Effect of saline water ionic strength on phosphorus recovery from synthetic swine wastewater

Highlights

• The effect of ionic strength on struvite crystallisation was studied with a 2^k factorial design.

• Synthetic nanofiltration retentate was as good as MgCl₂ for struvite recovery.

• Ionic strength showed a more positive effect on struvite purity than on P removal.

Abstract

Declining worldwide phosphate rock reserves has driven a growing interest in exploration of alternative phosphate supplies. This study involved phosphorus recovery from swine wastewater through precipitation of struvite, a valuable slow-release fertiliser. The economic feasibility of this process is highly dependent on the cost of magnesium source. Two different magnesium sources were used for phosphorus recovery: pure magnesium chloride and nanofiltration (NF) saline water retentate. The paper focuses on the impact of ionic strength on phosphorus recovery performance that has not been reported elsewhere. Experimental design with five numerical variables (Mg/P molar ratio, pH, PO₄³⁻-P, NH₄⁺-N, and Ca²⁺ levels) and one categorical variable (type of magnesium source) was used to evaluate the effect of ionic strength on phosphorus removal and struvite purity. The experimental data were analysed using analysis of variance (ANOVA) and principal component analysis (PCA). Results indicated that a magnesium source obtained from NF retentate was as effective as MgCl₂ for struvite precipitation. It was also revealed that ionic strength had a more positive effect on struvite purity than on phosphorus removal. Within the range of parameters studied in this research, high ionic strength, high pH and wastewater with high phosphate, high ammonium and low calcium contents were found to be the most favourable conditions for struvite precipitation. Findings from this study will be beneficial to determine the feasibility of using high ionic strength saline water, such as NF seawater retentate, as a magnesium source for phosphorus recovery from wastewater that is rich in ammonium-nitrogen and phosphate.

Introduction

Phosphorus is an essential element for plants and is present in proteins, phospholipids, and deoxyribonucleic acids. Phosphorus is usually produced from phosphate rock, a rare resource which is predicted to be depleted in the next century (Elser and Bennett, 2011; Li et al., 2017). Countries such as China and the United States have restricted phosphate export to secure their own demand (Cordell and White, 2014; USGS, 2017), indicating that that the current supply of natural phosphate ores may be insufficient for future demand worldwide. Furthermore, excessive phosphorus in waterways can cause eutrophication which results in the growth of algae, depletion of dissolved oxygen, an increase of turbidity, and threats to aquatic life (Smith, 2003). Eutrophication is a widespread environmental issue and has become more severe since the mid-20th century (Cai et al., 2013). Therefore, phosphorus recovery from different types of bio-waste (such as wastewater (Peng et al., 2018), sludge (Matsuo, 1996) and urine (Wilsenach et al., 2007)) could become a key approach to overcome the potential challenge of phosphorus shortage and to control eutrophication. Nevertheless, current attempts to reuse or recover phosphorus are limited due to a significant gap between the technologies and frameworks to guide and support the nutrient recovery strategies, including identifying the key drivers and defining system boundary (Cordell et al., 2011).

One common process for phosphorus recovery from wastewater is chemical precipitation, which can be achieved by using phosphate to react with calcium, magnesium, iron or aluminium salts (Bacelo et al., 2020; Peng et al., 2018); for example, the formation of struvite (magnesium ammonium phosphate/MAP or MgNH₄PO₄•6H₂O) using magnesium salts allows phosphorus to be recycled as a slow-release fertiliser (Bacelo et al., 2020). The recovered struvite can also be used for other purposes, such as a building material (Guo et al., 2020) or an adsorbent (Wang et al., 2017). The struvite formation reaction can be expressed in the following equation (Eq. (1)), where the n value can be 0, 1 or 2, depending on the solution pH.

$$\begin{array}{ccc}& & \mathrm{M}{\mathrm{g}}^{2+}\left(\text{aq}\right)+{\text{NH}}_4^{+}\left(\text{aq}\right)+{\mathrm{H}}_n{\text{PO}}_4^{\mathrm{n}-3}\left(\text{aq}\right)\hfill \\ & & +6{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \text{MgN}{\mathrm{H}}_4\mathrm{P}{\mathrm{O}}_4·6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+n{\mathrm{H}}^{+}\left(\text{aq}\right)\hfill \end{array}$$

Chemical precipitation of struvite from wastewater is at a higher cost (6.03 US$/m³), compared to other technologies such as adsorption (3.65 US$/m³) and fluidised-bed homogeneous granulation (1.58 US$/m³) (Le et al., 2021). The cost of commercial magnesium source is a major economic constraint for struvite precipitation (Liu et al., 2014). The suitability of a magnesium salt for struvite precipitation is highly dependent on its availability, solubility and reactivity (Romero-Güiza et al., 2015). Magnesium chloride and magnesium sulphate are common magnesium sources used for struvite formation (Liu et al., 2013; Liu et al., 2008; Peng et al., 2018; Ronteltap et al., 2007). However, the application of these salts is limited due to their high cost (Desmidt et al., 2015). The use of cheaper alternatives such as magnesia (magnesium oxide), brucite (magnesium hydroxide) and magnesite (magnesium carbonate) (Liu et al., 2013) may reduce the phosphorus removal efficiency due to their limited solubility (Stolzenburg et al., 2017). Additionally, magnesium-containing adsorbents (such as MgO modified palygorskite (Wang et al., 2019), MgO modified diatomite (Li et al., 2019a) and silicon-doped MgO (Li et al., 2020)) have also been used as magnesium source for nutrient recovery in recent studies. Phosphorus removal efficiencies with various magnesium sources were found to be in the following order: MgCl₂ > MgSO₄ > MgO > Mg(OH)₂ > MgCO₃ (Li et al., 2019b), which is highly related to their solubilities. This suggests that there is a trade-off between the cost of commercial magnesium salts and their performances. Therefore, it is necessary to find a cheaper and more reactive magnesium source for phosphorus recovery.

Studies on the use of saline water, including seawater and bittern, as a magnesium source for phosphorus recovery from various liquid wastes had been reported, such as from wastewater (Lahav et al., 2013; Nur et al., 2018; Quist-Jensen et al., 2016; Wongphudphad and Kemacheevakul, 2019), urine (Aguado et al., 2019; Rubio-Rincón et al., 2014; Ye et al., 2014), landfill leachates (Siciliano et al., 2013) and digestion effluent (Maaß et al., 2014; Siciliano and De Rosa, 2014). Former researchers estimated that the total cost of ammonia recovery from digested manure with seawater bittern as magnesium source was approximately 47% lower than the total cost of the process with pure reagents, such as magnesium chloride (Siciliano et al., 2014). Such cost reduction would make the whole process to become more economically feasible. This suggests that seawater and brine could potentially be used as magnesium sources for phosphorus recovery. The typical magnesium concentration in seawater is approximately 1300 mg/L (El-Dessouky and Ettouney, 2002), which can be further concentrated by techniques such as nanofiltration (NF), reverse osmosis (RO) and evaporation. The presence of calcium ions in seawater can reduce the purity of struvite (Lahav et al., 2013; Nur et al., 2018), causing the struvite crystals to become less homogeneous and thus less valuable (Lahav et al., 2013). Furthermore, high concentration of calcium in seawater can also reduce the size of struvite crystals, thus affecting the product quality (Aguado et al., 2019). Nevertheless, it was reported that such an effect was negligible when the Ca/Mg molar ratio was less than 0.5 (Le Corre et al., 2005). Both magnesium and calcium ions are divalent ions with similar hydrated radii (0.43 nm and 0.41 nm, respectively), thus the rejection rates of these ions by NF membranes are similar. Consequently, the Ca/Mg molar ratio in the NF retentate (0.16 at recovery ratio of 50%) is close to that of untreated seawater (0.17) (Lahav et al., 2013). However, this should not be an issue as the molar ratio of Ca/Mg is less than 0.5. In addition to the above-mentioned factors, seawater contains a high sodium concentration (approximately 11000 mg/L (El-Dessouky et al., 2002)), which will significantly increase the total ionic strength of the recovery stream. Although high ionic strength increases the solubility of struvite (Hanhoun et al., 2011; Tao et al., 2016), its impact on the struvite precipitation performance has not been reported. It should be noted that NF retentate contains higher concentration of magnesium ions than fresh seawater, while the concentration of sodium in the retentate is lower than that in seawater. Therefore, compared to fresh seawater, NF retentate has a higher Mg/Na molar ratio and the dosage required for struvite precipitation will be lower than fresh seawater. Since the majority of sodium ions are not retained by NF membrane, the ionic strength of the recovery stream is lower than fresh seawater. This suggests that the use of NF retentate as magnesium source may counteract the negative effect of increased ionic strength. Limited studies on the effect of concentration factor (Quist-Jensen et al., 2016), pH and reaction time (Lahav et al., 2013) of NF retentate on phosphorus recovery (or phosphorus removal) and purity have been reported. However, the effect of ionic strength of NF retentate is not known.

Due to the high magnesium concentration required at low phosphorus levels, struvite precipitation is only considered economically feasible for wastewater with a PO₄³⁻-P concentration greater than 50 mg/L (Desmidt et al., 2013); for example, swine wastewater or other agricultural wastewater. Global pork production was predicted to be approximately 9.5 × 10⁷ tonnes in 2020, led by China (36.5% of the total world production), the European Union (25.6%) and the United States (13.7%) (USDA, 2020). In New Zealand, approximately 620,000 pigs (44,000 tonnes) were slaughtered in 2018/2019 (Ministry for Primary Industries, 2019). This large pork market means there is a large amount of swine wastewater which requires effective phosphorus removal (or recovery) prior to discharge. Swine wastewater from pig farms is typically rich in ammonium nitrogen (406–985 mg/L NH₄⁺-N (Muhmood et al., 2019)), phosphate (31–161 mg/L PO₄³⁻-P (Muhmood et al., 2019)), and organics, suggesting that swine wastewater is suitable for struvite production with the addition of an external magnesium source. This study focused on investigating the effect of the high ionic strength of saline water on phosphorus recovery from swine wastewater. The effect of ionic strength on phosphorus removal and the purity of struvite was evaluated by using experimental design with six variables: five numeric variables: PO₄³⁻-P level (A), NH₄⁺-N level (B), Ca²⁺ level (C), Mg/P molar ratio (D) and pH (E) and one categorical variable: type of magnesium source. The experiment data were statistically analysed using ANOVA and PCA. Findings from this study will be beneficial to determine the feasibility of using high ionic strength saline water, such as NF seawater retentate, as a magnesium source for phosphorus recovery from wastewater that is rich in ammonium-nitrogen and phosphate.