2.1. Microalgal strains and culture conditions
Three strains of Chlorophyceae known for their productivity and their high biogas tolerance were selected as candidates for sustainable energy production and nutrient removal. All strains were obtained from stock cultures in Dr. Yuhong Liu’s laboratory and confirmed as highly biogas tolerant and fast growing (Li, 2012 and Li et al., 2013).
The strains were preserved in BG11 medium (Rippka et al., 1979). The culture conditions were as follows: cool-white LED light with a photosynthetic photon flux density (PPFD) of 100 μmol m−2 s−1, temperature of 25 ± 0.5 °C, and light–dark AG 555 of 12 h light:12 h dark; and artificial intermittent shaking thrice a day (8:00 AM, 2:00 PM, and 8:00 PM).
2.2. Growth medium
The growth medium was obtained from an anaerobic digester in Jiaxing, Zhejiang Province, China. The biogas was pretreated via chemical absorption to decrease the H2S concentration to <50 ppm (v/v) (Chung et al., 2006). The biogas slurry was treated using an ultraviolet sterilizer (KCJ-10 W, Konche Water Treatment Co., Ltd., PR China) for 2 min and then filtered using a glass microfiber filter (GF/C, Whatman, USA) to prevent interference from other sediments and microorganisms. Table 1 lists the characteristics of biogas slurry and crude biogas before and after pretreatments.