Anaerobic Digesters

anaerobic digesters

Anaerobic digesters are an emerging technology on modern farms.

As I discussed on another page ( Methane ) decomposition of manure leads to the release of methane. This is both a waste of a potential fuel but also a threat to the environment since methane is a potent greenhouse gas.

Anaerobic digesters resolve many of these issues.

They speed the processing of manure and as they do they collect the released biogas to be used as fuel while turning raw manure into an environmentally friendly end product.

The benefits of using anaerobic digesters include:

Reducing odor

Preventing release of green house gases

Providing quality compost than can be sold or used on the farm

Protecting ground water

Killing weed seed in the manure

Reducing disease-causing bacteria in the manure

Producing energy

There are three main types of digesters, each with its owm pros and cons.

The remainder of this page discusses each.

Covered Pond

This digester is basically a holding pond with a cover over it. Foam tubes under the cover hold it afloat. The methane is collected by pipes running under the cover.

Some farms use this system just to control odors, but it is possible to use the methane to produce heat or electricity.

These digesters are simple, low maintenance and relatively inexpensive.

On the down side, they can only be used in relatively warm climates because the bacterial action slows too much in cold weather. Also, they pose a risk or water contamination if ground water levels in the area are high.

Complete Mix Digesters

As the name implies, these digesters continually mix the manure slurry to keep the solids in suspension. They need to be warmed to work properly and different designs operate at different temperatures.

The digester is a sealed, above-ground circular tank with an agitator paddle. The biogas collects under the cover and is available for use.

The structures with green roofs in the picture at the top of the page are direct mix anaerobic digesters.

In a typical operation, the biogas is used to run a generator that provides electricity for the farm. In many situations, the farm can completely meet its electricity needs from its own generator and have some to sell to the electric grid.

To make the operation even more efficient, waste heat from the operation of the generator is collected and used to warm the digester.

These digesters are most suitable where the manure mixture has a relatively low solid content (3-10%). Operations that use water to flush away manure often have this type of waste.

Complete mix digesters are expensive, more complex to operate and more subject to difficulties. For example, the temperature of the mix needs to be monitored carefully and kept in a specific range.

Plug Flow Digesters

These digesters are usually long in-ground troughs made of concrete with an air-tight plastic cover. The farmers add manure on one end. It moves through the digester as a “plug” and emerges as compost at the other end. Along the way, methane collects under the cover and is used to generate electricity and heat as I described above.

These digesters work best with manure with a higher concentration of solids. They need to be warmed, but the temperature range is less critical than with complete mix digesters. Pipes running the length of the trough carry water heated by the generator running to keep the temperature at around 100 degrees.

The final product of all these systems is treated liquid and solid. Both are essentially odorless. The liquid can be spread on the farm by either manure sprayers of irrigation systems.

The compost can be spread as well, but often the land doesn’t require that level of nutrients. In that case, the compost is a salable product.

Practical Issues

Anaerobic digesters require a significant investment by the farmer, but can make economic sense for medium to large size farms. The reliability and support for digesters has improved greatly over the last 30 years and the systems are much more practical now.

If you’re interested in how this plays out in the real world Cornell University has some interesting case studies at: www.manuremanagement.cornell.edu For example, under “Lessons Learned” at the end of one case study was the fact that a generator running in a metal pole barn can be quite noisy. The farmer said that the neighbors who used to complain about the smell of the manure now complained about the noise. Making allowances for sound proofing might be a good idea.

Finally, farmers don’t always need to have their own digesters. In parts of Europe, central manure processing plants handle waste from surrounding farms. This adds the need to transport the manure to the center but allows smaller farms to handle manure appropriately.

Anaerobic digesters are not going to be a major source of energy, but every bit produced means less that needs to come from elsewhere. And there’s the very significant advantage of keeping that methane out of the atmosphere.

Biomass Energy Links

Click here to go to Biomass Energy Click here to go to Methane