Anaerobic Digestion Basics
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
