Struvite precipitation in anaerobic swine lagoon liquid: effect of pH and Mg:P ratio and determination of rate constant

Because of increased concern about surface water eutrophication from nutrient-enriched agricultural runoff, many swine producers are encouraged to decrease application rates of waste-based P. Precipitation and subsequent removal of magnesium ammonium phosphate (MgNH4PO4 • 6H2O), commonly known as struvite, is a promising mechanism for N and P removal from anaerobic swine lagoon effluent. The objectives of this research were to (i) quantify the effects of adjusting pH and Mg:P ratio on struvite precipitation and (ii) determine the rate constant pH effect for struvite precipitation in anaerobic swine lagoon liquid. Concentrations of PO4-P in liquid from two anaerobic swine lagoons were determined after 24 h of equilibration for a pH range of 7.5–9.5 and Mg:P ratios between 1:1 and 1.6:1. Struvite formation reduced the PO4-P concentration in the effluents to as low as 2 mg l−1. Minimum concentrations of PO4-P occurred between pH 8.9 and 9.25 at all Mg:P ratios. Struvite precipitation decreased PO4-P concentrations by 85% within 20 min at pH 9.0 for an initial Mg:P ratio of 1.2:1. The rate of PO4-P decrease was described by a first-order kinetic model, with rate constants of 3.7, 7.9, and 12.3 h−1 at pH 8.4, 8.7 and 9.0 respectively. Our results indicate that induced struvite formation is a technically feasible method to remove N and P from swine lagoon liquid and it may allow swine producers to recover nutrients for off-farm sale.