will lead to the inhibition of the anaerobic digestion system of ammonia (Antonio et al., 2011).
The level to which feeds are contaminated should be also considered as they affect the functionality of the digester. In case the materials are contaminated by indigestible materials such as plastics and metals, it will affect the normal functioning of the system. The contaminating materials should be eliminated before feeds are introduced into the system. This is aimed at preventing the system from getting blocked. This is followed by shredding of feeds which are then minced and pulped, either using hydraulic or mechanical systems so as to increase their surface area that will be acted upon by the bacteria. This will ensure that the process of digestion is speeded up.
The composition of substrates affects the production of methane gas and the rate at which it is produced. Various techniques can be used in the determination of the composition of substrates or feeds. The design of the digesters majorly depends on factors such as the solid elements and analysis of the organic substances.
The expected products in any anaerobic system are biogas, digestive and water wastes. Biogas is composed of methane gas (CH4), nitrogen gas (N2), hydrogen sulfide gas (H2S), carbon dioxide gas (CO2), oxygen gas (O2) and hydrogen gas (H2).
Biogas production is usually high when the population of bacteria is high. This majorly takes place in the middle of the digestion process. As the process comes to an end, the biogas production slows down. The produced gas can either be stored in an inflatable gas bubble or extracted and stored next to the facility. The produced methane is capable of being burnt to produce heat and electricity. The heat generated as waste can either be used in warming up the digester itself or in warming up buildings. The biogas production does not lead to the depletion of the ozone layer since the produced carbon dioxide is not directly released to the atmosphere. The carbon dioxide also has a short carbon cycle since it emanates from organic substances (Antonio et al., 2011).
There have been strict limits put by regulating authorities on the amount of gas containing the toxic hydrogen sulfide that can be produced. Environment protection agencies both in the US and Great Britain have put limits on a number of gases that contain hydrogen sulfide to be produced. Maintenance activities such as gas scrubbing are usually done so as to keep biogas at acceptable levels. Special equipment is used since the addition of ferrous chloride FeCl2 to the digestion tanks inhibits hydrogen sulfide is used so as to slow down the production of hydrogen sulfide gas. Methane produced can sometimes be compressed and used as vehicle fuel. This technology has been employed in such countries as Sweden, German and Switzerland. The only disadvantage with this technology is that it also requires the use of energy during the processing which in effect reduces the amount of energy that will be available to be sold.
These are the materials remaining after the anaerobic digestion. These materials are of a solid form and the microbes within the system cannot act on them. Digestates also consist of the various remaining materials of dead bacteria. Digestates are found in three major forms namely liquor, fibrous or a combination of the two.
Water found during anaerobic digestion usually comes from the moisture of the original feeds and also is produced during the process of microbial reactions within the digestive system. The water can be released through the process of dewatering the digestates or through the separation of the digestates.
The water that will be drawn from the digestive system of anaerobic digestion will be tested for two major things. First, it will be tested for biochemical oxygen demand (BOD) and also for chemical oxygen demand (COD). In most cases, there will be a high level of these oxygen demands that will clearly be an indication of the ability of water to pollute the environment. The water is then treated to avoid eutrophication when released in the water bodies. The treatment will generally involve an oxidation process through a reverse osmosis process.
Applications of the Anaerobic Digestion Technologies
Treatment of Waste
The anaerobic digestion technology is commonly used in the treatment of water to re-use sewage effluent for drinking water. It is the simplest method ever used in the reduction of organic matters that could otherwise be released to various water sources or deposited on open fields thus being contagious to human health. In case the digestible remnants are released to the environment, they would anaerobically break down thus emitting methane gas to the environment. Studies show that methane gas is 20 times more likely to cause negative environmental effects as opposed to the carbon dioxide gas that is produced in greenhouses. There are countries that have taken up the responsibility of controlling the waste products emitted. These countries use anaerobic facilities to centrally manage their waste through incineration (Chung, Yi & Hassell, 2009). Either, legislations are also made so as to regulate waste management.