Algal wastewater treatment: Photoautotrophic vs. mixotrophic processes

Photoautotrophic algal systems have been investigated as potentially greener and more sustainable alternatives to the traditional bacteria-based wastewater treatment (WWT) systems. This paper presents mixotrophic algal systems as better suited for WWT. Since the literature is void of mixotrophic WWT systems, a brief review of the laboratory results from the literature comparing the different types of algal metabolisms is presented first. Details of a mixotrophic WWT system driven by an extremophilic alga, Galdieria sulphuraria (pH = 1 to 4; temperature = 25 to 56 °C) are presented next. Results from pilot scale version of this mixotrophic system (700 L) are summarized to show that it can reduce biochemical oxygen demand (BOD) and nutrients in primary-settled wastewater in a single step to yield discharge-ready effluent in <3 days of fed-batch processing. Average volumetric removal rates of BOD₅ (16.5 ± 3.6 mg L⁻¹ d⁻¹) and ammoniacal nitrogen (6.09 ± 0.92 mg L⁻¹ d⁻¹) in this mixotrophic system were found to be not different from those reported for the photoautotrophic high rate algal ponds (significance level of 0.05); but, that of phosphates (1.40 ± 0.57 mg L⁻¹ d⁻¹) is greater than that reported for the photoautotrophic high rate algal ponds (significance level of 0.05). Additionally, the extreme culture conditions in this particular mixotrophic system enabled concurrent reductions of E. coli (>5 log) and other pathogenic bacteria.