Environmental Issues

Home

Archives

Article in Print

Animal Agriculture and the Environment
Your Top 5 Environmental Questions Answered

November 2004 - by Katharine Knowlton, Ph.D., Department of Dairy Science, Virginia Tech, Blacksburg

As producers feel increasing pressure to obtain a permit, develop and implement nutrient management plans, or participate in TMDL planning, there are several questions they always ask. Periodically, I’ll pull the “top 5” of these questions, and dig up answers to them for this column. Recently there’s been increasing attention to alternative methods of waste treatment, especially methods that convert manure to energy. This month’s “top 5 questions” focus on these.

1. How can energy be generated from dairy manure?

Dairy waste has a high moisture content (80-90 percent) which limits the available technologies compared to a drier material like poultry litter. Generally, waste can be converted into energy with either thermal or biological conversion.

Thermal conversion techniques include direct combustion (burning) of waste to produce energy, pyrolysis (thermal treatment in the absence of oxygen, resulting in the production of pyrolysis oil or “biodiesel”), gasification (similar to pyrolysis but at higher temperatures), and thermal hydrogen production. Generally, thermal technologies are not suitable for dairy waste because of the high energy cost required to dry the manure to acceptable moisture levels.

Biological methods are most suitable for energy production from dairy waste; these lead to the production of methane, ethanol, butanol, and hydrogen. The last three have promise for the future, but anaerobic digesters to produce methane are by far the most tested and currently feasible of these technologies.

2. How do anaerobic digesters/methane generators work?

Digesting liquid manure anaerobically converts most of the soluble fractions into a mixture of gasses including methane (60-70 percent), carbon dioxide (30-40 percent), and small quantities of hydrogen sulfide, ammonia, hydrogen and other compounds.

In 2001, there were 32 anaerobic digestion plants operating in the US for methane production; 14 of these used dairy waste. This number has certainly increased in the past few years. With high energy costs and an increased attention to the need for “green” energy sources, interest in methane generation has grown exponentially.

Four types of digester designs are currently used for methane production; the two most suitable for dairy manure are “complete mix” digesters, suitable for waste containing 2 to 10 percent solids (i.e., flushed or separated liquid dairy manure), and “plug-flow” digesters, suitable for scraped dairy manure with 11 to 14 percent solids. Research is underway on other systems that alter these basic designs to maximize methane production.

3. Are anaerobic digesters/methane generators economical?

Good question. The answer, as usual, is “it depends.” Economic studies on anaerobic digestion plants in Minnesota and Wisconsin indicate that a minimum of 400 cows are needed to make on-farm methane production economically viable.

The heating value of biogas is 600-800 Btu/ft3, and on average, ten dairy cows are required to power a 1kW-generator. A 1,000 cow herd would produce about 60,000 -100,000 ft3 biogas daily and support 100 kW generating capacity. The estimated cost of a digester plus power generation unit for a 1,000-cow dairy is $250,000, but there’s no such thing as a “standard” methane digester.

Some producers with on-farm digesters have established arrangements with the local power company to sell excess electricity generated. In most cases, however, the small amounts of energy involved make the economics unfeasible for the power company, and producers simply use the energy they generate to reduce their own power bill. Other benefits to the producer are reduced odor emissions and reduced emission of greenhouse gases.

4. These systems are so expensive and fairly complicated – do central digestion facilities serving many farms work? Is there government money available to support these?

While most anaerobic digestion/methane generation systems are now on-farm, there is significant interest in developing systems to serve multiple farms. There are now examples of operational central digestion facilities and several in the advanced planning stages.

One of the most well-known regional anaerobic digestion facilities is the dairy manure digester pilot project in Chino, California, owned by the Inland Empire Utilities Agency.

This plant has received substantial financial support from a variety of state and federal agencies, and processes the manure from ten dairies within four miles of the digester. The electricity produced is used to power a desalination plant and the biosolids are composted. The dairies involved use a “Honey-Vac” to collect manure daily, and deliver it to the digester.

The benefits to the dairymen are reduced odor, and reduced liability for nutrient pollution. Most important, these dairymen don’t have to deal with manure storage and land application. Although the revenues from the generation of energy are not returned to the producers, there’s a waiting list of dairies wanting to participate.

Three other high profile multi-farm projects are in the advanced planning stages. The Cove Area Regional Digester in Pennsylvania will process the manure from about 6,800 cows, generating methane and dried manure solids for use as a soil amendment and bedding. Also, a community-based cooperative waste treatment facility with power generation is planned in Wyoming County, New York. This project will combine manure from several thousand cows with food waste, and has received significant grant support from the state and from USDA. And in Tillamook, Oregon, a centralized methane digester is being built to process the manure from about 4,000 cows. This digester will be owned by the Port of Tillamook, and manure will be picked up by the facility. The plan is that transportation costs will be offset by sale of electricity and by sale of fiber recovered for use by a potting soil manufacturer.

The facilities differ in their business structure (some owned collectively by producers, others by state agencies or public-private partnerships) and their economic details (delivery, economic returns). Some of these facilities include components beyond the methane digester include composting facilities and fertilizer plants, while others actually return the residual manure solids to the producers.

Other factors affecting the potential profitability of the systems and the returns to producers (if any) include the extent of public financing, the proximity of farms to the central digester, and the quantity of manure processed.

5. Will an on-farm methane digester help me meet my nutrient management plan?

Generally, no. Anaerobic digestion does affect the nitrogen content of the waste, but the effect is highly variable. Most of the solids will not be converted to methane and are recovered for land application or composting. These solids will have a high potassium and phosphorus content because these nutrients are not affected by the anaerobic digestion.

6. How else can I make money with manure?

Yes, this makes six questions rather than five, but this is the one everyone asks. The short answer is that there are producers profiting in other ways from the manure generated by their cows. There are hundreds of research and demonstration projects underway on other “value-added” alternatives to traditional manure handling systems. These alternatives range from time-tested options like composting to generation of single-cell protein, fertilizer, and novelty “zoo-doo” type items. A future “top 5 questions” article will focus on these opportunities.