Many gases exhibit these “greenhouse” properties. Some of them occur in nature (water vapor, carbon dioxide, methane, and nitrous oxide), while others are exclusively human-made (like gases used for aerosols).

How Can We Decrease Greenhouse Gas Emissions?

Cities and counties with landfills and wastewater treatment plants generate large volumes or Biomethane - which is 21 times more harmful to the climate than carbon dioxide emissions. We can help your city or county recover this valuable biomethane which will significantly improve your air quality while providing a "renewable natural gas" for clean power generation! 

Call us at (512) 220 - 1498 for more information or send an email to:  
info @ RenewableEnergyInstitute . org    
for more information.

Other ways to reduce Greenhouse Gas Emissions include:

Cogeneration, for industrial customers, and trigeneration, for commercial applications, are the most efficient ways of producing energy for these applications. Cogeneration, at around 60-70% efficiency, is more than twice the efficiency of traditional power plants. Cogeneration is the simultaneous production of electrical and thermal energy, and is the best method of generating electricity and steam for industrial customers such as refineries, plastics, and paper/wood industries.  Trigeneration, at about 90% efficiency, is about 300% more efficient over traditional electric power plants. Trigeneration is the simultaneous production of cooling, heating and power, and the best method for generating power and energy for commercial customers like office buildings, schools, universities, military bases, shopping centers, radio/television stations, and casinos, among many other commercial applications.  

Why Are Atmospheric Levels Increasing?

Levels of several important greenhouse gases have increased by about 25 percent since large-scale industrialization began around 150 years ago (Figure 1). During the past 20 years, about three-quarters of human-made carbon dioxide emissions were from burning fossil fuels.

Figure 1. Trends in Atmospheric Concentrations and Anthropogenic Emissions of Carbon Dioxide

Concentrations of carbon dioxide in the atmosphere are naturally regulated by numerous processes collectively known as the “carbon cycle” (Figure 2). The movement (“flux”) of carbon between the atmosphere and the land and oceans is dominated by natural processes, such as plant photosynthesis. While these natural processes can absorb some of the net 6.1 billion metric tons of anthropogenic carbon dioxide emissions produced each year (measured in carbon equivalent terms), an estimated 3.2 billion metric tons is added to the atmosphere annually. The Earth’s positive imbalance between emissions and absorption results in the continuing growth in greenhouse gases in the atmosphere.

Figure 2. Global Carbon Cycle (Billion Metric Tons Carbon)

What Effect Do Greenhouse Gases Have on Climate Change?

Given the natural variability of the Earth’s climate, it is difficult to determine the extent of change that humans cause. In computer-based models, rising concentrations of greenhouse gases generally produce an increase in the average temperature of the Earth. Rising temperatures may, in turn, produce changes in weather, sea levels, and land use patterns, commonly referred to as “climate change.”

Assessments generally suggest that the Earth’s climate has warmed over the past century and that human activity affecting the atmosphere is likely an important driving factor. A National Research Council study dated May 2001 stated, “Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and sub-surface ocean temperatures to rise. Temperatures are, in fact, rising. The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability.”

However, there is uncertainty in how the climate system varies naturally and reacts to emissions of greenhouse gases. Making progress in reducing uncertainties in projections of future climate will require better awareness and understanding of the buildup of greenhouse gases in the atmosphere and the behavior of the climate system.

What Are the Sources of Greenhouse Gases?

In the U.S., our greenhouse gas emissions come mostly from energy use. These are driven largely by economic growth, fuel used for electricity generation, and weather patterns affecting heating and cooling needs. Energy-related carbon dioxide emissions, resulting from petroleum and natural gas, represent 82 percent of total U.S. human-made greenhouse gas emissions (Figure 3). The connection between energy use and carbon dioxide emissions is explored in the box on the reverse side (Figure 4).

Figure 3. U.S. Anthropogenic Greenhouse Gas Emissions by Gas, 2001
(Million Metric Tons of Carbon Equivalent)

 

Figure 4. U.S. Primary Energy Consumption and Carbon Dioxide Emissions, 2001

Another greenhouse gas, methane, comes from landfills, coal mines, oil and gas operations, and agriculture; it represents 9 percent of total emissions. Nitrogen oxides (5 percent of total emissions), meanwhile, is emitted from burning fossil fuels and through the use of certain fertilizers and industrial processes. Human-made gases (2 percent of total emissions) are released as byproducts of industrial processes and through leakage.

What Is the Prospect for Future Emissions?

World carbon dioxide emissions are expected to increase by 1.9 percent annually between 2001 and 2025 (Figure 5). Much of the increase in these emissions is expected to occur in the developing world where emerging economies, such as China and India, fuel economic development with fossil energy. Developing countries’ emissions are expected to grow above the world average at 2.7 percent annually between 2001 and 2025; and surpass emissions of industrialized countries near 2018.

Figure 5. World Carbon Dioxide Emissions by Region, 2001-2025
(Million Metric Tons of Carbon Equivalent)

The U.S. produces about 25 percent of global carbon dioxide emissions from burning fossil fuels; primarily because our economy is the largest in the world and we meet 85 percent of our energy needs through burning fossil fuels. The U.S. is projected to lower its carbon intensity by 25 percent from 2001 to 2025, and remain below the world average (Figure 6).

Figure 6. Carbon Intensity by Region, 2001-2025
(Metric Tons of Carbon Equivalent per Million $1997)

Energy Production and Carbon Dioxide Emissions

For over one hundred years, energy and power production have been generated around the world through the burning of fossil fuels, including;  fuel oil, coal, diesel, and natural gas.  Over the past decade, environmental science and research has discovered and linked global warming, and global climate change to the carbon dioxide emissions from the combustion of fossil fuels.  This has placed an increased need to reduce energy consumption and discover more environmentally friendly fuel sources. 

A Cogeneration powerplant produces heat and power simultaneously by burning a primary fuel like natural gas, or biomethane.   Cogeneration plants typically reach system efficiencies of 60% to 70% - or about double that of standard power plants.  Trigeneration plants produce 3 energies - cooling, heating and power - simultaneously, with one fuel input and combustion process (such as natural gas or biomethane) and is an environmentally-friendlier method of generating electricity. Trigeneration, at around 90% efficiency, is about 300% more efficient than typical power plants, and 50% more efficient than cogeneration plants.  Cogeneration and trigeneration power plants are much less expensive and costly in terms of both economic and environmental expenses, than traditional forms of power generation.  There are also far fewer carbon and carbon dioxide emissions generated through co/trigeneration.  

Trigeneration slashes carbon dioxide emissions by as much 80% and more.

In 1992, managers of the 2.8-million-square-foot McCormick Place Exhibition and Convention Center in Chicago were planning an addition that would double the size of their convention center. To avoid $27 million in capital costs for a new heating and cooling system, the McCormick Place managers selected a new trigeneration system under an energy outsource or energy services agreement. The new trigeneration system simultaneously provides the McCormick Place Convention Center with heating, cooling, and electricity and achieves an overall efficiency rating of 93%.  Besides the initial savings of not having to spend $27 million for the new system, McCormick Place also saves >$1 million annually in energy and operating expenses. The system produces about half the carbon dioxide emissions of a traditional system, as well as 24,000 tons of carbon dioxide and 59 tons of nitrogen oxides (NOx) each year when compared to a traditional system.  

Coors Brewing Company has a 90 percent efficient trigeneration system at its Golden, Colorado plant, the largest single brewing site in the world. The trigeneration system saves 250,000 tons of carbon dioxide annually, along with 125 tons of NOx and 900 tons of SO2. 

* A New Perspective on Energy

Integrated systems for cooling, heating and power (CHP) for buildings incorporate multiple technologies for providing energy services to a single building or to a campus of buildings. Electricity to such buildings is provided by on-site or near-site power generators using one or more of the many options: internal combustion (IC) engines, combustion turbines, miniturbines or microturbines, and fuel cells. In CHP systems, waste heat from power generation equipment is recovered for operating equipment for cooling, heating, or controlling humidity in buildings, by using absorption chillers, desiccant dehumidifiers, or heat recovery equipment for producing steam or hot water. These integrated systems are known by a variety of acronyms: CHP, Trigeneration and IES (Integrated Energy System). 

CHP systems provide many benefits, including:

reduced energy costs, 
improved power reliability, 
increased energy efficiency, and 
improved environmental quality. 

What is a CHP System?

A CHP System is an efficient, environmentally-friendly "cogeneration" system that provides power (electricity) and energy (hot water and/or steam) at the location the power and energy are needed also known as "distributed generation." Cogeneration systems are at least two times more efficient than typical power plants which average about 27% - 35% efficiency - meaning 65% to 73% of the energy is wasted. 

What is a CHP System with Absorption Chillers or "Trigeneration"? 

Even more efficient than a standard CHP system is a CHP system that incorporates absorption chillers, which is  then a "trigeneration" system, also referred to as an "Integrated Energy System" or "Cooling, Heating and Power."  Trigeneration systems can be up to 50% more efficient than cogeneration systems and many average about 90% or more efficiency.  Absorption chillers recover the additional waste heat from CHP Systems to make chilled water for air-conditioning, thereby providing the building or facility's electricity, hot water/steam and air conditioning.

Some of the above information courtesy of the U.S. Department of Energy with our thanks.

Are you doing your part to stop Global Warming and Climate Change? 

Learn more about the leading causes of Global Warming and Climate Change, which are Carbon Dioxide Emissions and Greenhouse Gas Emissions at the following websites:

Carbon Dioxide Emissions
www.CarbonDioxideEmissions.org
 

Greenhouse Gas Emissions
www.GreenhouseGasEmissions.org 

For more information on how your company can reduce, or eliminate Greenhouse Gas Emissions and Carbon Dioxide Emissions, visit one of our sponsors below. All of the following companies offer products and technologies that are "sustainable" and reduce Greenhouse Gas Emissions and Carbon Dioxide Emissions.

BIOMETHANE FACTS

1.  Biomethane is One of the Most Common and Harmful of All Greenhouse Gas
     Emissions.

2.  Biomethane is 21 Times More Harmful to Climate than Carbon Dioxide Emissions.
     Stated another way, Biomethane Causes Global Warming and Climate Change to
     Increase 21 Times Faster than Carbon Dioxide Emissions. 

3.  Biomethane Is A "Renewable Natural Gas."

4.  Biomethane is One of the Easiest and Most Profitable of all Greenhouse Gas Emissions
     to Recover and Control.

Biomethane - Best Renewable Fuel?
Anaerobic Digesters - Best Renewable Energy Technology?

What is an Anaerobic Digester?

An Anaerobic Digester is a device for optimizing the anaerobic digestion of biomass and/or animal manure, and to recover the BioMethane for energy production. Digester types include batch, complete mix, continuous flow (horizontal or plug-flow, multiple-tank, and vertical tank), and covered lagoon.

What is Anaerobic Digestion?

Anaerobic digestion is a biological process that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as BioMethane. These gases are produced from organic wastes such as livestock manure, food processing waste, etc. 

Anaerobic processes could either occur naturally or in a controlled environment such as a BioMethane plant. Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. Depending on the waste feedstock and the system design, raw biogas is typically 55 to 75 percent pure methane. State-of-the-art systems - after cleaning up the raw biogas, report producing biogas that is more than 95 percent pure BioMethane. 

The U.S. EPA AgSTAR Program Background

The U.S. EPA AgSTAR is an outreach program designed to reduce methane emissions from livestock waste management operations by promoting the use of BioMethane recovery systems. A BioMethane recovery system is an anaerobic digester with BioMethane capture and combustion to produce electricity, heat or hot water. BioMethane recovery systems are effective at confined livestock facilities that handle manure as liquids and slurries, typically swine and dairy farms. Anaerobic digester technologies provide enhanced environmental and financial performance when compared to traditional waste management systems such as manure storages and lagoons. Anaerobic digesters are particularly effective in reducing methane emissions but also provide other air and water pollution control opportunities. AgSTAR provides an array of information and tools designed to assist producers in the evaluation and implementation these systems, including:

• Conducting farm digester extension events and conferences

• Providing “How-To” project development tools and industry listings

• Conducting performance characterizations for digesters and conventional waste management systems

• Operating a toll free hotline

• Providing farm recognition for voluntary environmental initiatives

• Collaborating with federal and state renewable energy, agricultural, and environmental programs

Biomethane Emissions from Animal Waste Management

Biomethane emissions occur whenever animal waste is managed in anaerobic conditions. Liquid manure management systems, such as ponds, anaerobic lagoons, and holding tanks create oxygen free environments that promote Biomethane production. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of Biomethane. Currently, livestock waste contributes about 8 percent of human-related Biomethane emissions in the U.S. Given the trend toward larger farms, liquid manure management is expected to increase. For more information on international emissions, projections, and mitigation costs, see International Analyses.

Biomethane from Anaerobic Digesters

Biomethane is a gas that contains molecules of methane with one atom of carbon and four atoms of hydrogen (CH4 ). It is the major component of the "natural" gas used in many homes for cooking and heating. It is odorless, colorless, and yields about 1,000 British Thermal Units (Btu) [252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic meters) when burned. Natural gas is a fossil fuel that was created eons ago by the anaerobic decomposition of organic materials. It is often found in association with oil and coal.

The same types of anaerobic bacteria that produced natural gas also produce Biomethane today. Anaerobic bacteria are some of the oldest forms of life on earth. They evolved before the photosynthesis of green plants released large quantities of oxygen into the atmosphere. Anaerobic bacteria break down or "digest" organic material in the absence of oxygen and produce "BioMethane" as a waste product. (Aerobic decomposition, or composting, requires large amounts of oxygen and produces heat.) Anaerobic decomposition occurs naturally in swamps, water-logged soils and rice fields, deep bodies of water, and in the digestive systems of termites and large animals. Anaerobic processes can be managed in a "digester" (an airtight tank) or a covered lagoon (a pond used to store manure) for waste treatment. The primary benefits of anaerobic digestion are nutrient recycling, waste treatment, and odor control. Except in very large systems, BioMethane production is a highly useful but secondary benefit.

Biomethane produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The relative percentage of these gases in BioMethane depends on the feed material and management of the process. When burned, a cubic foot (0.028 cubic meters) of BioMethane yields about 10 Btu (2.52 kcal) of heat energy per percentage of Biomethane composition. For example, Biomethane composed of 65% methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).

Digester Designs

Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).

There are two basic types of digesters: batch and continuous. Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.

In a continuous digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Unlike batch-type digesters, continuous digesters produce BioMethane without the interruption of loading material and unloading effluent. They may be better suited for large-scale operations. There are three types of continuous digesters: vertical tank systems, horizontal tank or plug-flow systems, and multiple tank systems. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable BioMethane.

Many livestock operations store the manure they produce in waste lagoons, or ponds. A growing number of these operations are placing floating covers on their lagoons to capture the BioMethane. They use it to run an engine/generator to produce electricity.

The Digestion Process

Anaerobic decomposition is a complex process. It occurs in three basic stages as the result of the activity of a variety of microorganisms. Initially, a group of microorganisms converts organic material to a form that a second group of organisms utilizes to form organic acids. Methane-producing (methanogenic) anaerobic bacteria utilize these acids and complete the decomposition process.

A variety of factors affect the rate of digestion and Biomethane production. The most important is temperature. Anaerobic bacteria communities can endure temperatures ranging from below freezing to above 135° Fahrenheit (F) (57.2° Centigrade [C]), but they thrive best at temperatures of about 98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria activity, and thus Biomethane production, falls off significantly between about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F (35° to 0°C).

In the thermophilic range, decomposition and Biomethane production occur more rapidly than in the mesophilic range. However, the process is highly sensitive to disturbances such as changes in feed materials or temperature. While all anaerobic digesters reduce the viability of weed seeds and disease-producing (pathogenic) organisms, the higher temperatures of thermophilic digestion result in more complete destruction. Although digesters operated in the mesophilic range must be larger (to accommodate a longer period of decomposition within the tank [residence time]), the process is less sensitive to upset or change in operating regimen.

To optimize the digestion process, the digester must be kept at a consistent temperature, as rapid changes will upset bacterial activity. In most areas of the United States, digestion vessels require some level of insulation and/or heating. Some installations circulate the coolant from their Biomethane-powered engines in or around the digester to keep it warm, while others burn part of the Biomethane to heat the digester. In a properly designed system, heating generally results in an increase in Biomethane production during colder periods. The trade-offs in maintaining optimum digester temperatures to maximize gas production while minimizing expenses are somewhat complex. Studies on digesters in the north-central areas of the country indicate that maximum net Biomethane production can occur in digesters maintained at temperatures as low as 72°F (22.2°C).

Other factors affect the rate and amount of Biomethane output. These include pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting material, the particle size of the material being digested, and retention time. Pre-sizing and mixing of the feed material for a uniform consistency allows the bacteria to work more quickly. The pH is self-regulating in most cases. Bicarbonate of soda can be added to maintain a consistent pH, for example when too much "green" or material high in nitrogen content is added. It may be necessary to add water to the feed material if it is too dry, or if the nitrogen content is very high. A carbon/nitrogen ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the digesting material can aid the digestion process. Antibiotics in livestock feed have been known to kill the anaerobic bacteria in digesters. Complete digestion, and retention times, depend on all of the above factors.

Producing and Using Biomethane

As long as proper conditions are present, anaerobic bacteria will continuously produce Biomethane. Minor fluctuations may occur that reflect the loading routine. Biomethane can be used for heating, cooking, and to operate an internal combustion engine for mechanical and electric power. For engine applications, it may be advisable to scrub out hydrogen sulfide (a highly corrosive and toxic gas). Very large-scale systems/producers may be able to sell the gas to natural gas companies, but this may require scrubbing out the carbon dioxide.

Using the Effluent

The material drawn from the digester is called sludge, or effluent. It is rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen trace elements) and is an excellent soil conditioner. It can also be used as a livestock feed additive when dried. Any toxic compounds (pesticides, etc.) that are in the digester feedstock material may become concentrated in the effluent. Therefore, it is important to test the effluent before using it on a large scale.

Economics

Anaerobic digester system costs vary widely. Systems can be put together using off-the-shelf materials. There are also a few companies that build system components. Sophisticated systems have been designed by professionals whose major focus is research, not low cost. Factors to consider when building a digester are cost, size, the local climate, and the availability and type of organic feedstock material.

In the United States, the availability of inexpensive fossil fuels has limited the use of digesters solely for Biomethane production. However, the waste treatment and odor reduction benefits of controlled anaerobic digestion are receiving increasing interest, especially for large-scale livestock operations such as dairies, feedlots, and slaughterhouses. Where costs are high for sewage, agricultural, or animal waste disposal, and the effluent has economic value, anaerobic digestion and Biomethane production can reduce overall operating costs. Biomethane production for generating cost effective electricity requires manure from more than 150 large animals.

Below-ground, concrete anaerobic digesters have proven to be especially useful to agricultural communities in parts of the world such as China, where fossil fuels and electricity are expensive or unavailable. The primary purpose of these anaerobic digesters is waste (sewage) treatment and fertilizer production. Biomethane production is secondary.

Accomplishments

The AgSTAR Program has been very successful in encouraging the development and adoption of anaerobic digestion technology. Since the establishment of the program in 1994, the number of operational digester systems has doubled. This has produced significant environmental and energy benefits, including Biomethane emission reductions of approximately 124,000 metric tons of carbon equivalent and annual energy generation of about 30 million kWh. The graph below shows the historical use of Biomethane recovery technology for animal waste management.

The development of anaerobic digesters for livestock manure treatment and energy production has accelerated at a very fast pace over the past few years. Factors influencing this market demand include: increased technical reliability of anaerobic digesters through the deployment of successful operating systems over the past five years; growing concern of farm owners about environmental quality; an increasing number of state and federal programs designed to cost share in the development of these systems; and the emergence of new state energy policies (such as net metering legislation) designed to expand growth in reliable renewable energy and green power markets.

In the past 2 years alone, the number of operational digester systems has increased by 30%. For more detailed information on anaerobic digester use in the U.S. , go to the Guide to Operational Systems or see the AgSTAR 2003 Digest

The process of anaerobic digestion consists of three steps. 

The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to Biomethane gas. 

Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored. 

Many anaerobic digestion technologies are commercially available and have been demonstrated for use with agricultural wastes and for treating municipal and industrial wastewater. 

At Royal Farms No. 1 in Tulare, California, hog manure is slurried and sent to a Hypalon-covered lagoon for Biomethane generation. The collected Biomethane fuels a 70 kilowatt (kW) engine-generator and a 100 kW engine-generator. The electricity generated on the farm is able to meet monthly electric and heat energy demand.

Given the success of this project, three other swine farms (Sharp Ranch, Fresno and Prison Farm) have also installed floating covers on lagoons. The Knudsen and Sons project in Chico, California, treated wastewater which contained organic matter from fruit crushing and wash down in a covered and lined lagoon. The Biomethane produce is burned in a boiler. And at Langerwerf Dairy in Durham, California, cow manure is scraped and fed into a plug flow digester. The Biomethane produced is used to fire an 85 kW gas engine. The engine operates at 35 kW capacity level and drives a generator to produce electricity. Electricity and heat generated is able to offest all dairy energy demand. The system has been in operation since 1982. 

Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a Biomethane fuel that can be used for process heating and/or electricity generation. 

Technology assessment

This section describes the anaerobic digestion (AD) process, outlines guidelines for assessing the feasibility of AD and Biomethane usage at a swine facility and provides summary information on AD system performance and reliability.

Anaerobic Digestion Technology Description

AD promotes the bacterial decomposition of the volatile solids (VS) in animal wastes to Biomethane, thereby reducing lagoon loading rates and odor. The primary component of an AD system is the anaerobic digester, a waste vessel containing bacteria that digest the organic matter in waste streams under controlled conditions to produce Biomethane. As an effluent, AD yields nearly all of the liquid that is fed to the digester. This remaining fluid consists of mostly water and is allowed to evaporate from a secondary lagoon, land-applied for irrigation and fertilizer value or recycled to flush manure from the swine building to the digester.

The benefits of AD include:

• Odor reduction;

• Reduction in the biological oxygen demand of treated effluent by up to 90 percent, reducing the risk for water contamination;

• Improved nutrient application control, because up to 70 percent of the nitrogen in the waste is converted to ammonia, the primary nitrogen constituent of fertilizer;

• Reduced pathogens, viruses, protozoa and other disease-causing organisms in lagoon water, resulting in improved herd health and possible reduced water requirements; and

• Potential to generate electricity and process heat.

AD takes place in three steps: hydrolysis, acid formation, and Biomethane generation. During the first step, hydrolysis, bacterial enzymes break down proteins, fats and sugars in the waste to simple sugars. During acid formation, bacteria convert the sugars to acetic acid, carbon dioxide and hydrogen. Then the bacteria convert the acetic acid to methane and carbon dioxide, and combine carbon dioxide and hydrogen to form Biomethane and water.

Digester technologies that can be used to collect Biomethane from swine facilities include:

• Covered anaerobic lagoons,

• Complete mix digesters and

• Sequencing batch reactors.

Although a sequencing batch reactor has been used for AD at one swine facility in the United States , this technology is considered to be experimental, and thus is not included in this report. This report focuses on technologies that have verifiable performance characteristics, namely, covered anaerobic lagoons and complete mix digesters. 

Appendix B provides contact information that can help producers find AD system designers/installers, odor control technologies, generators, heating and cooling equipment, and other information to help manage air and water quality at hog facilities.

Covered lagoon digesters are the simplest AD system. These systems typically consist of an anaerobic combined storage and treatment lagoon, an anaerobic lagoon cover, an evaporative pond for the digester effluent, and a gas treatment and/or energy conversion system. Figure 1 shows a typical schematic for a floating covered anaerobic lagoon.

Source: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biomethane Systems at Commercial Farms in the United States . EPA 430-B-97-015. Washington , DC . pp. 1-3.

Figure 1 . Covered anaerobic lagoon digester

Covered lagoon digesters typically have a hydraulic retention time (HRT) of 40 to 60 days. The HRT is the amount of time a given volume of waste remains in the treatment lagoon. A collection pipe leading from the digester carries the Biomethane to either a gas treatment system such as a combustion flare, or to an engine/generator or boiler that uses the Biomethane to produce electricity and heat. Following treatment, the digester effluent is often transferred to an evaporative pond or to a storage lagoon prior to land application.

Climate affects the feasibility of using covered lagoon digesters to generate electricity. Engine/generator systems typically do not produce sufficient waste heat to maintain temperatures high enough in covered lagoon digesters in the winter to sustain consistently high Biomethane production rates. Using propane or natural gas to provide additional heat for the lagoon contents is typically not an economically viable option. Without that additional heat, most covered lagoon digesters produce less Biomethane in colder temperatures, and little or no gas below 39 FACE= "Symbol">° F. As a result, covered lagoon digesters are most appropriate for use in warm climates if the Biomethane is to be used for energy or heating purposes.

Complete mix digester systems consist of a mix tank, a complete mix digester and a secondary storage or evaporative pond. The mix tank is either an aboveground tank or concrete in-ground tank that is fed regularly from underfloor waste storage below the animal feedlot. Waste is stirred in the mix tank to prevent solids from settling in the waste prior to being fed to the digester. The complete mix digester is essentially a constant-volume aboveground tank or in-ground covered lagoon that is fed daily from the mix tank. Complete mix digesters with in-ground lagoons often employ covers similar to those used in covered lagoon digesters. In the digester, a mix pump circulates waste material slowly around the heater to maintain a uniform temperature. Hot water from an engine/generator cogeneration water jacket or boiler is used to heat the digester. A cylindrical aboveground tank, such as that shown in Figure 2, optimizes Biomethane production, but is more capital intensive than in-ground tanks. The only operating AD system in Colorado that recovers Biomethane for energy use is a complete mix digester, located at Colorado Pork LLC near Lamar , Colorado .

Source: EPA. (February 1997). AgStar Technical Series: Complete Mix Digesters – A Biomethane Recovery
Option for All Climates. EPA 430-F-97-004. Washington , DC .

Figure 2 . Complete mix digester schematic

Complete mix digesters have an HRT of 15 to 20 days, which means that complete mix digesters can reduce the overall lagoon volume required for waste storage and treatment. This makes complete mix digesters comparable to covered lagoon digesters in cost, despite the increased complexity of stirring, mixing and plumbing components. In addition, Biomethane production rates, and therefore heat and electricity production, are greater and more consistent than for covered lagoons. This can help reduce system payback periods compared to covered lagoon systems. Like covered lagoon systems, digester effluent from complete mix digesters is frequently stored in evaporative ponds or storage lagoons.

System Requirements

This section provides guidelines for conducting a preliminary assessment of the feasibility of using AD at a swine facility. Although AD system requirements will vary depending on the application and system design, there are some rule-of-thumb measures that should be noted when assessing the feasibility of AD at a given location. For AD to potentially be technically feasible and cost-effective, a swine facility should:

• Simultaneously house at least 2,000 animals with a total live animal weight of at least 110,000 pounds,

• Have no more than 20 percent variation in animal population throughout the year,

• Collect waste at one central location such as an underfloor pit,

• Collect waste daily or every other day, or can convert to an equivalent collection system,

• Have manure free of large amounts of bedding or other foreign materials, and

• Have some manure storage capability to maintain a steady digester feedstock supply

If the above characteristics are present, the facility is a possible candidate for AD. Many pre-existing waste storage and treatment lagoons are too large to practically or cost-effectively employ covers over their entire area. Partial covers may be an option to recover Biomethane from these older systems, as an alternative to installing a completely new storage and treatment lagoon system.

If energy recovery is to be employed, Biomethane production and gas quality should be considered and compared to energy requirements at the facility. Daily Biomethane production at installed farm-based anaerobic digesters in the United States varies from 24,000 to 75,000 cubic feet, or an energy equivalent of 13 to 42 million British thermal units (Btu) (assuming 55 percent methane content for Biomethane). Covered anaerobic from anaerobic digesters should be chosen according to the end-use objective for the system. Complete mix digesters can produce heat and electricity at a constant rate throughout the year because heat recovery can be used to heat the digesters in the winter. Covered lagoon digesters can consistently produce Biomethane only in months when the temperature exceeds 39 degrees Fahrenheit.

Facilities that are located south of the line of climate limitation in Figure 3 are usually warm enough for cost-effective energy recovery from covered lagoon digesters. In most cases, facilities north of the climate line in Figure 3 are too cold for cost-effective energy recovery from covered lagoon digesters. Complete mix digesters can be used in cold or warm climates. If odor control is the only objective, either covered lagoon or complete mix digesters may be used, but odor control will be less effective in the winter for covered lagoon digesters south of the line of climate limitation in Figure 3. In general, complete mix digesters are the most appropriate choice for use in Colorado . 

Source: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biomethane Systems
at Commercial Farms in the United States . EPA 430-B-97-015. pp. 4-12.

Figure 3 . Line of climate limitation for Biomethane energy recovery

Table 2 shows which digesters are appropriate for the waste collection strategies at covered swine facilities. Complete mix digesters can operate with a waste total solids (TS) percentage between 3 and 10 percent, while covered lagoon digesters can use waste with a TS percentage less than 2 percent.

Table 2 . Matching a digester to existing waste collection practices

Collection system	Percent TS required	Digester type	Suitable climate
Scrape	3-8	Complete mix	Warm or cold
Pit storage	3-8	Complete mix	Warm or cold
Flush	<2	Covered lagoon	Warm
Pit recharge	<3	Covered lagoon	Warm
Gravity drainage
Pull plug