Anaerobic DigestionThe Maryland Energy Administration is currently collaborating with the Maryland Department of Agriculture, Natural Resources Conservation Service, US Department of Agriculture and the Maryland Resource Conservation & Development Council to explore ways to combat nutrient waste issues, produce energy and promote energy efficiency on farms. One way of accomplishing this is through the process of anaerobic digestion. Anaerobic digestion is a biological process that produces gas mainly composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas in an oxygen-free environment. It is produced from the decomposition of organic wastes (i.e. manure, food waste, yard waste, etc.). Methane is the principle constituent in natural gas. Biogas is composed of 55%-80% pure methane with the remainder being carbon dioxide. Methane has a Btu content of 896-1069 Btu/ft3. This gas can be run through an ordinary reciprocating engine, used to fire a boiler or even run a vehicle. It can be burned in every way natural gas can be burned. The process of anaerobic digestion consists of three steps all occurring in an oxygen free environment:The first step is the decomposition of plant and/or animal matter. This step breaks down the organic material into smaller molecules such as sugar. The second step is the conversion to organic acids. And finally, the acids are converted to methane gas. All of these steps are carried out by various bacteria known as Anaerobes with the terminal anaerobe being a Methanogen. The ultimate yield of biogas depends on the composition and biodegradability of the organic feedstock. The rate of formation depends on the population of bacteria breaking down the organics, their growth conditions and the temperature. Process temperature affects the rate of digestion and should be maintained in the mesophilic range (95° to 105°F) with an optimum of 100°F. It is possible to operate in the thermophilic range (135° to 145°F), this will speed the process and allow for more material to be digested, but it does require more energy to maintain the thermophilic temperature range. Nutrient ManagementAll living matter is composed of Carbon. In addition, living matter requires a few essential elements, two of which are Nitrogen and Phosphorus. Nitrogen and Phosphorus are more important to some organisms than others. These two elements are extremely important to organisms in the Plant Kingdom. Their concentrations along with available sunlight and water determine the growth of organisms. Eutrophication is the process by which excess nutrients (Nitrogen and Phosphorus) on land are deposited in our water systems. Eutrophication is a natural process: terrestrial organisms die, decompose and the leftover nutrients are washed into lakes and rivers. Excessive eutrophication leads to hypoxic conditions. This is a reduction of dissolved oxygen in the water to a point that is detrimental to oxygen breathing organisms. This occurs because algae (one cell plants) absorb the nutrients that are flushed into the water system and begin to grow at an increased rate. These unicellular organisms then die, sink to the bottom and are decomposed by bacteria which consume oxygen, resulting in a hypoxic condition and killing fish, crabs, oysters…etc. Managing the volume of nutrients making there way into the water system will help ensure a natural balance in our aquatic ecosystems. The State of Maryland has a Nutrient Management Program that will help minimize nutrient runoff on farms. Most of the nutrients in organic matter are bound up in proteins and are not readily available to plants without undergoing a biological conversion. This conversion takes place every day in the natural world around us. Nonliving organic matter decomposes naturally. An alternative is to quickly break down organic matter using an anaerobic digester and, in the process release the nutrients into the liquid effluent. For example, during the digestion process, much of the nitrogen is reduced to dissolved ammonia. Plants can not take up ammonium directly; bacteria on the ground convert it to nitrates first, a form of nitrogen that plants can use. This is an example of a slow release fertilizer. Plants utilize more nitrogen from fertilizers when the fertilizers are slow release, thus keeping excessive nitrogen out of the water system. Many farmers are looking at anaerobic digesters as a solution to their odor problem. Many of the odors associated with manure are intermediate compounds of the anaerobic process breaking down organics. An anaerobic digester confines odorous intermediate compounds to fermentation tanks and then breaks them down to less offensive ones. Digesters convert these offensive odors into methane and carbon dioxide, both of which are colorless and odorless. Animal manure is also a source of pathogenic organisms (E.coli, & Salmonella). Digesters have the potential to run at high temperatures, thus making them effective hygienization vehicles. Anaerobic digesters convert organic matter into three usable products: biogas, fiber (used as a nutrient rich soil conditioner), and liquor (used as a slow release liquid fertilizer). More Information |
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Maryland Energy Facts
Wind farms in the U.S. have helped avoid nearly 62 million pounds of pollutants, such as sulfur dioxide (SO2) and nitrogen oxide (NOx), tied to global climate change.