Autoflocculation of microalgae, via magnesium hydroxide precipitation, in a high rate algal pond treating municipal wastewater in the South Australian Riverland

High rate algal ponds (HRAP) are considered a promising system for coupling wastewater treatment with the growth of microalgae biomass. However, the absence of a cost-effective harvesting method limits this application. In this study, in situ autoflocculation, via magnesium hydroxide precipitation, was assessed in a HRAP containing 33 m3 of HRAP treated municipal wastewater which had a total suspended solids concentration of 115.00 mg L−1. Autoflocculation was induced using MgCl2.6H2O to increase the magnesium concentration of the wastewater to 82.63 ± 0.09 mg L−1, followed by the addition of 182 mg L−1 of slaked lime to increase the pH to 11. A high flocculation efficiency, determined by the reduction in turbidity, of 91.52 ± 0.57% was achieved with good removal of total suspended solids, 84.93 ± 1.00%. Excellent nutrient removal, 91.10 ± 0.14% of total phosphorus, and pathogen removal, a 3 log10 reduction in E. coli MPN 100 mL−1, was also observed. Hydrochemical modelling (PHREECQC-2) identified improvements in operational conditions to reduce cost. Autoflocculation, via magnesium hydroxide precipitation, was shown to be a promising method for harvesting microalgae and nutrients.