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

Abstract The pre-treatment of microalgae cell walls is known to be a key factor to enhance methane (CH 4 ) 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 CH 4 yields of cooled (4 °C)  A. obliquus biomass were achieved through ultrasonication (+53% CH 4 ) and wet-milling (+51% CH 4 ). 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 CH 4 yields for A. obliquus biomass (323 dm 3  kg −1  CH 4 yield calculated at standard gas conditions of 273 K, and 101.5 kPa per unit volatile solids, +41% CH 4 ), and C. emersonii biomass (308 dm 3  kg −1 ; +35% CH 4 ). 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 CH 4 yields were achieved with untreated, freeze-thawed C. vulgaris biomass (406 dm 3  kg −1 ); pre-treatment did not enhance CH 4 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.