Biogas production from llama and cow manure at high altitude

Methane production from llama and cow manures from the Bolivian high plateau (The “Altiplano”) was studied using a parallel reactor set-up consisting of 10 lab-scale biogasifiers. The effects of pressure (495 and 760 mmHg), temperature (11 and 35∘C), hydraulic retention time (20 and 50 days), and manure content in the slurry (10%, 20% and 50%) were evaluated with respect to productivity and methane yields based on two 24-124-1 fractional factorial designs with 8 treatments for each kind of manure. The reactors were operated semi-continuously with daily manure feeding for periods between 50 and 100 days. Temperature was the main factor effect found, and the hydraulic retention time and the manure content in feed were also found significant whereas the effect of pressure was not significant in the range studied. The methane yield obtained with cow manure at 11∘C was between 6.4 and 33.6lCH4kg-1VS (volatile solids added) whereas at 35∘C the methane yield was between 49.6 and 131.3lCH4kg-1VS. The methane yield from llama manure was somewhat lower than for cow manure (between 3.3 and 19.3lCH4kg-1VS at 11∘C and between 35.6 and 84.1lCH4kg-1VS at 35∘C, respectively). However, overall llama manure was found to be the best raw material of the two for biogas production, due to its high content of volatile solid—higher than has been previously reported for most manures—and also its high nitrogen and phosphorous content.