MICROALGAL BIOMASS IN ANAEROBIC DIGESTION: EFFECTS OF PRETREATMENT AND DIFFERENT STORAGE METHODS

Anaerobic digestion of microalgal biomass is one option for sustainable energy production. Therefore, several microalgae i.e. Chlorella vulgaris, C. emersonii and Acutodesmus obliquus were produced in a sleevebag photobioreactor system (maximum volume 800 L) and investigated in so-called biochemical methane potential tests (BMP). Different pre-treatments were selected in order to enhance methane (CH4) yields. A. obliquus was selected to screen for effective pretreatment methods: a combined milling and enzyme approach (+57%), ultrasonication (+53%) and milling (+51%) resulted in significantly higher CH4 yields. In the follow-up experiments, milling and ultrasonication were applied to cooled biomass of C. emersonii and A. obliquus. In both cases, ultrasonication led again to significantly higher CH4 yields (+41%) for A. obliquus and (+35%) for C. emersonii. In experiments with frozen biomass, highest CH4 yields were reached with freeze-thawed C. vulgaris biomass i.e. 406 Nm (3) t(-1) VSharvest. Freeze-thawed A. obliquus biomass yielded 337 Nm (3) t(-1) VSharvest, an increase of 47% if compared to unfrozen biomass i.e. 229 Nm(3) t(-1) VSharvest. Ultrasonication of frozen A. obliquus biomass increased CH4 yields for only 20% indicating that freeze-thawing strongly affects CH4 yields and provokes cell disruption by ice crystals. In addition, our work proposes that pretreatments seem to act strain-specific.