Factors contributing to bacteria inactivation in the Galdieria sulphuraria-based wastewater treatment system

Abstract

Our previous pilot scale studies showed concurrent removals of biochemical oxygen demand and nutrients, and inactivation of pathogens in urban wastewaters by an extremophilic alga, Galdieria sulphuraria. The current work evaluated the following as potential factors contributing to the observed pathogen inactivation, using E. coli as a surrogate: bacterial toxicity of algal metabolites, culture pH and temperature, sunlight, dissolved oxygen and adsorption to algal biomass. Microtox toxicity results implied that algal metabolites were free of bacterial toxicants as the toxicity in the reactor decreased from 38.8% to zero in 4 days. Low cultivation pH of 4 was identified as the primary factor influencing E. coli reductions. At neutral pH, under sunlight, E. coli were reduced to undetectable levels in 24 h. But, in the presence of live algal biomass and sunlight, E. coli were reduced to undetectable levels within 12 h. We attribute this accelerated reduction of E. coli to the synergistic effect of sunlight and elevated dissolved oxygen levels generated by biomass in the reactor. Temperatures below 45 °C and adsorption of bacteria to algal biomass did not appear to cause inactivation of E. coli. Simulation of pilot-scale conditions confirmed that the synergistic effects of pH, sunlight, algal biomass, and dissolved oxygen contributed to the superior fecal coliform inactivation observed in the algal wastewater treatment system.