Ionic effects on microalgae harvest via microalgae-fungi co-pelletization

Microalgae harvesting is a difficult and costly unit operation, hampering commercial microalgae based processes. Various innovative approaches have been under research investigations. Algae harvesting using microalgae/fungi co-pelletization where microalgae cells are attracted to filamentous fungus and form co-pellets during their co-culture has been proved to be an innovative approach that might be both cost effective and sustainable. Yet, the mechanism behind the co-pelletization based algae harvesting is still not known. Understanding its mechanism will significantly facilitate future developments to decrease the processing cost for generating biofuel and other microalgae based bio-products. Considering that the ionic conditions greatly affect the flocculation performance of microalgae cells, studies were conducted to compare the co-pelletization performance at different pH values and ionic strengths in order to evaluate the surface charge changes at the different conditions tested. Zeta-potential measurements indicated that (1) both microalgae and fungi have low negative zeta-potentials regardless of the pH of the bulk solution (<−10 mV) (2) fungi can have a positive electric charge at low pH (pH=3). The results suggest that it might be possible that the degree of repulsion and dispersion between these organisms is low at certain conditions, for instance, at higher concentrations of Ca²⁺ (>0.1 g/L) the surface charge of fungal and microalgae cells were less negative (>−5 mV for fungal hyphae and >−12 mV for microalgae) at higher concentrations of magnesium (0.5 g/L), which might have facilitated the attraction between them.