Harvesting of algae in municipal wastewater treatment by calcium phosphate precipitation mediated by photosynthesis, sodium hydroxide and lime

The cultivation of microalgae within municipal and agricultural wastewater offers the opportunity to treat the water and partition nutrients into valuable biomass. Harvesting of those algae is a challenge, as conventional coagulants (iron and aluminum salts) contaminate algae and lock up phosphorus preventing beneficial reuse. We show that flocculation by calcium phosphate precipitation is capable of harvesting algae and phosphate from municipal wastewater in a form suitable for reuse. However, the initiation of flocculation by natural pH increase was infeasible. We harvested 93% of algal solids with an initial concentration of 113 mg VSS/L and 96% of total phosphate from a waste stabilization pond effluent by initiating flocculation with 129 mg-P/L of soluble phosphate and addition of 364 mg/L quicklime to achieve a pH of 10.0, at a calculated cost of $0.05 USD per kL for alkali addition, which is similar to the chemical addition costs of conventional coagulation.