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Act, 2002 and the Environmental Protection Act is out of
date.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.
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
For more information:
Toll Free: 1-877-424-1300
Local: (519) 826-4047
E-mail: ag.info.omafra@ontario.ca
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