Struvite separation from wastewater and its use with sulfur-oxidizing bacteria improves phosphorus utilization in alkaline soil

Phosphorus (P) is a major limiting nutrient for plant growth, with an average concentration of 0.05% (w/w) in soil, of which only 0.1% is available as phosphate (Zou et al. 1992). Phosphate fertilizers are commonly used to maintain soil fertility, but this practice is unsustainable due to the limited availability of high-quality reserves (Isherwood 2000). Phosphate fertilizers are commonly used to maintain soil fertility, but this practice is unsustainable due to the limited availability of high-quality reserves. The current global reserves of phosphate rock are estimated to be 7,000 million tons as P₂O₅, with 40 billion tons being utilized annually for fertilizer and human use. If current consumption rates continue, reserves may last for only 125 years (Cordell et al. 2009). Recycling P in nature, especially from wastewater, can help overcome the shortage of rock phosphate. Municipal and livestock wastewater, which contains a significant amount of P, can be a valuable source for recycling P (Rahman et al. 2014).

Wastewater containing high levels of nitrogen and phosphorus can be a valuable source of struvite. To reduce water contamination, wastewater treatment plants employ various techniques to remove excessive phosphorus, as high levels of phosphorus contribute significantly to water body pollution, eutrophication, and algal blooms. Elevated concentrations of phosphorus in wastewater also harm aquatic life and result in the production of toxic compounds. Therefore, it is crucial to remove nitrogen and phosphorus from wastewater before discharge into the main stream to maintain a pollution-free and eco-friendly environment. Phosphorus recovery from wastewater is a potential solution for addressing the depletion of phosphorus reserves. The struvite crystallization process is a promising method for simultaneously removing nitrogen and recovering phosphorus from wastewater such as biogas fermentation residues indicating promising economic results for struvite formation (Maroušek & Gavurová 2022). Various chemical and biological procedures, such as metal precipitation, improved biological phosphorus removal processes, and engineered wetland systems, have also been developed for P removal (Suzuki et al. 2007). However, among these methods, the struvite crystallization process is considered the most ideal because the cost of such captured P is now closer than ever to meeting the pricing of P from conventional P fertilizers that are made of P rocks (Stavkova & Marousek 2021). The process produces struvite crystals that can be used for the removal or recovery of nitrogen and phosphorus from wastewater (El Diwani et al. 2007). Adding more P to the wastewater could increase the recovery of the NH₄-N and 1:1 M ratio was suitable (Jin et al. 2018).

Struvite, also called magnesium ammonium phosphate (MAP) hexahydrate (MgNH₄PO₄·6H₂O), is a white inorganic crystalline mineral with a density of 1.71 g cm⁻³ and limited solubility in water, alkali, and ethanol. Struvite contains 12.6% phosphate and 5.7% nitrogen by weight, with completely citrate-soluble phosphate and an oxide form of nitrogen fertilizer with 9.9% magnesium (Bridger et al. 1962). It is commonly used as a slow-release fertilizer (Le Corre et al. 2009). To improve its competitiveness against triple superphosphate (TSP) and diammonium phosphate (DAP) fertilizers, researchers have proposed treating struvite with phosphoric acid (Gaterell et al. 2000). This modification has been shown to enhance the struvite's technical and commercial feasibility (Shu et al. 2006). All in all, these technologies should be sustainable because in long run life cycle affect earnings management (Durana et al. 2021).

From an economic standpoint, this process can be highly beneficial for both wastewater treatment plants and farmers. By recovering phosphorus from wastewater, treatment plants can reduce the costs associated with traditional methods of phosphorus removal and disposal. Additionally, struvite production generates a valuable fertilizer product that can be sold to farmers, reducing their dependence on expensive chemical fertilizers. This process also has environmental benefits by reducing the amount of phosphorus pollution in waterways, which can lead to harmful algal blooms and other ecological problems. Overall, the economics of wastewater-based struvite production make it a promising technology for improving both the sustainability and profitability of wastewater treatment and agricultural industries.

Limited research has been conducted on the evaluation and optimization of struvite crystallization processes in local wastewaters and its application on wheat plant as the utilization of phosphorus by crops is restricted to phosphates that are based on calcium (CaP) and, to a lesser extent, on aluminum (AlP). Conversely, phosphorus that is present in ferric-based phosphates (FeP) and other forms is deemed unattainable for human use (Kulhánek et al. 2019). To address this gap, we aim to compare the effectiveness of struvite as a fertilizer for major cereals under alkaline soil conditions with other chemical fertilizers. Our study aims to optimize struvite production conditions in local environments and evaluate its effectiveness as a phosphorus (P) source under alkaline conditions.

The present research was planned and executed keeping in view the limiting natural P resource and finding its alternate to overcome the possible deficiency of P fertilizers in the future. All the local wastewater streams in the vicinity were evaluated for their nutrient content and as viable source for struvite production. Struvite precipitation conditions were optimized and the precipitated struvite was evaluated as nutrients source (majorly P) and wheat (Triticum aestivum L.) as a test crop in greenhouse trials. At Pir Mehr Ali Shah, Arid Agriculture University Rawalpindi, Pakistan, in the Institute of Soil and Environmental Sciences, the current study was carried out. The study details are provided below.

To create a representative sample, replicated samples were taken from each site and mixed together. The selected sites included the I-9 wastewater treatment plant, Dhoke Kala Khan (DK), Nala Lai (NL), and H-8 sector. Magnesium chloride hexahydrate (MgCl₂·6H₂O) and trisodium phosphate were used to maintain an acceptable ratio for Mg²⁺:-N: -P = 1.2:1:1 in order to optimize the precipitation conditions for struvite (Na₃PO₄). Seven pH levels (8.0, 8.5, 9.0, 9.5, 10.0, 10.5, and 11.0) and two stirring speeds of 250 and 300 rpm were selected for struvite precipitation.

Struvite separation was conducted by adding predetermined amounts of Mg (MgCl₂·6H₂O) and PO₄-P (Na₃PO₄) ions based on stoichiometric calculations that considered the initial ion concentrations in the samples (Table 2) to maintain the desired Mg:N:P ratio. A combination of Mg:N:P ratio of 1.2:1:1, pH values ranging from 8.0 to 11.0, and stirring speeds of 250 and 300 rpm were used for MAP/struvite precipitation. During each MAP precipitation process, 100 mL of wastewater was mixed with the required dosages of magnesium and phosphate sources in a 100 mL flask, which was stirred continuously on a magnetic stirrer until a stable pH was achieved, typically after 15 min. Adjustments to the pH were made using 2 N or 5 N NaOH solution after the chemicals were added. The samples were then stirred for another 30 min before allowing the mixture to settle for 60 min to separate the precipitated crystals at the bottom from the liquid. The supernatant samples were collected for further analysis.

In addition to being analyzed for its nutritional content, struvite was put to the test in a greenhouse pot experiment with the test crop (wheat, Triticum aestivum L.) to see how well it worked as phosphatic fertilizer. For the processes happening in soil for P release, the struvite and the soil both were investigated of an alkaline nature. Struvite was used in five different combinations in 5 kg of soil plastic pots to assess the release of P, including I control (no P source), (ii) all P from SSP fertilizer, (iii) all P from struvite, (iv) all P from struvite + sulfur (100 mg kg⁻¹ of soil), and (v) all P from rock phosphate. In all treatments, regardless of source, the application rate of phosphorus was 100 mg kg⁻¹ soil, with the exception of control. Additionally, the sulfur-oxidizing bacteria (SOB) inoculation was applied to all five of the treatments once again. During all stages (tillering, booting, and maturity) of the crop, soil and plant samples were taken, and information was examined and documented for various soil and plant characteristics. Height, spike length, quantity of grains per spike of the plant, chlorophyll content, fresh and dry matter biomass yield, and grain yield were all noted. P analysis was performed on the plant samples, and P uptake was computed. Measurements were made of the soil's pH, EC, and available P during the tillering, booting, and maturity stages.

By using a pH meter, rinse the electrode with distilled water and then immerse it into the wastewater sample. Stir the solution gently with a stirring rod and wait for the pH reading to stabilize on the pH meter display (Page et al. 1982).

Collected a representative wastewater sample using a grab sampling technique. Prepare a series of phosphorus standards in the range of 0.1–1.0 mg/L by diluting a stock solution with phosphorus-free water. Pipette 5 mL of the wastewater sample and transfer it into a 50 mL volumetric flask. Add 5 mL of ammonium molybdate solution to the flask and mix well. Add 5 mL of ascorbic acid solution to the flask and mix well. Dilute the mixture to the mark with phosphorus-free water and mix thoroughly. Allow the mixture to stand for 15–30 min at room temperature to allow the color to develop. Measure the absorbance of the solution at a wavelength of 880 nm using a spectrophotometer. Construct a calibration curve by plotting the absorbance versus the phosphorus concentration of the standards. Determine the phosphorus concentration of the sample using the calibration curve. If the phosphorus concentration of the sample is above the detection limit of the method, dilute the sample and repeat the analysis.

By distillation apparatus about 100 mL of distilled water was added to 50 mL of the sample. The boric acid indicator was prepared by mixing 20 g of boric acid with 20 mL of mixed indicator in 900 mL of hot distilled water and volume was then calculated. The distillate was titrated against 0.02 N H₂SO₄ after distillation until the pinkish color appeared.

Precipitates were dried at 450 °C in an oven for 2 days following MAP crystallization, and then they were weighed, tested for -P by dissolving in 0.1 M HCL, and P levels were quantified using the ascorbic acid technique and a spectrophotometer.

Financial analysis refers to the process of evaluating the financial health and performance of a system using various financial tools and techniques and it also identifies areas for improvement.

The information gathered for soil and plant characteristics at each sample stage was statistically analyzed using a two-factor factorial design, and least significant difference (LSD) at a significance threshold of 5% was used for treatment means (Shen et al. 2017). Statstix 8.1 was used to analyze the data.

The mean pH of the collected wastewater from four locations ranged from 7.28 to 7.81 (Table 1). The site-2 (NL) (Rawalpindi) source's wastewater has the highest pH of any of the other sources at 7.81. In wastewater samples, phosphorus concentrations varied and have a mean value of 42.0 mg L⁻¹ and a range of 21.2–59.5 mg L⁻¹. Wastewater from the site-1 (I-9S) area (Islamabad) has the greatest phosphorus concentration (59.5 mg L⁻¹) compared to all other sources. Wastewater sample's concentrations varied with a mean value of 40.8 mg L⁻¹ and ranged from 35.0 to 46.0 mg L⁻¹. Wastewater from the site-1 (I-9S) area had a higher level than any other source, at 46 mg L⁻¹. Wastewater samples had Mg²⁺ concentrations ranging from 32.0 to 42.0 mg L⁻¹, with a mean value of 37.5 mg L⁻¹. The Mg²⁺ concentration of the wastewater from the site-1 (I-9S) was 42 mg L⁻¹, which was higher than the Mg²⁺ level of any other locations.

Table 1

Wastewater characteristics collected from four sites (Site-1; I-9 Sector, Site-2; Nala Lai, Site-3; H-8 Sector, and Site-4; Dhoke Kala Khan). Different chemical compositions in wastewater such as phosphorus -P, ammoniacal nitrogen -N, and magnesium Mg²⁺

Locations .	pH .	-P (mg L−1) .	-N (mg L−1) .	Mg2+ (mg L−1) .
Site-1 (I-9S)	7.51	59.5	46 .0	42.0
Site-2 (NL)	7.81	56.0	45.0	39.0
Site-3 (H-8S)	7.71	31.2	37.0	37.0
Site-4 (DKK)	7.28	21.2	35.0	32.0
Average	7.59	42.0	40.8	37.5

Locations .	pH .	-P (mg L−1) .	-N (mg L−1) .	Mg2+ (mg L−1) .
Site-1 (I-9S)	7.51	59.5	46 .0	42.0
Site-2 (NL)	7.81	56.0	45.0	39.0
Site-3 (H-8S)	7.71	31.2	37.0	37.0
Site-4 (DKK)	7.28	21.2	35.0	32.0
Average	7.59	42.0	40.8	37.5

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The findings showed that despite the presence of several acidic and basic elements delivered from various industries or residences, the pH of all wastewater was within the normal range.

Additionally, effluent containing organic matter is regularly injected into the waste system. Phosphorus and magnesium concentrations were medium to high range in wastewater; this might be because wastewater is continuously enriched with municipal and industrial wastes that contain organic ingredients. The ejected P either infiltrates into the groundwater or dissolves into wastewater that is then carried to the wastewater system (Moreno et al. 2008). Wastewater had a low quantity of because it was outside of the typical range. This may be because rainwater was introduced to the system a few days before the sample day to dilute the effluent. All of these physicochemical process parameters are also influenced by the source of the wastewater effluent and the kind of treatment used. In order to give a fresh data, set under diverse process circumstances and to optimize the settings for struvite crystal formation, this study was conducted.

The necessary chemical doses for the crystallization of struvite were determined using stoichiometric calculations (Mg²⁺:-N:-P = 1.2:1:1) (Table 2).

Table 2

Wastewater collected from four sites (Site-1; I-9 Sector, Site-2; Nala Lai, Site-3; H-8 Sector, and Site-4; Dhoke Kala Khan). With the following molar ratio: Mg²⁺:-N:-P = 1.2:1:1

Samples .	Amount of -N in wastewater (400 mL) (g) .	Mg and P sources .	Molecular weight .	Chemicals amount (g/mL) .
Site-1 (I-9S)	0.0184	MgCl2·6H2O	203.30	0.321
		Na3PO4	164.0	0.216
Site-2 (NL)	0.018	MgCl2·6H2O	203.30	0.314
		Na3PO4	164.0	0.211
Site-3 (H-8S)	0.0148	MgCl2·6H2O	203.30	0.258
		Na3PO4	164.0	0.173
Site-4 (DKK)	0.014	MgCl2·6H2O	203.30	0.244
		Na3PO4	164.0	0.164

Samples .	Amount of -N in wastewater (400 mL) (g) .	Mg and P sources .	Molecular weight .	Chemicals amount (g/mL) .
Site-1 (I-9S)	0.0184	MgCl2·6H2O	203.30	0.321
		Na3PO4	164.0	0.216
Site-2 (NL)	0.018	MgCl2·6H2O	203.30	0.314
		Na3PO4	164.0	0.211
Site-3 (H-8S)	0.0148	MgCl2·6H2O	203.30	0.258
		Na3PO4	164.0	0.173
Site-4 (DKK)	0.014	MgCl2·6H2O	203.30	0.244
		Na3PO4	164.0	0.164

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Different mechanisms for recovering phosphorus as struvite have been studied to assess their potential at a bench and pilot scale, while few processes, already integrated in wastewater treatment plants, are being applied effectively in the Netherlands and Italy (Battistoni et al. 2005). The P removal efficiency at different levels of pH indicated higher precipitation of struvite and higher P was removed at elevated pH conditions. The trend for P removal over different pH levels and at varying stirring conditions has been reported in Figure 1(a). These variations in removal effectiveness were caused by the properties of the wastewater, as well as varied pH and P:Mg:N ratios. The increase in pH level altered the P removal significantly and the highest removal (85%) was achieved at a pH of 10.5 and stirring conditions of 250 rpm. The increase in P removal continued till pH 10.5 and declined thereafter. Phosphorus recovery as struvite has been one of the interested way of researchers in the past decade (Doyle & Parsons 2002). Stirring speed contributed significantly toward P removal and ranged between 47 and 49% at 250 and 300 rpm, respectively. Slower stirring conditions proved better struvite formation from wastewater. The supernatant after struvite precipitation showed lower P levels at higher stirring speed. The pH levels employed had a key role in formation of struvite. The residual P content in wastewater ranged from 4 to 85% in response to pH condition changes from 8 to 11. The difference in P removal under stirring was displayed better at elevated pH. Since the molar ratio applied was maintained at Mg²⁺:-N:-P = 1.2:1:1.

A non-significant difference existed for P removal efficiency at pH levels of 10.5 and 11 and ranged from 82 to 85%, respectively. However, the residual P concentration in the supernatant was lower at elevated pH levels and ranged between 5.72 and 39.75 mg L⁻¹. According to Zhang et al. (2012), the raw swine wastewater's phosphorus removal effectiveness ranged from 40.3 to 88.5% at a P:Mg:N ratio of 1:1.5:1.67 and a pH range between 7.5 and 9.0. According to Stratful et al. (2001), increasing pH from 7.5 to 8.5 increased removal efficiency from synthetic wastewater from 42 to 85%, whereas increasing pH further to 10.0 only increased removal efficiency from synthetic wastewater to 87%.

Mechanically stirred reactors have proved efficient for P removal. Korchef et al. (2011) achieved 60% P removal in synthetic liquors and determined up to 90% P removal in anaerobic digester. However, the high P removal can also be attributed toward co-precipitation of calcium phosphate with struvite under high calcium levels (Yee et al. 2019).

There was no significant change in NH₄-N removal through MgCl₂ and MgSO₄, however MgO hindered NH₄-N removal from wastewater significantly by increasing total suspended solids (TSS) (Yetilmezsoy & Sapci-Zengin 2009). The residual N contents ranged from 12.2 to 31.3 mg L⁻¹. As the pH rose, the residual N contents decreased and MAP precipitation and N removal increased significantly. The molar ratio of 1:1.5:1 (P:Mg:N) at pH 9.0 increased the removal efficiency of -N up to 89.4% from sewage sludge effluent (Uysal et al. 2010). The trend for N removal was not similar to that of P removal. The pH alteration from 9.5 to 10.5 increased the MAP precipitation and N removal was enhanced. Moreover, the stirring speed effect was more evident at the pH range of 9.5–10.5. The pH values greater than 10.5 declined the N removal and MAP precipitation. The lower shaking speed utilizes less energy and was more economical for struvite precipitation. Stratful et al. (2001) achieved a mere 18% NH₄-N removal at pH 7.5 by maintaining ratio 1.0:1.0:1.9 (P:Mg:N), due to the lower pH value of the wastewater.

A significant positive correlation existed for struvite production with P and N removal. The highly positive correlation for P removal (r = 0.86) and struvite production indicated toward the direct relationship for struvite production and removal of P from wastewater. The maximum struvite production was achieved when maximum P was removed and the least residual P contents were present in the wastewater (Figure 1(c) and 1(d)). The phosphorus from wastewater cannot be removed through gaseous exchange like nitrogen and therefore needs to be precipitated (Beckinghausen et al. 2020). When the water temperature increased from 25 to 45 °C, the treatment time decreased to 60 min, and the EC reduced to 42.7 kJ/L. Different reactive oxygen species (¹O₂, ·O₂⁻, and ·OH) had been detected and played important roles during the disinfection process. Similarly, the struvite precipitation was directly affected by N removal (r = 0.86) from wastewater and with increased N removal the weight of struvite increased indicating toward the dependence of N and P removal toward struvite production. Moreover, the struvite production was greater at lower stirring speeds of 250 rpm rather than higher (300 rpm), as lower shaking resulted in better crystal formation and high struvite production. High mixing speeds decreased the crystal growth and elongation (Ohlinger et al. 1999).

The relationship of struvite precipitation and pH levels indicated highly significant results (r = 0.86). At elevated levels of pH, the conditions favored the MgNH₄PO₄·6H₂O precipitation (Le Corre et al. 2009). The maximum level of production of struvite was depicted by a 10.5 pH level (Figure 2(c)). At pH levels of 8.3–8.5 61%, P removal was achieved (Battistoni et al. 2005). The process of nucleation is followed by crystal growth at elevated pH levels for struvite production (Jones 2002). Synthetic solutions showed a detrimental effect on crystal size at elevated pH 8–11 levels (Matynia et al. 2006). Furthermore, the stirring speed also contributed toward struvite precipitation. Lower speeds not only proved economical in respect of energy consumption but also provided the suitable shaking conditions for crystallization of struvite.

The elevation of pH and an appropriate saturation of the involved ions increased the crystallization process and up to 30% increase in struvite production was achieved. Further, the agitation of the influent increased struvite production up to 9% (Rahman et al. 2014). The rise in pH at wastewater treatment plants occurs due to CO₂ stripping (⁠ → CO₂↑ + OH⁻) which is responsible for struvite production (Neethling & Benisch 2004). The crystal size is decreased by increased stirring conditions as a result of increased nucleation (Durrant et al. 1999).

Throughout crop growth period, soil pH and nutrient availability were greatly influenced by the type of phosphorus source applied. In the control, no phosphorus source was provided, and there was no SOB, the pH of the soil did not dramatically alter over time. Sulfate was added with P as part of the single superphosphate (SSP) addition, which helped manage soil pH and led to the lowest pH values at all phases of monitoring. The addition of struvite raised the pH level, which continued to rise dramatically from the beginning to the end of the crop cycle. The steady rise in pH level was brought on by the struvite's increasing solubility over time. Regarding the change in soil pH, rock phosphate did not substantially vary from the control (no P supply). The SOB application significantly influenced the soil pH at every sampled stage. It is correlated with the production of sulfuric acid under the influence of SOB in the presence of sulfur, which serves as a food source for SOB (Ullah et al. 2013). An essential substrate for SOB is elemental S (Pokorna & Zabranska 2015) and its oxidation by SOB is a biological process in which sulfuric acid production is carried out. In treatments where SOB was supplemented, a significant influence of SOB addition was recorded. However, the most promising results regarding soil pH control was attained with sulfur addition with struvite as P source. The soil pH approached toward normality which increased the nutrients supply to crop with the addition of SOB. The treatment with struvite alone had highest pH value (7.57) among SOB-added treatments, as struvite had very high initial pH.

The higher N-P₂O₅ content in struvite was always favoring its use as a phosphatic fertilizer but its ability to provide nutrients over the growing period was the key concern. The usage of SOB inoculum was employed as a mediated measure for the projected rise since the use of struvite in an alkaline soil environment elevated soil pH. Without SOB inoculation, struvite alone and in conjunction with sulfur did not vary substantially from the chemical P source, SSP. 100% of the P uptake from struvite has been studied and published (Khan et al. 2020). However, sulfur-added struvite therapy, where superior growth conditions and food sources promoted bacterial proliferation and boosted P bioavailability, reacted better to the usage of SOB. The majority of phosphatic fertilizers are produced using rock phosphate. However, using raw rock phosphate was ineffective in giving plants P and did not materially vary from the control, which had no P source applied. Rock phosphate is less soluble in calcareous soils because of the high calcium content. However, under alkaline calcareous circumstances, struvite was more soluble than rock phosphate. Additionally, the P absorption dramatically rose from tillering to booting and continued to rise till maturity. The treatments set without microbial inoculation SSP performed best in terms of providing P to plants because the sulfate in SSP aided in regulating soil pH levels, which improved P and other nutrient absorption. The outcomes were consistent with those reported in the VUNA experiment because struvite delivered adequate nutrients for absorption. According to Ganrot et al. (2007), struvite and other chemical P fertilizers showed to be effective sources of P for plants, and P absorption in wheat crops was unaffected by changes in P source. When the straw was harvested, the P absorption from it ranged from 7.96 to 1.34 mg kg⁻¹. The P uptake increased throughout time. For the SOB inoculated treatments, the maximum P uptake was figured. Struvite + sulfur showed the greatest P uptake at 7.96 mg kg⁻¹, followed by struvite and SSP at 6.71 and 6.43 mg kg⁻¹, respectively. When struvite was added with sulfur in addition to SOB, P absorption by the crop at the maturity stage was increased compared to the SSP-modified treatment without SOB addition. Rishi & Goswami (1977) discussed the increased P uptake with increasing P solubility. However, the poor solubility and soil environment disintegration of rock phosphate meant that its application did not contribute to substantial P absorption. So, the rock phosphate was very close to control and was below with SSP and struvite in both with or without SOB inoculated set of treatments. Dhillon et al. (1993) showed increased P uptake by using good P fertilizer like SSP. Our results were found in close relation with those of Setia & Sharma (2007).

By analyzing various crop development characteristics, the application of wastewater precipitated into struvite and its capacity to deliver nutrients equivalent to SSP were examined (Table 3). In terms of supplying nutrients to agricultural plants, struvite alone and in combination with sulfur were determined to be statistically comparable and provide greater growth and yield which was statistically comparable with the chemical P fertilizer, SSP. The treatment of struvite demonstrates numerically superior results, as evidenced by its effectiveness in all stages of growth, including tillering, booting, harvesting, and grain yield, compared to all other treatments except for struvite + SSP. This indicates a positive outcome for the struvite treatment. The struvite, a precipitated wastewater P fertilizer, showed better growth and produced more effectively in alkaline soil conditions. The chemical fertilizer SSP along with struvite alone and as comparison to the raw rock phosphate and the control where no P source was supplied, in conjunction with sulfur, demonstrated superior in the growth of tillers. Morison et al. (2008) reported an increase in plant height along with other agronomic parameters like leaf area index and plant vigor with the use of different ratios of N and P fertilizers. Elser & Bennett (2011) proved the function of phosphorus (P) in plant development and connected plant growth metrics, such as plant height, with the P content of the soil.

The inoculation of SOB increased the microbial activity in soil and helped in production of organic acids and lowering soil pH. The SOB inoculation in an alkaline soil environment increased nutrient availability and microbial diversity, which improved the use of P sources. The behavior of struvite, which has an alkaline nature, expected the use of SOB inoculum as a pH regulating agent. In terms of crop development, the use of sulfur as a medium for the inoculated SOB produced the greatest results. The supply of P was put under pressure in these treatments, and the solubility of raw rock phosphate in alkaline soils remained a major problem. Greater biological and grain yield were the result of better root proliferation under microbial inoculation, which increased nutrient accessibility. Prabhu & Mutnuri (2014) indicated in their study that the additional amount of N and Mg along with P increased the chlorophyll content in the struvite applied treatments. Similar to this, plant biomass rises when Olsen P levels are high in the soil (Afzal & Bano 2008). Better nutrient absorption by the crop, notably N and P, which play a role in vegetative development, was associated with an increase in biological yield (Ma 2004). Phosphorus has a positive correlation with crop growth when applied with the required amount of N (Pederson et al. 2002). The absence of P source also significantly decreased the biological and grain yield of wheat. In comparison to the rock phosphate and control treatments, the tillers produced more under the SSP and struvite-amended treatments. Height and biological output were both boosted overall due to the use of a suitable P fertilizer source. Higher biological yield, better growth, and more vigor are the effects of using struvite plus a suitable amount of DAP (Barak & Stafford 2006). Soil P availability under SSP and struvite proved better while rock phosphate did not improved availability of P under alkaline conditions because of low dissolution. Hassan (2013) concluded in their research that optimum P level improved crop yield parameters like height, biomass, and grain yield. Basak et al. (2020) reported the importance and role of P in increasing biological yield of wheat. Elser & Bennett (2011) also confirmed the increased growth of wheat by enhanced P supply. The number of grains per spike increased when P₂O₅ was more readily available at the same levels of N, demonstrating the significance of P for seed production and grain filling. The crop received the necessary amounts of potassium and nitrogen but the P absence resulted in lower vigor and biomass production which influenced in lesser output in grain yield perspective.

Increased nutrient availability boosts soil fertility, which in turn enhances plant nutrient absorption and boosts crop vigor and height (Imtiaz et al. 2003). The grain production showed better and well-formed grains where P availability and uptake remained high at critical growth stages. The SOB-infected treatments had higher and better crop responses, and sulfur administration significantly decreased soil pH and increased nutrient availability. Not only did the application of struvite increase the availability of P, but it also promoted crop growth and productivity by containing nitrogen and magnesium. Furthermore, the grain weight and number of grains were more and better in the treatments where P stress was not applied. Higher grain yield of wheat was found by Kumar & Yadav (2005) as influenced by increasing P levels. Additionally, Villar-Mir et al.(2002) discovered that using the correct P fertilizer dosages boosted the grain production of wheat crops. The use of struvite as a fertilizer has been described by many studies over the years (Liu et al. 2011). Struvite has mostly been employed on turf grasses, vegetables, and decorative plants since its delayed release makes large amounts safe for the plants. The high Mg demand in sugar beet and other crops can be supplemented by struvite (Gaterell et al. 2000).

Struvite, a naturally occurring phosphate mineral, has gained attention as a potential alternative to fossil-based fertilizers due to its potential to recover and reuse phosphorus from wastewater and other organic waste streams. The industrial importance of struvite can be quantified in financial terms by considering its cost-competitiveness with fossil fertilizers.

The cost of struvite production can vary depending on the method used for recovery and the characteristics of the wastewater or waste stream from which it is recovered. However, it is generally more expensive to produce struvite compared to fossil-based fertilizers such as DAP and TSP. According to a study published in the Journal of Environmental Management, the cost of struvite production ranges from $200 to $800 per ton, while the cost of DAP and TSP is around $500–$600 per ton. This suggests that struvite is currently less cost-competitive than fossil-based fertilizers. However, it is important to note that the cost of fossil-based fertilizers does not reflect the environmental and social costs associated with their production and use, such as greenhouse gas emissions, water pollution, and soil degradation. In contrast, the production and use of struvite can contribute to reducing these impacts by recovering and reusing nutrients from wastewater and organic waste streams. Struvite has gained attention as a sustainable fertilizer due to its ability to recycle phosphorus from wastewater. While it is cost-competitive with fossil fertilizers in terms of nutrient content, the high capital cost of building struvite recovery systems limits widespread adoption. Nevertheless, the global market for sustainable fertilizers is projected to reach $3.5 billion by 2027, providing an opportunity for struvite to capture a share of this growing market. The economics of struvite production in this study is similar to other commercial production, provided that the production costs calculated during the pilot testing are only slightly lower than the lowest struvite wholesale prices (Stavkova & Marousek 2021).

Therefore, while struvite may not currently be cost-competitive with fossil-based fertilizers, its potential environmental and social benefits should also be considered in assessing its industrial importance. As concerns over the sustainability and long-term availability of phosphorus continue to grow, struvite and other alternative sources of phosphorus are likely to become increasingly important in the future. Due to its capacity to recycle phosphorus from wastewater, struvite is poised to emerge as a sustainable fertilizer that is attracting growing attention. As a result, there is an opportunity for struvite to capture a share of this expanding market.

Optimization of pH level and stirring speed enhances the quality as well as production of struvite from wastewater. The results indicate that the highest removal of P and N was achieved in the wastewater at a pH of 10.5. Struvite was discovered to be an outstanding P source for the key cereal crops like wheat because it maintained enough phosphorus accessibility due to its slow-release mechanism throughout the growth season. Wheat growth and yield was more when struvite was used as phosphorus fertilizer, and phosphorus availability and absorption were equivalent to those of a SSP. Therefore, this technology provides opportunities to recover phosphorus sustainably from wastewater and preserve phosphorus reserves. Consequently, in order to ensure the maximum benefit of struvite as a fertilizer for wheat crops, it is recommended to study the solubility product of struvite and its dependence on various factors, including pH levels. This task accessibility can only be improved through partnerships with other companies, researchers, and public authorities. Collaborating with these entities can provide the resources, knowledge, and support needed to overcome any challenges and ensure that the task is accessible to all and will be beneficial for economy as well.

We acknowledge the Institute of Soil and Environmental Sciences, PMAS-Arid Agriculture University Rawalpindi, Pakistan for all the laboratory and technical assistance.

S.R. designed the research and manuscript writing. S.S.I. and M.A.R. have given their inputs for the interpretation of the results. S.F., N.J., and M.I. performed experimental analysis. S.H., A.S., and M.L. reviewed, corrected, and formatted the manuscript. A.R.K., R.A.K., and T.A. helped to collect the data and processed it in the laboratory.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.