Effect of mixing duration and vacuum on methane production rate from beef cattle waste

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Abstract
The effects of mixing duration and vacuum on methane production rates from anaerobically fermented beef cattle wastes were discussed. The results showed that continuously mixed fermentors produced significantly (P < 0.05) higher methane production rates than fermentors mixed two hours per day. However, the rates from the continuously mixed fermentors were only 8–11% higher than the intermittently mixed fermentors at six and four days hydraulic retention time (HRT), respectively. There was no significant difference between the vacuum and conventional fermentors at six days HRT, but there was a significant difference at four days HRT. The CH4 production rate of the vacuum fermentors was 5% higher than the conventional fermentors at four days HRT. The results of these experiments compared well with predicted CH4 production rates. These results suggest that there is little potential for increasing the fermentation rates of livestock wastes by increased mixing or vacuum.