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.
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