Anaerobic
Digestion Basics
We are updating this page to reflect current regulations.
Information
on this page about regulations under the Nutrient Management Act, 2002
and the Environmental Protection Act is out of date. On September 18,
2009 new regulations affecting those applying non-agricultural source materials
on agricultural lands were filed. Please visit www.ontario.ca/nasm-omafra
for more information.
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Table of Contents
- Introduction
- What
is Anaerobic Digestion?
- Use of Agricultural Anaerobic
Digestion Systems
- Types of Anaerobic Digestion Systems
- Temperature
Ranges
- Scale of AD Systems
- Challenges
of Anaerobic Digestion of Manure
- Challenges When Planning
an Anaerobic Digestion System
- Making the Economics Make
Sense
- What Types of Input Materials are Suitable?
- On-Farm
Mixing of Off-Farm Source Material
- Resources
Introduction
Agri-food
anaerobic digestion has the potential to:
-
reduce odour and pathogen
levels in manure
-
reduce greenhouse gas production from a
farmstead
- produce renewable energy
- utilize food byproducts
and other organic materials sourced off-farm
-
improve the fertilizer
value of the manure
This Factsheet outlines some of the
factors involved in deciding whether to add an anaerobic digester to a farming
or food processing operation.
What is Anaerobic Digestion?
Anaerobic
digestion (AD) is the process by which organic materials in an enclosed vessel
are broken down by micro-organisms, in the absence of oxygen (Figure 1).
Anaerobic digestion produces biogas (consisting primarily of methane and carbon
dioxide). AD systems are also often referred to as "biogas systems."
Depending
on the system design, biogas can be combusted to run a generator producing electricity
and heat (called a co-generation system), burned as a fuel in a boiler or furnace,
or cleaned and used as a natural gas replacement.
The AD process also produces
a liquid effluent (called digestate) that contains all the water, all the minerals
and approximately half of the carbon from the incoming materials.
Many
agri-food AD systems are located on farms. Farm-based AD systems work well with
liquid manure. AD systems provide a valuable manure treatment option, since most
other economically effective manure treatment systems (such as composting) require
solid materials with dry matter greater than 30%.
Figure 1. An on-farm anaerobic digester in Ontario.
Use
of Agricultural Anaerobic Digestion Systems
Biogas from biomass has historically
been used in Asia as a fuel for household uses such as cooking. Denmark and Germany
have many modern digesters operating on farms and in central locations using materials
such as manure, energy crops, and food-based products and byproducts. These systems
typically use biogas to produce electricity and heat.
Manure-based anaerobic
digesters built in Ontario in the 1980s failed due to poor economic returns or
operational difficulties. However, new technologies and control systems have seen
a new deployment of agri-food anaerobic digesters. There are currently four new
anaerobic digestion systems operating in the province with several in the development
or construction stages. These four anaerobic digesters spread the resulting effluent
(called digestate) on agricultural land. There are several other systems based
on food byproducts that release their effluent to municipal sewage systems for
further treatment. This Factsheet will focus primarily on systems that produce
digestate for land application or capture value from the digestate in other ways.
Types
of Anaerobic Digestion Systems
There are two general AD system configurations suitable
for agri-food systems in Canada: completely mixed and plug flow.
Completely
Mixed
Completely mixed systems, as the name implies, consist of a large
tank where fresh material is mixed with partially digested material (Figure
1). These systems are suitable for manure or other agri-food inputs with lower
dry matter content (4%–12%). Material with higher dry matter content will
work in completely mixed systems by recirculating the liquid effluent.
Plug
Flow
Plug flow systems typically consist of long channels in which the
manure and other inputs move along as a plug (Figure 2). These
systems are suitable for thicker materials such as liquid manure with 11%–13%
dry matter or higher.
Figure 2. Plug flow digester for dairy manure.
Temperature
Ranges
There are three main temperature ranges for AD systems.
Thermophylic
(50°C–60°C)
Thermophylic systems operate at a high temperature.
The micro-organisms rapidly break down organic matter and produce large volumes
of biogas. The quick breakdown means that the digester volume can be smaller than
in other systems (average retention times in the range of 3–5 days). Greater
insulation is necessary to maintain the optimum temperature range, and more energy
will be consumed in heating the system. While these systems may be more sensitive
to nitrogen levels in the incoming materials and to temperature variations, they
are more effective in pathogen removal.
Larger, centralized systems, with
more material to handle and a need for a higher level of pathogen removal, will
typically run at thermophylic temperatures. Heat exchangers used to pass the heat
from the effluent to the influent are more efficient at the higher temperatures
(Figure 3).
Figure 3. Heat exchanger used in a centralized thermophylic AD
system in Denmark.
Mesophylic (35°C–40°C)
Mesophylic
systems need a longer treatment time (retention times of at least 15–20 days
or more) in order for the lower temperature micro-organisms to break down organic
matter. In general, these systems are reported to be more robust when considering
temperature upsets. Small and mid-sized agri-food systems will typically operate
in this temperature range. Some AD systems are specifically designed to concentrate
the solids content to reduce the average overall retention time needed in a mesophylic
system.
Psychrophylic (15°C–25°C)
AD systems running
in Quebec and Manitoba have been designed to operate in this temperature range.
These systems are very stable and easy to manage, however, longer retention times
are required to achieve equivalent gas production and pathogen removal.
Scale
of AD Systems
There are three conventional options for the scale of the
AD system.
Farm-Based Systems
These systems are typically designed
for one farm's manure, for the manure from several nearby small farms or for the
use of energy crops from local fields.
They may use lower cost components
and often involve a lower level of control or complexity. Farm-based systems have
been successfully operated throughout North America and Europe. Farm-based systems
at large farms may come closer to approximating centralized systems.
Some
farm-based systems accept off-farm input materials such as commercial food processing
byproducts. Farm-based systems will be sized to utilize the farm-based byproducts,
to provide sufficient heat or power for the farm (such as at a greenhouse), or
to provide surplus power to the local electrical lines.
Food Processing
Systems
AD systems located at a food processing site may have similar characteristics
and designs to farm-based systems, or they may be designed for removing organic
matter from wastewater. Food processing systems will likely be sized to meet either
the heating requirements of the facility or to manage the byproducts produced
on-site or from several food processing facilities.
Centralized Systems
Centralized
AD systems are found throughout Europe (Figure 4). Material
from many farms and food processing plants is hauled to a centralized facility
operating with a high biosecurity hauling process. Other materials, such as source-separated
organics, are often added to boost gas production. Often the digestate is immediately
transferred to remote field storages to allow for easier handling for land application.
In many instances, heat from the centralized AD system is used nearby at another
commercial facility or for heating residences.
Figure 4. Centralized
AD Plant in Europe.
Challenges of Anaerobic
Digestion of Manure
Although the fundamentals of AD systems are very simple,
the operation and control can be complex. Management considerations include:
-
mixing primarily fresh organic material (<1 week old) so that optimum
organic matter is available for digestion
- maintaining a narrow temperature
range suitable for digestion – adding material that has already cooled down
in the barn or storage will increase the heating requirements
-
completing
proper physical design of the system to eliminate plugging, crusting or foaming
problems
- optimizing the "recipe" to generate sufficient
and consistent biogas production to make the economics work
-
installing
and managing an interrelated group of systems to safely handle heating of the
tank, material flow, hydrogen sulphide reduction, methane transfer, heat production,
electrical production, inter connection with the electrical grid and surplus heat
management (
Figure 5)
Figure 5. Anaerobic digestion systems involve
a variety of control systems.
Challenges When Planning
an Anaerobic Digestion System
When planning to build an anaerobic digestion
system, considerations include:
-
Obtaining insurance:
Insurance companies may approach these systems with uncertainty – work with
a known manufacturer or installer with proven designs. In Ontario, the Technical
Safety Standards Association (TSSA) sets standards and processes for properly
managing the biogas when it is utilized.
-
Obtaining
a building permit: Digesters are not common to the rural landscape, so obtaining
necessary building permits may involve delays, including the need for possible
zoning changes. Allow adequate time to address these issues.
-
Obtaining permission to accept materials: A new AD system that accepts
off-farm material may need to receive a Certificate of Approval under the Environmental
Protection Act or an approved Nutrient Management Strategy under the Nutrient
Management Act. Allow sufficient time for those processes as well. See below
for more information.
-
Obtaining agreements to utilize
energy produced: Information below discusses options to utilize the energy.
-
Siting the facility adequate distances from conflicting uses: An AD system
is totally enclosed, and the produced biogas is typically contained, stored and
utilized. However, there may be some slight odours from feedstocks and other sources.
These odours may be more evident if the biogas system is constructed in an area
not used to livestock production. Because the AD system reduces the odour in the
effluent, the net effect for livestock facilities is anticipated to be a reduction
in odour. AD facilities have been built in the U.S. for the prime reason of effective
odour reduction.
Making the Economics Make
Sense
For agri-food AD systems to be economical, there are several key
considerations.
Electricity Considerations
Interconnection to the
Electricity Grid
When AD systems are designed for electrical production,
the system typically generates more energy than can be used on that one site.
Even in cases where energy production matches on-site energy needs, an interconnection
with the grid is useful (Figure 6). Energy demands at most
facilities are not typically static or linear. Under normal conditions, there
are peaks in energy demand that the AD co-generation system may not be responsive
enough to supply. Instead, the grid essentially acts as a large battery, with
the AD system putting energy in and the local facility drawing energy out.
Figure 6. Interconnection
with the electricity grid.
Net Metering
Net metering
is an agreement where the energy generator (the AD operator) pays the electricity
distributor only for the net amount of electricity consumed. This allows the AD
facility to generate electricity at any time, send it into the grid and then use
electricity at any other time. The net billing or reconciliation is typically
within a specified period of time (1 year in Ontario). The electricity distributor
bills the facility for the net amount used. See the Ministry of Energy's Net Metering
brochure for more information.
Standard Offer Program
The Renewable
Energy Standard Offer Program (RESOP) gives some renewable energy system operations,
including AD system operators, the option to sell or replace electricity at fixed
rates for a period of 20 years. At the time of writing, the value of the electricity
is around 11¢/kWh for non-peak electrical consumption periods and around
14.52¢/kWh for peak periods (2,000 hr/yr). These values will inflate at 20%
of the Consumer Price Inflation Index. For more details, see the OMAFRA Factsheet
Anaerobic Digestion and the Renewable Energy Standard Offer Program,
Order No. 07-051, or visit the Ontario Power Authority (OPA) website.
Use
of Surplus Heat
Some manure AD systems are designed exclusively to combust
biogas for heat. In addition to heating the digester, buildings or hot water,
it is sometimes used to heat, dry or process agricultural feeds. The heat may
have excellent application in greenhouses. Finding a use for surplus heat can
be a significant contributor to making AD systems economical.
Sale of a
Nutrient Byproduct
Many project proponents include the sale of the digestate
nutrient end product in their budget. Niche marketing of nutrient products may
be possible (Figure 7). As more systems are implemented, this
economic stream may diminish. Additionally, the cost of processing the end product
to a point where it is a marketable product (e.g., solid-liquid separation, evaporation,
composting, nutrient blending, etc.) is an additional budget item that must be
accounted for.
Figure 7. Generating
a nutrient-rich byproduct, such as a dry compost product, may provide an economic
opportunity for agri-food AD systems.
Tipping Fees
If off-farm source materials
are being added to the system, AD system operators may benefit from tipping fees
for those materials. These materials may also boost biogas production.
Pathogen
and Odour Reduction Benefits
Agri-food AD systems remove pathogens and
odour from the input materials. In the case of farm-based AD systems, the removal
of pathogens and/or odour may result in a digestate that's more acceptable to
neighbours than conventional, untreated manure. There is difficulty in assigning
a dollar value to these intangible benefits.
Changes in Nutrient Availability
Due
to the removal of readily available carbon through the breakdown of organic compounds,
the effluent (digestate) contains nutrients that are more readily available for
crop uptake (similar to commercial fertilizer). On the positive side, it means
that the effluent will be more predictable in nutrient release, allowing the farmer
to possibly reduce commercial fertilizer use more than they would have using raw
manure. Conversely, the increased nutrient availability may increase nutrient
loss if crops are not available for uptake. This may require longer storage or
use of cover crops to hold the nutrients.
Reduction in Weed Seeds and Improvement
in Effluent Handling
Farmers in Europe claim high reduction in weed seeds
compared to raw manure. The reduction of weed seeds is of special importance to
organic farming systems. The digested material is also easier to agitate, pump
and move through small distribution pipes used in a liquid application system
because of the breakdown of organic matter.
Volume Reduction
There
is a volume reduction in the effluent from a digester. Approximately 1.1 kg of
mass is removed from the effluent per cubic metre of gas produced. This volume
reduction is relatively insignificant for manure with high moisture content. However,
if manure with high dry matter content is used or other materials, such as off-farm
wastes, are added, the effect can be significant.
What
Types of Input Materials are Suitable?
In general, many organic materials
can be digested, particularly feed for animals or humans or byproducts from that
feed.
Manure
Manure is simply animal feed that was not fully digested,
as well as additional water and bedding. It contains significant energy that can
be harvested in an anaerobic digester. The following rules of thumb should be
considered:
- Digestion of dairy and cattle manure has been successfully
implemented in many jurisdictions.
-
Digestion of only poultry or
swine manure may present more challenges because of their higher nitrogen levels
– other materials may be added to optimize the blend.
-
Sand or other inorganic materials will settle out in the digester and must be
considered in the design. Many digesters will require shutdown and removal of
built-up materials after 10 years of usage.
-
AD systems work
best with fresh manure – manure stored under a barn may not be as suitable.
- AD
systems are not effective with highly diluted manure. Processes such as bypassing
milkhouse wash water should be considered.
- AD systems can use solid manure,
however, floating material and non-digestible material from livestock bedding
may make the system difficult to operate.
Food Byproducts
Typically,
food byproducts (and unmarketed food products) can be secured for the digester
at little cost or for a tipping fee. In addition to the considerations below,
see the section in this document on off-farm source material for more details.
-
Most food byproducts break down rapidly in the digester.
- Optimizing
the carbon:nitrogen ratio will be necessary, especially for materials with higher
protein levels.
-
When introducing different food byproducts, it is
important to make changes to the recipe slowly to allow the micro-organisms to
adapt to the new menu.
-
When food byproducts come from a variety
of sources (for instance, a blend of processing facilities, restaurants and retail
food store materials), there may be less certainty about the consistency or quality
of material compared to material from one consistent source. Agreements with reputable
material handling companies are key to success.
Energy
Crops
-
Energy crops such as corn silage, haylage and
grasses typically require on-site storage (conventional ensiling systems).
-
The addition of solid energy crops to liquid systems requires specially designed
solid input devices that prevent gas or liquids from escaping. These systems are
commonly available in Europe.
-
Energy crop addition can result
in floating material, often called the floating or swimming layer. This floating
layer can dry out, forming a crust that can take up digester capacity. With insufficient
mixing, this crust can present a serious headache to digester operators.
-
Unlike manure or many food byproducts, energy crops need to be purchased (or the
cost of crop production needs to be covered by energy production).
On-Farm Mixing of Off-Farm Source Material
Mixing
of off-farm source material with manure in an "on-farm mixed anaerobic digester"
may increase biogas production. Some European jurisdictions allow mixing of up
to 25% of off-farm source materials such as fats, oils and greases, pre-consumer
food wastes, and other food products or byproducts. As a result of the high carbon
content of these materials, biogas production can be doubled or tripled depending
on the quantity and quality of the feedstock.
Proper storage of off-farm
source materials is necessary to minimize the potential for odour nuisance. In
addition, a blend tank may be necessary, depending on the type of AD system used.
There are two regulatory systems to bring most off-farm source materials to a
farm for mixing with manure in a digester: a Certificate of Approval under the
Environmental Protection Act, or an approval under the Nutrient Management
Regulation 267/03. Both of these regulatory systems have requirements for the
facility and for the land to receive the end product.
Certificate of Approval
Off-farm
sourced inputs, such as food processing byproducts, boost biogas production. In
Ontario, to treat many of these materials, a Certificate of Approval (C of
A) for both the on-site treatment facility and the land-application of the digestate
(the digester's end product) may be required.
Securing a C of A for the
treatment facility will designate the farm or a component of the farm as a waste
disposal site. This may trigger concern from neighbours who are unaware of the
standards and operation of an anaerobic digester, and also has the possibility
of affecting the zoning of the farm. Requiring a C of A for land application of
the digestate will add additional criteria not required for manure application.
New Nutrient Management Act Regulation
A new regulation has been
developed to streamline this regulatory process. This change to the Nutrient
Management Act Regulation 267/03 allows the mixing of limited amounts of
specified off-farm source materials into farm-based digesters for the purposes
of manure treatment and energy production without the requirement of a C of A.
Off-farm
material limits in this regulation include:
This regulation has three lists of off-farm source materials.
The first list specifies materials that are allowed to enter into a digester without
any additional treatment. The second list specifies material requiring a guaranteed
time/temperature treatment of 70°C for 1 hr or 50°C for 20 hr. The
third list specifies materials that cannot be accepted for use in a digester.
If
an operation wants to exceed the limits in the regulation or take materials not
in the list, then a C of A will still be required for the treatment facility.
However, this regulation allows the digestate from farm-based digesters to be
treated as equivalent to manure as long as at least 50% of the inputs are agricultural-sourced
material. This situation is true even if the system operates with a C of A.
Nutrient Management Implications of Anaerobic Digestion Systems
The end
product of AD systems has less organic matter than the material that was used
as an input. As a result, the nitrogen that was tied up in organic matter converts
to the ammonium form (Figure 8). When spreading this product
in autumn, take up the ammonium-N with a standing crop or cover crop to prevent
the nitrogen from leaching below the root zone.
Figure 8. Covering
the digested manure minimizes N loss from the storage. This system in Denmark
uses a layer of small floating balls as the cover.
Resources
