Generation of Magnesium Potassium Phosphate Using Synthetic Livestock Wastewater

Livestock wastewater is mainly treated with activated sludge, but ions such as phosphate, potassium, ammonium, nitrate, and sulfate remain in the effluent. In this study, the effects of residual ions on phosphorus recovery using the magnesium potassium phosphate (MPP) crystallization were investigated. If coexisting ions affect the products, the potassium (K) to phosphorus (P) molar ratio (K/P) of the precipitate will deviate from equimolar. Artificial wastewater test solutions containing 5.6–20.3 mM ammonium, 25.6 mM potassium, 6.5 mM phosphorus, 0–7.35 mM nitrate, and 0–3.06 mM sulfate were used. The optimum operating pH and amount of magnesium required to give a high phosphorus removal and recovery rates were determined. The experimental apparatus used a 10-L aerated and stirred reactor, and a 5-L settling tank. The K/P ratio in the precipitate was approximately 1 after using optimum conditions. Continuous 2-h treatment generated a white precipitate containing about 30 g of needle-like crystals. The effects of varying the ammonium, nitrate, and sulfate ion concentrations in the artificial effluent were also investigated. Ammonium concentration of 8 mM or more and sulfate concentration of 3 mM or more caused the K/P ratio to decrease to about 0.7 and 0.5, respectively. Varying the nitrate concentration did not affect the K/P ratio, even at a nitrate concentration of 7.35 mM. We also investigated the generation of products in the reaction tank and the behavior in the settling tank. With mixing in the reaction vessel by air agitation, the generation of primary nuclei occurred within 60 s, and growth to secondary nuclei was confirmed. The suspension containing the particles overflowed into a settling tank and separated into particles and treated water. It was found that K? in the MPP crystal was eluted by allowing the MPP crystal to stand for 7 h or more in the settling tank. Therefore, it was concluded that management of the interval between solids removal was important in controlling the composition of the solid.