- February, 2011
- January, 2011
- December, 2010
- November, 2010
- October, 2010
- September, 2010
- August, 2010
- July, 2010
- June, 2010
- May, 2010
- April, 2010
- March, 2010
- February, 2010
- January, 2010
- December, 2009
- November, 2009
- October, 2009
- September, 2009
- August, 2009
- July, 2009
- June, 2009
- May, 2009
- April, 2009
- March, 2009
- February, 2009
- January, 2009
- December, 2008
- November, 2008
- October, 2008
- September, 2008
- August, 2008
- July, 2008
- June, 2008
- May, 2008
- April, 2008
- March, 2008
- February, 2008
- January, 2008
- December, 2007
- November, 2007
- October, 2007
- September, 2007
- August, 2007
- July, 2007
- June, 2007
- May, 2007
- April, 2007
- March, 2007
- February, 2007
- January, 2007
- December, 2006
- November, 2006
- October, 2006
- September, 2006
- August, 2006
- July, 2006
- June, 2006
Most read articles
|0406 ANM: Anaerobic digestion: Biogas production and odor reduction|
|Archives - Past Articles|
|Monday, 26 June 2006 02:58|
Anaerobic digestion (the decomposition of organic matter by bacteria in the absence of oxygen) occurs naturally in liquid manure systems. The lack of oxygen and abundance of organic matter in liquid manure provide the proper conditions for anaerobic bacteria to survive. Unfortunately, uncontrolled anaerobic decomposition can cause the foul odors sometimes associated with liquid manure storage and spreading.
However, controlled anaerobic decomposition not only can reduce the odors in liquid manure systems, but also can turn odorous compounds and organic matter into energy. The effluent remaining after controlled anaerobic decomposition, equal in volume to the influent material, is liquefied, low in odor and rich in nutrients. This digested material is biologically stable and will resist further breakdown and odor production, when stored under normal conditions.
Anaerobic bacteria transform manure and other organic material into biogas and a liquefied effluent during the three stages of biogas production (see Figure 1*). In the liquefaction stage, liquefying bacteria convert insoluble, fibrous materials such as carbohydrates, fats and proteins into soluble substances.
However, some fibrous material cannot be liquefied and can accumulate in the digester or can pass through the digester intact. Water and other inorganic material can also accumulate in the digester or pass through the digester unchanged. Undigested materials make up the low-odor, liquefied effluent. Most of the liquefied, soluble compounds are converted to biogas by the acid- and methane-forming bacteria during steps 2 and 3 of biogas production.
In the second stage of anaerobic digestion, acid-forming bacteria convert the soluble organic matter into volatile acids (the organic acids that can cause odor production from stored liquid manure).
Finally, methane-forming bacteria convert those volatile acids into biogas (a gas composed of about 60 percent methane, 40 percent carbon dioxide and trace amounts of water vapor, hydrogen sulfide and ammonia). Not all volatile acids and soluble organic compounds are converted to biogas; some become part of the effluent.
Methane-forming bacteria are more sensitive to their environment than acid-forming bacteria. Acid-forming bacteria can survive under a wide range of conditions, while methane-forming bacteria are more demanding (see Figure 2*). Under the conditions typical of liquid manure storages, more acid-forming bacteria can survive than methane-forming bacteria. Therefore, acids are formed and are not converted to biogas. This excess of volatile acids can result in a putrid odor.
In a controlled, optimum environment, methane-forming bacteria survive and convert most of the odor-producing volatile acids into biogas. Conditions that encourage activity of both acid- and methane-forming bacteria include:
•An oxygen-free environment
For consistent operation of an anaerobic digester, the manure that “feeds” the bacteria should be:
•A flowable liquid, about 12 percent solids or less (for pump or flow requirements)
Anaerobic digestion is simply a continuation of the animal’s digestive system – a process to turn manure into energy and effluent, just like an animal turns feed into energy and manure.
Anaerobic digestion system
Liquid manure-handling system
A bypass line routes manure around the digester when the manure is unsuitable for digestion or the digester is not operating.
Rigid or flexible covers have been used. They are designed to hold about 20 days of manure and a small supply of biogas. Manure, added daily to the digester, remains inside for about 20 days (the retention time) before flowing to the storage facility or spreader. Because there is no volume reduction with anaerobic digestion, the same amount of material added daily to the digester is also removed daily. While manure is flowing through the digester, the bacteria convert organic matter to biogas and effluent.
During the retention time, lightweight materials such as bedding or animal hair can float to the top of the digester, forming a crusty scum, and heavy or insoluble materials such as dirt can settle to the bottom. Settling reduces the effective volume of the digester and can cause incomplete digestion and odor problems, while crusting can keep gas from escaping the surface of the digesting manure. To control settling and scum formations, material in the digester can be agitated by a slurry pump, a mechanical stirrer or strategic placement of the heating pipes.
Slurry pumps are an effective way to keep material in the digester well-mixed. Mechanical mixing adds complexity to the system, but can aid thermal uniformity, reduce settling and break up crust formation. Mechanical mixing may be necessary for certain manure-handling systems (such as flush systems) where solid and liquid portions may separate easily into distinct layers within the digester. Strategic placement of the heating pipes will encourage thermal circulation and reduce settling problems.
The heating system is a critical part of the anaerobic digester. Heating pipes in which hot water circulates must be able to heat all material entering the digester to 95ºF and to resist corrosion from manure. Adding manure to the digester as soon as possible after it is excreted from the animal will help minimize heating requirements.
Gas utilization equipment
Biogas utilization equipment typically consists of either an engine generator set with electric utility hook-up, an engine operating hydraulic or air pumps or a gas boiler. Utilization equipment should be housed in a separate equipment shed apart from the digester to prevent corrosion.
Operating biogas-powered equipment continuously keeps the equipment temperature high enough to prevent condensation and sulfuric acid formation. Sulfuric acid is highly corrosive and can ruin expensive engines or boilers. Because biogas is a gas and not a liquid fuel, it is not practical for fueling vehicles. It would take 240 square feet of biogas to produce the same energy as one gallon of fuel oil. Biogas cannot feasibly be compressed to a liquid fuel due to its low-energy density.
For electricity production, biogas is piped to an internal combustion engine. The engine drives a generator to produce electricity that can be used on the farm or sold. To maintain continuous operation, the engine throttle is adjusted to balance biogas use with production. Waste heat from the engine is used to heat the digester and for other farm heating needs.
Most systems produce about 2 kilowatt-hours per day per 1,400-pound cow. Many utility companies pay much less than the consumer price for a kilowatt-hour. Therefore, maximizing the replacement of purchased energy with farm-produced energy will improve the economics of on-farm electricity generation.
Natural ventilation is not enough to remove toxic gases from the digester or to provide sufficient breathable air. Dense hydrogen sulfide gas will sink to the bottom of the tank, lighter ammonia will linger in the top of the tank and neither gas will escape without mechanical ventilation. Moreover, methane is explosive when mixed with air in concentrations of 5 to 15 percent. A leak in a gas line will create a fire hazard.
Potential advantages of controlled anaerobic digestion
•Substantially less odor with digested manure than with stored liquid manure.
Potential disadvantages of anaerobic digestion
•Initial investment may be costly for a digestion system.
Alternatives to electric generation from anaerobic digesters
Another option is to remove carbon dioxide and hydrogen sulfide from the biogas and sell it as natural gas. Scrubbing the gas, finding a market, providing the buyer with a dependable supply of gas and maintaining the distribution equipment require money, time, maintenance and management. Additionally, natural gas will sell for a much lower price than electricity. Although other options are available for biogas utilization, electricity is the most versatile and valuable energy product from biogas.
Planning for future changes
Options with solids separation
The solids can be field-applied, sold or composted and used for animal bedding. Separation and marketing of solids can generate farm income. Replacing bedding with composted solids could be a money-saver if a substantial amount of bedding currently is purchased and a solids separator is owned. However, if a solids separator needs to be purchased, the savings in bedding costs may not cover the cost of solids separation. ANM
References omitted due to space but are available upon request.
Figures omitted but are available upon request to
—From Penn State University, Agricultural and Biological Engineering website Copywrite 2006 Progressive Dairy Publishing. All Rights Reserved.
Jeannie Leggett, Extension Assistant; Robert E. Graves, Professor of Agricultural Engineering; and Les E. Lanyon, Associate Professor of Soil Fertility; Penn State University