Impact of Magnesium Sources for Phosphate Recovery and/or Removal from Waste

Abstract

As the population continues to rise, the demand for resources and environmentally friendly management of produced wastes has shown a significant increase in concern. To decrease the impact of these wastes on the environment, it is important to utilize the wastes in producing and/or recovering usable products to provide for the sustainable management of resources. One non-renewable and rapidly diminishing resource is phosphorus, which is used in several products, the most important being its use in manufacturing chemical fertilizer. With the increase in demand but reduction in availability of naturally occurring mineral phosphorus, it is important to investigate other sources of phosphorus. Phosphorus is most commonly recovered through struvite (magnesium ammonium phosphate) precipitation. The recovery of phosphorus from various wastewater has been well established and documented with recovery rates mostly above 90%. However, one of the major drawbacks of the recovery is the high cost of chemicals needed to precipitate the phosphorus. Since the external magnesium needed to achieve struvite precipitation accounts for around 75% of the total chemical cost, applicability of low-cost magnesium sources, such as bittern or seawater, can help reduce the operational cost significantly. This paper investigates the different magnesium sources that have been used for the recovery of phosphorus, highlighting the different approaches and operating conditions investigated, and their corresponding phosphorus recovery rates. An investigation of the economic aspects of the magnesium sources used for removal/recovery show that costs are dependent on the raw waste treated, the source of magnesium and the location of treatment. A review of published articles on the economics of phosphorus removal/recovery also indicates that there is a lack of studies on the economics of the treatment processes, and there is a need for a comprehensive study on life cycle assessment of such processes that go beyond the technical and economical aspects of treatment processes.

1. Introduction

The rapid population growth is expected to cause a more rapid increase in freshwater and food production demand in the future [1,2]. In fact, a 70% raise approximately in food production is expected between 2005 and 2050 [3]. As the need for food rises, agricultural production is also expected to increase. Another negative aspect associated with the increase in population, urbanization and economic growth is the rise in the amount of wastewater produced. The presence of untreated or semi-treated sewage water, industrial wastewaters, and agricultural runoffs has been causing major contamination to the waterbody in which it is discharged, and consequently leading to eutrophication. The eutrophication of waterbodies cannot be isolated as an environmental impact, as it impacts human health, the real-estate market of surrounding properties, the tourism sector, in addition to increasing the cost for drinking water treatment [4]. A case study by Dodds et al. [4] regarding the economic damage of eutrophication in the Great Lakes (USA) concluded that a loss of over USD 1 billion occurs annually due to the closure of recreational areas, 15.6% loss in waterfront property values with every 1m reduction in clear water, USD 44 million lost annually due to species endangerment, and a USD 150.9 billion investment is needed for drinking water treatment. These costs are in addition to USD 1.3 billion needed annually to mechanically harvest the algal blooms on the side of the lake, or USD 105 million needed annually to treat them chemically. In 1998, a drastic algal bloom occurred in Lake Tai (China), and the economic loss was estimated to be USD 6.4 billion [5]. Therefore, sustainable wastewater management, either through reuse, recycle, or recovering of materials is one of the key aspects of ensuring future adequacy of water, maintaining water quality, and eliminating the threats to marine life and human health.

Phosphorus is a major component of industrially produced fertilizers [6,7]. Phosphorus is usually obtained through the mining of phosphate rocks that are considered the natural reserves of phosphorus [2]. However, with the constant increase in the demand for phosphorus in the production of fertilizers for agriculture and other household products, the non-renewable natural phosphorus reserves are depleting at an unsustainable rate [8,9] and are expected to diminish within the next 50 to 100 years [10,11]. The decrease in the available phosphorus sources has caused an increase in the price of phosphorus, whereas the quality of the produced phosphorus has declined [12]. Another aspect of phosphorus mining on the environment is the large amount of water used in the beneficiation stage to separate the refined ore from the clay. It is estimated that China uses 1.8 billion m³ of ground water annually in the phosphorus mining process, where only 4.23% is reused and the rest of the contaminated water (phosphorus, cadmium, and fluoride contamination) is discharged into water bodies, leading to eutrophication [13]. Considering the latter, finding alternative sources of phosphorus is a key step to avoid future phosphorus-fertilizer scarcity and their impacts on the environment. Wastewater is a large source of phosphorus, but as mentioned earlier, the direct use of wastewater as a nutrient (nitrogen and phosphorus) for agricultural lands may lead to the contamination of water bodies. Therefore, several processes to recover phosphorus from different wastewaters in the form of a fertilizer known as struvite (MgNH₄PO₄·6H₂O) are either being experimented or already employed in different regions [8,14].

Struvite, also known as magnesium ammonium phosphate (MAP), is a white crystal compound that commonly accumulates in post anaerobic digestion pipes in wastewater treatment plants (WWTPs) [15,16]. MAP is considered a slow-release fertilizer [2,17,18], hence providing minimum run-off, enhanced settling and better nutrient uptake by crops [6]. As such, MAP has good potential as a substitute for the rock phosphate used to produce fertilizers.

Table 1. Chemical reactions occurring during struvite formation.

Pure struvite formation chemical reactions
${\mathrm{Mg}}^{+2}+{\mathrm{NH}}_4^{+}+{\mathrm{PO}}_4^{-3}+6{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{NH}}_4{\mathrm{MgPO}}_4·6{\mathrm{H}}_2\mathrm{O}$	(1)
${\mathrm{Mg}}^{+2}+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{MgOH}}^{+}$	(2)
${\mathrm{Mg}}^{+2}+{\mathrm{HPO}}_4^{-2}\leftrightarrow {\mathrm{MgHPO}}_4$	(3)
${\mathrm{Mg}}^{+2}+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\leftrightarrow {\mathrm{MgH}}_2{\mathrm{PO}}_4^{-}$	(4)
${\mathrm{Mg}}^{+2}+{\mathrm{PO}}_4^{-3}\leftrightarrow {\mathrm{MgPO}}_4^{-}$	(5)
${\mathrm{H}}^{+}+{\mathrm{H}}_2{\mathrm{PO}}_4^{-}\leftrightarrow {\mathrm{H}}_3{\mathrm{PO}}_{4\left(\mathrm{aq}\right)}$	(6)
${\mathrm{H}}^{+}+{\mathrm{HPO}}_4^{-2}\leftrightarrow {\mathrm{H}}_2{\mathrm{PO}}_4^{-}$	(7)
${\mathrm{H}}^{+}+{\mathrm{PO}}_4^{-3}\leftrightarrow {\mathrm{HPO}}_4^{-2}$	(8)
${\mathrm{NH}}_3+{\mathrm{H}}^{+}\leftrightarrow {\mathrm{NH}}_4^{+}$	(9)
${\mathrm{H}}^{+}+{\mathrm{OH}}^{-}\leftrightarrow {\mathrm{H}}_2\mathrm{O}$	(10)
Chemical reactions when competitive ions are present
${\mathrm{Mg}}^{+2}+{\mathrm{Na}}^{+}+{\mathrm{PO}}_4^{-3}\leftrightarrow {\mathrm{MgNaPO}}_4$	(11)
${\mathrm{Mg}}^{+2}+{\mathrm{K}}^{+}+{\mathrm{PO}}_4^{-3}\leftrightarrow {\mathrm{MgKPO}}_4$	(12)
${\mathrm{Mg}}^{+2}+{\mathrm{CO}}_3^{-2}\leftrightarrow {\mathrm{MgCO}}_3$	(13)
${\mathrm{Mg}}^{+2}+2{\mathrm{HCO}}_3^{-}\leftrightarrow \mathrm{Mg}{({\mathrm{HCO}}_3)}_2$	(14)
${\mathrm{NH}}_4^{+}+{\mathrm{HCO}}_3^{-}\leftrightarrow {\mathrm{NH}}_4{\mathrm{HCO}}_3$	(15)
$5{\mathrm{Ca}}^{+2}+3{\mathrm{PO}}_4^{-3}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{Ca}}_5{({\mathrm{PO}}_4)}_3\mathrm{OH}+{\mathrm{H}}^{+}$	(16)

summarizes the chemical reactions that take place during struvite formation and the formation of compounds that hinder the formation of struvite. Equation (1) is the struvite formation formula, showing the equimolar concentrations of magnesium:phosphorus:nitrogen (Mg:P:N) (1:1:1) [19,20]. Equation (2) through Equation (10) represent the reaction equations for the species formed in a synthetic aqueous solution containing only magnesium (Mg), ammonium (NH₄⁺) and phosphate (PO₄⁻³) [21]. However, in real wastewater, there are competitive ions, such as calcium (Ca⁺²), sodium (Na⁺), potassium (K⁺), CO₃⁻², and HCO₃⁻, which react with Mg⁺², NH₄⁺ and PO₄⁻³ according to the reactions in Equation (11) through Equation (16) [22].

As observed earlier in the equation for struvite formation (Equation (1)), equal molar ratios between the nutrients are needed; therefore, if there is a deficiency in one or more of the nutrients, an external source to provide the missing ion is required [20,23]. In most cases, and since the aim is to recover phosphorus, the limiting nutrient is magnesium [19], and according to multiple studies, Mg is usually supplemented using magnesium chloride or magnesium hydroxide. However, these sources contribute to around 75% of the total cost of the process [6,7,24]. To simultaneously reuse wastes, avoid future shortages of resources, reduce the cost of phosphorus recovery, and produce a local fertilizer, low-cost magnesium from waste products/wastewater is needed [7,25]. In fact, over 75% of the total recovery cost can be reduced by simply using a low-cost magnesium source [6,9].

This review paper summarizes the different magnesium sources used for the physiochemical process of struvite precipitation and phosphorus recovery from different types of wastewaters. Section 2 of this paper reviews the magnesium sources mentioned in the literature; Section 3 explains the methodology of this review; and finally, Section 4 discusses the technical and economic aspects of phosphorus recovery in the literature. The conclusions of this review are presented in Section 5.

2. Literature Review

Several studies have conducted experiments to test the efficiency of different wastes or abundant resources as a source of magnesium, as shown in

Table 2. Waste and abundant magnesium sources and estimated concentrations.

References	Magnesium Source	Approximate Magnesium Concentration
[9]	Bitterns	2000–3000 mg/L
[6,27,35]	Seawater	1276–1300 mg/L
[20]	Leachate	~722 mg/L
[36]	Wastewater from shale oil and gas production	764 mg/L
[29,30]	Desalination reject brine	9010 mg/L
[37]	Fertilizer industry’s wastewater	1178 mg/L

. One of these sources is bittern, which is the solution remaining after the crystallization of sodium chloride from brine or seawater [9,26]. As a by-product of the sea salt industry, bittern contains extremely high concentrations of Mg⁺² [9]. Seawater is also considered an unlimited and feasible option, since it contains abundant concentrations of magnesium, is cheap to use as a magnesium source, and has negligible impacts on the environment when acquired as a magnesium source [6,27]. The growing population and urbanization have also led to a rapid increase in municipal solid waste (MSW); therefore, sanitary landfills are considered an easy and economical technique for the disposal of MSW. However, the decomposition of landfill waste results in landfill leachate that is rich in a number of valuable minerals, including magnesium [20]. The production of shale oil and gas acts as an alternative energy source, but the produced wastewater from the operation contains organic and inorganic contaminants and is highly saline. The produced wastewater, nevertheless, also contains useful minerals, such as magnesium. Dolomite (CaMg(CO₃)₂), as well as dolomite lime (CaMg(OH)₄), are cheap abundant minerals that are naturally available [28]. Desalination processes to produce potable water produce reject brine, which is also considered a viable source of magnesium [29,30]. Sacrificial magnesium anodes [31,32], magnesium oxides [33], as well as cryptocrystalline magnesite [34], are also considered viable sources for magnesium. The efficiencies of all the mentioned sources have been compared to the commonly used expensive magnesium sources, such as magnesium chloride in this study.

3. Methodology

Peer-reviewed publications were reviewed in this study to discuss the methods of phosphorus recovery using different magnesium sources and to review the optimal conditions for efficient phosphorus recovery. The studies were also reviewed to synthesize the economic advantages of different magnesium sources to provide insight on reducing the cost of the recovery process. The literature included herein was published between 2002 and 2020. Forty-eight papers found across different databases and an online search were reviewed for this study.

4. Results and Discussion

4.1. Technical Aspects of Magnesium Sources for Phosphorus Recovery

Various experimental procedures have been carried out globally to assess the efficiency of different waste/wastewaters in acting as a reliable magnesium source to produce struvite. X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques were mostly used to analyze and identify the purity of the formed struvite. Even though the precipitation of struvite depends on multiple factors, the two major parameters identified in the literature that affect the process—pH and the molar ratio between the reactants (magnesium and phosphorus)—are discussed in detail in this study. Moreover, in almost all cases, sodium hydroxide was used to increase the pH and maintain it for the experiments.

Table 3. Optimum conditions and efficiencies for phosphorus removal and/or recovery.

References	Magnesium Sources	Wastewater Influent	pH	Molar Ratio Mg:P	Phosphorus Removal
[9]	Bittern	Swine wastewater	8.5	1.5:1	71%
[26]	Bittern	Animal manure anaerobic digester effluent	8.52		79%
[27]	Seawater	Dewatering of biological sludge	8.8	1.67:1	99%
[38]	Seawater	Synthetic wastewater	9	Mg:PO4 1.72:1	95%
[1]	Seawater	Toilet flushing water	9	3.3:1	99%
[35]	Seawater	Urine	8.8	1:1	87%
[7]	Synthetic seawater	Synthetic swine wastewater	9.2	Ca:Mg 0.3:1.3
[37]	Fertilizer industry wastewater	Fertilizer industry	9	N:P * 2:1	98%
[39]	Magnesium oxide Supernatant containing magnesium	Swine wastewater	- 9	3:1 1.2:1	98% 98.5%
[25]	Rift lake	Synthetic and real urine	9	1.6:1	98%
[15]	Pyrolysate magnesite (53% Mg)	Piggery wastewater	8/8.5	2.5:1	96%
[31]	Magnesium anode	Synthetic wastewater	8.8		65%
[17]	Magnesium chloride hexahydrate	Urine	10	1.5:1	98.4%
[40]	Magnesium chloride	Swine wastewater	10	1:1	93%
[18]	Magnesium chloride hexahydrate	Incinerated municipal sewage sludge	9.63	1.59:1	97.7%
[19]	Magnesium chloride Sea salt	Urine	11 9	1 1.25	97% 91.6%
[41]	Magnesium chloride hexahydrate	Semiconductor wastewater	9	1:1	NH4—N recovery 89%
[20]	Leachate	Phosphoric acid	9.5	Mg + Caresidual:P 1:1.5	Mg recovery 98.6%
[29]	Reject brine	Phosphoric acid	9–10	1:1
[36]	Produced water (wastewater)	Sodium phosphate	9.5	Mg:N:P 1.5:1:1.5	Mg Recovery 96.8%
[14]	Dolomite	Synthetic livestock wastewater	9		92%
[28]	Magnesium chloride	Hydrolyzed urine		1	95%
[16]	Magnesium chloride	Urine	9	Mg:PO4 1.2:1.1	98.9%

* In the fertilizers industry, the limiting nutrient that must be controlled is the ammonium.

summarizes the results for the optimum conditions and corresponding processes’ efficiency obtained from the literature. The effectiveness of different magnesium sources in the recovery of phosphorus are discussed in this section based on magnesium source.

4.1.1. Magnesium Chloride

Although expensive, magnesium chloride is commonly used to effectively recover phosphorus for struvite production. An experiment was conducted by Le et al. [17] using magnesium chloride hexahydrate to recover phosphorus from urine through struvite precipitation. Mg:P (0.5 to 2 with 0.25 increments), pH (7 to 12 with 1 level increments), and reaction time (0 to 240 min with 15-min increments) were the main varying parameters. Their results indicated that 98.4% of phosphorus removal was achieved using pH 10 and Mg:P ratio of 1.5:1. In a study by Rahman et al. [40], up to 93% of phosphorus was removed from swine wastewater when running the experiment at a pH of 10 and Mg:P ratio of 1:1. Similarly, Zin et al. [18] recovered 97.7% of phosphorus from incinerated municipal sewage sludge ash using magnesium chloride hexahydrate, while running the experiment at a pH of 9.63 and P:N:Mg ratio of 1:0.6:1.59. An experiment conducted by Pinatha et al. [19] compared the efficiency of magnesium chloride and sea salt to recover phosphorus from urine. The experiment was performed at pH values of 7, 9, 11 and Mg:P ratios of 0.75:1, 1:1 and 1.25:1. The optimal pH for struvite recovery was found as 11 and 9 for magnesium chloride and sea salt, respectively. Meanwhile, the optimal molar ratio was found as 1.25 for both Mg sources; however, there were no significant differences in phosphorus recovery rate for Mg:P ratios of 1 and 1.25 for magnesium chloride. Up to 97% and 91.6% were removed using magnesium chloride and sea salt, respectively. Ryu et al. [41] considered the effluent from treating semiconductor wastewater, collected from Korea, a potential phosphorus and ammonia nitrogen source to recover struvite by adding magnesium chloride hexahydrate and maintaining a pH of 9. Approximately 89% of ammonia nitrogen was recovered through the process. Kim et al. [16] utilized magnesium chloride to recover struvite from swine wastewater treatment effluent using a 5L airlift reactor. The pH was maintained at 9 and several Mg:PO₄⁻³ ratios were tested. The optimum phosphorus recovery of 98.9% was achieved when a ratio of 1.2:1.1 was used.

Tao et al. [28] attempted to recover struvite from hydrolyzed urine, which acted as a phosphorus source. The authors used magnesium chloride as a source of magnesium and varied the Mg⁺²: PO₄⁻³ ratio between 1 and 2.5. Tao et al. [28] reported that increasing the molar ratio increased the amount of produced crystals. However, as the residual Mg⁺² concentrations were increased, the purity of the produced struvite decreased at increasing ratios, and an optimum ratio of 1 was determined. At this molar ratio, the pilot-scale air-lift crystallizer used in the study recovered up to 95% of the phosphates in 1–5 h.

4.1.2. Bittern

Wang et al. [9] tested the efficiency of bittern as a source of magnesium to recover struvite from swine wastewater. Several experiments, utilizing a fluidized bed reactor, were performed to compare the efficiency of bittern to the efficiency of typical magnesium sources, such as MgCl₂, MgSO₄, MgO, and Mg(OH)₂. The parameters controlled and/or variated were pH and magnesium to phosphorus ratio with values of 8.0, 8.2, 8.5, 9.0 and 0.8, 1.2, 1.5, 2.0, respectively. It was found that under optimum conditions of pH 8.5 and Mg:P ratio of 1.5, up to 71% of phosphorus could be recovered. Similarly, a study by Orner et al. [26] proved bittern to be an effective magnesium source by recovering up to 79% of phosphorus from animal manure anaerobic digester effluent. The experiment was performed using a 200L batch struvite precipitation reactor, and pH of 8.52 (achieved using soda ash). A stirring time of 5 min and stirring speed of 60 rpm were the conditions for the experiment. An amount of 100 mL of bittern containing approximately 1.7 g of magnesium was used.

4.1.3. Seawater

Another study performed by Shaddel et al. [27] compared the efficiency of seawater collected from the Norwegian Sea to the efficiency of the pure magnesium chloride often used in recovering struvite from wastewater. The wastewater used as a source of phosphorus was collected from the dewatering of biological sludge process of a treatment plant employing biological phosphorus removal. The researchers utilized a bench scale crystallization system consisting of 1L reactor and a mechanical stirrer (150 rpm). They studied the effect of pH, ammonium concentration, magnesium concentration and the type of wastewater on struvite crystallization. The temperature was maintained at 20 °C using a water bath throughout the experimental procedure and each reaction was given a total time of 60 min. With a pH of 8.8 and a Mg:P ratio of 1.67, up to 99% of phosphorus was recovered. Similarly, a pilot scale struvite crystallization reactor by Aguado et al. [35] was used to test the efficiency of seawater in recovering struvite from urine. The performance of seawater was compared to magnesium chloride. The pH and Mg:P ratio were kept constant at 8.8 and 1, respectively, while the duration (15 to 34 days) and the hydraulic retention time (HRT) were varied. The results from the experiments showed that up to 87% of phosphorus was recovered during the process. Kumari et al. [38] tested the efficiency of seawater in recovering struvite from synthetic distillery wastewater. The varying parameters were pH, Mg:PO₄⁻³, and NH₄⁺:PO₄⁻³ with ranges of 5.98 to 11.02, 0.66 to 2.34 and 0.32 to 3.68, respectively. Under optimum conditions of pH of 9 and Mg:PO₄⁻³ of 1.72, up to 95% of phosphorus was recovered. Another study by Rubio-Rincón et al. [1] tested using seawater for flushing toilets to precipitate struvite as it mixes with urine. The experiments showed that up to 99% of phosphorus was recovered, while maintaining the seawater to urine ratio below 3.3:1 at a pH of 9.

Although recovering struvite using raw seawater as a source of magnesium showed success, disadvantages associated with the method include the need for more water, which leads to the dilution of phosphorus, eventually lowering the efficiency of struvite precipitation. Therefore, Ye et al. [7] experimented with electrodialysis using two titanium electrodes coated with ruthenium oxide to fractionate magnesium from synthetic seawater. The experiments were conducted at room temperature and the effects of voltage, current density and the availability of calcium ions were tested. At a constant voltage of 13V, 80% of Mg⁺² ions were recovered from the synthetic wastewater and then used to recover struvite. Additionally, the results showed that a recovery rate of up to 100% was achieved for a Ca:Mg ratio of 0:3 and 1:3. Increasing the calcium content to a Ca:Mg ratio of 4:3 decreased the struvite recovery rate by almost 61%, due to the increase in formation of ACP.

4.1.4. Wastewater

Several studies have investigated the use of different types of wastewaters abundant in magnesium as a source of the mineral. Ouchah et al. [37] studied the possibility of using fertilizer production plant wastewater for struvite recovery, since the wastewater generated from fertilizer industry contains both magnesium (1178 mg/L) and phosphates (685.23 mg/L). However, the limiting nutrient nitrogen was added in the form of ammonium chloride. The pH and N:P ratios were varied and 98% of the phosphorus was removed under the optimum conditions of pH 9 and N:P ratio of 2:1. Wu et al. [20] investigated the efficiency of leachate samples collected from a landfill in Virginia, USA as a source of magnesium to recover struvite. Phosphorus was added in the form of phosphoric acid (H₃PO₄) in this study. To account for calcium concentration, the researchers conducted three different experiments—with and without calcium pre-treatment. In experiment one, Mg:P ratios of 1:1 and 1:1.2 were tried, each coupled with pH 8, 8,5, 9 and 9.5. The second experiment included taking into account calcium concentration by using the (Mg + Ca):P ratios of 1:1 and 1:1.2, each with pH 8, 8.5, 9 and 9.5. Finally, an experiment employing calcium precipitation using sodium carbonate (Na₂CO₃) was conducted, and the (Mg + residual Ca):P ratios of 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.8, 1:2.2 and 1:2.4 were tested at a pH of 9.5. It was found that pre-treatment improved the struvite recovery process by precipitating calcium compounds. Additionally, a ratio of 1:1.5 was found to be optimal for struvite recovery; at this ratio, 98.6% of the magnesium was recovered from the wastewater sample and the dosage of phosphorus added was minimized. Heraldy et al. [29] also tested desalination reject brine from Indonesia as another magnesium alternative, with H₃PO₄ as the phosphorus source to precipitate struvite. During the experiments, Mg:N:P ratios of 1:1:1, 1:2:1 and 1:1:2 were varied along with pH values from 9 to 11. The results show that reject brine could successfully act as a magnesium source to recover phosphorus at optimal pH values of 9–10. Additionally, a higher N:P ratio improves the production rate of struvite. It is important to note that the calcium ions present in the reject brine could form calcium compounds and affect the quality of the produced struvite, particularly if the pH increased beyond 10. Another research by Hu et al. [36] investigated the difference between the process with and without calcium pre-treatment, while assessing the performance of produced water (oil and gas industry wastewater) as a source of magnesium to recover struvite. Na₂PO₄ was used as a source of phosphorus. Mg:N:P ratios of 1:1:1, 1.2:1:1.2, 1.5:1:1.5, and 1.8:1:1.8 and pH values of 8 to 10 were tested. Pre-treatment was performed using both Na₂CO₃ and the carbonation process. The results emphasize that pretreatment is necessary for struvite recovery. The optimal conditions were at a pH of 9.5 and Mg:N:P ratio of 1.5:1:1.5, at which 96.8% of Mg⁺² ions were recovered through struvite precipitation.

4.1.5. Other Magnesium Sources

Other less commonly used magnesium sources were studied in the literature to recover phosphorus. These sources include anodes, metals, fresh water, and pyrolysate of magnesite. Magnesium anode was used by Cai et al. [31] to remove phosphorus from synthetic wastewater and precipitate struvite through an electrodialysis process. Up to 65% of phosphorus was removed when the pH used was 8.8 and the flow rate was 200 L/hr. Huang et al. [39] compared three different magnesium sources, including magnesium alloy, magnesium oxide, and a magnesium rich supernatant, to remove phosphorus and recover struvite from swine wastewater collected from a farm in China. The magnesium alloy plate was placed in wastewater directly and the struvite precipitation was achieved through the chemical corrosion of the alloy. Low-cost magnesium oxide was added to the wastewater and up to 98% of phosphorus was recovered, while maintaining the MgO:P ratio at 3:1. Finally, the supernatant collected from the ammonia stripping process resulted in the best phosphorus recovery of 98.5%, while maintaining the pH at 9 and the Mg:P ratio at 1.2:1. Huang et al. [15] attempted to recover phosphorus from anaerobically digested piggery wastewater of a farm in Guangdong using the pyrolysate of magnesite containing 53% of magnesium. A total of 96% of phosphorus was removed when a pH of 8 to 8.5 was used, along with the Mg:N:P ratio of 2.5:1:1. Yin et al. [14] tested the efficiency of dolomite in recovering phosphorus from livestock wastewater of a pig farm in Linyi, China. The concentration of phosphorus in the wastewater was around 100 mg/L and the parameters varied were pH (7.5 to 10 with 0.5 increment), stirring speed (20 rpm to 320 rpm with 60 rpm increment), time (200 s to 1200 s with 200 s increment), and extract dose (of calcium and magnesium) of 4, 8, 12, 16, 20, and 24 mL. Under optimal conditions of pH 9, stirring speed 200 rpm, stirring time 600 s, and extract dose of 20 mL (corresponding to a magnesium concentration of 122.86 mg/L and a calcium concentration of 207.62 mg/L), the removal rate of phosphorus was 92%. Guadie et al. [25] tested the efficiency of the rift valley lake Afrera as a potential source of magnesium to recover phosphorus as struvite from real and synthetic urine and compare it to the commonly used magnesium chloride and magnesium oxide. The variation in factors, such as pH, Mg:P ratio, mixing speed, dilution of urine, and Ca:Mg ratio, was essential to study their effect and find the optimum conditions. A pH range of 5.5–10.5, Mg:P ratio range of 0.8 to 2, speed of 30 to 180 rpm, dilution of 0.11 to 9 and Ca:Mg ratio of 0.3 to 2.5 were used. In addition, the effect of storing urine was taken into consideration. With a pH of 9 and Mg:P ratio of 1.6, up to 98% of phosphorus was recovered.

4.2. Economical Aspects of Magnesium Source for Phosphorus Recovery

As discussed in earlier sections, different magnesium and phosphorus sources can be used to recover phosphorus in the form of struvite precipitate. The choice of these sources depends heavily on their process and availability, since the chemicals’ costs constitute around 75% of the total process cost. Cost analyses were conducted for different magnesium and phosphorus sources for the treatment of landfill leachate, and the results are summarized in

Table 4. Struvite precipitation’s materials cost.

Reference	Type of Wastewater Treated	Nutrient Source	Unit Price	Amount Needed	Cost (USD/m3)	Total Cost USD/m3
[42]	Landfill leachate	MgO	700 USD/ton	15 kg/m3	10.2	27.4
		H3PO4	900 USD/m3	15.4 L/m3	13.9
		NaOH	220 USD/m3	13.9 L/m3	3.3
[43]	Landfill leachate	Bone meal (P source)	79 USD/ton	24.4 kg/m3	1.8	8.8
		Bittern (Mg source)	17 USD/ton	71.5 kg/m3	1.1
		NaOH	266 USD/m3	15.8 L/m3	4.2
		H2SO4 (96% w/o)	209 USD/m3	8.2 L/m3	1.7
[44]	Pre-treated leachate for enhanced nitrogen removal	Bone meal (P source)	-	-	1.1–2.3	16.2–18.6
		Bittern (Mg source)	-	-	1.1–2.3
		NaOH	226 USD/m3	24.5 L/m3	5.5
		H2SO4 (96% w/o)	199 USD/m3	8.5 L/m3	1.7
		H2O2	334 USD/m3	25.7 L/m3	6.8

.

A study on treating municipal landfill leachate using only chemical reagents, such as MgO as a magnesium source, H₃PO₄ (75%) as a phosphorus source, and NaOH (30%) for pH stabilization, showed that the total cost of chemicals was 27.4 USD/m³ of treated leachate [42]. In comparison, another study using low-cost reagents found the cost to be 8.8 USD/m³ of treated leachate, achieving a 67% reduction in struvite production cost. The latter reagents were bone meal (by-product of thermal treatment of meat waste) as a phosphorus source, and seawater bittern (by-product of salt production) as a magnesium source [43]. The amount of struvite produced was estimated to be 63 kg/m³, and the production cost was estimated to be around 144 USD/ton, which is cheaper than the commercial fertilizer composed of ammonium and phosphorus [43]. To optimize the treatment of the leachate and to increase the nitrogen removal, the leachate can be pre-treated by oxidizing it through the addition of hydrogen peroxide [44]. This additional step results in an increase in the total chemical costs, which is around 16–19 USD/m³ of treated leachate.

Shaddel et al. [6] compared the costs of using MgCl₂ and raw seawater for struvite precipitation using reject wastewater from a municipal wastewater treatment plant. In their study, they concluded that using seawater as a source of magnesium would decrease the operational costs by 30 to 50%, the rest of the costs being incurred by the energy needed to pump the seawater and the NaOH used for pH adjustments. It was noted that increasing the pH above 8 when Mg is the limiting factor is not cost effective and can increase the dosage of NaOH needed by 2.2 and 7.7 times for pH 8.5 and 9, respectively [6].

A review paper by Li et al. [45] compared the cost of using MgCl₂ to the cost of using MgO as a magnesium source, and it was found that the latter prices varied between 0.3 and 0.6 USD/m³ of leachate, whereas for MgCl₂, prices ranged between 2.03 and 2.85 USD/m³ of leachate or digestate. It is important to note that there is lack of information with consistent functional units, as there might be a difference in phosphorus concentration between the leachate and the digestate, making it difficult to compare the cost of additional magnesium needed to treat 1 m³ of each wastewater.

Another review paper by Krishnamoorthy et al. [46] compared the costs of using different magnesium sources for phosphorus recovery as struvite and it stated that MgSO₄ and Mg anodes have approximately the same cost of around 4.73 USD/kg of struvite recovered, MgO being the cheapest with a cost of 0.89 USD/kg struvite, whereas MgCl₂ is the costliest, with 7.16 USD/kg struvite. It was also mentioned that 1 kg of struvite recovered from wastewater treatment plant would reduce the cost of sludge handling and disposal by USD 0.8. In addition, the paper mentioned saving USD 640,000 annually if 20% of the phosphorus in sewage was to be recovered through struvite crystallization.

A case study by Sena et al. [47] looked into the economics of introducing the Ostara Nutrient Recovery Technology to the Madison Metropolitan Sewage District, looking into the initial investment, operation and maintenance costs in addition to the financial revenues from selling struvite and the environmental revenues from reducing the amount of phosphorus released into the environment. It was found that an initial investment of USD 22,770,000 was required, in addition to the annual operation and maintenance cost of USD 225,500. Selling nearly 1950 tonnes of struvite yearly resulted in a revenue of USD 205,600 per year. Comparing the costs incurred with the revenue, one can concluded that phosphorus recovery through struvite crystallization is economically not feasible. It is important to note that the revenues from recovering phosphorus from wastewater is not limited to the financial revenues from selling it, but the environmental benefit can be factored economically from the burdens the citizens carry from the excessive abundance of phosphorus in the water bodies.

While phosphorus removal has taken precedence over recovery in the past, the future demands sustainable removal processes that consider the recovery and reuse of the nutrients and other waste products. Current recovery methods may not always seem to be cost effective if only the capital and operational costs are considered. A detailed study is required to consider the cost estimates related to the environmental impacts that the non-recovery of nutrients can pose, for example, the increase in eutrophication and its economic impact. This research team is currently carrying out a life cycle assessment (LCA) study on nutrient recovery to take into consideration all the factors that impact the removal and recovery of nutrients from different sources and the use of different chemicals [48]. This LCA study will be the first step in an attempt to show the importance of the circular economy when the treatment of wastes is considered. The status quo thinking and perception of treating waste to only reduce environmental pollution must be changed to a more comprehensive thinking that targets the concept of zero waste, the impact of treatment on the environment and the reuse of the products obtained from the treatment processes. In short, how can we as practitioners, planners and administrators help to improve the overall sustainability of treatment processes.

5. Conclusions

Struvite and phosphorus recovery were successful using different magnesium sources; including chemicals and wastewater/by-products using optimum conditions of a pH ranging between 8 and 9 and a molar ratio of Mg:P ranging between 1 and 1.67 for phosphorus recovery mostly above 90%. The use of previously discussed magnesium alternative sources, such as bittern or seawater, decrease the cost of struvite crystallization by 18% to 81%, making it more economically feasible. Although the source of magnesium did not affect the quantity nor the quality of the struvite formed in most instances, the type of wastewater used as a phosphorus source may contain high concentrations of competing ions. Wastewaters containing high concentrations of calcium can hinder struvite crystallization, as calcium phosphates have lower solubility compared to struvite. Thus, the pre-treatment of such wastewaters is suggested to optimize struvite crystallization, leading to additional costs. Another factor that can affect the total operational costs of struvite production is the location of the plant, which is due to the transportation of chemicals or wastewaters used as the nutrient source. Although studies have been conducted regarding the chemicals and operational costs of struvite crystallization, the most recent costs mentioned refer back to 2016; thus, it is important to conduct newer studies with updated costs, including also the new technologies to investigate the feasibility of wastewater pre-treatment and phosphorus recovery through struvite crystallization. A thorough investigation regarding the costs of different magnesium sources and the savings achievable is needed to be able to compare different nutrient sources and to assess the economic feasibility of phosphorus recovery. In addition, studies regarding the social, health and economic burdens of phosphorus leaching/dumping into water bodies is needed; such studies would help compare the costs and benefits of phosphorus removal and recovery.