Biologically mediated phosphorus precipitation in wastewater treatment with microalgae.

A lab-scale continuous microalgal culture was grown on sterile-filtered wastewater in order to clarify the phosphorus removing mechanisms in a microalgal treatment step that treats residual phosphorus from a hydroponic wastewater treatment pilot plant. The phosphorus assimilation was dependent on algal biomass production, whereas the chemical precipitation was dependent on phosphorus load, i.e. an increase in average precipitation rate with decreased hydraulic retention time was observed. The chemical precipitation was mainly a result of the increased pH, which was biologically mediated by the photosynthesising algae. The precipitate was composed of a calcium phosphate with magnesium included, magnesium hydroxide and calcite. A significant nitrogen removal was also experienced, which implies that the microalgal wastewater treatment is appropriate both for phosphorus and nitrogen removal.