Evaluating the resilience of photobioreactors in response to hazardous chemicals

Photobioreactors are increasingly being considered for wastewater and biotechnology industries, due to their capability to simultaneously treat wastewater and produce biomass that can be converted into food, fuels and high-value chemicals. However, the resilience of these algal systems during hazardous events such as shock loads of harmful chemicals remains underexplored. This study provided an in-depth analysis of cellular response to different chemical shocks, including sodium chlorite (20–40 g/L), sodium hypochlorite (20–150 mg/L) and 2,4-dinitrophenol (200–1000 mg/L), in photobioreactors acclimated to domestic wastewater influent. Increased stress levels exerted detrimental effects in most cases. For instance, applying 40 g/L sodium chlorite led to significantly lower cell count (0.61 × 10⁷ cells/mL) and mixed liquor suspended solids (0.37 g/L) relative to the control (1.53 × 10⁷ cells/mL and 1.06 g/L) at the end of the trials. The flow cytometry results indicate that the chemicals impacted the photobioreactor cultures differently due to their varying biological effects. Applying a chemical shock through excessive sodium chlorite (40 g/L) increased the proportion of smaller cells from 2 to 18% as well as greater cellular lipid accumulation. A shock of sodium hypochlorite (150 mg/L) resulted in an immediate drop in the chlorophyll content in up to 55% algal cells, followed by a notable recovery. Addition of 1000 mg/L 2,4-dinitrophenol resulted in irreversible reduction in cell growth and viability. Co-culturing microalgae and activated sludge provided improved system performance, but this benefit was eliminated during the hazardous chemical events. Overall, the photobioreactors have displayed certain degrees of resilience to lower levels of chemical shocks; these systems have also shown their ability to regain some of their original characteristics under highly unfavourable conditions.