PUB - Publikationen an der Universität Bielefeld

Worldwide depletion of fossil fuel reserves advanced the search for environmental friendly and sustainable alternatives. The fact that microalgae perform very efficiently photosynthetic conversion of sunlight into chemical energy has moved them into the focus of regenerative fuel research, especially since algae cultivation, in contrast to land plants, is not restricted to arable land. Renewable fuel generation via anaerobic fermentation using microalgae biomass for biogas production, compared to biodiesel and bioethanol, is less intensive investigated.

This thesis provides a systematic analysis of parameters influencing the degradability of microalgae biomass in an anaerobic digestion process, with respect to algae species, biomass composition and culture conditions. The biodegradability of twenty different freshwater microalgae species possessing different cell wall characteristics, cultured under comparable conditions and harvested in the same growth phase, was observed to be relatively similar, corresponding to rather low conversion efficiencies of less than 53 % of the theoretical maximum. These findings suggested that the recalcitrance of the cell wall is not the only factor influencing anaerobic digestion, since not every algal species contains a rigid cell wall, further indicating that other parameters must influence the accessibility of algae cells towards decomposition by anaerobic microorganisms.
Naturally occurring nutrient starvation is a direct consequence of algae blooms in late summer, and therefore this natural phenomenon was simulated under controlled conditions and the impact on algae biomass degradability was investigated. Three scientifically and industrially relevant algae strains Chlamydomonas reinhardtii, Parachlorella kessleri and Scenedesmus obliquus were therefore cultured in low-nitrogen media (containing insufficient nitrogen source for extensive cell proliferation) and subjected at different growth stages to anaerobic fermentation in batch test. The results revealed a strong correlation of the cell starvation status and biodegradability to biogas, towards complete biomass disintegration at the maximum starvations level (indicated by max. C:N ratio). The feasibility of fermentation of “nitrogen starved” vs “nitrogen replete” microalgae biomass was furthermore investigated in a long term (160 days) continuous lab-scale simulation of an industrial biogas plant. The results of “nitrogen replete” biomass fermentation revealed low conversion efficiency and subsequent fermentations failure caused by high protein content in the biomass. The fermentation of “low nitrogen” biomass, on the contrary, was characterized by very stable process parameters and highly efficient biomass to methane conversion efficiency of 84 %. In comparison to “energy crops” (e.g. maize), usually used for biogas generation, the achieved methane yield was 37 % higher on biomass basis and approximately 4.5 times higher based on areal productivity (conservative estimation).

In conclusion, this PhD work provides a simple and effective microalgae cultivation method for subsequent use of biomass as mono-substrate for anaerobic fermentation to methane. Highly efficient and stable fermentation process of this biomass was demonstrated in a continuous long-term experiment within this work and enables therefore an efficient industrial scale application.