Anaerobic Digestion as an Alternative Way of Recycling Biowaste

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Chynoweth and Isaacson (1987) describe the process of anaerobic digestion as follows: The process begins with the separation of household waste into biodegradable and nonbiodegradable waste. The biodegradable material is shredded, slurried, and then screened and pasteurized to start the process of killing harmful pathogens. It is then pumped into the digester where bacteria break down the material and form biogas, leaving a digestate. The three main process stages in anaerobic digestion are: hydrolysis, acidogenesis, and methanogenesis. Hydrolysis: Insoluble organic polymers such as carbohydrates, cellulose, proteins, and fats are broken down and liquefied by enzymes produced by hydrolytic bacteria. Carbohydrates, proteins, and lipids are hydrolyzed to sugars which then decompose further to form carbon dioxide, hydrogen, ammonia, and organic acids. Proteins decompose to form ammonia, carboxylic acids, and carbon dioxide. During this phase gas concentrations may rise to levels of 80% carbon dioxide and 20% hydrogen. Acidogenesis: Organic acids formed in the hydrolysis and fermentation stage are converted by acetogenic microorganisms to acetic acid. At the end of this stage carbon dioxide and hydrogen concentrations begin to decrease.

Methanogenesis: Methane (60%) and carbon dioxide (40%) are produced from the organic acids and their derivatives are produced in the acidogenic phase. Methane is a useful fuel source and methanogenic bacteria play a further role in maintaining wider breakdown processes. Efficient mixing of the contents of the digester improves the contact between the material and the resident bacteria. Mixing of the waste slurry in the digester is important in maintaining a high rate of anaerobic biodegradation and a high production level of gas. The mixing process disperses the incoming waste within the digesting sludge, improving contact with the microorganisms. Monitoring the acidity within the digester is necessary to provide optimum conditions for the balanced growth of bacteria. Monitoring takes place in the reactor using probes. The concentration of volatile fatty acids is an important parameter for monitoring as this can be the first indicator that digestion is not progressing normally.

Biogas plants have two products: biogas and digestate. Digestate is regarded as an organic fertilizer. The main issue in the Swedish regulations for organic fertilizer is coverage of the storage; field application is prohibited during winter months and also during early spring and late autumn in coastal areas, maximum 110 kg P/ha during a 5-year period for a single application (Palm 2008).The waste status of the outputs of anaerobic digestion has been identified as a key barrier to the development of the industry to treat waste in the UK. Because of that a standard for anaerobic digestion outputs should be developed with a certification scheme and quality protocol insisting of the clarity of the regulators and regulated, the confidence in a product delivered to the right market, and removing a barrier and allowing development of the industry (Verma 2008). The soil improvement challenges with digestate are opportunities to improve source separation and the digestate quality and the threat is lack of lignin or wood (Pires 2008). Dry digestion (specifically developed for the anaerobic digestion of organics derived from household waste) becomes particularly attractive when the production of excess wastewater can be avoided by particular stream digestion or by drying the digestate with waste heat coming from the biogas engines. Dry digestion is more easily integrated on existing composting sites and can be used to expand the capacity on the site with the use of limited amount of surface area (Baere 2008). Wet digestion systems are operated at a lower solid concentration compared to dry digestion systems. Taking into account all areas in which the biogas technology is used worldwide the wet technology is the most prevailed biogas technology (Korz 2008). In composting plants based on the percolation technique the waste is first percolated to deliver easily accessible organic matter to the methane reactor. This ensures the existence of two different microbial communities, which are totally dependent on each other. When the percolate from the module becomes low in organic matter the composting of the energy-poor waste in the module is started (Bloch 2008). The biogas potential of kitchen waste is because of the high fat and carbohydrates content, so with a partial stream digestion the waste is to be divided into a stream perfect for both anaerobic digestion and composting (Mayer 2008). Cuhls (2008) determined gaseous emissions from different types of large-scale treatment plants for biowaste in Germany and found that CO2-equivalent (methane and nitrous oxide) from biological treatment of biowaste is in the waste gas ~30-40 kg/Mg and in the clean gas after biofilter ~70-80 kg/Mg. The estimated emissions for methane and ammonia are overvalued so far, specific contingent of composting or digestion is rather low (<0.5% of total national emission). Anaerobic digestion is increasing. The importance of compost suppressing plant diseases is growing. Results of public RFP (province Utrecht,

2007) granted on price and CO2 performance indicate results in savings on greenhouse gases up to 160 kton CO2-eq/ton of biowaste to be realized in 2009 (Elsinga

2008). Through co-fermentation, i.e., through the joint treatment of biogenic wastes (co-substrates) in the digesters of the wastewater treatment plant, the digester gas production cab has to be increased considerably (Reipa and Schmelz 2008). Some authors have concluded that there is a need for the cost-effective solutions in the integrations of the anaerobic digestion plant technologies (Hogg 2008; Kajan et al. 2008; Persson 2008; Santen and Fricke 2008; Turk and Kern 2008; Vasconcellos 2008). In comparison with other biotreatment methods biogas production is more expensive. The environmental degradation effects are not sufficient, economically tested and they are not compared with the investments planned for biogas production (Bendere 2008). Uriate (2008) and Siebert (2008) have concluded that the product quality is a very important aspect and because of that quality standards are needed. The highly developed European techniques of anaerobic digestion having a standard quality have good chances in the developing countries, and also in the agricultural production through composting and the use in aqua culture of the residues from the anaerobic digestion (Bildlingmaier 2008).

Biomass (including organic residues and biowaste) represents a continuously renewable potential source of methane and thus offers a partial solution to the eventual prospect of fossil fuel depletion. Processes for conservation of biomass to methane may be classified into two categories: thermal and biological. Thermal processes have the ability to effect total conservation of organic matter at rapid rates. The biological gasification, better known as anaerobic digestion, is a low-temperature process that can process wet or dry feeds (with added water) economically at a variety of scales. The process is based on methane fermentation. The product gas is composed primarily of methane and carbon dioxide with traces of hydrogen sulfide. The major limitation of this process is that conservation is usually not complete, often leaving as much as 50% or more of organic material unconverted.

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