Conversion and use as an energy source: Anaerobic digestion

Gregory Beatty and Hank Zygmunt, Office of Wastewater Management, Environmental Protection Agency

Anaerobic digestion is the decomposition of manure in an oxygen-free (anaerobic) environment. Two types of systems are available for on-the-farm anaerobic digestion: anaerobic digesters and anaerobic lagoons.

Anaerobic digesters work in much the same way as an animal’s digestive tract; microorganisms break down or digest the manure. One of the last phases of digestion is the conversion of the manure into biogas by methane-forming bacteria. Biogas is a combination of methane, carbon dioxide, nitrogen, hydrogen, carbon monoxide, oxygen and hydrogen sulfide.

Between 55 and 70 percent of biogas is methane, and the remainder consists mostly of carbon dioxide. Usually, the nitrogen, hydrogen, carbon monoxide, oxygen and hydrogen sulfide are found in trace amounts. The methane in biogas is similar to natural gas, and after scrubbing it can be used to fuel internal combustion engines that run generators and produce electricity.

Anaerobic digestion is not limited to manure products and can be applied at any site that produces organic material. In this [article], however, only farm-based anaerobic digestion is addressed. The on-farm digester system typically involves an animal facility where manure is produced; a manure-handling system to transport manure to and from the digester; a reactor tank, where anaerobic digestion occurs; and an apparatus for the collection, pretreatment and use of biogas.

There are several types of digesters which are made for specific types of manure management situations. The complete-mix digester and plug-flow digester are the most common types of on-farm digesters. Other advances in this area include the loop digester and the advanced integrated pond system.

The complete-mix digester has an above-ground or below-ground tank that uses heat to treat swine or dairy manure with a 3 to 10 percent solid concentration.

The plug-flow digester uses a below-ground tank to hold manure and a gas-tight, expandable top to capture biogas. Plug-flow digesters work only with high-solid-content (greater than 11 percent), scraped dairy manure and cannot be used with other manures. Each day a new “plug” is added, which gradually pushes the old manure down the tank.

The covered lagoon digesters typically consists of a floating impermeable cover placed over the surface of a manure treatment lagoon. Covered lagoons can be used at both swine and dairy operations. They work best when the manure is handled as a liquid and the climate is temperate to warm year-round.

The loop digester uses a circular design and a fabric cover to collect the biogas. The circular design allows for convective currents to form in the digester, eliminating crusting problems common to plug-flow digesters. A loop digester can operate with 11 to 13 percent total solid slurry.

The advanced integrated pond system uses a submerged canopy to cover a facultative pond. The cover captures biogas. Effluent is discharged to a second pond to be used as a growth medium for algae.

Anaerobic lagoons function exactly like anaerobic digesters with one exception: the lagoons are usually not covered to capture gas. They are much larger than digesters – up to 100 times larger in some cases. Anaerobic lagoons reduce the volume of manure by breaking the manure down in an anaerobic environment. All types of livestock manure can be digested in an anaerobic lagoon.

Incentives for using this technology
There are five reasons why a farmer or producer might want to install an anaerobic digester:

1. Anaerobic digesters reduce unpleasant odors from waste management systems which are often offensive to surrounding communities. Digesters reduce odors because biogas-producing bacteria consume the odor-causing compounds.

2. Anaerobic digesters produce a high-quality fertilizer. During the digestion process, nitrogen is converted to ammonia which is a common component of commercial fertilizer and easily used by plants.

3. The digestion process treats the manure, making it a uniform and predictable product. Digested manure is easy to spread during land application. When it is applied properly, it can reduce the chance of contamination of surface and ground water.

4. Heated anaerobic digesters reduce the amount of pathogens in manure, thus reducing the amount of pathogens entering a storage unit or land-applied after digestion. Anaerobic lagoons also help reduce the amount of pathogens by allowing time for the pathogens to die off during the digestion process.

5. Biogas recovered from anaerobic digesters can be used to generate electricity to use on the farm or to sell to a local power grid. The biogas can be used on site to fuel a boiler, space heater or refrigeration equipment or as cooking and lighting fuel. Gas recovered from digesters is approximately 60 to 80 percent methane and has a heating value of 600 to 800 Btu per cubic foot. Most equipment that uses natural gas, butane or propane as fuel can be modified to use biogas.

Technical and social barriers
The perception among the farming community that anaerobic digesters fail is one of the largest social barriers to acceptance of the technology. Anaerobic digesters require high-level management time, and when farmers do not have the skill or time to manage the digesters, the systems tend to fail. Farmers are reluctant to use digesters because the operation and maintenance costs are too high compared to the financial returns from energy production.

Another reason for digester failure is that producers select systems that are not compatible with their type of manure handling method and the layout of their farm. Poor design and installation also contribute to the failure of digesters.

One of the largest technological problems for anaerobic digesters is sand clogging the digester. Sand bedding material is typically preferred to organic material because it prevents bacteria from growing and infecting the livestock. When farmers use sand as bedding material, their anaerobic digesters have a high probability of failing. In addition, warmer climates are better for anaerobic digesters used to produce biogas; in cooler climates, the production of biogas decreases. ANM

References omitted but are available upon request at editor@progressivedairy.com

—Excerpts from EPA report “Alternative Technologies/Uses for Manure”