Anaerobic digestion of high ash, open lot beef cattle manure

Abstract Text: Nine, 56.8-L anaerobic digesters were utilized to study biogas generation from beef cattle manure with varying OM content.  Manure was collected from the settling basin of a finishing cattle barn with a sloped floor and flush system.  Treatments consisted of pure manure (65% OM) or manure with soil (90% DM, 97% ash) added to create 40% and 15% OM manure.  For all treatments, water was added to the manure-soil mixture to equal 9% DM when fed to digesters.  All digesters received the same amount of OM each day (i.e. varying amount of soil and constant amount of manure).  Digesters were on each treatment for a 41-d acclimation period after which measurements were taken for 5 d; 3 periods allowed all treatments to be evaluated on all digesters.  Measurements included weight, DM, and OM of all material fed into and removed from each digester in order to calculate OM degradation of manure within digesters.  A cone-bottom tank and intermittent mixing was used to aid in separating organic and inorganic particles within digesters and increase daily ash removal within the effluent stream.  Concentration of methane within a known flow of N₂ gas was measured and total methane production was calculated.  Increased ash contamination of manure linearly decreased OM degradation from 63.2 to 54.1% for the 65 and 15% OM treatments, respectively (linear P = 0.02).  The high level of ash contamination also decreased daily methane production from 0.589 to 0.425 L CH₄ per L digester volume daily for the 65 and 15% OM treatments, respectively  (linear P < 0.01).  This is equal to 0.187 and 0.139 L CH₄ per g of OM fed (linear P = 0.02) for the 65 and 15% OM treatments, respectively.  The 40% OM treatment was intermediate for both L CH₄ per L digester volume daily and L CH₄ per g of OM fed. Of ash added to digesters, 20.5, 18.3, and 9.5% was not removed from the 65, 40, and 15% OM treatments, respectively (linear P = 0.12).  This resulted in ash buildup (mineral or inorganic material that was added to the digester, but not removed in the effluent and not degraded within the digester) of 17.0, 45.5, and 64.7 g/d, respectively, as % OM in the manure decreased (linear P< 0.01).  With adequate daily cleanout of ash from digesters, lower OM manure can be used for anaerobic digestion, although methane yields may be slightly reduced.

Keywords: anaerobic digestion, beef cattle, methane