Recovery of nitrogen and phosphorus from anaerobically treated wastes using struvite precipitation

Abstract

Struvite, or magnesium ammonium phosphate hexahydrate, is a mineral that commonly precipitates when ammonium, phosphate, and magnesium ions are released during the anaerobic digestion of wastewater. Due to the high concentrations of dissolved orthophosphates and ammonia and magnesium ions encountered, there is a strong potential for forming struvite in the biological treatment of hog wastes. The mass of struvite formed could be deleterious to many waste operations because of its tendency to form damaging scale formations which are extremely difficult to remove. Thus, much of the past research has focused on a way to prevent the precipitation of struvite. This research investigated the positive potential of struvite, mainly as a method to recover soluble ammonia and phosphorus from anaerobically treated swine wastes to be used as a slow release fertilizer.

Optimum conditions for struvite precipitation were determined, including mixing and settling times, pH, molar ratio, chemical dosing conditions, and most effective supplemental chemicals to use in order to alter the pH and molar ratio. Laboratory results were compared with results from the MINTEQA2 equilibrium speciation computer model. Ammonia concentrations in effluent from a laboratory-scale anaerobic sequencing batch reactor treating swine wastes were reduced by as much as 98% using struvite precipitation. Following struvite precipitation, the supernatent was successfully recycled to the raw feed to be used as dilution water. Studies showed that the phosphate concentration in anaerobically treated waste activated sludge could also be reduced using struvite precipitation.