Optimization and modeling of process parameters for nutrient recovery from sewage wastewater

Phosphorus (P) stands as an indispensable component of life, without any viable replacement (Chowdhury et al. 2017). Phosphate is typically derived from phosphate mineral rock, a scarce resource projected to be exhausted shortly (Li et al. 2019a). Simultaneously, the efficiency of current phosphorus utilization is maximum with a minimal amount of phosphorus being reused or recycled (Li et al. 2019b). During the leaching, the phosphorus enters rivers, lakes, and the ocean, resulting in eutrophication and red flood (Numviyimana et al. 2020). This large-scale discharge of nutrients can be mitigated by P removal techniques such as physical methods like adsorption, filtration, chemical precipitation, or ion exchange as well as biological methods using plant uptake or microbial degradation techniques (Ramasahayam et al. 2014; Perera et al. 2019). Recent past growing interest in economically justified concepts for P recovery using low-cost biochar adsorption (Maroušek et al. 2020). However, the recovery rate of nitrogen (N), and P from wastewater using these available methods did not contribute to the efficient use of nutrients for food security (Szymańska et al. 2020). Sometimes the suitability of recovered products like sludge as a fertilizer is debated due to the contaminants. Among the technological solutions for nutrient recovery from waste streams available in the literature (Ren et al. 2016), struvite precipitation and ammonia stripping are identified in systematic maps of technologies from domestic wastewater/agriculture waste streams/livestock wastes (Rodríguez Arredondo et al. 2015).

A notable feature in the precipitation of struvite is the simultaneous retrieval of ammonia and phosphate from nutrient-rich wastewater (Rufí-Salís et al. 2022). This method entails crystallization in the presence of magnesium, resulting in the formation of magnesium ammonia phosphate, commonly referred to as struvite (MgNH₄PO₄·6H₂O) (Yu et al. 2017). Struvite is a magnesium-rich, slow-releasing fertilizer rich in magnesium, ammonium, and phosphate and has a mica-like crystal with a white to yellowish plate appearance. Additionally, it finds application as both a construction material and an adsorbent (Uysal et al. 2014). On the one hand, the cost of N, P fertilizer has increased several times in the last 5 years for various reasons (Krishnamoorthy et al. 2021). India is primarily aligned with agriculture, being the foundation of the country's economy (Talboys et al. 2016). The government is allocating a budget for supplying N, P fertilizer at a subsidized rate. Therefore, the traditional fertilizers can be substituted with struvite which would simultaneously support a circular economy.

Literature reports on the studies of the economy of struvite have been discussed by Josef Marousek. The report states that more than 2.5 kg of P can be recovered from the 80 mg P L⁻¹ environment per 100 kg of modified biochar (Maroušek et al. 2020). The cost of production (213 € 1 kg Pe^{− 1}) is approximately 3% less than what the farmers are ready to pay for the same product (Maroušek & Gavurová 2022). In another report on the effect of change of struvite sale price on the profit share had examined for the optimum conditions, and the investment and operating costs were determined. The net profit is predicted when the struvite sale price is raised to 560 V/ton, with a payback period of approximately 6 years (Yetilmezsoy et al. 2017). Few reports on the sale of recovered struvite at higher prices in the range of US$ 100–450 depending upon the operating conditions and environmental discharge standard.

However, the recovery of phosphorus and nitrogen through struvite crystallization exhibits significant promise in addressing the diminishing of mineral phosphorus resources mitigating environmental pollution in nearby streams (Sun et al. 2023), and completing the P loop. Another advantage of struvite as fertilizer over conventional P mineral fertilizer is its slow-releasing rate which can completely avoid run-off when rain falls soon after the application (Shalaby et al. 2015). Low P availability to plant nutrition is associated with the complexity of struvite crystal formation that is driven by process conditions. Furthermore, the decomposition process is mainly from a form of struvite, a complex mineral present during the preparation, and conditioning of soil, crop response to struvite, etc. (Stávková & Maroušek 2021). Compared to traditional fertilizer, P solubility is limited in struvite that can avoid the P adsorption or immobilization into soil particle surfaces (Latifian et al. 2012). Concurrently, it increases the P concentration for early crop growth and later stages when crop P demand is high (Zin et al. 2020).

In light of these facts, an alternate source of fertilizer production is necessary to fulfill the demand for fertilizer (Lavanya & Ramesh 2021). In this regard, the beneficiation of nutrients from waste streams to form P-rich compounds as fertilizer is attractive in economies as well moving to fulfill sustainable development goals (SDG) 2 and 6 (Wang et al. 2021; Moyo et al. 2023). Therefore, from the beneficial use of struvite under a reduced application rate of P whilst maintaining the yield with minimum environmental impact, this study aims to produce struvite from wastewater with a method of low-cost, efficient, and convenient operation (Martinon 2023; Trotta et al. 2023).

Simple precipitation techniques can produce struvite, but it is important to understand the key factors of its nucleation and growth, in addition to the particular mechanics of crystallization that lead to its formation (Campos et al. 2023). Widely investigated studies in the literature are on optimal pH (Moussa et al. 2011), temperature, stirring rate, and supersaturation level (Zhang et al. 2018) including the effects of external ions like calcium ions (Ca²⁺) (Capdevielle et al. 2013) and K⁺. Similarly, the sources of waste streams are different, including municipal wastewater (Qiu & Ting 2014), agro-industrial wastes, swine wastewater (Huang et al. 2014), and human urine (O'Neal & Boyer 2013). Of all of these sources, sewage contains a lot of nitrogen and phosphorus but little magnesium (Mg). Therefore, amending the Mg source is necessary to get a substantial recovery of P. Magnesium obtained from a variety of salts, including seawater, brucite, MgCl₂ (magnesium chloride) (Zhou et al. 2021), and magnesite. According to reports utilizing bittern as a source of Mg (Addagada 2020) can reduce the cost as well, struvite precipitation was able to reach as much as 98% of the phosphate from sewage effluent (Wang et al. 2023).

From the process point, it is clear that to achieve the largest output from wastewater, it is practically necessary to optimize the struvite precipitation process. However, the standard procedure for figuring out the ideal circumstances for struvite precipitation is incredibly time-consuming and labor-intensive because only one parameter is changed at a time while the others are kept constant (McIntosh et al. 2022). Given the above, the response surface methodology (RSM), which is one of the most used techniques is followed in this study to examine the impact of pH, ammonium, phosphate, magnesium, and calcium concentrations on the recovery of phosphate from actual sewage wastewater.

The economic viability of struvite recovery is pivotal for its widespread adoption. Studies emphasize the dual importance of profitability and environmental sustainability in attracting investment and overcoming the traditional barriers to technology adoption (Martinon 2023; Trotta et al. 2023). By closing the phosphorus loop and supporting global SDG related to food security and water quality (Wang et al. 2021; Moyo et al. 2023), struvite precipitation aligns with broader sustainability agendas.

In conclusion, struvite recovery from wastewater represents a compelling solution to the urgent challenges posed by phosphorus depletion and environmental degradation. Its potential to transform wastewater into a valuable resource underscores the need for continued research and investment to optimize its efficiency and economic feasibility.

In the current study, six beakers, each with a working volume of 500 mL were utilized for the precipitation of struvite, magnesium ammonium phosphate (MgNH₄PO₄^.6H₂O). The trials were conducted following the experimental plan. The sewage wastewater was added, and mixed with the MgCl₂ solution at room temperature, and sodium hydroxide solution was utilized for obtaining the required pH. The Whatman filter paper with a pore size of 40 μm was used to screen the contents once the struvite precipitation had completed. The precipitate was dried at room temperature until it reached consistent weights. A sample of the reaction was taken, and it was promptly stopped and acidified with a 0.1 M hydrochloric acid (HCl) solution to evaluate the remaining phosphate concentration (Hussain et al. 2022).

The optimization of chemical reactions and the influent parameters are common applications of the RSM, which is a blend of statistical and mathematical methodologies. The Plackett–Burmann design (PBD) model was used in this study (Shim et al. 2020), and the experimental data collected was analyzed using the Minitab software (Minitab Inc., USA) to determine the factors that affect the result, specifically the percentage of phosphate removal. The statistical model takes into account the interaction between the various parameters that affect the recovery of phosphate (Song et al. 2018).

The results from experiments performed in this study were examined with respect to the chemical combinations, which met the fertilizer producer industry's receiving environment discharge standards and were economically compared based on the operating parameters summarized in Table S6.

Struvite synthesized using batch mode was subjected to mineral phase identification through X-ray diffraction (XRD) and Micro Raman spectroscopy. Field emission scanning electron microscopy (FESEM) with energy-dispersive X-ray (EDAX) was used to examine the morphological phase arrangement of the crystals that had formed. The crystalline content and phase composition of struvite were obtained using the XRD technique. The X-ray Diffractometer (Shimadzu, Japan), test model XRD-6000 is used for XRD at 40 kV and Cu K-alpha wavelengths: 1.54060 and 1.54439 Å (BRUKER USA D8 Advance, Davinci). Thermal stability was studied from thermogravimetric analysis/differential thermal analysis (TGA/DTA) in the temperature range from 30 to 350 °C at different heating rates under the N₂ atmosphere. All the experiments were conducted in duplicate to achieve the best accuracy. STA 2500 regulus-TGA-differential scanning calorimetry (DSC) thermal analyser is employed for thermograms and used in estimating the kinetics of the disintegration. Scanning electron microscopy (SEM) was performed using a high-resolution field emission electron microscope (Thermo-scientific Apreo S) to investigate the microstructure of struvite at elevated temperatures. The EDS study was done to recognize the composition and purity of the struvite obtained. It is a chemical microanalysis approach utilized in conjunction with SEM. The measurement of the wavelength and intensity of inelastically scattered light from molecules is known as Raman spectroscopy. The energy of the molecule vibrations causes the wavelengths of the Raman scattered light to differ from the incident light. The technique is used to find out the various functional groups that are Raman active. The BRUKER USA D8 Advance with Davinci served as the instrument model for the study, utilizing laser wavelengths of 325, 532, 633, and 785 nm, with a resolution of 0.4 cm⁻¹ at 532 nm.

The experiments were conducted using the Plackett–Burman Design (PBD) model, and the resulting data were analyzed with the Minitab software package (Minitab Inc., USA) to identify the factors influencing the outcome, particularly the degree of phosphate removal. This statistical model considers the interconnected relationships among the various parameters affecting phosphate recovery. Statistical analyses were conducted using Design Expert 7.1.6 statistical software (Stat-Ease, Inc., Minneapolis, MN, USA), which included the analysis of variance (ANOVA) and multiple regressions (Sathiasivan et al. 2019).

In the Plackett–Burman design experiments, 12 distinct trials were conducted to investigate 11 factors associated with 9 process parameters, including two dummy factors. Trial-7 had the maximum phosphate removal while Trial-9 had the lowest, according to the results shown in Table S3. To determine the confidence level and calculate the p-value, an ANOVA was performed on the response data. According to Table 2, the p-value at a 90% confidence level was determined to be the benchmark to identify the significant and comparatively non-significant parameters. Statistical analysis indicates that pH, Mg:P ratio, additive concentration, and precipitation time have a significant impact on struvite yield. According to their p-values, stirring rate, revolutions per minute (rpm), seed dosage (g/L), and temperature (°C) are insignificant.

The results of the ANOVA demonstrate that the linear and squared terms in the second-order polynomial model have a high significance (p ≤ 0.005), which accurately represents the association for phosphate recovery. The findings show that the factors (pH, additive concentration, pH with additive concentration, and Mg:P ratio with additive concentration) have a major effect on the recovery of phosphate. Notably, a reasonable agreement is indicated by a correlation coefficient of 0.97 between the predicted and experimental values.

The combined effects of pH and Mg:P on the response are shown in Figure 2(b), with the other two variables held constant. Phosphate recovery is seen to increase up to around half an hour into the precipitation period. Phosphate recovery does not significantly increase with further time extensions, indicating that the maximal phosphorous recovery can be reached in around 30 min at a pH of 10.5 and an additive concentration of 350 ppm. The response surface plot, shown in Figure 2(c), illustrates the effect of pH and additive concentration. It shows that up to 10.5 pH and 350 ppm additive concentration rises simultaneously, and phosphorous recovery increases. After that, the yield percentage decreases.

Based on the aforementioned observations, it can be concluded that the CCD predicts a maximum yield of approximately 89% phosphate recovery under conditions of pH 10.5, additive concentration of 350 ppm, Mg/P ratio of 1.2, and a precipitation time of 30 min. In agreement with these predictions, a recovery of about 93% under similar conditions of about 26 °C was reported (Kumari et al. 2019). The significance of pH 10.5 is highlighted, as it favors the formation of magnesium phosphate, thereby diminishing struvite formation. Moreover, an increase in additive concentration beyond 350 ppm results in reduced struvite formation, possibly due to excess ammonium ions forming soluble ammonium phosphate, thereby reducing phosphate recovery as struvite. A validation experiment conducted with sewage wastewater yielded approximately 85% phosphate recovery, as indicated in Table 3. These findings suggest promising phosphate recovery from sewage wastewater.

The following conclusions were drawn from a comparison of RSM cum CCD techniques and traditional optimization. In the classical method, stirring was employed without considering additive concentration. Stirring likely facilitated the escape of ammonia, thereby diminishing phosphate recovery as struvite. Additionally, stirring may have favored the formation of ammonium phosphate, as struvite nuclei could potentially be sheared. Consequently, the optimum parameters differed between the two approaches. Nevertheless, for continuous production, the conditions derived from CCD have been adopted.

The parameters examined included pH (ranging from 7 to 11), Mg/P ratio (from 1 to 2), and precipitation time (spanning 20–60 min), and the optimal point was identified. The CCD experimental design matrix, following the PBD experiments to identify significant variables, revealed that, in addition to precipitation time, pH, Mg:P ratio, and additive concentration also influence struvite formation. Accordingly, 30 distinct experimental sets were conducted within the predefined range of values. The discussed results covered the Mg:P ratio (1–2), pH (8–11), additive concentration (100–500 ppm), and precipitation time (20–60 min). With a statistically significant correlation factor of 0.92 considered, the observed findings were as follows: (i) At pH 10.5, 350 ppm of additive concentration, 1.2 magnesium-to-phosphate ratio, and 30 min of precipitation duration, the highest struvite yield (89%) was attained. (ii) Higher pH values than 10.5 promoted the production of magnesium phosphate, which decreased the quantity of struvite that formed. (iii) The synthesis of struvite was inhibited by an increase in additive concentration above 350 ppm, presumably due to an excess of ammonium ions.

Struvite precipitation from wastewater presents a promising economic opportunity for the recovery of nutrients from wastewater by reducing operational costs, generating revenue from recovered products, and contributing to environmental sustainability. However, its economic feasibility depends on several factors, including initial capital investment, operating costs, and market demand. The precipitation cum crystallization technique would be a suitable method of producing struvite fertilizer where N and P source from the raw feed of wastewater with zero cost. Since the relationship between phosphate content in raw feed and struvite production was non-linear as well, the ion concentration and ion–ion interaction became significant. For this reason, optimization studies must be conducted to maximize the N and P recovery as struvite with minimal amendment of magnesium source and alkali. With this background, the process conditions reported in this study and by other authors (Maroušek & Maroušková 2021) that the economic analysis was done for input chemical costs and the mass balance. The current report findings imply that the struvite precipitation modeling score R² is close to 1 with no negative impact on the final analysis suggesting the constructed model depicted better P removal efficiency compared to other results reported. The operating cost considering the chemical prices for the present study was found to be 0.36 USD as shown in Table S6. The production cost of diammonium phosphate (DAP) is approximately 0.44 USD, which is higher than that of recovered struvite. Commercial DAP relies on non-renewable rock phosphate as its raw material, whereas struvite production utilizes diversified raw feedstock, with a potential benefit in the physio-chemical treatment of wastewater containing nutrients. Furthermore, by optimizing the maintenance practice, improving industrial operation performance, and shifting standards toward supply–demand in this area, the adaptation of artificial intelligence (AI) and related technologies can help to transform positive cash flow change to project Industry 4.0 (Dvorský et al. 2023; Kliestik et al. 2023).

According to reports, struvite decomposes at a temperature of 125 °C. Because of this, the total mass loss was ascribed to the volatile emission of H₂O (g) and NH₃ (g) during heating in the 100–300 °C range.

The production of magnesium ammonium phosphate, struvite was performed from the phosphate-containing wastewater. RSM cum CCD model was applied to investigate the impact of four different independent factors after screening on the removal of P. The experimental data were interpreted using the polynomial regression model which has a coefficient of determination R² value of 0.97 and the maximum P removal was estimated to be up to 85% within the range studied. The Pareto solutions based on the removal of P comprised a minimum precipitation time of 30 min Mg: P ratio of 1:2 and the additive concentration of 350 ppm. According to the analysis, pH and Mg:P parameters are essential in P removal efficiency. Considering the economic viability of the present study, despite DAP remains a widely used fertilizer with a well-established market, struvite offers a more economical and environmentally friendly alternative. Struvite's lower production costs, reduced hazardous considerations, and sustainable use of raw materials with optimized process conditions make it an increasingly attractive option in the context of growing environmental awareness and cost-efficiency demands.

The authors extend their sincere gratitude to the management of SRM Institute of Science and Technology for their invaluable support in facilitating this research.

In the future, the optimization algorithm which precisely determines the P recovery can be implemented.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.