Fast cultivation and harvesting of oil-producing microalgae Ankistrodesmus falcatus var. acicularis fed with anaerobic digestion liquor via biogranulation in addition to nutrients removal.

Abstract This study examined the feasibility of cultivation and harvesting of oil-producing microalgae (i.e. Ankistrodesmus falcatus var. acicularis) via biogranulation in two identical sequencing batch reactors (SBRs) fed with synthetic anaerobic digestion liquor. Easily settled algae granules with compact structure appeared around day 90 and mature granules were obtained after 150 days' operation. The microalgae settleability was remarkably improved, signaling by the substantial decrease of sludge volume index (SVI30) from initially >3000 to 53.44 ± 3.31 mL/g, with settling velocity correspondingly increased from nearly 0 to 18.47 ± 0.23 m/h. Although the percentage of the target microalgae (Ankistrodesmus falcatus var. acicularis) decreased along with the granulation process, the biomass concentration (2–4 g/L) and biomass productivity (130–270 mg/L/d) using biogranulation were 10–20 times and 16–34 times that by the traditional suspension method. Compared to the seed microalgae cells, more extracellular polymeric substances (EPS) (162.54 ± 3.60 mg/g-mixed liquor volatile suspended solids (MLVSS)) with a higher proteins/polysaccharides ratio (7.62) were excreted from the mature algae granules. Moreover, the mature microalgae granules showed comparable nutrients removal, averagely 96% and 86% of dissolved organic carbon (DOC) and NH4+-N from the digestion liquor, respectively, reflecting its great potential for simultaneous microalgae cultivation, harvesting and wastewater treatment.