Methane Production from Manure Digestion

By John Hibma

The hydrocarbon methane (CH4) has the dubious distinction of being both a readily available source of energy and a potent greenhouse gas. Methane is highly volatile and poisonous, and it is the smallest organic compound created from the decomposition of organic matter in an anaerobic environment. Virtually all carbon-based material (e.g., plants, leaves, household garbage, sewage and manure) can be a source of methane when conditions are correct.

Louis Aragi Jr. stands in front of the Guascor engine that powers the methane digester.
Photos by John Hibma.

Manure from dairy cows will, when anaerobic conditions exist, produce significant amounts of methane. Herein lies one of environmentalists' largest criticisms of the dairy industry - and all animal agriculture for that matter. The resulting methane generated from manure is a significant contributor to greenhouse gas (GHG) emissions.

Methane is reported to be more than 20 times more powerful than carbon dioxide in its ability to absorb and trap heat in the earth's atmosphere, thereby making it a significant contributor to atmospheric warming. However, when methane is burned, it matches the energy output of coal and oil.

The purpose of this article will not be to take part in the controversy of how much dairy farms may be contributing to GHG and global warming. Rather, it will look at the potential value of manure and the methane generated by it as an untapped revenue stream for commercial dairy farms.

For large dairy herds numbering in the hundreds and even thousands of cows, management and disposal of waste is now well established on the radar screen of the EPA as more attention is turned to the growing concern and prevalence of GHG emissions. Nutrient management of nitrogen and phosphorus - almost all of that coming from manure as well - is already being regulated.

With the capture of methane on dairy farms, the dairy industry can continue to present itself as being environmentally responsible and part of the solution instead of part of the problem. Methane capture also offers the potential to bring in additional revenue as a renewable energy source for every dairy farm.

Historically, animal manure has been valued and regarded mostly for its fertilization attributes; it adds nitrogen to the soil for plant growth and can be used as a soil amendment to improve soil structure. Some cultures regard manure for its therapeutic properties.

Disposing of manure has always been a cumbersome and undesirable part of animal agriculture. As dairy farms continue to get larger, manure management has become integrated into the total management program. Dairy facilities must be designed not only for milk production, cow comfort, cow movement and cow feeding, but also for manure removal, storage and management due to the volume that's generated each day.

The digestion process creates these partially dried manure solids.
Today, most large dairy farms handle manure in a semiliquid form, or slurry. In freestall barns it's mechanically scraped or, when water is available, flushed to holding tanks or storage ponds (lagoons). Naturally occurring bacteria almost immediately begin the process of decomposition in either an aerobic or anaerobic environment. Due to space considerations on most dairies, much of the decomposition of manure occurs anaerobically, resulting in the generation of methane. In ponds and storage tanks, methane bubbles to the surface and escapes into the air.

Even though the volatility of methane has long been known, and there have been enterprising farmers making use of its energy potential, the dairy industry as a whole has been slow to tap the energy potential of methane, mostly due to the costs associated with building the systems, the management involved in operating them, and the relatively low cost of conventional energy sources.

However, as environmental concerns and regulations increase and the costs of complying with those regulations add more expense to the construction and operation of a dairy farm, dairy farmers are now taking a closer look at manure for its potential as a revenue source that will help offset operating costs. No longer does it need to be regarded merely as fertilizer to be spread on the cornfields every year. Methane from manure has the potential to generate energy, reducing reliance on the traditional energy grid. And when the leftover manure solids are dried, it can be used as cow bedding in freestalls.

In Sheffield, Mass., Louis "Chico" Aragi Sr. and his son, Louis Jr., milk about 470 cows at their Pine Island Farm. Like so many dairies in the Northeast, the Aragis have had their share of struggles trying to stay profitable. At the dawn of the 21st century, milking even more cows than they do today, the Aragis were going backward as debt was piling up.

A dairy this size produces many thousands of pounds of manure every day from the milk cows and the heifer herd. In 2005, knowing that he wanted to keep the dairy going, Louis Aragi Jr. began searching for alternative ways for the dairy to generate more revenue, including the possibility of constructing a methane generation system.

Manure is pushed to this catch basin by the freestalls three times a day. It's then pumped underground to the storage tank.

He found that there was state and federal money available through grants for renewable energy systems and went to work designing and developing a manure digester. It took a lot of work, time and patience applying for the various grants and loans and getting it all set up. Permits had to be applied for, and arrangements had to be made with the Massachusetts power utility, National Grid, to purchase the power that would be generated from the dairy. Various EPA regulations also had to be satisfied.

Methane Generation at Pine Island Farm, Sheffield, Mass.

  • Manure is collected into a receiving pit from freestalls and other areas of the dairy three times per day.
  • Additional effluent is added from squeezed manure solids to keep manure adequately liquefied; additional enzymes are then added to aid the methane-generating process when the slurry is in the digester.
  • Slurry is pumped from the receiving pit through a grinder, making sure all contaminants, such as rocks, pebbles, wood scraps, etc., are completely pulverized before being pumped into a 4.1-million-gallon storage tank.
  • Slurry is pumped into an anaerobic (no oxygen) digester tank with a capacity of 650,000 gallons, where methane is produced by anaerobic bacteria action on the organic material.
  • Digester contains heating pipes that have warm water circulating to maintain a consistent temperature of 99 to 101 degrees Fahrenheit, which is the optimal temperature for methane production. Heat for the water is supplied by the engine.
  • Methane drawn off the top of the slurry is piped from the digester to the generating shed, where it is routed through filters to reduce contaminants, including hydrogen sulfide gas.
  • Methane is cooled to remove water vapor before it is fed to the generator engine.
  • Six-cylinder Guascor engine runs at a constant 1,200 RPM and turns a generator that produces a maximum of 225 kilowatts of electricity that is metered by the electric utility and fed directly into the local utility's power grid.
  • Digested manure solids are fed to a press, where effluent is drained and returned to a receiving pit at the freestall barn.
  • Some of the solids are collected and used for freestall bedding.
  • Waste heat is utilized to heat water that will be used to mix milk replacer for the calves.
  • Future plans are to use the waste heat to provide heat to houses on the dairy property.
  • Waste heat will also be used in an absorption refrigeration system that will help cool milk produced at the dairy.

The system went up in stages over several years. All told, the digestion and storage system cost about $2.2 million. During this time, the Aragis had to reduce the herd size to help pay down debt. About half of the cost of the digester and power generator was paid for by grants. A 4.1-million-gallon manure storage tank was paid for with other funding. Pine Island Farm has been generating electricity from manure digestion since November 2011.

A six-cylinder engine, from the Italian company Guascor, consumes the methane produced in an anaerobic digester that holds about 650,000 gallons of manure slurry. The engine is designed to run on methane. The generator connected to the engine has a maximum output of 225 kilowatts and can run continuously around the clock, feeding electricity directly into the utility system's grid.

The 4.1-million-gallon manure storage tank is on the right; the 650,000-gallon manure digester is in the background. The tall stack beyond the digester is a methane burner that will ignite like a safety valve when there's too much methane pressure in the digester.

Because of the complexities of power generation and usage, none of the electricity produced on the dairy is consumed on the dairy. The dairy's daily power comes from the grid, and when power goes down, the Aragis must hook up a tractor to run a generator off the PTO until the power comes back on.

A dairy the size of Pine Island uses about 45 to 50 kilowatt-hours on average for the milking barn, refrigeration, lights, shops, houses, pumps, etc. The generator, when it's running at maximum output, generates over four times more electricity than the farm uses and is metered as it goes into the grid. According to Aragi, the system has a return-on-investment period of about five years.

In order for all of this to happen, Massachusetts law and National Grid stipulate that in order for a company or individual to generate a "renewable form of energy" and feed it into the grid, they must locate and partner with a third party who agrees to purchase the electricity that's produced. The Aragis partnered with the Red Lion Inn in nearby Stockbridge, Mass., which purchases all the excess or net electricity the dairy produces. The process is called "net metering," and the Aragis are paid by Red Lion Inn for the electricity produced at the dairy, minus the dairy's usage from the grid to operate the dairy.

There are additional benefits coming from the digestion system. Used manure solids coming out of the digester have the water pressed out of them and are transported via conveyor to a storage area. Those partially dried solids are used for bedding in the freestalls, replacing sand that had to be purchased and trucked in.

The manure digestion process, which must occur at about 100 degrees Fahrenheit (37 degrees Celsius), provides adequate heat over a long enough period of time to kill harmful pathogens, making this material safe for bedding. Aragi initially had some trepidation regarding possible mastitis issues and high somatic cell counts (SCC) using the semi-moist manure, but the opposite occurred. He explained that before the manure solids were used in the freestalls, the milk SCC was in the 300,000 range. It now stays in the 200,000 range.

The Guascor engine produces a great deal of heat, which also has potential value. Aragi has installed a small stainless steel tank storing heated water, held at 122 degrees Fahrenheit (50 degrees Celsius), which is used to mix with milk replacer powder for the calves. He has also installed heat exchangers that will eventually heat the homes on the property, eliminating the need for the heating oil that is currently used.

Aragi is also looking into absorption refrigeration technology to cool the milk in the bulk tank. Once again, the heat from the generator engine is used to heat water that is then used to evaporate refrigeration coolant. Reducing the need for compressors running on grid electricity will decrease the amount of electricity used at the dairy, sending that much more salable electricity back into the grid.

With a properly designed, sized and constructed digester, any dairy farm produces enough manure to be used to generate more than enough electricity to power their operation. Obviously, economy of scales can shorten the payback time of a project.

It's also important that dairy farmers understand the environmental concerns related to methane combustion - it's not a perfect system. Byproducts of methane combustion are carbon dioxide (CO2) and water (H2O), along with the heat-energy (kilocalories) that's produced. Critics of methane combustion will point out that there are better ways to handle manure on a dairy (i.e., composting).

During methane combustion, one GHG is being traded for another. For every pound of methane burned, about 2.75 pounds of CO2 are produced, so the question then arises as to whether manure digestion and methane production is a responsible way to manage manure. A logical answer lies in the fact that, even though over twice as much GHG, by weight, is produced from the CO2, we are entirely eliminating the presence of another GHG (methane) that's over 20 times more potent a contributor to global warming. Ideally, the CO2 should be captured as well. When methane is consumed, it's entirely eliminated, and in the case of a manure digestion system on a dairy farm, additional economic value is realized from a waste product.

Aragi agrees that his dairy business must be environmentally responsible, but he admits that he invested in a manure digestion system mainly to generate another stream of revenue that would offset costs for his struggling dairy farm.

"I didn't have a dream about saving the world, but I did have a dream about saving the farm," he said.

Dairy farms everywhere struggle to remain profitable in an economic climate of mediocre milk prices and continually rising production costs. Dairies are also being continually confronted with new costs associated with environmental regulations and waste and nutrient management. While dairy farmers don't necessarily have to become radical environmental activists, they do need to recognize that their dairies are being eyed more critically from an environmental standpoint. While there's no question that installing a manure digestion system constitutes a large expense for any dairy farm, manure digestion offers a means to be environmentally proactive while also enabling dairy farms to remain profitable.

The author is a dairy nutritional consultant and works for Central Connecticut Cooperative Farmers Association in Manchester, Conn.