ANAEROBIC DIGESTION OF COW MANURE: INFLUENCE OF HYDRAULIC RETENTION TIME ON BIOGAS YIELD AND PROPERTIES OF DIGESTATE
This research paper assessed the influence of hydraulic retention (HRT) time on biogas yield and the properties of digestate during anaerobic digestion. Anaerobic digestion of cow manure was simultaneously digested in a custom built 20L digester at different HRT’s of 10, 18, 28, 38, 50 and 60day. The study showed HRT had substantial influence on biogas yield and on the properties of digestate. An average biogas yield of 1.6m3/KgVSadded to 3m3/KgVSadded was obtained. The elemental content of carbon, hydrogen, and nitrogen of the resultant digestate reduced from 35.6w/w% to 19.9w/w%, 6.3w/w% to 2.8w/w%, and 2.1w/w% to 0.8w/w%, respectively, after a 60day HRT. Consequently, the higher heating values reduced from 15.1MJ/kg to 3.5MJ/kg. Also, the carbon-to nitrogen ratio was between 17.9 and 24.8 when compared to that of cow manure, while the hydrogen-to carbon ratio was in the range of 1.4 to 1.8. The digestate could be used as a fertilizer, alternatively as a precursor for biofuel production.