BENEFITS OF ANAEROBIC DIGESTION 

            Collected biogas can be used for process heating, electrical and steam generation, and transportation. A rule of thumb for electricity production is on kilowatt for every seven to ten cows.

            Odor and fly control. Bacteria in the digester significantly reduce odor-causing compounds. In addition, fly eggs are killed during anaerobic digestion, reducing this disease-causing pest.

            Pathogen control. Pathogens like E. coli, Salmonella and Cryptosporidium can't survive the high temperature of a heated digester.

            Disposal. Anaerobic digestion destroys more volatile organic compounds and produces more gas than aerobic digestion does (65 percent to 75 percent of volume), resulting in less solid waste.

            Environmental. Where animal manure is stored in pits or lagoons, methane is released into the atmosphere. An anaerobic digesters reduces the damaging effects of methane, which is 21 times more potent than carbon dioxide in causing global warming.

            Valuable byproducts. In addition to the biogas produced, most of the organic nitrogen present in the manure is converted to ammonia, a fertilizer readily utilized by plants. Liquid effluent can be spread on fields, and the solids can be sold amendment.

 What is anaerobic digestion?

 Anaerobic digestion is the bacterial decomposition of organic matter that occurs in the absence of oxygen. Anaerobic bacteria exist naturally at the bottom of ponds, swamps and other moist and airless places, and even in the digestive tracts of termites and large animals. These bacteria are among the oldest life forms on earth. Thousands of years ago, anaerobic decomposition of organic matter formed the earth's coal and oil deposits and created the natural gas we currently use for cooking and heating.

    The same process can be duplicated today with a mechanical digester that re-creates the ideal natural conditions for decomposition. Three primary reasons for use of mechanical digesters managing organic waste are nutrient recycling, waste treatment, and odor control. The methane produced in the process is a useful and valuable byproduct.

    Anaerobic digestion differs from composting, which is an aerobic, or oxygen-utilizing, process. Composting organisms produce the high temperatures the process requires by consuming oxygen. Their efficiency is maintained by providing the proper mix of air and types of organic matter. Creating a thriving anaerobic climate, however, requires maintaining a consistent temperature and quality of organic matter within a sealed and airless container. The anaerobic digestion process is more chemically complex and technically demanding than the composting process is, but it requires less space. In addition, its products make more efficient use of the organic resource.

 DEFINITIONS

 Methane: the principal component of natural gas. It also a major element of biogas, or gas produced during the decomposition of organic materials in the absence of oxygen.

 Methane Recovery: the capturing and harnessing of the methane generated from the breakdown of livestock, landfill or municipal wastes to energy such as heat or electricity.

 AT A GLANCE

Environmental Benefits of Methane Gas Recovery  

• Biogas can produce electricity and heat offsetting a farm's energy costs and reducing the need for other fuels. This, in turn, reduces pollution that comes from drilling, mining, transporting and burning and reduces carbon dioxide-another major contributor to global climate change.
• Digestion reduces the potential for surface and ground water contamination.
• Collecting biogas prevents the methane from releasing into the atmosphere and becoming a powerful greenhouse gas.
• Anaerobic digestion typically decreases the volume of manure solids by more than 90 percent. The remaining biosolids are an extremely high quality fertilizer.

 What is an Anaerobic Digester?

 Anaerobic means "without oxygen," therefore, an anaerobic digester is an enclosed tank that excludes oxygen. Naturally occurring bacteria break down manure into a variety of gasses, including methane. Since methane-producing bacteria are most active in a range of 95 to 105 degrees F, some digesters circulate hot water through pipes to heat the manure and maintain the desired temperature range. A flexible, impermeable cover on the digester traps the biogas, which is either burned off in an open flame or passed through a electrical generator.

There are several types of generators, but most use a modified internal combustion engine. The electricity can then be used on the farm.

Environmental Benefits

Anaerobic digestion typically decreases the volume of manure solids by more than 90 percent. The biosolids that remain contain higher concentrations of NPK and trace elements than manure and are, therefore, an excellent soil amendment. They also contain relatively high concentrations of ammonium-N, making the N rapidly available, once it is spread on fields.

Because manure digestion is anaerobic, most weed seeds and pathogens are killed during the process. Fecal coliform bacteria numbers in the biosolids are only about one percent of those in fresh and restored manure, lowering the potential for this source of water pollution.

Odor Control

As housing developments sprout up on what was once exclusively farmland, and the trend toward larger confined livestock operations continues, manure odors are pitting the public against livestock producers. Anaerobic digesters offer a solution. Anaerobic microorganisms break down many odor-causing compounds in manure as it moves through the digester. The basically eliminates odor problems. Research has shown anaerobic digestion reduced odor by 97 percent over fresh manure. In farm expansion projects, odor control can be a primary reason for installing a digester.

Is It Best for Your Farm?

The cost of a digester depends on specific farm conditions and pay back can range from a few years to more than 10 years. Systems typically use 30 percent of the biogas to heat the digestion system. Many systems are able to supply farm's complete electricity or heating needs. Typically a minimum herd of 300 dairy cows or 2,00 swine is needed to make such a system feasible.

Money isn't the only consideration. On average, about 45 minutes of daily maintenance is required to keep a digester working smoothly. This includes system inspection, mixing and pumping manure into the digester twice a day, and checking and recording gauges to measure biogas and electricity output. Generator engines also require routine monthly maintenance including oil change, valve adjustment and spark plug