Livestock wastewater is mainly treated with activated sludge, but ions such as phosphorus, 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 crystallization method were investigated when magnesium was added to increase the pH. If co-existing ions affect the products, the phosphorus to potassium molar ratio (K/P ratio) of the precipitate will deviate from being 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. First, the optimum operating pH and amount of magnesium added to give a high phosphorus removal rate and recovery rate were determined. The experimental setup was a 10 L aerated and stirred reactor, and a 5 L settling tank. The K/P ratio in precipitate was approximately 1 using the optimum conditions. Continuous 2 h treatment allowed a white precipitate containing about 30 g of needle-like crystals to be obtained. Next, the effects of varying the ammonium, nitrate, and sulfate ion concentrations in the artificial effluent were investigated. Ammonium and sulfate ion concentrations of 8 mM or more and 3 mM or more, respectively, 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.
Modern agricultural systems require phosphorus compounds to be applied in fertilizers, but the raw materials required (phosphorus ores) are likely to be exhausted at current rates of use. According to Kuroda et al., The 80% of the phosphorus ore that is extracted is currently used in fertilizers [
The ammonia concentration is high at the start of the biological treatment process, so phosphorus can be recovered as magnesium ammonium phosphate (MAP; MgNH4PO4) using coexisting Mg. The crystallization of MAP is caused by a reaction among PO 4 3 − , NH 4 + , and Mg2+ in aqueous solution. The reaction is shown in formula (1).
Mg 2 + + NH 4 + + PO 4 3 − → MgNH 4 PO 4 (1)
The MAP precipitation method is used for liquids containing P and ammonium. Mg2+ is added to generate MAP in weak alkaline solutions. The MAP method gives a high crystal formation rate, so crystallization occurs without seeding. This MAP method can be used at pH 8.5 to pH 9. The MAP method is of-
ten used for solutions containing relatively high P or ammonium concentrations, such as livestock wastewater and side stream wastewater produced during activated sludge treatments [
The ammonium concentration will decrease and MAP treatment will be difficult when a long treatment time is used and the nitrification efficiency is high. In this study, we investigated the recovery of phosphorus as magnesium potassium phosphate (MPP) through the reaction shown in formula (2) using a high potassium concentration.
Mg 2 + + K + + PO 4 3 − → MgKPO 4 (2)
The MPP crystallization method has been used for wastewater with a high phosphorous concentration (30 mM or more, e.g., in urine) and a high potassium concentration [
The experimental setup is shown in
The pH of the solution in the reactor was varied between pH 8 and pH 12 using a pH controller. The dosage Mg/P ratio was changed from 1 to 2 by changing the rate at which magnesium chloride solution was added.
Tests were performed using ammonium concentrations of 8.36 mM, 11.4 mM, and 22.3 mM at the standard conditions. Tests were also performed at nitrate concentrations of 0 mM, 4.02 mM, and 8.04 mM, and at sulfate concentrations of 1.02 mM, 2.04 mM, and 3.06 mM.
The white precipitate that formed in the reaction tank was moved to the settling tank in the overflowing solution and settled in the cone of the sedimentation tank. After a reaction, the mixture was allowed to stand for a specified time, then the reaction solution in the settling tank was passed through a filter. The white precipitate adhering to the filter paper was dried for 24 h in flowing air at 60˚C. The filtrate was passed through a 0.45 μm membrane filter.
The Magnesium concentrations in the samples were determined using an AA- 6300 atomic absorption spectrometer (Shimadzu, Kyoto, Japan). The potassium and sodium concentrations were measured by flame photometry using the same atomic absorption spectrometer. The phosphate and ammonium concentrations were measured using the molybdenum blue and ascorbic acid method and using the indophenol method, respectively, and a V-530 ultraviolet-visible spectrophotometer (JASCO, Halifax, Canada) [
The MPP crystallization method can be used as a post-treatment for effluent treated with activated sludge, so we assessed the effects of the presence of nitrate, ammonium, and sulfate ions on the MPP production process.
Needle-like crystals not seen at pH 8 became visible at pH 9, and the needle length increased and more needle-like crystals were present at higher pH values. The decreases in the P and K concentrations in the liquid increased as the pH increased from pH 8 and reached a maximum at pH 11, but then decreased when the pH was increased to pH 12.
The amount by which the Mg concentration decreased did not decrease between pH 11 and pH 12, and the ammonium concentration remained almost constant.
At pH 11, the decreases in the phosphorous potassium, and magnesium Concentrations in the liquid were almost equimolar. The molar ratios of the precipitates, determined by EDS, were 0.84 and 0.95, and the ammonium and sodium contents were 4% and 8%, respectively. It therefore seems that the contents of the components of the precipitate reflected the decreases in the concentrations of the components of the solution. We will examine this using the removal rate and recovery rate using the equations below [
Recovery rate of PO 4 -P = PO 4 -P in white precipitate PO 4 -P decrement (3)
Recovery rate of PO 4 -P = PO 4 -P initial − PO 4 -P equilibrium PO 4 -P initial (4)
The results are shown in
The effects of the magnesium dosage on the three components are shown in
Further tests were performed at pH 11 using an Mg/P molar concentration ratio of 1. The results obtained when the water quality was varied are shown in
An electron microscopy photograph of the precipitate is shown in
In our simple batch tests, ammonium concentrations up to about 5 mM did not affect the formation of MPP, and the K/P molar concentration ratios in the crystals were close to 1 [
initial ammonium concentration on the K/P ratio are shown in
Electron microscopy photographs of the crystals produced at initial ammonium concentrations of between 0 mM and 22.4 mM are shown in
An initial ammonium concentration of 22.3 mM gave crystals with a K/P ratio of 0.73, but the ratio was only 0.52 when the test was performed at room temperature. It is therefore possible to decrease the effects of the presence of ammonium even at high ammonium concentrations by aerating and stirring the solution and controlling the temperature of the system. Even though the production of MPP was affected by the higher ammonium concentrations, needle-like crystals were found in the precipitate formed at each ammonium concentration that was tested.
The effects of sulfate on the components of the solutions and precipitate and an electron micrograph of the precipitate are shown in
The effects of sulfate on the components of the solutions and precipitate and an electron micrograph of the precipitate are shown in
but MAP production has been found to be affected by the presence of SO 4 2 − at a concentration of 1300 mg/L (13.54 mM) [
The aim of the study was to further improve livestock wastewater treatment and the recycling of phosphorous resources. The MPP crystallization process was applied to model wastewater containing 25.6 mM potassium, 6.5 mM phosphorus, and 5.6 mM ammonium at pH 11 using a Mg dosage equimolar with the P concentration. Crystals containing equimolar K/P ratios were obtained after 2 h of continuous treatment. Small amounts of ammonium and sodium were found in the crystals. Nitrate did not affect the K/P ratio, but ammonia and sulfate did affect the ratio, 1 mM sulfate in particular decreasing the K/P ratio to 0.7.
This work was supported by JSPS KAKENHI (grant no. 15K00605).
Harada, H., Katayama, Y., Afriliana, A., Inoue, M., Teranaka, R. and Mitoma, Y. (2017) Effects of Co-Existing Ions on the Phosphorus Potassium Ratio of the Precipitate Formed in the Potassium Phosphate Crystallization Process. Journal of Environmental Protection, 8, 1424-1434. https://doi.org/10.4236/jep.2017.811086