Anaerobic digestion is a naturally occurring biological process that uses microbes to break down organic material in the absence of oxygen. In engineered anaerobic digesters, the digestion of organic waste takes place in a special reactor, or enclosed chamber, where critical environmental conditions such as moisture content, temperature and pH levels can be controlled to maximize gas generation and waste decomposition rates.
Landfills generating noxious odors demonstrate the impact of organic waste digestion in a semi-enclosed environment with little or no oxygen. However, by using anaerobic digestion technology, odors are greatly reduced because the gases are captured. Commercial anaerobic digestion systems can replicate this natural process in an engineered reactor that produces methane gas much more quickly, in as little as two to three weeks compared to the 30 to 100 years required by the anaerobic conditions in a landfill.
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Digester Prevalence
Anaerobic digestion systems designed to process animal manure have been in widespread use for years in parts of the developing world. Several hundred thousand digester systems are estimated to operate in India, and several million are in use in China. In Europe, government incentives in the form of grants, low- and no-interest loans, and mandates that utility companies purchase the energy produced at a premium (often two to four cents per kilowatt above market value), combined with rising energy prices have encouraged the development of anaerobic digestion plants, with more than 1,000 now in place. These digesters mostly serve waste management and odor control needs and provide limited energy generation, though several in Europe and Asia are net suppliers of energy to utility companies. Examples include the Kompogas plants in Kyoto, Japan, and Rostock, Germany, as well as the Valorga International plants in Barcelona, Spain, and Hanover, Germany.
The use of anaerobic digestion technology is rapidly growing in the U.S. It is already a developing market within the agricultural industry. The technology is economically and environmentally beneficial. The country’s high demand for energy coupled with a concern for reducing its dependence on imported oil has driven the expansion in the use of electric power generated from methane. Other incentives include the desire to redirect organic waste from landfills. Anaerobic digestion optimizes the benefits of organic waste used for methane production and helps with the landfill shortage problem. Anaerobic digesters have a financially attractive payback period (dependent on energy prices, subsidies and a number of other factors) when the methane gas is used to generate energy in the form of heat, steam or electricity. A proposed 10,000 tons per year plant servicing the industries at the Brooklyn Naval Yard had an anticipated return on investment of just seven years as a result of significant subsidization by the New York Sustainable Energy Research and Development Authority. Larger plants can be even more profitable.
The Anaerobic Process
When using a thermophilic process (a higher temperature and more efficient bacteria), digestion takes place in four stages (Figure 1) plus a preliminary stage over 10 to 14 days.
Prior to digestion, the feedstock enters the buffer or pretreatment tank where its temperature is raised and microbial activity begins. After one day of pretreatment, the feedstock is released into the main digestion tank where the first of the four steps—hydrolysis—occurs, during which complex organic molecules are broken down into simple sugars, amino acids and fatty acids with hydroxyl groups. The second stage is known as acidogenesis, during which further breakdown occurs producing ammonia, carbon dioxide and hydrogen sulfide.
The third stage is acetogenesis during which the products of acidogenesis are further digested to produce carbon dioxide, hydrogen and acetates, along with some higher-molecular weight organic salts.
Methanogenesis, the fourth and final stage, produces methane, carbon dioxide and water. Methane and carbon dioxide are the main components of biogas (Figure 2). Approximately 55 percent to 70 percent of the gas composition is expected to be methane.
Environmental, Other Benefits of Anaerobic Digestion
From an environmental standpoint, anaerobic digestion has three main benefits. First, it is a waste-to-energy technology, meaning that it converts waste materials and not food supplies or other usable products into energy. As a result, demand for power generated from anaerobic digestion will not affect resource markets or lead to poor land management practices as producers attempt to produce more of a resource on a given area of land to satisfy increased demand. In addition, as anaerobic digestion uses waste as its fuel source, it has the potential to divert large quantities of biodegradable waste away from landfills.
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