Solar energy heating of dairy-manure anaerobic digesters

Abstract A 110-litre dairy-manure anaerobic digester was operated for 200 days using solar energy as the major heating source. Theoretical considerations indicate that up to 90% of the heat required for large-scale digester operation is used to raise the temperature of the influent to digester operating temperature (35°C), the remaining 10% being utilised for temperature maintenance. Since digesters require a constant temperature for satisfactory bacterial performance and since the magnitude of solar heating varies throughout the day, only the influent was solar heated in this experiment. A minimal amount of supplemental electric heating was used to maintain the constant temperature of the fully insulated digester. The quantity of dairy-manure slurry fed to the digester on alternate days established a 20-day retention time and a loading rate of 4·4 kg VS m−3 day−1. Two solar collectors, breadbox and solar pond, were tested for influent heating capability. Although the breadbox type was the more efficient of the two, the solar pond type appears more suitable for farm use.