Acutodesmus obliquus as a benchmark strain for evaluating methane production from microalgae: Influence of different storage and pretreatment methods on biogas yield

Acutodesmus obliquus (SAG 276-1), a microalga with a resistant cell wall, was chosen as a benchmark strain for testing the effect of storage and pretreatment methods on methane yields during anaerobic digestion. A. obliquus was cultivated in sleevebag photobioreactors (batch cultivation). Biomass was centrifuged to a final total solid concentration of 91gL-1 and subjected to different treatments. The biomass was then subjected to biochemical methane potential tests. Among the different storage methods tested, freezing and freeze-drying resulted in significantly higher CH4 yields, i.e., 350 Nm3t-1 VSharvest (=normalized gas volume in m3 corrected to norm temperature and pressure per unit harvested volatile solids) (+53% CH4) and 291Nm3t-1 VSharvest (+29% CH4) compared to the fresh biomass (229 Nm3t-1 VSharvest). For pretreatments, the combined milling and enzyme approach (300 Nm3t-1 VSharvest, +57% CH4), ultrasonication (292 Nm3t-1 VSharvest, +52% CH4) and milling (289 Nm3t-1 VSharvest, +51% CH4) resulted in significantly higher CH4 conversion compared to the untreated control (191Nm3t-1 VSharvest). We found a significantly strong positive correlation between viable cell counts and CH4 yields, and a correlation with the solubilized chemical oxygen demand. Our study revealed that mechanical pretreatments were effective and resulted in highest CH4 yields. In addition, the effect of different storage methods must not be neglected.Austrian COMET program under grant number 834018. Republic of Austria and the Federal Provinces of Styria, Lower Austria and Burgenland. Co-funding from the industry partner Erber Group shall be highly acknowledged.Scopu

Associated effects of storage and mechanical pre-treatments of microalgae biomass on biomethane yields in anaerobic digestion

The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH4) yields during anaerobic digestion. This study investigated the combined effects of two different biomass storage methods and physical pre-treatments on the anaerobic digestion for three different microalgae species. Acutodesmus obliquus, Chlorella vulgaris and Chlorella emersonii were cultivated in 80 L sleevebag photobioreactors (batch mode), and then subjected to different storage (cooling and freezing) and pre-treatment methods prior to anaerobic digestion using the biochemical methane potential (BMP) test. A. obliquus was selected to evaluate pre-treatment methods for further experimentation. Significantly higher CH4 yields of cooled (4 °C) A. obliquus biomass were achieved through ultrasonication (+53% CH4) and wet-milling (+51% CH4). These methods were then applied in follow-up experiments to cooled (4 °C) biomass of C. emersonii and A. obliquus. Ultrasonication again led to significantly higher CH4 yields for A. obliquus biomass (323 dm3 kg−1 CH4 yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH4), and C. emersonii biomass (308 dm3 kg−1; +35% CH4). In a third experiment series, frozen A. obliquus and C. vulgaris biomass were thawed prior to pre-treatment and BMP-testing. Among all BMP tests, the highest CH4 yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm3 kg−1); pre-treatment did not enhance CH4 yields for C. vulgaris, but for A. obliquus (ultrasonication +20%). Pre-treatment was more effective for cooled than freeze-thawed microalgal biomass and combined effects acted strain dependently.This work was performed in the frame of the Austrian COMET program under grant number 834018. It has received funding from the Republic of Austria and the Federal Provinces of Styria, Lower Austria and Burgenland. Co-funding from the industry partner Erber Group shall be highly acknowledged