A covered lagoon digester is a large anaerobic lagoon (not a manure storage pond or basin) with a long retention time and a high dilution factor. Typically covered lagoons are used with flush manure management systems that discharge manure at 0.5 to 2 percent solids. The in-ground, earth or lined lagoon is covered with a flexible or floating gas tight cover. They are not heated and considered ambient temperature digesters. Retention time is usually 30-45 days or longer depending on lagoon size. In climates that have elevated year round temperatures, such as southern and western U.S., these digesters can produce stable, reduced odor, nutrient rich effluent for application on fields and crops; pathogen and weed seed reduction and; produce biogas for farm energy use. Heat recovery from the biogas can be used to heat nurseries on swine farms and warm milking parlors on dairy farms. Very large lagoons in hot climates may produce sufficient quantity, quality and consistency of gas to justify use in an engine generator. In areas with cooler climates, waste digestion, odor control and gas production will be less consistent and the low quality gas may need to be flared off much of the year for odor control and greenhouse gas reduction.
Biogas
from any size or type of digester is flammable, explosive and corrosive.
Example
The covered lagoon at Royal Farms in Tulare, CA is a farrow-to-finish
swine operation and has been in operation since 1982. The farm has
a water flush manure management
system with a three cell lagoon. In the beginning only part of the first
cell was covered, but in 1987 the entire first cell was covered. The
covered lagoon
has operated to provide odor reduction, reduce pathogens, produce biogas
and provide a stabilized nutrient rich effluent. The biogas produced
on Royal Farms
is used to make electricity to off-set power costs and any excess is sold.
Heat is also recovered from the engine generator and used to supplement
the heat in
the nursery barns. Some of the effluent from the third lagoon is recycled
for use as part of the barn flush water with the remainder irrigated
on corn, wheat
and silage crops.
For more information on the covered lagoon digester system on Royal Farms please see: Methane Recovery From Animal Manures: The Current Opportunities Casebook, Case Study 4.7, Royal Farms pg. 4-23