Semicontinuous outdoor cultivation and efficient harvesting of marine Chlorella vulgaris BDUG 91771 with minimum solid co-precipitation and high floc recovery for biodiesel

The present study demonstrated contamination free, semicontinuous mode cultivation of marine Chlorella vulgaris BDUG 91771 at 5 kL open pond. Culture cultivated in seawater supplemented with inexpensive urea and superphosphate exhibited specific growth rate, biomass productivity, lipid content, lipid productivity, and doubling time of 0.703 d−1, 0.056 g L−1 d−1, 20%, 0.012 g L−1 d−1 and 37-40 h respectively at 30 °C average system temperature and 30o/00 salinity. With an aim to develop an appropriate harvesting technique based on maximal floc recovery and less solid co-precipitation, various chemical, natural, electroflocculation methods were investigated. Of the flocculants, 70 mg L−1 aluminium sulfate, 20 mg L−1 polymer flocculant, Cu electrode mediated electroflocculation showed ∼98% floc recovery at 120 min flocculation time whereas natural flocculant Moringa seed powder even at a high dose of 700 mg L−1 divulged low floc recovery (63.06%). Notably, polymer flocculant exhibited the minimum co-precipitation of solid and high biomass content of 13 mg L−1 and 147 mg L−1 respectively compared to other flocculants, and it also had stable, intact viable cells in flocs reflected through dye exclusion staining. This study further stated that the critical elements namely Ca2+ (1.4 mM), and Mg2+ (0.27 mM) in the marine medium are not required for polymer based flocculation and were essential for chemical based flocculation. Eventually, the fatty acid pool of laboratory and outdoor grown biomass reflected the same pattern and possess C16:0 C16:1, C18:1 for high-quality biodiesel.