Improving the Methane Production in the Co-Digestion of Microalgae and Cattle Manure

The objective of this thesis is to evaluate the effects from various treatments in the anaerobic digestion of cattle manure when mixed with microalgae. The analysis would focus on two primary subjects: the effects of different treatments on the microalgae sludge, and the balancing of the carbon-to-nitrogen ratio. The results of this experiment would give a viable estimate on the possible methane production from co-digestion of these resources.

At the conclusion of the experiment, it was found that biogas production increased when algae was added to the digester. The highest methane production in the control groups, containing only manure, digestion sludge, and newsprint was 48120 L, while the highest in the mixtures containing algae and pretreated algae were 71170 L and 87715 L, respectively. Based on volatile solids, the highest production in the control groups was0.36 (L CH_(4))/(g VS), while the production rates in the algae and pretreated algae mixtures were 0.22 (L CH_(4))/(g VS) and 0.44 (L CH_(4))/(g VS), respectively. This shows that the presence of algae increases the overall methane production, but is hindered by inhibitory factors contributing to ineffectiveness in the overall digestion process. The effects of carbon balancing for the carbon-to-nitrogen ratio also showed that overall, mixtures balanced at 25:1 carbon-to-nitrogen yielded more biogas. The exception is the normal algae mixture, in which the optimal ratio was 20:1. In conclusion, the anaerobic co-digestion of cattle manure with pretreated algae, when balanced for carbon and nitrogen, can severely increase methane production rates.