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.