Phosphate Uptake by Precipitation in Model Animal Wastewaters: Adjusting Ionic Strength and Ionic Composition to Maximize Phosphorus Removal

Abstract

While phosphorus is a limited resource that is essential for agriculture, its release to the environment adversely impacts water quality. At the same time, animal wastewaters contain significant quantities of phosphorus and nitrogen that can be recovered for beneficial use. Phosphorus uptake experiments were performed with magnesium-treated corn-cob char and with magnesium silicate prepared using silicate from rice straw at pH 8 and 9. The concentration of dissolved phosphorus as a function of total added ammonium chloride (NH4Cl) was determined, and chemical equilibrium modeling was used to investigate the concentration trends of dissolved and mineral species. According to chemical equilibrium modeling, carbonate alkalinity exerted a significant magnesium demand, with approximately half of all added magnesium forming magnesite (MgCO3(s)). As total added NH4Cl increased, excess Cl− complexed with dissolved Mg2+ in competition with orthophosphate, freeing orthophosphate to precipitate, mainly as the mineral struvite (NH4MgPO4·6H2O(s)). As the concentration of added NH4Cl increased by a factor of ten, measured concentrations of dissolved phosphorus decreased by a factor of ten, meaning that ionic composition has the potential to significantly impact the amount of phosphorus that can be recovered from wastewaters for beneficial use.