Struvite (MgNH4PO4·6H2O) Solubility and Its Application to a Piggery Effluent Problem

Abstract

Struvite (MgNH4PO4·6H2O) solution chemistry was studied in order to understand a struvite scaling problem which exists in the pipe network of a series of 4 effluent lagoons at a piggery. The conditional solubility constant (PS=SMG·SNH3−N·SPO4−P, equilibrium S values (M)) was determined over a range of pH values (6.8-8.5) and solution concentrations of Mg, NH3−N and PO4−P close to field conditions, and at a temperature of 30°C. A comparison of field and laboratory data indicates struvite precipitates from the second lagoon onwards. The solubility data obtained also indicate potential for precipitation as a method of nutrient removal from wastewaters.

Crystallization of struvite from metastable region with different types of seed crystal

“…Presence of base in solution provides increased free ions of phosphate (Eq. [11]), thereby bringing on supersaturation.…”

“…The literature suggests that various types of materials are used as seed materials in di¤erent types of crystallization. Phosphate rock, bone charcoal, magnesia clinker, zirconium hydroxide, pumice stone, borosilicate glass, struvite, quartz sand, marble, calcium carbonate, activated carbon, bone char, and phosphorus rock, mother crystal (struvite) are the possible candidate seeds [3,[11][12][13][14][15]. Chemical inactivity in the mother liquor, isomorphism with mother crystal, and adequate surface area in suspension are the predominant properties of ideal seeds [1,2,16].…”

Influence of airflow rate and substrate nature on heterogeneous struvite precipitation

“…This scale reduces pipe diameter and increases pumping costs. An extensive number of investigations [1][2][3][4] has reported this problem in the wastewater treatment industry. On the other hand, struvite exhibits ecological and economical benefits [5,6].…”

“…Under certain conditions, water pH is lower than (pK 1 +pK 2 )/2. In these conditions HCO 3 − is the prevalent carbonate species. The transformation of HCO 3 − ions to CO 2 by a proton capture (Equation (1)) is responsible for the immediate increase in the pH of the solution [12].…”

“…In these conditions HCO 3 − is the prevalent carbonate species. The transformation of HCO 3 − ions to CO 2 by a proton capture (Equation (1)) is responsible for the immediate increase in the pH of the solution [12]. Crystalline growth of the CaCO 3 preferentially involves Ca 2+ and HCO 3 − with the release of H + (Equation (2)) rather than Ca 2+ and CO 3 2 − .…”