Farm methane digesters are large tanks, or covered lagoons, where organic materials from the dairy farm are stored and broken down by bacteria and organisms called archaea to make renewable methane gas. These are also known as anaerobic digesters: facilities that harness the power of methane produced by decomposing cow waste and convert it into renewable energy.

In the process, flammable gas is produced, which can be used to heat water and the barns on the farm. It may also power a generator to provide the farm's electricity. This allows dairies to mostly eliminate their use of fossil fuels. The result is lower energy bills, reduced greenhouse gas emissions, and healthier cows because the quality of the grass is often improved.

For the cows, life goes on as normal. They produce their organic waste for free and are fed up to a certain level. Assuming the value of the output from the digester is greater than the combination of capital and operating costs, dairy farmers can reduce their feed costs and increase gross margins.

Why Farm Methane Digesters are important

Dairy or Farm Methane Digesters are important for:

• the generation of clean energy
• protecting our environment and lakes from farm waste
• giving local farmers a way to safely dispose of waste
• generating energy for their operations.

Anaerobic digesters will play an increasingly crucial role throughout US farming communities.

This is especially in the finger lakes region, where one-third of New York state's Farm Methane Digesters are located. Farmers rely on this technology to generate energy while limiting farm waste and protecting our precious water resources.

In 2016, California passed a law requiring dairy farms to cut methane production from slurry lagoons and other storage by 40% by 2030. Farm methane digesters have helped farmers already achieve a 25% reduction in three years, according to Mitloehner. A patchwork of other state regulations also aims to cut methane emissions from various sources, including agriculture.

There were 221 AD systems processing dairy cow manure in the United States as of April 2021, and these systems reduced approximately 4.29 MMTCO2e per year. 1 Furthermore, over 50 dairy AD systems are currently under construction, providing a significant opportunity to scale AD capacity in the coming years.

It is important to continue to encourage the deployment of anaerobic digesters on dairy farms as a critical component of environmental stewardship as well as creating opportunities for new systems.

The legislative commission on rural resources' senate chair collaborates with the NY Cow Power Coalition, the Northeast Dairy Producers Association, and Cornell Pro-dairy to ensure that farmers who use farm methane digesters have the resources they need to continue developing this technology.

How does a dairy digester work?

An increasing number of dairies in the U.S. Are adding methane digesters to their operation to turn manure into renewable natural gas. For example, the High Plains Ponderosa Dairy has partnered with Shell Oil for their digester, and CEO Greg Bethard says it’s really an exciting opportunity.

The primary types of farm methane digesters globally are covered lagoon digesters, complete mix digesters for slurry manure, plug-flow digesters for dairy manure, and dry digesters for slurry manure and crop residues. In the US, including California, the most common commercial farm digesters are usually large circular concrete or steel tanks with heavy plastic membrane covers.

The most common U.S. dairy farm methane digesters are covered lagoon digesters. These work as large-volume, liquid manure lagoons which fill over the wet season from the manure discharged from barns, farm yards, and milking parlours. The lagoons often have less than 2% solids. A plastic cover traps the biogas made during decomposition. To collect that gas which is, to a large extent, methane, a floating lagoon cover is used. The system also needs a gas pump. This system is used in both swine and dairy operations. It works best where the manure is handled as a liquid. The climate should be temperate to warm all year. This is the least expensive system. But, it's not fit for use in cold climates. Farmers in Wisconsin and the Upper Peninsula need a different system tank-based system.

The Lagoon Type AD Plant (cover removed at the time of photo): Better mixing at Butler Farms has increased methane levels for its biogas plant.

That anaerobic digesters work for dairy farms can be seen from the manner in which they are growing in popularity across the United States. In July 2010, the EPA estimated that just 157 farm methane digester projects were operating on a commercial scale nationwide. Of those 157 digesters, 22 were located in New York state, making it the second leading state in operating digesters in the country.

Frequently asked questions include: “How does a dairy methane digester work?” By covering slurry ponds, farms capture the biogas that is produced from the natural breakdown of manure during storage. You can actually see the plastic tarp on the pond rise as it fills up with biogas.

Lochmead Farms Inc. Low-Temperature Sequential-batch Anaerobic Digester

Lochmead Farms Inc. installed a patented, psychrophilic (low temperature), sequential-batch anaerobic digester on their 3,000-acre, 750-cow dairy operation near Junction City, Oregon. They did the design and the build, and now own and operate it. The fact is that every dairy or other livestock farm in the United States already has a complex manure management system that is existing and paid for. Therefore to make ad projects work they must enhance what’s already in place.

What the process does to manure

Anaerobic simply means “without oxygen”. An anaerobic digester is made up of a closed tank that excludes oxygen. Naturally occurring bacteria break down the manure. The carbon in the manure is converted into a range of gases. One such gas is methane.

Bacteria and archaea that produce methane are most active from 95 to 105°F. For this reason, some digesters run hot water through the pipes. It heats the manure, to stay in the ideal temperature range. A cover on the digester traps the biogas. The cover is both flexible and impermeable. When trapped, the biogas can be burned off in an open flame. It could also pass through an electrical generator. There are numerous types of generators. However, the most common is the modified internal combustion engine. Electricity produced can then be used on the farm.

After treatment in the digester, the manure becomes what is known as a digestate. Both fluid and solid digestates are rich in Nitrogen (N), Phosphorus (P), and Potassium (K) as natural organic fertilizer.

Natural Synthetic Fertilizer Made on the Farm

Therefore,  they can provide a sustainable substitute for synthetic fertilizers (Vaneeckhaute et Al., 2013) and recycle nutrients for organic crop production to offset the use of synthetic fertilizers.

All organic fertilizers produced by this process, when studied, have stimulated potential organic food production for farm ecosystems. However, different feedstocks and AD treatments generate digestate of differing chemical compositions compared to undigested animal manures, which may affect the soil microbial ecosystem differently and plant growth when used as fertilizers (Abubaker et Al., 2012, 2013). Bertora et Al. (2008) showed that N2O and CO2 emissions as well as denitrification processes and nitrate availability are potentially reduced.

Pennsylvania Department of Environmental protection general permit for dairy farm methane digesters

The Pennsylvania Department of Environmental protection has created a new general permit for dairy farm digesters (wmgm042) that authorizes anaerobic digestion of animal manure on a farm:

• mixed with grease trap waste (collected from restaurants or grocery stores) and
• preconsumer and postconsumer food waste from commercial or institutional establishments.

The permit states that the by-products must be beneficially used as follows:

• methane gas is produced by anaerobic digestion as fuel, including in the production of electricity
• waste solids removed from the digester are used as an animal bedding material at the farm; and
• liquid waste and solids are removed from the digester as a soil additive for agricultural purposes.

If fats, oils, and grease are added to the digester, the liquid waste and solids may not be beneficially used as a soil additive if the concentration of fats, oils and grease exceeds 15,000 mg/l (milligram per litre).

Anaerobic Digestion on US Dairy Farms

Farm operations in the USA and Europe have seen a radical change in the last decades: small-sized farms are disappearing, and farm size and total livestock on larger farms are increasing.

The resulting high spatial density of animals causes several environmental impacts.

Anaerobic digestion is one promising technical solution to alleviate most of these impacts while simultaneously providing a regional energy source. Analyses have been done to assess the economic viability of using dairy-cow manure for either:

(i) the on-farm production and use of biogas to generate electricity and heat or
(ii) upgrading biogas to biomethane, a natural-gas substitute.

A non-linear optimization model was developed to optimize plant capacity for anaerobic digestion and maximize the net present value for each option by farm size.

The process of anaerobic digestion allows for the conversion of methane, by upgrading (purification) into renewable natural gas (RNG). Domestic American dairy farms produce high volumes of methane, and this process means that it can be turned into RNG which is very useful in:

• fueling vehicles,
• powering businesses, and
• heating homes.

The market for this renewable gas is insatiable, and at 2022 prices, very profitable.

Many dairy farmers from coast to coast are becoming aware of this, and are considering the addition of anaerobic digesters to their broader operations.

Penn State Research on Farm Methane Digesters

Penn State has done research to determine the technical requirements and economic feasibility of producing methane gas by anaerobic digestion of dairy-cow manure. A review of the literature revealed a substantial amount of laboratory experience with methane digestion from farm wastes, mainly dairy manure.

However, about 10 years ago, information on experiences with full-scale digesters in operation on commercial-size farms in the United States was not found. As a consequence, it was decided to build a digester of sufficient size to study the engineering problems related to the use of digesters on dairy farms. The university dairy barns have facilities for 50 cows each, which is representative of the size of an average Pennsylvania dairy herd. The results of the research will be vital to the implementation of the technology on smaller farms.

Dairy Farms in Merced County California

For example, a program in California to turn cow manure into renewable natural gas will help dairy farmers reduce methane emissions while potentially creating new economic opportunities. The project will produce RNG by capturing methane from 15 dairy farms in Merced County and then conducting anaerobic digestion. The project is being funded primarily through the California public utilities commission’s dairy biomethane pilot program, which was established as part of the state’s strategy to reduce emissions, including from methane gases.

The fundamentals of anaerobic digestion of dairy manure should not be overlooked though. Since 2004 there have been researched studies done in the U.S. on the reduction of selected pathogens during anaerobic digestion. Care is needed to avoid the possibility of infection spreading through the AD process.

Farm Digesters at Cornell University

It has been almost 40 years since Cornell University first became involved with anaerobic digestion on dairy farms in New York state. What created the initial interest? Cornell university pioneered on-farm anaerobic digestion of dairy manure starting in the mid-1970s when alternative energy solutions were being pursued as a result of the 1973 oil embargo.

Professor Emeritus William Jewell developed, tested and demonstrated plug-flow anaerobic digestion of dairy manure on local collaborating farms. Lower energy prices prevailed soon afterwards and that, along with relatively few farms of sufficient size to justify an anaerobic digester at that time resulted in a loss of interest by the dairy industry.

Pro-Dairy New York State

Pro-dairy became a major player in manure-based anaerobic digestion in the late 1990s, which was when the New York state dairy industry’s interest in anaerobic digestion was resurrected.

Membrane Covered AD Lagoons at Butler Farms in North Carolina, where they have always tried to minimise the impact on the environment.

Where Cow Power Dairy Farms Are Located

Reinford Farms, Pennsylvania

A second digester at Reinford Farms, about 3 times the size of the first, began operating in late 2019. Reinford Farms is located in Mifflintown in central Pennsylvania. Steve and Gina Reinford purchased the farm in 1991, starting out with 57 dairy cows and 144 acres. Their 3 sons are all involved in the family business. In 2008, they installed a 526,000-gallon complete-mix anaerobic digester supplied by RCM (now known as Martin Construction Resources (MCR)), sized to treat manure from 800 dairy cows. There were about 400 cows at the time. Capital costs for the digester and related equipment were about $ 1 million.

Among those companies that currently offer mini biogas plant versions is Biolectric of Belgium. The firm has installed 300 minis, almost all of them (295) in Europe and the rest in North America. Almost all (285) are located on dairy farms, with the remainder mostly on pig farms and a few installed at wastewater treatment plants and small food factories.

Straus Dairy Farms California

Renewable energy is one of the focuses of Straus Dairy Farm’s sustainable model. At the dairy farm, the methane biodigester captures methane (a potent greenhouse gas) from the cows’ manure and transforms it into electricity. Operating since 2004, the methane digester provides enough renewable energy to power the entire dairy farm, and charge Albert Straus' electric car and other farm vehicles.

Current Manure Management Practices

Current manure management practices are far from sustainable. The accepted norm for farm fertilizer requires the constant input of mined fertilisers to retain N:P:K level high enough to sustain current crop production.

Muck is placed on the land while still highly organically active without prior processing.

Not only is this a huge additional carbon emission burden on the environment, but it raises nutrient run-off. This jeopardises all aquatic ecologies and fails to maintain soil structure which, as soil deteriorates, means that yet more mineral-based fertilizer has to be used.

Ryan Flaherty is the director of business partnerships at sustainable conservation. In this position, Flaherty leads a team to identify and test manure management solutions that reduce the environmental impact of California's dairy industry while keeping it economically viable.

Current initiatives include assessing non-digester practices to reduce methane and other emissions; identifying economically viable options for exporting excess nutrients; and demonstrating how liquid manure can be used with subsurface drip irrigation to:

• improve crop yields,
• save water,
• reduce greenhouse gases, and
• cut nitrogen leaching.

However, such approaches are rare and most farmers globally are moving toward the anaerobic digestion process and farm digesters.

The three main designs for farm-based digesters are:

• the covered anaerobic lagoon,
• plug-flow, and
• complete mix (or continually stirred tank reactor).

The solids content of the material to be digested is an important criterion in the choice of digester design.

Obstacles to Good Manure Management Practices on Dairy Farms in the U.S.

Before environmentalists, energy companies, and the government can leverage the benefits of  Good Manure Management Practices, to create a more sustainable industry, they must rebuild trust with the dairy farming community.

When we started this, there were a handful of dairy digesters, maybe ten or 15, and a lot of stories of, “well we’ve tried this before. It didn’t work then, why will it work now?” admits one guy involved. “And the answer, they said, is that we weren’t really committed to making it work then and we are now. And there are now all these programs in place that can make it work and we want to work to figure it out.”

NYSERDA Funding for New York State Fram Methane Digesters

To that end, last year, the New York State Energy Research and Development Authority (NYSERDA) announced $16 million in funding for new and existing on-farm anaerobic digesters. An additional windfall for a sector that has already received more than $26 million from the agency. A biogas plant is a big investment, but NYSERDA is just one government agency bankrolling the digester industry.

A database of renewable energy policies and programs across the country lists 96 financial incentives for anaerobic digesters, including property tax reductions, corporate tax credits, loan programs, grant programs, and performance-based incentives.

Most materials digest well at the lower range, the thermophilic bacteria are very sensitive to any changes in the digester, the sludge they produce is of poor fertilizer quality, and it is difficult to maintain such a high temperature, especially in temperate climates. The bacteria that produce methane in the “normal range” 90-95°f (32-35°c) are more stable and produce a high-quality sludge. It is not difficult to maintain a digester temperature of 95°f (35°c).

California has used a carrot approach by awarding grants to dairies to capture and recycle manure methane gas for energy, helping offset millions of dollars of costs.

So most digesters are in California, the nation’s top dairy state, which has a substantial grant program. For Texas producers to benefit from installing a methane digester, we believe we need a better option than the current digester process.

2022 energy price increases

Before the 2022 energy price increases, it simply didn’t make financial sense for most farms to build or operate a digester without outside funding. The question was, do the environmental benefits justify significant—and ongoing—public investment? But, now, in 2022 this has all changed due to energy price rises since the start of 2021.

Can Dairy Be Sustainable? Yes, And Here Are Examples

Goodrich farm Salisbury, Vermont

Goodrich farm in Salisbury, Vermont, has been recognized with a 2021 us dairy sustainability award for outstanding dairy farm sustainability. The farm was one of three farms chosen nationally to receive the award and was selected for its many sustainable initiatives including its role in a unique regional partnership to create renewable energy, recycle food waste, and reduce greenhouse gas emissions through the creation of one of the largest on-farm anaerobic digesters in the U.S.

Goodrich Farm is leading the way in New England. Alongside its partners, the farm launched a 1.32-million-gallon anaerobic digester in 2020 that can produce 180,000 mcf of renewable natural gas annually. This partnership brings together Goodrich Farm, which hosts the digester, Vanguard Renewables, developer, owner, and operator of the digester, Vermont Gas Systems, and Middlebury College, which will purchase much of the renewable natural gas produced at the digester.

Pine Island Farm, Sheffield, MA

Pine Island Farm, Sheffield, MA is a family-owned and operated dairy farm in Berkshire county, pine island farm spans 1,300 acres of cropland, houses approximately 1,000 head of Holstein cattle, and produces and sells 12 million pounds of milk from 750 cows annually. A fully-grown milking cow can produce upwards of 100 pounds of manure per day, which can create odours and – due to carbon dioxide and methane produced during decomposition – greenhouse gas emissions.

Prior to 2011, manure at the Pine Island Farm was stored in a 4. 1-million-gallon slurry tank and spread on cropland as fertilizer. That changed in November 2011, when the farm began using manure as feedstock for its new anaerobic digester with a combined heat and power (CHP) energy system.

How Dairy Farmers Are Reducing Methane And Greenhouse Gas Emissions

While dairy cattle contribute just 1.3% of total GHG emissions in the U.S., there is a sense of urgency around reducing emissions, especially in California where new legislation requires dairy farmers to cut methane emissions by 40% before 2030.

Biodigesters turn poop into power and bring the smackdown on smell.

A dairy digester is a device that captures methane resulting from the decomposition of manure. Why do we care about decomposing manure?

The Agstar Project

The Agstar project of the environment protection agency describes anaerobic digesters as a solution to a problem dairy farmers have always had to solve but that has become more acute with the innovation of larger scale, confined animal feeding operations developed in response to the growing food demands.

Digesters take cow manure and convert it into energy while also eliminating manure odour.

The Low Carbon Fuel Standard (LCFS)

Since 2011, California has been running a policy called the Low Carbon Fuel Standard (LCFS), which now includes incentives for dairy farms to convert methane into energy to fuel vehicles by enabling them to sell offset credits. This is intended to be a win-win: reducing farm emissions while allowing fossil fuel companies to mitigate their own greenhouse gas emissions by buying these offsets. The number of anaerobic digesters used to produce the biogas has surged in the state, especially among large dairy farms.

Renewable electricity renewable vehicle fuel (biomethane) renewable natural gas that can be can be injected into existing pipelines for a wide range of uses (heating, power plants). Electricity from methane digesters on dairy farms can be used on site (thereby reducing the farmer’s energy costs), and the surplus can be sold to a public or private utility. The electricity can replace some power from fossil-fuel plants, which account for approximately 20% of California's greenhouse gas emissions.

Achieving Revenue from Farm Biogas

Revenues from California's market-based programs have made dairy digesters one of the most efficient ways to reduce methane emissions, state officials say. But that efficiency, environmental justice advocates counter, comes at the cost of dumping even more pollution on neighbourhoods already shouldering the burden of hazardous industrial operations.

That’s because California's market-based climate policies allow polluting companies like power plants and oil companies to buy offset credits rather than reducing emissions at the source. The first study to evaluate potential disparate air quality impacts of California's climate policies, published in 2018, found that emissions of greenhouse gases and co-pollutants from industrial facilities were more likely to increase in disadvantaged neighbourhoods with more than their share of pollution.

On Farm Anaerobic Digesters

Anaerobic digesters compost (or “digest”) organic waste in a machine that limits access to oxygen (hence the “anaerobic” part), encouraging the generation of methane and carbon dioxide by microbes in the waste. This digester gas (which also comes contaminated with hydrogen sulfide) is then burned as fuel to make electricity.

Farm methane digesters aren't widely used yet but tend to be used for sewage sludge at sewage treatment plants and for animal waste on farms. Digesting organic waste doesn't avoid the need to handle the digested material (a mostly solid, but wet, byproduct known as “digestate”).

New laws to help with climate control are Driving the Sector Forward

Many farmers are motivated by more than new laws to help with climate control. They are often called the original environmentalists for spreading manure from their own livestock to fertilize their fields, as opposed to buying synthetic or mined fertilizers, which require additional energy and resources to produce and transport. Consistent with that tradition of practical solutions, farms across the country, in addition to building anaerobic digesters, are implementing other practices to address climate change.

New York’s 4,300 dairy farms could easily support a tenfold increase in anaerobic digesters, yielding a variety of environmental benefits through the production of renewable natural gas. The challenge is establishing a business model that makes such efforts attractive to farmers, investors and the consumers that would use such fuel.

A converted sceptic gains from installing a dairy digester

At first, California dairy farmer Felix Echeverria was sceptical about installing a dairy digester on his 12,000-cow operation. The process, which involved covering a pit of liquid manure and capturing the methane emissions it releases before “digesting” it anaerobically, is expensive and complex, and we understand that he said himself that it was not something he was qualified to run.

But he saw the benefits neighbouring farmers in the Bakersfield Area reaped from their farm methane digesters and decided to get ahead of a state law that would require him to reduce emissions by 2030.

High-rate digesters for Hog Farms

On hog farms, the anaerobic digestion process must be designed to reduce HRT (Hydraulic Retention Time) while maintaining SRT long enough to sustain the microbial communities. This is what drives the demand for high-rate digesters. Digesters such as fixed-film, anaerobic sequencing batch reactor (ASBR), and Upflow Anaerobic Sludge Blanket (UASB) types work well with dilute hog manure.

There really is money in manure for California dairy farmers with Farm Methane Digesters that capture methane, a potent greenhouse gas, from their cattle’s manure. Each cow on a farm with a digester can generate $2,827 a year in air pollution and biofuel credits for methane that would otherwise go into the atmosphere, calculated Aaron Smith, a professor at UC-Davis.

Dairy manure is the U.S.’ largest source of methane from livestock, yet it remains uncommon for farmers to deploy anaerobic digesters (ADs) to convert biogas into renewable power, cutting tons of emissions in the process. The environmental protection agency estimated that just 221 AD systems processing dairy cow manure were in operation in the U.S. in 2021.

The Potential for Decentralized Biogas Technology

Long-term economic and environmental concerns have resulted in a great amount of research on renewable sources of biomass and bioenergy to replace fossil fuels in the past decades. Decentralized biogas technology is one of the most potential technologies for biomass and bioenergy by using agricultural waste materials (E.G., animal manure, crop straw, and by-products from food industries) as feedstocks.

By-products from biogas production, called digestate, are nutrient-rich, which could potentially be reused as green fertilizers in agriculture, thereby providing a sustainable substitute for synthetic fertilizers for the farm ecosystem. Thus, the biogas production of anaerobic digestion is a win-win option for livestock and crop producers to address issues of waste management and energy supply, and to avoid contamination of surface and ground waters and emissions of odours and greenhouse gases.

More examples of the US methane digesters working

The potential for methane production from livestock waste depends on the size of the farm operation, the freshness of the waste, and the concentration of digestible materials in the manure. Free-stall dairy operations with daily-scraped alleys work well with digesters because the manure does not get mixed with dirt or stones and is moved into the digester while fresh.

Olean Energy Nebraska

Nebraska's first on-farm power station is powered by methane produced by an anaerobic digester of swine manure. Danny and Josie Kluthe of Dodge, Nebraska, run the farm. Olean Energy is the company's name in Nebraska. The video, titled “complete mix swine manure digester,” explains the entire system's operation.

The operation's manure is fed into an in-ground concrete tank with an insulated flexible cover. As it stirs and heats waste, the complete mix digester system produces methane. The gas is used to power an internal combustion engine, which generates electricity and sells it to NPPD under a buy-all, sell-all agreement. The farm generates 730,000 kilowatt-hours of energy per year, enough to power 53 homes. The system's recovered heat is used to keep the digester temperature stable. The Kluthes used the anaerobic manure digester to capture natural gas in 2013, which they used to supplement fuel for their farm's vehicles and for other purposes.

Crave Brothers Farm Waterloo

A manure digester at Crave Brothers Farm in Waterloo produces electricity for the township and bedding for the cattle. It also reduces methane emissions. Tom crave says his family’s dairy farm is leading the way in sustainability by operating at 100% green power. Since adding a manure digester in 2006, the 2,200-cow farm is producing enough electricity for farm operations, Crave Brothers Farm cheese and the roughly 300 homes in the township.

Joseph Gallo Dairy Farm

Cows at the Joseph Gallo dairy farm in the central valley in 2007. The farm’s use of dairy Farm Methane Digesters was the subject of a television feature. A third major focus of sb1383 is the development of a research working group to fill gaps in industry knowledge. Least is known about fixing the problem of enteric fermentation- the technical name for methane produced during digestion and released in cow burps. In 2015 bovine gas made up almost 30% of state methane emissions in the agricultural sector.

Anaerobic digesters work by sealing manure into a giant pit or tanks to keep oxygen out while microbes feed on the contents and produce methane that's captured and refined into renewable natural gas or burned to generate electricity. The liquid and solid materials that come out of the process can be applied to fields as fertilizer in a more precise, focused way than raw manure, reducing but not eliminating the risk of pollution from phosphorous and nitrates that trigger algae blooms in waterways and contaminate drinking water in private wells.

Dairy Farmer in Central Massachusetts

Randy Jordan, a fifth-generation dairy farmer in central Massachusetts, looked into turning manure from his 300 cows into natural gas more than a decade ago, he just wanted to find a way to lower his increasingly painful electric bill. He knew that on a farm methane digester, a sort of modern alchemy that transforms poop into profits, the possibility of profit been around for decades. But many of the tanks, where microorganisms digest manure and turn it into methane gas that can be burned as fuel or converted to electricity, had been abandoned. They proved too complicated to manage. “it was challenging,” he remembered, “and the money didn’t work. But now, with such high gas prices globally the tables have been turned completely.

Biogas Amounts and Composition

Biogas consists of 50–60% ch4, 40–45% CO2, and trace amounts of hydrogen sulfide (h2s). As a result of the AD process, a number of changes in slurry composition can be expected. Both fluid and solid digestates are nutrient rich in n, phosphorus (p), and potassium (k) as organic fertilizers free of seed, pathogens, and odours.

That is because the digestate resulting from the anaerobic process uses only the c, but the n, p, k, and micronutrient components remain intact (Kryzanowski, 2013 ). Therefore, the nutrient profile of the digestate is much the same as the nutrient profile of the feedstock sources and digestate are excellent sources of nutrients.

Use of the Digester Effluent

The size of the storage area for the farm methane digester effluent depends on how the manure is to be managed after processing. If it is to be spread daily on land, the storage space needs to be only large enough to hold a few days' outputs, which will allow for delays because of equipment breakdown or extremely severe weather. On the other hand, if manure is to be stored until the best time for land application, larger storage will be necessary. With long-term storage, dewatering might be advantageous. In this way, the liquid could be distributed by sprinkler irrigation, and the solids stockpiled and distributed whenever convenient.

The manure digestion process itself isn't the end of the story. The liquids and solids that come out of a farm methane digester can be subjected to a number of other post-treatment practices, depending on what the operator is trying to accomplish. These actions can include additional disinfection, or processes to filter out nutrients. And regardless of those practices, there's also the need to make choices about how the effluent is used, like spreading post-digester manure in a way that doesn't cause excessive nutrient runoff.

The above discussion has been concerned with the energy aspects of anaerobic digestion. A farm methane digester will also be an integral part of the waste management system. The advantages and disadvantages should be reviewed from a waste management viewpoint.

More Digester Advantages

A primary advantage of an anaerobic digester is its ability to nearly completely stabilize raw manure. As a result, the effluent from a properly operating farm methane digester is relatively odour-free and odour problems usually associated with production facilities and disposal operations may be reduced.

Another advantage of anaerobic digestion is nearly complete retention of the fertilizer nutrients (n,p,k) that were in the raw manure. Nutrient losses may occur in the subsequent handling of the effluent. This advantage may become more significant in the future if fertilizer shortages become more acute.

Another advantage of the anaerobic digester is its ability to stabilize more waste per unit volume than other treatment facilities such as lagoons. This advantage is offset in most cases by the fact that a lagoon will probably be required for storage of farm methane digester effluent until such time that it can be used in irrigation or otherwise distributed over the land.

Digesters Convert Carbon but Do Not Achieve Disposal

This brings up the point that a farm methane digester is not a complete disposal tool in itself. The volume of liquid effluent from a digester is the same as the liquid volume of waste introduced into the digester. Hence there is no reduction in the liquid volume of waste to be handled due to the action of the digester. A farm methane digester does reduce the number of solids to be handled and provides a relatively odour-free treatment.

Digester effluent is not suitable for discharge into streams. There usually are less expensive systems that conform with the pollution regulations set forth by the Missouri clean water commission.

Capital Costs for Farm Methane Digesters

The capital cost for the locally centralized farm methane digester system specified by carb is about $4. 8 million for a typical farm. Capital cost is the total of the one-time expenses when the project is initiated, which include collective costs for building the digester itself, pipeline construction, manure transportation equipment and interconnection (costs to connect to and inject renewable natural gas into the main utility pipeline).

Capital cost also includes the investments that each farm must make to convert to a dry-scrape system that will allow dry manure to be collected and transported to the central location. (this article evaluates the accounting and financial data that carb presents for a system of this kind.

Converting gas energy to other forms

The energy of the produced gas can also be saved by converting it to other forms of energy. Electricity seems to be the less efficient alternative when compared to using biomethane direct but has been used in many systems.

Another approach is to store the energy as heated water. This can be as hot water for domestic use, for space heating, or for producing a temperature increase in the digester itself. The heat energy contained in 1 ft3 (0. 03 m3) of biogas is sufficient to increase the temperature of 1 gal (3. 8 litres) of water to approximately 46°f or 25°c. Converting the methane into methanol is not feasible on the average farm, as it involves a catalytic process requiring high pressure and high temperature.

Processed manure as fertilizer

The nitrogen content of the farm methane digester effluent is essentially the same as that of the input material; its fertilizer value is quite comparable to that of unprocessed manure. However, part of the organic nitrogen in the manure has been converted to ammonia forms that are more readily available to plants but also more susceptible to losses through volatilization.

Loss of nitrogen from the farm methane digester effluent may be reduced by the addition of small amounts of phosphoric acid to the stored effluent (Bartlett et al. 1978). Digester effluent is partly stabilized so that it does not have an offensive odour and does not attract pests, such as flies.

Dairy Digester Research & Development Program

In California, the state government wants to reduce greenhouse gas emissions levels by 80% below 1990 levels by 2030. This is where anaerobic digesters on dairies come into play. The technology helps change how manure and urine are managed to decrease greenhouse gas emissions.

Overall, efforts are going well. According to Dairy Cares, there are now a total of 185 farm methane digesters servicing 194 dairies across all of California – including functioning digesters and those under construction. The investment in dairy Farm Methane Digesters started more heavily in 2015, spearheaded by the dairy digester research and development program (DDRDP) from the California department of food & agriculture.

Calbiogas Kern County Biogas Cluster

The Calbiogas Kern County Biogas Cluster – or kern cluster – was developed by a joint venture between California Bioenergy, and Chevron U.S.A. Incorporated, and several California dairy farmers.

The dairy digester research and development program has awarded grants totalling over $17. 6 million from California climate investments to the dairy operations in the kern cluster, which is comprised of eight family‑owned dairy farms. Thanks to these funds, these dairies now capture and prevent the release of methane – a greenhouse gas 25 to 80 times more potent than carbon dioxide – and are creating a renewable source of fuel.

A dairy digester word of caution (Opinion)

I know some of the deals being offered are tempting producers to strongly consider placing a farm methane digester on their farms.

But remember that most of the nutrients will still be in the remaining waste product, just at a more concentrated level than before. Nationally, the connection of a digester on a dairy farm has been sold as our best technology.

But truthfully, we are still considering the same technology that has been used for decades with little advancement other than better materials to build each of the units.

Even though this process has not improved, we are still selling this as our best effort to become “greener” in our dairy waste processing.

California's cap-and-trade and low carbon fuel standard (LCFS)

The financial incentives and energy market opportunities created by California's cap-and-trade and low carbon fuel standard (LCFS) programs are the main sources of revenue for dairy digester projects. The risk of policy change to the cap-and-trade program and the LCFS is low in California, but the risk may be higher for projects outside of California trying to capitalize on credits.

A key driver behind the growth of manure/ farm methane digesters has been consumers’ increasing concern about GHG emissions. They are putting that concern into action by demanding products have smaller carbon footprints, the pressure is being felt by state governments, retailers and dairy supply chains.

In turn, California has implemented two measures beyond the grant program that are driving farm methane digester development: the cap-and-trade program and the low carbon fuel standard (LCFS). These programs are now the main sources of revenue for dairy digester projects.

Farm Methane Digester Feedstocks

The feedstocks used for methane gas from cows used for energy include waste whey from milk product manufacture, and crop residues left over after the food has been harvested from it.

Watch the video below, which we made about this subject. For a change, we used an animation format. Please comment on this new cartoon style. Did it improve your experience or not? Thanks for commenting!

— Video Not Currently Available —

Find out more at the Sustainable Conservation website here.

Coming Soon: Watch this (Cow Power!) video on YouTube here.

[Page first published September 2018. Updated with new article August 2022.]