Biogas - Will it Provide Energy and Odor Free Manure?

Posted: April 4, 2006

Energy and odor combined with state and federal cost sharing programs have increased the interest in using controlled anaerobic digestion to stabilize dairy manure and produce biogas.

Biogas production using anaerobic (oxygen free) digestion is a biological treatment process that reduces odor, produces energy and improves the storage and handling characteristics of manure. A biogas production system must be specially designed and requires regular attention by someone familiar with the needs and operation of the digester. Biogas is a low grade form of natural gas, is flammable and explosive and must be handled accordingly!

To find out more about biogas production visit the new biogas website on the Department of Agricultural an Biological Engineering web site select extension and then click on biogas. This site has brief discussions on many topics related to biogas and also links to many other sources of technical, operational and financial information.

A constant elevated digester temperature is conducive to the optimal growth and productivity of the desired microbial workers. These workers break down the organic material and convert it to the desired biogas and low activity low-odor effluent. The most common agricultural digesters operate in the mesophilic temperature range (95°F - 105°F). Retention times for mesophilic digesters are typically 10 – 20 days. Higher temperature, thermophilic digesters (125°F - 135°F) will require more energy for heating and more careful management but may have retention times as low as 5 days. Heated digesters also have the added benefits of greater pathogen and odor reduction, higher biogas production, and
better weed seed destruction.

A well designed and operated digester will require modest daily attention and maintenance but a successful operator must understand the process. The care and feeding of a digester is not unlike feeding a cow or a pig; it responds best to consistent feeding and the appropriate environmental (temperature and anaerobic- oxygen free) conditions. The earlier a problem in operation is identified the easier it is to fix and still maintain productivity.

Common digester misconceptions include that they will reduce the quantity of manure and the amount of nutrients that remain for utilization or disposal - an anaerobic digester DOES NOT MAKE MANURE DISAPPEAR! Often the volume of material handled from a digester increases because of required dilution with water for satisfactory pumping or digester operation. On an average, only 4% of the influent (volatile solids) is converted to biogas. The remaining 96% leaves the digester as a stable, nutrient rich, reduced or pathogen and weed seed free and nearly odorless effluent. This means that a farm loading 1000 gallons per day into a digester can expect to have 960 gallons of material to store and ultimately utilize. Depending on digester design and operation, solids can also settle out in the bottom of the digester and/or form a floating scum mat. When evaluating the actual performance and operation of a digester it is important to determine and account for the amount and type of material retained in the digester and the cost of lost digester volume and ultimate cleaning.

Robert E. Graves, Agricultural and Biological Engineering Extension