Phosphorus Removal from Liquid Swine Manure by Chemical Precipitation

Over application of phosphorus (P) by the land application of manure from animal feeding operations is an increasing threat to surface water quality. Regulations limiting manure application rates based on P are expanding and many operators will be land limited when forced to follow these standards. The excessive loading of P onto agricultural lands is a serious environmental issue because it can be transported to surface waters where it can cause eutrophication. It has been shown that P content in swine manure can be reduced by precipitation of magnesium ammonium phosphate hexahydrate, MgNH 4PO4·6H2O (struvite). In this research, the fundamental objective was to reduce the P concentration of swine wastewater by precipitation methods. This task was divided into three phases. Phase 1 investigated the optimum pH, and the molar ratios of magnesium (Mg2+), ammonium (NH4+), and phosphate (PO43-) needed to enhance precipitation. Laboratory experiments were conducted using magnesium chloride (MgCl2·6H2O, 64% solution) to increase the Mg2+ concentration and sodium hydroxide (NaOH) to increase the pH. Phase 2 of this project investigated the usefulness of a chemical equilibrium model, Visual Minteq, for prescribing the amendments needed to maximize struvite precipitation from liquid swine manure and thus reduce the orthophosphate phosphorus (OP) concentration. The actual concentrations of Mg2+, calcium (Ca2+), potassium (K+), OP, NH4 +,alkalinity and pH from a liquid swine manure system were used as inputs to the model. The model was modified to remove species with extremely low formation rates, because they would not realistically form during a short retention-time process such as those envisioned for swine manure struvite-formation reactors. Using the model’s output, a series of bench-scale reactors were used to verify the results. Finally, phase 3 involved the development of a laboratory-scale, continuous-flow reactor that was used to investigate the hydraulic retention time (HRT) needed to optimize struvite precipitation. Using the values for retention time, the Mg2+:PO43-ratio, and pH adjustment that were determined by this research, a full-scale economic analysis was conducted to estimate the cost of using struvite precipitation to reduce the P concentration of a liquid swine manure during the land application process