Anaerobic Digestion Basics
Table of Contents
Anaerobic digestion (AD) of manure and agri-food by-products is becoming more common in Ontario. There are a number of reasons farmers and developers consider building AD systems, including:
AD systems also provide benefits to others, including creating new destinations for managing food waste and creating economic opportunities in rural areas.
This Factsheet discusses some of the factors to consider when deciding whether to build an anaerobic digester for a farming or food processing operation.
Anaerobic digestion 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, which consists primarily of methane and carbon dioxide. AD systems are often referred to as "biogas systems."
Depending on the system design, biogas is 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 (called "upgrading") and used as a natural gas replacement. Pipeline-quality upgraded biogas is often referred to as renewable natural gas (RNG) or biomethane.
Figure 1. A manure-based anaerobic digestion system on a dairy farm.
The AD process also produces a liquid effluent (called digestate) that contains all the water, nutrients and minerals, and approximately half of the carbon from the incoming materials. A solid-liquid separator is commonly used to produce a solid digestate component, which is used as livestock bedding or as a solid nutrient for land application.
In Ontario, the main ingredient in a farm-based AD system is manure. AD systems are attractive as manure treatment systems because they offer multiple benefits while providing a good economic return from energy production. AD systems can handle both liquid and solid inputs, thereby providing a good solution to liquid agri-food wastes such as food-processing by-products.
Ontario has a growing biogas marketplace, with anaerobic digestion systems located on farms and in urban areas. While most farm-based AD systems are located on large dairy farms, a number of systems have been built at other livestock farms and at greenhouses. In addition, several large non-manure digesters have been built in urban areas to handle primarily food processing waste and some post-consumer food waste. Most Ontario AD systems produce electricity for sale to the electrical grid. Most agricultural AD systems return their digested nutrients to the land as an agricultural nutrient.
Farm-based AD systems in Ontario are all completely mixed or continuously stirred tank reactor (CSTR) systems. Completely mixed systems consist of a large tank where fresh material is mixed with partially digested material. These systems are suitable for manure or other agri-food inputs with a lower dry matter content (4%-12%). Material with higher dry matter content can also be added to a completely mixed system since the anaerobic bacteria consume the carbon, liquefying the input material. Very watery materials can also be added to a completely mixed AD system, although there are other digester configurations better suited for wastewater treatment.
There are three main temperature ranges for AD systems:
Scale and Location of AD Systems
There are three options for the scale of the AD system:
Figure 2. A load of vegetable by-products is received at a farm-based anaerobic digestion system.
When planning to build an anaerobic digestion system, consider the following:
Figure 3. Most biogas systems produce electricity by burning biogas in a co-generation system.
Although the fundamentals of AD systems are simple, the operation and control can be complex. Design and management considerations include:
For agri-food AD systems to be economical, there are several key considerations. The biggest is the sale of energy from the AD system. Other benefits, including revenue from tipping fees or sales of by-products will vary, depending on each project's specific conditions.
Interconnection to the Electricity Grid
AD systems designed for electrical production usually generate more energy than is used at the site. Even in cases where energy production matches on-site energy needs, an interconnection with the grid is useful. Energy demands at most facilities are not 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 acts as a large battery, with the AD system putting energy in and the local facility drawing energy out. When farms add non-agricultural materials to the AD system, the result is significantly more energy than the farmstead can use.
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 to the grid and then use electricity at any other time. The net billing or reconciliation is within a specified period of time (1 year in Ontario). The electricity distributor bills the facility for the net amount used. Generally, offsetting the farm's energy use will provide insufficient savings to cover the capital and operating costs of an AD system, meaning other revenue streams are needed. Contact the local electrical distribution company for more details on net metering.
Sale of Electricity
Ontario has some of the best rates in North America for renewable electricity from biogas. The former Renewable Energy Standard Offer Program (RESOP) and the Feed-In Tariff (FIT) Program have provided AD operators a guaranteed revenue stream. They ensure the long-term economic stability needed for a project of this nature. For more details on the sale of electricity, visit the website of the, Independent Electricity System Operator (IESO).
Sale of Renewable Natural Gas
Currently, economical pathways for renewable natural gas (RNG) sales still have to be clearly demonstrated. RNG from biogas is more expensive than conventional natural gas, meaning a premium price is required for good project economics. Some of the factors that influence the economics of an RNG project include:
Like renewable electricity, RNG is attractive because it fully integrates into the existing natural gas system without impact on the end user.
Use of Surplus Heat
While some manure AD systems are designed exclusively to combust biogas for heat, the system cost and the availability of low cost conventional fuels means that biogas for heat alone is not economically viable. However, with an electrical co-generation system, there is often significant excess heat available, even after heating the digester vessel. In addition to heating the digester, buildings and farmstead hot water, finding other users for hot water nearby can improve project economics. In Ontario, several greenhouses operate biogas systems and use the surplus co-generation heat to reduce their fuel use for hot water heating.
Off-farm feedstocks are added to farm-based AD systems to boost biogas production. Ontario allows the mixing of up to 50% off-farm feedstocks while still maintaining the agricultural designation of the digested materials that are produced. If off-farm source feedstocks are added to the system, AD operators may benefit from tipping fees for those materials.
Several factors influence tipping fees, including:
Tipping fees make up a small portion of the revenue stream of an on-farm AD system relative to energy production.
Figure 4. Digestate solids are commonly reused as dairy bedding.
Sale of Nutrient By-Products
Sometimes project developers include the sale of the digestate nutrient end product in their budget (Figure 4). Niche marketing of nutrient products is possible. As more systems come into operation, this economic stream may diminish. Also, 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 another budget item to account for. Several dairy farms in Ontario use their digested solids as bedding on their own farms. In the U.S., there are examples of farms selling their dried digestate solids to neighbouring dairy farms for use as bedding.
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 from manure may result in a digestate that's more acceptable to neighbours than conventional, untreated manure. It is difficult to assign 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 digestate contains nutrients that are more readily available for crop uptake (similar to commercial fertilizer). This means that the effluent is more predictable in nutrient release, allowing the operator to 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. When spreading digestate in autumn, it is best to take up the ammonium-N with a standing crop or cover crop to prevent the nitrogen from leaching below the root zone. Longer digestate storage is required to ensure proper nutrient application timing compared to conventional manure.
Reduction in Weed Seeds and Improvement in Effluent Handling
Farmers in Europe claim a high reduction in weed seeds compared to raw manure. The reduction of weed seeds is of special importance to organic farming systems but also contributes to reduced herbicide costs on conventional farms. The digested material is easier to agitate, pump and move through small distribution pipes used in a liquid application system because of the breakdown of organic matter in the digester.
Reduction of Inputs
A digester will remove approximately 1.1 kg mass/m3 of gas produced. This results in a significant reduction in volume if primarily dry input materials are used. However, since most digesters bring in off-farm feedstocks, the total digestate volume and total nutrients handled on the farm are higher compared to the original manure quantities.
Many organic materials can be digested, particularly feed for animals, food waste or by-products from that feed.
Manure is animal feed that was not fully digested, plus additional water and bedding. It contains energy that can be harvested in an anaerobic digester. Consider the following:
Food by-products and unmarketed food products are used as feedstock for the digester at little cost or for a tipping fee. Mixing of off-farm feedstocks with manure in an "on-farm mixed anaerobic digester" may increase biogas production and make the economics more attractive. A variety of off-farm source feedstocks are received at farm-based AD systems in Ontario, including fats, oils and greases, pre- and post-consumer food wastes, grease trap waste and other food-processing products or by-products. There are a variety of considerations that influence the suitability of off-farm feedstocks:
For some farms, feeding energy crops makes sense:
AD systems present a variety of potential safety risks that must be considered in the design and operation of these systems. While combustibility of the methane in the biogas is the obvious risk, another key risk is the production of dangerous gases that can lead to serious health impacts or death. If materials are mishandled, there are also risks to air, water and land from spills or discharges.
A number of design and construction standards must be followed, including regulatory requirements from the municipal and provincial governments, technical standards from the Technical Standards and Safety Authority and Electrical Safety Authority, and workplace health and safety rules. In addition, an online biogas safety awareness e-learning package for biogas workers is offered by the University of Guelph-Ridgetown College to create awareness about risk management at an AD system.
As with all farming systems, proper design and training are key to minizing risks. Information on the various rules are found at the Biogas Association website.
Anaerobic digestion systems are becoming more common in Ontario. AD systems provide a viable option for manure treatment, with real economic returns and a variety of other societal benefits. Many resources are available to help inform a potential developer about anaerobic digestion systems. For more information, visit the websites of the Province of Ontario, The Biogas Association, the American Biogas Council and the US AgSTAR program.
This Factsheet was revised by Jake DeBruyn, P.Eng., New Technology Integration Engineer, OMAFRA, Guelph, and Don Hilborn, P.Eng., By-Products Management Engineer, OMAFRA, Woodstock (retired). The Nutrient Management Act, 2002, (NMA) outlines rules and requirements for the handling and storage of manure and other nutrients on farms. The specific rules governing liquid manure management are found in the Nutrient Management Regulation O. Reg 267/03. Some of these rules are discussed in this document.
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