Exploring Pellet Forming Filamentous Fungi as Tool for Harvesting Non-flocculating Unicellular Microalgae

Recently, fungal pelletization-assisted microalgal harvesting has emerged as new research area for minimizing the harvesting cost and energy inputs in the algae-to-biofuel approach. The present study attempted to optimize this process in terms of substrate inputs, process time and pH conditions through selection of a robust fungal strain. A total of eight fungal strains were screened for their pelletizing efficiency in fresh/spent BG11 supplemented with selected media nutrient (glucose, nitrogen and phosphorous). Aspergillus lentulus was found most efficient for pelletizing in the nutrient supplemented spent BG11 in spite of its alkaline pH (8.6–9.2). Moreover, the selected fungus was found equally good under neutral (7.0) and acidic pH (5.0). Interestingly, A. lentulus was able to harvest nearly 100 % of the microalgal cells (at 1.58 g l⁻¹ initial working algal concentration) within only 24 h at supplementation of 10, 0.5 and 0.5 g l⁻¹ of glucose, NH₄NO₃ and K₂HPO₄, respectively. The process kinetics in term of harvesting index (HI) as well as the variation of residual glucose and pH with time was also studied. Surprisingly, A. lentulus also performed well at lower glucose level (5.0 g l⁻¹) resulting in 92 % harvesting within 24 h and up to 98 % harvesting within 52 h. The mechanism of harvesting process was studied through microscopic examination. A. lentulus strain investigated in this study could emerge as an efficient, sustainable and economically viable tool in microalgae harvesting for biofuel production. Further, the estimated stoichiometric methane potential (SMP) of pelletized microalgal–fungal biomass (0.756 m³ kg⁻¹ volatile solid [VS]) showed substantial enhancement over algal biomass alone (0.709 m³ kg⁻¹ VS). Hence, the investigated process not only overcomes the hurdles associated with microalgal harvesting but also improves its potential and suitability for bioenergy generation by anaerobic digestion. Further scale-up and pilot level testing is warranted for commercializing the investigated technique.