title: Struvite precipitation in anaerobic swine lagoon liquid: effect of pH and Mg:P ratio and determination of rate constant.
abstract: 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 (MgNH(4)PO(4) x 6H(2)O), 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 PO(4)-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 PO(4)-P concentration in the effluents to as low as 2 mgl(-1). Minimum concentrations of PO(4)-P occurred between pH 8.9 and 9.25 at all Mg:P ratios. Struvite precipitation decreased PO(4)-P concentrations by 85% within 20 min at pH 9.0 for an initial Mg:P ratio of 1.2:1. The rate of PO(4)-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.
authors.affiliation: Department of Soil Science, North Carolina State University, Box 7619, Raleigh, NC 27695-7619, USA. nonelson@unity.ncsu.edu
authors.forename: Nathan O
authors.initials: NO
authors.lastname: Nelson
authors.forename: Robert L
authors.initials: RL
authors.lastname: Mikkelsen
authors.forename: Dean L
authors.initials: DL
authors.lastname: Hesterberg
mesh_terms: D000693:Anaerobiosis; D000818:Animals; D011232:Chemical Precipitation; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008274:Magnesium / Q000032:analysis / Q000737:chemistry*; D017616:Magnesium Compounds / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D010758:Phosphorus / Q000032:analysis / Q000737:chemistry*; D000069877:Struvite; D013552:Swine; D018505:Waste Management / Q000379:methods*; D014867:Water / Q000737:chemistry*
chemical_list: D017616:Magnesium Compounds; D010710:Phosphates; D014867:Water; D010758:Phosphorus; D000069877:Struvite; D008274:Magnesium