Both the digester and the mixing pit are heated with waste heat from the engine cooling system. Larger volumes are usually handled by multiple digesters. These plug-flow digesters were adopted with some success in the cooler climate of the Northeast, where dairy farms primarily use scraping systems for manure removal. The 1979 project at the Mason Dixon Dairy Farms in Gettysburg, Pennsylvania, was the first plug-flow digester operated on a commercial farm. At the Mason Dixon project, the plug-flow digester was originally sized for a manure volume produced by a milking herd of 600 Holstein cows.
Manure is collected daily and added to one end of the trough.
Each day a new plug of manure is added, slowly pushing the other manure down the trough. The size of the plug-flow system is determined by the size of the daily plug. As the manure progresses through the trough, it decomposes and produces methane that is trapped in the expandable cover. To protect the flexible cover and maintain optimal temperatures, some plug-flow digesters are enclosed in simple greenhouses or insulated with a fiberglass blanket. Many systems use a mixing pit with a capacity roughly equal to 1 day s manure output to store manure before adding it to the digester.