Biogas / Anaerobic Disgesters Energy Self Assessment
The biogas self assessment tool was developed to increase awareness and utilization
of anaerobic digestion processes and renewable energy production in agriculture.
Biogas is a product of the anaerobic digestion process comprised of methane, carbon
dioxide, and trace amounts of other compounds. Biogas can be used for generating
electricity, thermal heating or cleaned and sold as renewable natural gas. The biogas
self assessment tool will assist farm producers in evaluating their farm’s potential
for producing biogas to generate electricity and methane as renewable energies.
The tool will assist producers in determining biomass feedstocks that are available
on-farm and calculate the biogas production from the different feedstock sources.
This tool does not provide a site-specific renewable energy assessment. It evaluates
thermal energy use based on user input.
Digester Basics
What is anaerobic digestion?
Anaerobic Digestion is the biological, physical and chemical breakdown of organic
materials in the absence of oxygen. The most common type of organic matter used
in anaerobic digestion on farms is animal manure. Anaerobic digesters have been
used at larger sewage treatment plants for many years. Anaerobic digestion can be
broken down into temperature and bacterial phases. Anaerobic digestion can happen
over a wide range of temperatures but there are three principle temperature ranges
which are classified as, psychrophilic, mesophilic, and thermophilic anaerobic digestion.
Different species of bacteria thrive depending on the temperature range. The bacterial
phases in the anaerobic digestion process contain four to five specific bacteria
classes; the acidogenic (acid forming) and the methanogenic (methane forming) are
the most commonly managed bacterial classes throughout the anaerobic digestion process.
A by-product of anaerobic digestion is methane gas which can be used as an energy
source. Methane gas is the main component of natural gas, often distributed as a
fuel source by utility companies.
What are the common types of anaerobic digester systems?
Hydraulic Retention Time: The hydraulic retention time refers to
the length of time it takes for organic materials input to an anaerobic digester
to be processed into methane gas, carbon dioxide gas, and other trace products.
Close
The differences between digester systems vary greatly. It is important to choose
an anaerobic digester system that can efficiently process the feedstock to be digested.
The following are common digester systems used in agricultural applications. The
most common types of digesters for on-farm operations at this time are plug-flow
and complete mixed.
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Covered Lagoon Digester
A Covered Lagoon Digester is an anaerobic lagoon fixed with an impermeable, gas
and air tight cover designed to capture biogas. These systems can produce biogas
naturally at ambient temperatures. Since covered lagoon digesters are not heated,
their biogas production varies as ground temperatures vary by season. It is recommended
to use feedstock with a solids content of 0.5 – 3% in a covered lagoon digester.
Covered lagoon systems tend to be the lowest cost system but also have the lowest
biogas production of all digester systems. The hydraulic retention time (HRT) of
covered lagoon digesters range from 40 to 60 days or longer, much longer than the
HRT of other digester types. (image from http://www.epa.gov/agstar/images/press)
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Complete-Mix Digester
A Complete-Mix Digester is a digestion reactor with a controlled temperature, constant
volume, and mechanical mixer. Temperature management and mixers are used to maximize
anaerobic bacteria growth and methane production. The feedstock is periodically
or constantly flowing in and out of the reactor. Unlike a plug-flow reactor, organic
material is uniformly mixed throughout the reactor. A benefit of a complete-mix
digester over a plug-flow digester is that it can be used to treat more diluted
wastes, ranging from 2 to 12% total solids because the mixing action keeps the solids
suspended in solution. Hydraulic retention times for complete—mix digesters are
usually 10 to 20 days.
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Fixed Film Digester
A Fixed Film Digester is basically a heated tank filled with an inert media that
a consortium of bacteria attach to and grow on as a biofilm. It has a constant volume,
constant flow-through, maintained under controlled temperatures and is designed
with a fixed media. The media is designed to provide an environment for anaerobic
bacterial growth. In this system, bacteria colonize on porous rock or synthetic
material media while high liquid/low solids content feedstock pass over the material.
The bacteria feed on the nutrients that contact them and convert it to methane and
carbon dioxide. This system will work only for diluted feedstocks with 0-2% total
solids, such as flushed manure systems. A short hydraulic retention time is used,
3 to 5 days, which reduces the size and expense of the reactor vessel. Biogas production
from a fixed film digester is high quality in comparison with other types of anaerobic
digesters, with methane content of about 80% methane. Solids must be separated from
feedstock or plugging can be a problem.
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Plug-Flow Digester
A Plug-Flow Digester has a constant volume and flow-through. Plug flow digesters
operate under controlled temperatures which maximize methane production. Unlike
a complete-mix digester, a plug flow digester often does not utilize any form of
mechanical mixing. If the plug-flow digester does have mixing, it is usually done
by pumping digester gas through a pipe at the bottom of the tank and allowing the
rising bubbles to provide some agitation. This mixing will tend to be across the
width of the tank not the length. This type of digester can be thought of as a tube,
substrate is fed into one end and forces digestate out of the other end. Because
materials in the digester are not mixed, the feedstock entering a plug flow digester
must be more viscous to ensure the solids do not settle out. A plug flow design
is recommended for feedstocks with 11% to 14% total solids. Hydraulic retention
times are typically 15 to 30 days.
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Community Digester
A community digester receives feedstocks from multiple sites and is centrally located
to minimize feedstock transportation. Feedstock is typically transported to a community
digester via truck but could be pumped by pipeline depending on distance and topography.
The philosophy of a community digester offers farm producers the opportunity to
participate in an anaerobic digester project even if it is not economical for their
farm to run an independent system. Community digesters require cooperation of the
involved producers for success and longevity of this type of system. Transportation
distances can affect the economics of community digesters because manure has to
be brought to the digester and the effluent returned to the farm to utilize nutrients.
Most community digesters currently in operation are being financially subsidized
to maintain operations.
What is an anaerobic digester feedstock?
An anaerobic digester feedstock or substrate is an organic based material that can
be converted to methane by anaerobic bacteria. Traditional feedstocks include sewage
sludge from municipal waste water treatment plants, industrial and food processing
wastes. Wastes classified with a high biological oxygen demand (BOD) work best as
anaerobic digester feedstocks. For agricultural systems, a variety of feedstocks
have been used for anaerobic digestion including bovine, swine, and poultry manures.
Many European systems utilize corn silage (ensilage), greases and fats, and locally
available food and yard wastes. Many systems are using multiple feedstocks into
one digester to optimize gas production and utilize locally available organic matter.
What do the terms psychrophilic, mesophilic, and thermophilic mean?
Psychrophilic, mesophilic, and thermophilic refers to a temperature class and/or
specific bacteria active within each temperature class.
Psychrophilic bacteria can be described as “cold loving” bacteria.
Psychrophilic production of bacteria and methane occur at temperatures less than
70°F with optimal conditions ranging from 39°F to 50°F. The least efficient class
of bacteria, psychrophillic bacteria are often the bacterial class found in natural
and ambient conditions. A lagoon will operate in this range in much of the U.S.
Mesophilic bacteria live at moderate temperatures. Mesophilic digesters
are usually operated at a range of 68°F to 105°F, with an optimal range between
95°F and 98°F. This is the most common temperature range used for anaerobic digestion.
Thermophilic bacteria live at a relatively high temperature. Thermophilic
digesters can operated at a range of 110°F to 160°F with the optimal range between
140°F and 145°F. The bacteria found in this temperature range will have the highest
theoretical gas yield but tend to be more sensitive to temperature and environmental
changes.
What are the roles of methanogenic and acidogenic bacteria?
Anaerobic digestion is a two-stage process that is mainly facilitated by acid-forming
and methane-forming bacterial.
Acidogenic (acid producing) bacteria break down organic materials and sugars into
volatile fatty acids.
Methanogenic (methane producing) bacteria produce methane during the anaerobic digestion
process by breaking down the volatile fatty acids produced by acidogenic bacteria.
These bacteria are the most sensitive to condition changes in the anaerobic digester
environment of the different types of bacteria involved in anaerobic digestion process.
Usable Energy From Digesters
What is the difference between methane gas, natural gas, and biogas?
Biogas is a byproduct of anaerobic decomposition. It is a mixture of 55-70% methane
(CH4), 30-45% carbon dioxide (CO2), and various trace gases including hydrogen sulfide
(H2S), hydrogen gas (H2) and nitrous oxides. Methane gas (CH4) is the pure gas molecular
compound. Natural Gas is methane gas piped from the earth and is almost 100% methane
in content with very few impurities. In order to convert biogas to pure methane,
biogas must undergo process scrubbing and cleaning to remove carbon dioxide, hydrogen
sulfide, and other trace gasses.
The energy content of a biogas mixture is proportionate to the amount of methane
contained in the biogas. Natural gas and pure methane have an average energy value
of about 1000 Btu per cubic foot. Biogas with a concentration of 55-70% methane
will have corresponding energy values between 550 and 700 Btu’s per cubic foot.
What can biogas be used for?
Since biogas is mixture of methane gas and other gases, biogas can be utilized in
applications where natural gas or liquid propane is used. Currently, most biogas
is used to generate electricity by using it as a fuel for an engine driven generator
or a microturbine. It can also be used in boiler applications, water heating, and
other combustion systems. However, in most cases, biogas cannot be used as a direct
gas replacement unless alterations are made to the natural gas or liquid propane
combustion system. Burner alterations are required for using biogas directly because
it contains compounds which can erode metals, and the energy content is less on
a per volume basis than natural gas. In order to use Biogas for combustion, it may
need to be scrubbed to remove hydrogen sulfide and moisture so it will combust efficiently
and to reduce equipment maintenance. Biogas can be purified to be equivalent to
natural gas by scrubbing and removing the carbon dioxide and trace gas impurities.
The purified gas may then be injected into a natural gas pipeline as renewable natural
gas.
What is the efficiency of electrical production from biogas?
Traditionally, biogas has been used to fuel engine-driven generators. These engines
are modified gasoline, diesel, or natural gas-fueled engines. The electrical conversion
efficiency is the amount of electrical energy from the total energy available. On
average, generation systems achieve 30-35% energy efficiency. Older systems achieve
only 20-30% electrical energy efficiency. Modern engine generator systems have improved
and will achieve 35 to 40% electrical energy efficiency. In recent years, microturbines
have been tested and used in biogas applications. These systems are much like a
jet engine and are currently being tested in various biogas systems to evaluate
use and durability.
What is the difference between an engine-generator and a microturbine?
A microturbine is a small-scale gas turbine generation systems that directly combusts
gas. During combustion the gas expands and is directed through a nozzle and over
turbine blades causing the turbine to rotate and drive an electrical generation
unit.
An Engine generator is an internal combustion engine that drives an electrical generator
to produce electricity.
What is combined heat and power (cogeneration)?
Combined heat and power is often referred to as cogeneration. Cogeneration is a
process for generating electricity and heat energy from biogas. The cogeneration
can use an engine generator or microturbine to generate electricity. The efficiency
of producing electricity is only 25 to 40% efficient, with remainder of the energy
(60-75%) converted to heat in the exhaust and cooling systems. In a cogeneration
system the heat energy is captured and used for process or space heating, increasing
the total energy efficiency of the process. Common uses for cogenerated heat include
space heating of farm buildings, animal housing facilities, greenhouse heating,
and used for industrial process heat such as drying wood or byproducts or heating
a reactor vessel.
What is the difference between a synchronous and an induction generator?
An Induction generator is similar electrically and mechanically to a 3-phase motor.
They require a voltage from the utility to supply a rotating magnetic flux for the
generator stator; therefore they can only operate if there is voltage from the utility.
Induction generators need less sophisticated controls because the voltage signal
from the utility aligns the frequency of the generator with the utility grid. Induction
generators are the most common type used with anaerobic digesters. A Synchronous
Generator is a generator that can operate either isolated (stand-alone) or in parallel
with the utility (i.e., it can run even if utility power is shut down). It requires
an expensive and sophisticated utility interconnection to match generator output
to utility phase, frequency, and voltage when compared to an induction generator.
This type of generator is typically only used if the generator is used for backup
power as well as power generation.
What goes in? What comes out?
What is left after anaerobic digestion is complete?
The feedstocks used for anaerobic digestion contain 75% to 99% water. The solid
portion will consist of some solids that can potentially be reduced or digested
by bacteria, referred to as volatile solids. Those solids that can not be broken
down further by bacteria are lignin, or plant fibers. During the anaerobic digestion
process bacteria will reduce some or most of the volatile solids. The effluent leaving
the digester is comprised of volatile solids that have not been reduced, non-volatile
solids, and water. Only a small volume of the material entering a digester is reduced
or removed.
For example, dairy manure collected in a free stall barn may consist of 10% total
solids and 90% water. Of the total solids, 8% may be volatile solids and 2% may
be non-volatile solids. Most anaerobic digestion processes will only reduce 35-50%
of the volatile solids, converting them into biogas. With 50% volatile solids reduction,
the feedstock in this digester will only be reduced by 4% volume.
Can the solids be removed from digested effluent or manure? How?
Solids can be removed from digester effluent by settling or mechanical separation.
The effluent mixture leaving the digester can be allowed to settle so that liquid
and solids form separate layers in a settling tank. Solids can be scraped or pumped
from the settling tank. Solids can be dried via various means including centrifugation
and/or screen separation. Solids can often be used as animal bedding, or as a soil
amendment to increase the carbon content of soils. Liquid digester effluent can
be land applied or irrigated onto field crops as a fertilizer during the growing
season.
What can the separated solids be used for?
Dried solids from digester effluent have been successfully used as animal bedding.
The easily digestible organic matter has been removed during anaerobic digestion
so the solids are a poorer growth media for bacteria that promote disease or infection.
Anaerobic digester solids can also be spread on fields as a soil amendment, providing
added carbon and nutrients to field soils. Dried digester solids contain higher
amounts phosphorus than the liquid effluent. Therefore, phosphorus loading of fields
can be controlled according to how much digester solids are applied to farm fields.
Digester solids have also been used to manufacture building materials. The U.S.
Forest Products lab in Madison, Wisconsin has developed fiber board utilizing dried
solids from a manure-fed anaerobic digester. (Picture at right is from U.S. Forest
Products Lab)
Does the digestion process kill weed seed or the organisms that cause diseases like
Johne’s ?
Digester effluent is considered safe for bedding and land spreading, because most
weed seeds and more than 90% of pathogens, such as E. coli are destroyed. Pathogens
are often destroyed due to the temperature and lack of oxygen in anaerobic digesters.
Anaerobic digestion of animal manure has shown to reduce Johne’s disease pathogens,
but may not completely eliminate Johne’s bacteria.
What happens to the major nutrients during anaerobic digestion (carbon, nitrogen,
phosphorous, and potassium)? Can the effluent be applied to crop land?
Depending on the anaerobic digestion system, only about 50% of the total carbon
is reduced into methane gas and carbon dioxide. Nitrogen found in the substrate
does not change by volume. However, anaerobic digestion tends to change nitrogen
into a form more readily available to plants. Phosphorous and potassium do not change
in volume and very little in form as a result of anaerobic digestion.
Nutrients added to an anaerobic digester are not destroyed during the digestion
process. In effect, all the nutrients added to a digester are retained in the effluent.
The liquid portion of digester effluent contains nitrogen while the solid portion
contains about 70% of the phosphorus. The nitrogen that is input to an anaerobic
digester is in the form of organic ammonia (NH3), and this nutrient leaves the digester
effluent as inorganic ammonium (NH4OH). Inorganic ammonium is the primary component
of commercial fertilizer. Since the inorganic ammonium is more readily available
to plants, and more volatile than manure-based ammonia, it must be injected or incorporated
into soil so that the fertilizer value is not lost. The digester effluent can be
irrigated or land applied to crops during the growing season to take advantage of
the nitrogen without burning crops.
Do anaerobic digestion systems affect the odor associated with manure storage?
Anaerobic digestion will reduce the offensive odors associated with raw manure and
manure stored in lagoons. Odors associated with manure occur from decomposition
of manure and other organic materials (bedding). There are between 80 and 200 odorous
compounds that have been identified from raw and decomposing manure. Odors can be
dispersed as gases or odorous compounds carried by dust particles. Some odors can
only be detected in high concentrations while others can be detected by humans at
very low levels. The anaerobic digestion process is a biological and chemical process
which converts volatile organic solids in the manure into biogas (methane and carbon
dioxide) and other trace compounds (hydrogen sulfide and others). The products of
biogas and trace compounds are biologically stable and do not have an offensive
odor. The more effective the anaerobic digester is at decomposing volatile solids,
the less odor the effluent will have.
Financial Considerations
How much does an anaerobic digester cost?
Cost estimates for installing anaerobic digester systems have been based on manure-fed
on-farm digesters. The estimated cost for a digester alone is between $400 and $700
per 1000 pounds of livestock weight to install. For dairy farms producing electricity,
the installed cost is estimated at $800 to $1200 per cow for anaerobic digester
system installation. The engine-generator can be up to half the cost of the project.
Besides installation, it is important to consider insurance, operation and maintenance
costs. Annual operation and maintenance costs can range from $11,000 for a small
digester to $51,000 for a large system. The U.S. E.P.A.’s AgSTAR program has advised
that anaerobic digester installation may not be economical for farms with less than
500 animal units (an animal unit is defined as 1000 pounds live weight), based on
energy payback and using solids for bedding or selling as a soil amendment. If generating
electricity, the electricity purchase price will have a large influence on the payback
rate. Most systems can not be justified on producing electricity alone. Avoided
costs of using digested solids as bedding and off setting some heating costs are
usually necessary to justify the investment. As anaerobic digester technology continues
to improve, it may become more feasible to install anaerobic digester systems on
smaller farms.
What are the labor and monitoring requirements of an anaerobic digester system?
An anaerobic digester system does not run itself, it is not self-maintaining. The
system requires continuous monitoring, which is often done using computer-operated
sensors. An anaerobic digester system requires temperature and pH regulation, as
well as feedstock consistency. It will usually require 30 minutes to an hour per
day to make adjustments and perform maintenance. Most systems that have failed were
in part because of a lack of oversight and a person to champion the system.
What is the difference between net metering and a power purchase agreement?
Power Purchase agreement is a contract between a utility and a power producer that
states the utility will purchase the net or excess energy generated by a distributed
generation system. The contract will state the price to be paid and quality requirements
that have to be met. The excess electricity will flow through an electric meter
and onto the electrical grid. Net-metering typically only applies to smaller systems
but varies widely by state. It allows a business or individual generating the electricity
to be paid retail rate for the electricity produced. Some states do not include
biogas in net-metering laws, See the following links for limits.
For more detailed information go to the
Database of State Incentives for Renewable and Efficiency
What types of incentives or tax credits are available for biogas generation systems?
The USDA’s Rural Energy for America Program (REAP) provides funding for renewable
energy and energy efficiency projects through the current farm bill.
Contact your state’s USDA rural development office for more
information on REAP funds. Many states and utilities have incentives for renewable
and energy efficiency projects. Some tax and incentive programs require an application
be submitted before purchasing a renewable energy system to take advantage of incentives
and tax credits so it is important to understand the rules for receiving the credit.
For information about state or utility programs, and for more specifics about the
federal tax credit, please visit
dsireusa.org.
Can I invest in renewable energy without owning my own equipment?
Many local utilities have Green Power or renewable energy programs, which allow
you to purchase electricity generated by a variety of renewable sources, without
having to install any renewable energy generation system or interconnection equipment.
Some of these programs charge an extra fee per kWh used, others charge a monthly
or annual fee to use green power. Utilities may also offer various levels of subscriptions,
allowing you to choose how much green power you would like to purchase. Check with
your electrical utility supplier to see how you can subscribe to a Green Power program.
Installation Information
I am interested in installing an anaerobic digester. How do I get started?
- Determine areas where you can conserve electricity (and natural gas) use. Visit
the energy assessment tools to calculate
potential energy savings for energy efficiency measures you could employ on your
farm.
- Educate yourself. Determine which type(s) of digester would be appropriate according
to the feedstock you will be using. See the frequently asked question, “What are
the common types of anaerobic digester systems?” to determine what type of digester
would be appropriate for your farm’s manure or other feedstock consistency. It is
important to understand how an anaerobic digester system operates, its limitations,
and how to manage the system.
- Contact anaerobic digester installers. Ask questions. Each digester equipment provider
will specialize in a particular type of digester. Digester installation companies
will often provide a feasibility assessment for your farm to determine the potential
biogas and electricity generation potential for your farm. There may be a fee associated
with this feasibility study. If possible, request recommendations for other systems
they have installed and question prior customers on their experiences with the installer,
system maintenance, and issues they have found. Contact your state’s Agricultural
Extension service for contact information of farms in your state with anaerobic
digesters. Go visit farms with digesters and ask question about installer, operational
issues and things they would have done different.
- Consider the financial investment of anaerobic digester system installation. System
installation is a significant business investment, and usually requires loan approval.
- Do you have the management skills and time to manage a new enterprise? These systems
are not “Plug and Play”. They will require daily adjustments and maintenance to
keep the digester operational. Typically, operators report that they spend one to
two hours per day. The digester is basically another animal enterprise except the
animals are microscopic. Over the past 25 years, digesters that have failed were
often due to inadequate management.
- Determine the manure or biological management regulations which may govern the installation
of your digester system. Your state’s Department of Natural Resources or Department
of Environmental Quality will likely require a permit for digester installation,
and some communities will require further approval.
- Determine the availability of tax and grant assistance programs, including their
application and payment processes. Some incentive programs may require paperwork
or other steps to take before the system is purchased or installed.
- Consult with your tax preparer to ensure that you can take advantage of any state
and federal tax credit available.
- Check with your insurance carrier to see if your proposed anaerobic digester system
is insurable.
- Order the system to be installed by a professional and reputable anaerobic digester
system installer.
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Once the system is installed, complete all applicable grant or tax incentive forms.
Submit appropriate agency regulatory paperwork.