ASSESSING EFFECTS AND INTERACTIONS AMONG KEY VARIABLES AFFECTING THE GROWTH OF MIXOTROPHIC MICROALGAE: pH, INOCULUM VOLUME, AND GROWTH MEDIUM COMPOSITION

A 2(3) + 3 full factorial experimental design was used to evaluate growth rate and biomass productivity of four selected, high-biomass-yielding microalgae species, namely, Chlorella vulgaris (CV), Scenedesmus acutus (SA), Chlamydomonas reinhardtii (CR), and Chlamydomonas debaryana (CD), in mixtures of growth medium (MWC) and wastewater at different proportions (from 20 to 50% of MWC) and at different pH (from 7 to 9). Multilinear regression analysis of the biomass productivity data showed that for SA and CD the biomass productivity was independent of the proportion of medium (MWC), while the growth of CV and CR slowed down in mixtures with high proportions of wastewater. However, the biomass productivity of SA was dependent on pH, while the growth of the other microalgae was independent of pH (7-9). When evaluating the influence of pH and proportion of medium, CD appeared most robust among the algae species, despite its lower biomass productivity. All the four species reduced 80-90% of the nitrate (NO3-) and 60-70% of the ammonia (NH4+) initially present in the wastewater: medium mixture, although the extent of the reduction was dependent on the initial NO3- : NH4+ ratio. Both SA and CV reduced similar to 20-25% of the chemical oxygen demand (COD) contained in the wastewater. This study shows the remarkable influence of certain variables that are often ignored in the search for optimal conditions of microalgal growth and also reveals the importance of considering interactions among growth variables in potential applications at large scale, particularly in the field of bioremediation.