Anaerobic Digestion

Submitted by:

Submitted on:

Overview

This essay studies the present status of the technology in Anaerobic Digestion in US. In the first section the general process of anaerobic digestion is described. Anaerobic Digestion is basically the decomposition of organic wastes in the absence of oxygen. A brief appraisal is provided about the current position of Anaerobic Digestion in the US energy Production. Then in the next section, the technological challenges of the Anaerobic Digestion are considered. The example of the usage of Anaerobic Digestion technology is discussed and the benefits of this technology. Also the demerits of this technology for the production of biogas are examined. Finally the current improvement in anaerobic digestion technologies in US is analyzed. Introduction

The Anaerobic Digestion is defined as the biodegradation of organic material that takes place in the absence of oxygen and in the presence of anaerobic microorganisms. The whole process proceeds in the series of metabolic reactions with the help of microorganisms. The organic material which is decomposed is generally the Municipal Solid Waste (MSW). This is the waste produced by the society excluding agriculture and industrial wastes. So, the waste that comes under MSW is commercial waste like from shops, markets, stores, hotels or residential waste from houses or institutional waste from hospitals, schools. The process of anaerobic Digestion completes in four stages namely Hydrolysis, Acidogenesis, Acetogeneis and Methanogenesis. In the first stage enzymes are secreted by microorganism and produced amino acids and glucose. Then these amino acids are converted into acetic acid by acetogenic bacteria. Finally a group of methanogenic bacteria convert CO2 , H2 and acetate to methane. The process is carried out in digester at temperature 300C to 650C and bio gas is produced.

Hydrolisis Acidogenesis Acetogeneis Methanogenesis

Figure 1: four stages of anaerobic digestion

Anaerobic Digestion in US

The major benefit of anaerobic digestion is environment friendliness and the biogas production which is a form of energy. In U.S. the use of anaerobic digestion was limited to senior citizen until recently. But from certain years the advancement in technology has resulted in the increase in the production of energy by anaerobic digestion. However to build a large scale market there must be rich quantities of natural resources and fossil fuels. And this result in high energy prices which is in contrast with U.S. from times the prices of energy in U.S has been low which hinders the development of anaerobic digestion. The recent trend in this technology in U.S is appreciable. According to the Bioenergy Report of the International Energy Agency (IEA), in U.S.A one plant of AD is in operation and two plants are under construction. In recent times U.S has taken serious outlook on this sector. The BTA new market plant in Canada has a capacity of 150,000 metric tons of organic waste every year. In US the 70 percent age of waste composition consists of Municipal Solid Waste. This shows us that the US has enough scope for the development in this field.
Current Technology

The benefits of anaerobic digestion have led to the consideration by many governments and institutions in recent times. The new rules are being formulated for efficient treatment of municipal solid waste. The recycling of waste is done by the new concepts of waste management. Incineration is used for the hazardous wastes like hospital waste which can easily be burned but cannot be recycled and the byproduct is disposed off to the landfills. In the recent years the special plants are designed which are used for separating the mixed waste known as Mechanical Biological Treatment Plant (MBTs). The most impressive technology recently employed by BioSTARTM is a efficient way to produce biogas by organic wastes. “BioSTARTM systems make use of a secure, web-based, data attainment and procedure control strategy to incorporate proprietary third-generation thermophilic anaerobic digestion technology, modern biogas consumption strategies, and verified fertilizer production processes to convert high-strength organic waste streams to green energy and premium organic fertilizer.”(Crabtree .D,n.d)

[pic]
Source: BioSTARTM Systems Although the principles of BioSTARTM systems are invariable, system designs vary from individual to individual depending on the stream of waste, the value of the by-products, the cost of waste disposal vary according to the specific sites, and the cost of energy in the local..

Three examples: Due to the various benefits of anaerobic digestion it is becoming the renowned source of energy. For example The United States consumed 0.6 percent of the total consumption of natural gas as BTU which accounts for about 147 trillion of energy in total. Due to its positive effect on environment it is being used more now. The cow manure can generate 100 billion (KWH) which can light up millions of house of U.S. It has been tested by the country that it can reduce around 4 percent of greenhouse gases. Also the biogas generated on various farms in Vermont comes under the CVPS Cow Power Program. The program is formulated by Public Service Corporation to provide charitable fare. In this program the premium paid by the customers is transferred to the farms which are part of the program. The biogas produced by the digester here can power almost 400 homes apart from the farms. The anaerobic digester was installed which produced biogas by the waste from the cow and the milking center. There were almost 950 cows used for this purpose. The capacity of this generator is around 300 (KW). This is the most suitable example of the “best out of waste”. Another place where the manure of cow is used for a good purpose is Hereford in Texas. Here it is used to power an ethanol power plant. This technology of production of methane bio-gas has conserved almost thousand barrel of oil per day in ethanol power plant which reduced the cost of transportation.
Challenges of biogas production

Not even after so much research and exploration of this field, the producer has no clear-cut distinction between various designs that are being implemented for the production of biogas. This is so because the different types of digesters are required according to the conditions. There are various digesters which provide maintenance and production problems for the use in farms. Sometimes when the technology is employed in the farms they import problems in cleaning up, gas collection of the digester and generator. Moreover the digester is still not safe to use. The various concerning factors are fire, explosion and asphyxiation related with the production of biogas. Biogas contains large amount of dangerous gases like hydrogen sulfide and ammonia which is harmful for the living beings. Therefore extra care must be taken on entering the anaerobic digester. Moreover the dependence on natural ventilation is not safe. There are some gases like carbon dioxide and hydrogen sulfide that are heavier than air which could be hazardous. The methane and air when mixed with concentration of 5 percent age to 15 percent age could lead to explosion. There are also chances of major accidents due to the leak in the gas line. So the major concern that must be taken into account is the safety and security of the anaerobic digester used for the production of biogas. The same problems are also there in electric generator and large engines which needs to be resolved for the better use of this technology.

Technological challenges

The return from digester is very low and requires large capital cost which is a major barrier to this technology. Also the main reason for the failure of already installed digester is the lack of management. Moreover the use of digester is not easy, as the change in the biological process disturbs the working of whole system. Let’s discuss the other technical challenges like complexity in maintaining the temperature of the system in detail. If the temperature of heat pipe is too high then the manure is cooked up more which decreases heat transfer efficiency. When the manure is cold then it takes large amount of heat to setup the manure at operating temperature. The environment is corrosive inside the digester here one metal act as anode and other as cathode which results in rapid depletion metallic layer due to corrosion. “During the 1970s energy crisis extra power production was welcomed and encouraged. Lately the price paid to non-utility generators has been under $0.025 per kw. Specific requirements for insurance, demand charges for the use of electricity when the on site generator is down, and other rules may be difficult to meet.” One more problem which needs to be considered is the use of digested effluent which contains lots of nutrient even after digestion. Also odor control needs to be done through some technique which is economical to implement. Until now the share of bio-energy is only 1.26% of total consumption of energy which needs an improvement for more use of this technology.
Conclusion:
The market of anaerobic digestion is yet not developed in US until now organic waste are generally treated aerobically which consume lot of energy and amid hazardous green house gases. Moreover the consumption of energy in US is mostly through fossil fuel which has negative impact on environment. So, the government should concern anaerobic digestion as the leading energy production technique.
REFERENCES

Morales, A. Why Don't We Hear Much About Anaerobic Digestion in the U.S.?, September 2007, Retrieved from:

http://www.renewableenergyworld.com/rea/news/article/2007/09/why-dont-we-hear-much-about-anaerobic-digestion-in-the-u-s-49970

Scruton, Daniel, S. Weeks, R. Achilles "Strategic Hurdles to Widespread Adoption of On-Farm Anaerobic Digesters" Presented at the 2004 ASAE Annual International Meeting Ottawa, Canada August 1-4, Paper No. 04-4164 ASAE 2950 Niles Road St. Joseph, MI 49085-9659 Evans. S, New Waste Technology retrieved from: http://anaerobic-digestion-news.blogspot.com/2011/02/new-waste-technologies-recycling-and.html Crabtree D. “Anaerobic Digestion Systems Tackle Growing Clean Water and Renewable Energy Challenges” http://www.biostarsystems.com/whitepaper/BioStar_Whitepaper_093009_.pdf -----------------------
Carbohydrates

Fats

Proteins

Sugars

Fatty Acids

Amino Acids

Carbonic acids & alcohols

Hydrogen
CO2 & Ammonia

Hydrogen
Acetic Acid &
CO2

Methane CO2