Production of slow release crystal fertilizer from wastewaters through struvite crystallization – A review

4.1 Molar ratio

Recently, researchers have tried to identify the factors that influence the struvite precipitation. A lot of factors such as types of chemicals added, PO₄:Mg molar ratio, pH, type of the reactors and the removal efficiency of P and N by several researchers are shown in Table 2. Most of the researchers used MgCl₂ as a source of magnesium for struvite precipitation, but only few of them used MgSO₄, MgO and Mg(OH)₂. Yetilmezsoy and Zengin (2009) found no significant differences between MgCl₂ and MgSO₄ in case of NH₄-N, COD and color removals. However, a much lesser NH₄-N, COD and color removal occurred when MgO was used as a source of magnesium. Kim et al. (2007) stated that magnesium hydroxide attributed to increased TSS in the solutions that lead to a lower NH₄-N removal. From the esthetic point of view, it might be stated that the addition of MgCl₂ as a source of magnesium is more suitable to achieve highest efficiencies of ${\text{NH}}_4^{+}$ -N, COD and color removals. Therefore, MgCl₂ may be proposed as the best option to achieve the maximum removal of ${\text{NH}}_4^{+}$ -N through recovery from wastewater. The presence of calcium ion (Ca²⁺) in the wastewater might preferentially react with PO₄ to produce hydroxyapatite, dicalcium phosphate, and octacalcium phosphate. Correspondingly, low level of residual PO₄ was observed by Kim et al. (2007) where Ca²⁺ concentration was approximately 224 mgL⁻¹. Le Corre et al. (2005) found an amorphous calcium phosphate in struvite when high concentration of Ca²⁺ ions was presented in the solution. Doyle and Parsons (2002) also stated that struvite formation can be inhibited by the formation of calcium phosphates. The presence of residual Ca²⁺ in the solution might be the cause of impure struvite. Therefore, it might be stated that highly pure struvite can be achieved by the addition of magnesium and orthophosphate in an ammonia rich wastewater.

It is found that a wide range of PO₄ and Mg ratio was applied for struvite precipitation, but in most cases, the effective ratio was 1:1 or 1:1.2 (Rahman et al., 2011). The addition of chemicals to the wastewaters would be needed to provide an equimolecular condition of PO₄ and Mg. Yetilmezsoy and Zengin (2009) conducted a series of experiments to see the effect of Mg, NH₄ and PO₄ ratio on struvite precipitation and nitrogen removal efficiency. They found a lesser N and COD removal at under dose conditions of molar ratios of Mg:NH₄:PO₄ (0.5:1:1, 0.8:1:1, 1:1:0.5, 1:1:0.8) compared to higher molar concentrations (1.2:1:1, 1.5:1:1, 1:1:1.2, 1:1:1.5). Actually, lesser N and COD removal occurred when the molar concentrations of Mg²⁺ or ${\text{PO}}_4^{3-}$ were lower than 1, compared to higher molar concentrations (1.5:1:1 and 1:1:1.5). They concluded that the Mg²⁺: ${\text{NH}}_4^{+}$ -N: ${\text{PO}}_4^{3-}$ -P molar ratio of 1:1:1 would be enough for the residual N removal and struvite precipitation. Zhang et al. (2009) stated that suitable Mg²⁺: ${\text{NH}}_4^{+}$ : ${\text{PO}}_4^{3-}$ would be 1.15:1:1 in order to remove ammonium effectively and avoid higher concentration of ${\text{PO}}_4^{3-}$ in the effluent. Kozik et al. (2011) found that an average crystal size was 42 μm when the pH was 9.0 and the PO₄:Mg molar ratio was 1:1, but the crystal size increased at 58 μm when the molar ratio was increased to 1:1.2 within 30 minutes. The crystal growth was 67 and 80 μm when the PO₄:Mg was 1:1 and 1:1.2, respectively in one-hour reaction at the same operational conditions. Larger crystals were also found by several other researchers with a higher molar ratio (1.2 mole) of magnesium based on the PO₄ of the wastewaters (Koralewska et al., 2009; Matynia et al., 2013; Hutnik et al., 2012, 2013a,b).

Struvite crystallization also occurred in swine manure composting when 1.2 mole Mg was added based on the TN of the manure (Lee et al., 2009, 2010). Lee et al. (2009) found an interesting behavior of the ortho phosphate/total phosphate (OP/TP) ratio in struvite formation during composting of swine manure. They found that the OP/TP ratio decreased when both Mg and PO₄ were added for struvite formation, and the decreasing rate was proportional to the amounts of Mg and PO₄ up to 0.05 molar ratio of Mg. The added Mg and PO₄ react with aboriginal NH₄ or the NH₄ produced during organic matter decomposition, resulting from the formation of struvite and subsequently the OP/TP decreased. In case of addition of only Mg, continuous struvite formation was not found during composting when Mg was below 0.06 molar ratio due to Mg deficiency. Whereas, Lee et al. (2010) stated that a decrease of OP/TP and a clear increase of TP content in the finishing compost might be due to the struvite formation in the finishing compost; because, they added 1.2 mole Mg according to the TN of the swine manure. Finally, they concluded that continuous formation of struvite during composting can be achieved if more than 0.06 molar ratio of Mg according to TP of the manure was added. However, further study is needed to identify the maximum amount of Mg for optimum struvite formation.

4.2 pH

The pH plays an important role during the struvite precipitation process. Struvite or MAP can be precipitated at a wide range of pH (7.0–11.5), but the suitable pH ranges between 7.5 to 9.0 (Hao et al., 2008). Efficiency of MAP precipitation depends on the concentration and molar ratios of Mg²⁺, ${\text{NH}}_4^{+}$ , & ${\text{PO}}_4^{3-}$ , pH, aeration rate, temperature, and presence of Ca²⁺ in the reacting media (Stratful et al., 2001; Le Corre et al., 2005; Hao et al., 2008; Yetilmezsoy and Zengin, 2009). The rate of pH decrease reflects the speed of crystal growth rate and also influences the quality of precipitated crystals. So, pH might be used as an indicator of struvite nucleation (Bouropoulos and Koutsoukos, 2000; Kabdasli et al., 2004). Influent pH is also important in transforming of ${\text{NH}}_4^{+}$ ions into gaseous ammonia through ammonia stripping. Various researchers have investigated the effect of pH on the removal efficiency of P and N (Ohlinger et al., 1999; Celen and Turker, 2001; Stratful et al., 2001; Battistoni et al., 2001; Dastur, 2001; Adnan et al., 2003). All these investigations have found a higher P and N removal with the increasing of pH. This might establish the fact that the solubility of struvite decreases with increasing of pH (Snoeyink and Jenkins, 1980), thus facilitating higher P & N removal and recovery for a given feed strength. Adnan et al. (2003) also stated that the operating pH should be raised up to 8.3 to get more than 90% P removal. They increased the pH gradually to a higher value and found a smoother operation of the reactor. Theoretically, there should be a direct relationship between pH and the P removal to some limiting value. Steady operation condition in struvite crystallization could be obtained by maintaining the pH, molar ratios, influent flow rate, and temperature. Systematic studies on successful steady-state operation of struvite were obtained both in bench scale operation (Lee et al., 2010; Adnan et al., 2003) and continuous reaction crystallization process (Matynia et al., 2013; Hutnik et al., 2012; Hutnik et al., 2013a,b). It can also be observed that comparable removal efficiencies were obtained in low and medium phosphorus concentrations, since for both feed strengths it was possible to achieve high P removal rates at more or less similar operating conditions.

Solution pH also influences the growth rate of struvite crystal. Ohlinger et al. (1999) showed that the accumulation of struvite on stainless steel coupons in contact with supernatants of anaerobically digested sludge was influenced by the pH. An increase in pH caused an increase of supersaturation and a resultant increase in the growth rate (Le Corre et al., 2009). Ali (2007) stated that the formation of struvite depends upon the reactant concentration and solution pH. He (Ali, 2007) found a mean particle size of 127.77 μm and 133.25 μm where the growth rate of struvite was 1.625 and 0.83 μm h⁻¹ and the operating supersaturations were 0.23 and 0.17, respectively. Thermodynamic modeling and simulation along with crystal growth are a function of solution supersaturation. Moreover, pH has also a significant effect on struvite crystal size and purity. Matynia et al. (2006) demonstrated that an increase of pH from 8 to 11 could decrease five times the mean crystal size of struvite formed in synthetic solutions (NH₄H₂PO₄/MgCl₂/NaOH). Matynia et al. (2013) also stated that an increase of pH from 9 to 11 decreased the crystal size by more than 2-times in a continuous flow struvite crystallizer (from 20.2 to 9.2 μm, in 15 min where V_w was 1.2 dm³). It is established through several researches that a comparative higher pH reduces the struvite crystal size within the same reaction time (Kozik et al., 2011; Hutnik et al., 2011, 2012). Struvite precipitation potential significantly increases due to its solubility and decreases with the increasing of pH. Increase in pH produces higher nuclei population densities that might be the cause of producing smaller crystals. Higher pH in the crystallizer is not only responsible for smaller particles, but also higher size diversity. On the other hand, prolonged contact time of crystal suspension with supersaturated condition increased the struvite crystal size significantly.

Le Corre et al. (2007) have also shown that pH was responsible for the change in struvite zeta-potential, hence influencing struvite agglomerative properties. They showed that the crystal size increased from 33.57 ± 0.7 to 84.07 ± 11.2 mm as the zeta-potential decreased in magnitude from −23.37 ± 0.6 to −17.57 ± 1.1 mV. Crystal’s size is not only depended on the pH, but also on other factors such as aeration rate (Liu et al., 2011b,c), influent concentration and crystal retention time (Adnan et al., 2003). They (Adnan et al., 2003) furthermore, stated that larger struvite crystal can be achieved when the crystals spend sufficient time within the reactor. They found a linear increase in the mean crystal size with an increase in crystal retention time (CRT). Optimum CRT of 8–12 d is sufficient to achieve the required size (3.5 mm) and structural strength of the crystals.

Hao et al. (2008) stated that transparent rods like crystals were obtained in the ultra-pure water solutions within the pH range of 7.5–9.0 and in the tap water solutions within the pH range of 7.0–8.5. In both solutions with higher pH (more than 9.0), the precipitations became white and even colorful powders, but not crystals. Therefore, the precipitations at higher pH values are not pure struvite. When pH was higher than 9.0, some impurities such as Mg(OH)₂ and Mg₃PO₄ probably emerged in the precipitations, which indicates more noisy XRD patterns with reduction in peak intensities and change in peak positions and which also shows more irregular patterns in the microscopic images. In other words, the existence of Ca²⁺ in the tap water solutions at higher pH values would contribute to more impurities like Ca₃PO₄ and even CaHPO₄ (Yigit and Mazlum, 2007), besides Mg(OH)₂ and Mg₃PO₄ found in the ultra-pure water solutions.

Very recently, Hao et al. (2013) described a little different statement about the pH properties during struvite formation. They found that XRD data misled the researchers into believing that the harvested crystals were struvite, but in fact, it was amorphous phosphorus. Struvite precipitation is a pH dependent reaction and pure struvite is form near neutral pH. From the elemental analysis of the crystals, they confirmed that the highly purified (99.7%) struvite was formed at pH 7.0–7.5 at a longer period of time (three months) under ambient temperatures (25–30 °C) and controlled conditions. They also found that the struvite content decreased to around 30–70% at pH 8.0–9.0, and over pH 9.5, the content decreased sharply to <30%. Higher pH levels (>10.5) resulted in the complete disappearance of struvite in the precipitates.

4.3 Aeration rate

Air flow plays an important role in the removal of NH₄-N from the solution. Air flow agitates the solution and creates a removal pathway for dissolved ammonia to volatilize from the solution. Moreover, air flow dilutes the concentration of gas-phase NH₄-N and increase the driving force for dissolving NH₄-N to separate the gaseous phase. So, there is an increasing tendency for ammonia volatilization with increasing air flow rate. Yetilmezsoy and Zengin (2009) stated that a sufficient aeration time should be provided to achieve high removal efficiencies. They obtained about 93.4% NH₄-N removal with an aeration rate of 0.6 L min⁻¹ within a period of 24 h. They also found the highest NH₄-N removal (95.3%) in 12 h reaction time with an aeration rate of 10 L min⁻¹. Lei et al. (2006) found about 60.2% ammonia removal with an aeration rate of 0.6 L min⁻¹ in a reaction time of four hours. On the contrary, they achieved the same removal efficiency without aeration in a period of 24 h. Liu et al. (2011b,c) found that struvite formation is proportional to the aeration rate and reached a plateau at around 0.73 L min⁻¹. Suzuki et al. (2007) stated that aeration rate enhanced the CO₂ stripping that helps to raise the pH of the aeration column. They found the pH ranged between 7.5 and 8.0 when the aeration rate was 12 m³ h⁻¹, but a higher pH (range between 8.0 and 8.5) was found when the aeration rate was 16 m³ h⁻¹. Higher aeration also enhances the foam formation in the aeration column during crystallization process, and it is necessary to use defoaming agent.

4.4 Operation mode of crystallization process

The operation mode of crystallization process has an important role on the performances of struvite production. Researchers are trying to get the highest removal of PO₄ and NH₄ through struvite precipitation from wastewaters through different types of reactors (Table 2 and Fig. 5). Cost involvement in preparation of reactors also influenced the production cost of struvite. Le Corre et al. (2007) designed a pilot-scale crystallization reactor continuously fed with synthetic liquors with two concentric stainless steel meshes that were immerged in the upper section (Fig. 5a). They found that the system combining two concentric meshes placed in the enlarged section of the pilot-scale crystallization reactor could accumulate struvite at a rate of 7.6 g m⁻² h⁻¹ under optimum conditions of precipitation and also to achieve 81% PO₄ removal. Suzuki et al. (2007) constructed a continuous flow reactor with three layers of stainless steel wire mesh providing aeration and settling zones (Fig. 5b). They produced 65.0 kg struvite in 70 days operation where influent loading rate was 5.3 m³ day⁻¹ and the struvite recovery rate was 171 g m⁻³ wastewater.

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Figure 5

Recently adopted struvite crystallizers.

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Bhuiyan et al. (2008) precipitated a comparative larger struvite crystal (0.5–3.5 mm) with a fluidized bed reactor consisting of an external clarifier, a storage tank for receiving anaerobic digester supernatant (PO₄ and NH₄ rich wastewater), several pumps for controlling feed materials, MgCl₂ and NaOH solutions. MgCl₂ and NaOH solutions were added into the reactor through the injection ports, just above the feed and recycle flows. Ali and Schneider (2008) designed a batch reactor with auto control of feed and temperature (Fig. 5c). Volume of the reactor was 44 L, made with clear perspex allowing the seed to keep in uniform and full suspension. A pH controller controlled the pH set point and triggered feed addition when solution pH dropped below the set point. Two dosing pumps were operated for the addition of feed solution based upon the output signal sent from the pH controller. They controlled the temperature through encompassing a plastic-coated copper-cooling coil, resistive temperature device and solenoid valves. They try to estimate the struvite growth kinetics using four different approaches with this highly controlled reactor. They also establish the kinetics of struvite growth and the mathematical relation between mean particle size and the solution supersaturation.

A simple reactor was designed by Rahman et al. (2011) where continuously feeds were added in a Plexiglas reactor (12.3 L). Influent wastewater and MgCl₂ solutions were added to the reaction zone through the pump where air flow rate was 0.73 L min⁻¹ (Fig. 5d). Irregular white crystals were produced with 93% PO₄ and 31% NH₄ removals with this crystallizer. A glass made continuous flow type cylindrical tank equipped with heating/cooling jacket was developed by Hutnik et al. (2013a,b). The process was also equipped with a draft type circular tube with a three-paddled propeller (Fig. 5e). PO₄, Mg and NH₄ were added in a ratio of 1:1:1 and 1:1.2:1 in a varied pH and fixed temperature. High-quality elongated crystals were produced through this crystallizer. It was also found that the unit process yield was small with this process condition and thus, a lower economic efficiency of a whole production plant was reported. Successful steady-state operation in the continuous reaction crystallization process was found where continuous struvite reaction crystallization process ran in original jet-pump crystallizers of various scales, which operate successfully in a continuous regime (Matynia et al., 2013).

5.1 Nutrient leaching rate

Struvite is an effective fertilizer as its nutrient releasing rate is very slow. Rate of N release also depends on the size of the crystals. Nelson (2000) studied the N releasing pattern by measuring N uptake rate from three different sizes of struvite crystals (<2 mm, 2–3 mm, and 4–8 mm) on ryegrass grown as a greenhouse. He found that the struvite particle size affected N release in the first 3–6 weeks after planting, when the smaller particle sizes released more N than the coarser particles did. After 6–9 weeks, the N release was similar for all particle sizes. Johnston and Richards (2004) also stated that struvite was the typical control released fertilizer, after testing the growth of potted ryegrass. It was found from the investigation that the uptake rate of P of struvite was 100% (Westerman et al., 2009).

Struvite has been successfully used on turf grass, tree seedlings, ornamentals, vegetables, flower boards and garden grass as fertilizer (Munch and Barr, 2001; Nelson et al., 2003; Gonzalez Poncer and Garcialopez, 2007; Yetilmezsoy and Zengin, 2009), and found it as an excellent fertilizer. Moreover, struvite would be the most alternative fertilizer for a few crops, like sugar beet that needs magnesium (Gaterell et al., 2000). Over application of struvite does not burn the roots due to its slow releasing characteristics, which is common with traditional ammonium–phosphate fertilizers (Scope Newsletter, 2003). It has a long residual effect on the root zone of plant that makes it more advantageous on soluble fertilizers. Highly soluble fertilizers are undesirable in grasslands and forests, where fertilizers are applied once in every several years. A slow-releasing fertilizer would be effective in such environments. According to Bashan and Bashan (2004), struvite may have a low leaching rate and slowly releases nutrients during the plant growing season. Small quantities of recovered struvite are currently being tested on ornamental plants as fertilizer. It is essential to evaluate the efficiency of struvite on different types of crops and plant growth as an alternative source of fertilizer. The fertilizer properties of struvite have been investigated by several researchers (Schuiling and Andrade, 1999; Li and Zhao, 2003; Yetilmezsoy and Zengin, 2009; Liu et al., 2011a). From the last decade, struvite had been commercially produced in Japan, and sold to fertilizer companies (Ueno and Fujii, 2001). It is very effective for those crops, which needs low-soluble fertilizers. The presence of magnesium makes struvite useful for sugar beets because sugar beet needs magnesium ions (Bashan and Bashan, 2004). This slow-release behavior is ideal for coastal agriculture.

Rothbaum and Rohde (1976) found that struvite showed the higher growth of grass, fruit and various crops, compared with conventional fertilizers. It is well suited for fertilizing turf grass not only providing the N and P, but also for the presence of the Mg, because it is the vital element of chlorophyll, which is responsible for the green coloration of the plants. Fernandez-Escobar et al. (2004) found that less than 20% of the urea N was recovered by fruit trees. They also stated that the annual application rate of N fertilizer for olive orchards is 350 kg ha⁻¹. Low N recovery by the fruit trees might be due to higher N loss by leaching and evaporation that results in a negative impact in the environment. Thus, urea fertilization increases the global warming issue. In that situation, struvite would be an effective eco friendly fertilizer for fruit trees.

5.2 Soil type and struvite

Gonzalez Poncer and Garcialopez (2007) studied the effectiveness of struvite as a P source compared to that of phosphate rock, mono ammonium phosphate and calcium superphosphate on potted ryegrass. The doses applied with each source were 0, 8, 16, 24, 32 and 40 mg of P kg⁻¹ of soil. They found that plant P concentration was greater than all of these fertilizers compared to no fertilizer applied. Moreover, struvite treated ryegrass achieved greater P concentration than that of any other P sources. It might be revealed that the P uptake rate is higher from struvite than that of any other PO₄ fertilizers.

Calcareous soil contains high concentrations of Ca phosphate that substantially decreases the solubility of rock phosphate. Manure and compost could be used for organic production in calcareous soil; rock phosphate and bone meal might also be used as fertilizer, but they are ineffective as fertilizers in calcareous soil. So, development of a sustainable, effective alternative P fertilizer for calcareous soils is important for organic agriculture in semi-arid environments. According to Lindsay (1979), struvite is substantially more soluble than Ca phosphates at alkaline pH, and hence it is a potential P fertilizer in calcareous soils. Massey et al. (2007) conducted a greenhouse trial for organic wheat production in calcareous soil to investigate the effectiveness of recovered struvite as a fertilizer in different types of soil conditions. They found that struvite is very effective in moderately acidic and moderately alkaline soils. Increased P concentration within the plant as well as higher yield was found in struvite applied plants in semiarid environment (Massey et al., 2009). Pastor et al. (2008) also stated that struvite was an effective fertilizer source in acidic and hilly lands. The presence of access heavy metals in the fertilizers reduced the crop yield (Islam et al., 2010). Struvite also contains some heavy metals, because wastewater contains a significant amount of heavy metals. Heavy-metal ions can be incorporated into the struvite crystalline network not only by nucleation, but also during the crystal growth process. Although the struvite contains heavy metals, it is perfectly maintained the legal limits for fertilizers (Munch and Barr, 2001; Antakyali et al., 2006; Guney et al., 2008; Qiu et al., 2011; Liu et al., 2011b,c). So, it is also an eco-friendly fertilizer regarding heavy-metal situation. Pretreatment with membrane filtration might be an effective method of reducing heavy metal contamination in struvite. Use of the Donnan membrane process can successfully remove metal ions from P rich wastewaters by the help of a Nafion 117 membrane after dissolution in the acidic media (Prakash et al., 2004; Guney et al., 2008). Table 3 shows the heavy-metal situation of struvite with its legal limit.

Application of struvite in the agricultural sector would be a profitable investment. Generation of 1 kg of struvite per day is enough to fertilize 2.6 ha of arable land by applying phosphorus (as P₂O₅) at the rate of 40 kg ha⁻¹ per annum (European Fertilizer Manufacturers Association, 2000; Zheng et al., 2004). Shu et al. (2006) stated that 100 m³ wastewater could recover 1 kg MAP per day that would lead to an increase of nine tons in dry grain yield. Furthermore, it helps to create an eco-friendly environment by reducing the need of rock phosphate.

5.3 Eco-friendly crop/agricultural production

Agricultural soil contributes only 1% of the global CO₂ whereas, CH₄ and N₂O emissions were 39 and 60%, respectively (OECD, 2000), and nitrogen fertilization is the chief source of NO, N₂O and CH₄ emission from the soil (Matson et al., 1998; Perala et al., 2006; Chu et al., 2007; Aguilera et al., 2013). Greenhouse gases (CH₄, CO₂, NO, N₂O) are the key elements for global warming and presently efforts are going on to reduce these gases. Urea is widely used as nitrogen fertilizer worldwide that released N rapidly to the soil, from which only 40% is recovered by plant and 60% is lost by different ways (Liang et al., 2007) and maximum loss was estimated as evaporative loss (26.5–29.4%) that contributed to greenhouse-gas emission. Methane contributes to climatic change by trapping outgoing terrestrial infrared radiation 20 times more effectively than CO₂ (Getachew et al., 2005), whereas N₂O is 300 times more potent than CO₂ in absorbing terrestrial thermal radiation (IPCC, 2001) in the troposphere that enhances global warming. Application of struvite as fertilizer may reduce the risk of global warming, although it contains only 6% N. Due to slow releasing characteristics (Nelson, 2000) of struvite, plant can uptake most of the N without any wastes (Lee et al., 2009; Westerman et al., 2009) after application in the soil. A controlled-release N fertilizer is the best solution to minimize the N₂O emission from soil (Chu et al., 2007). Nitrous oxide production by nitrification and denitrification of agricultural nitrogen that is lost from the field through leaching and runoff is directly influenced on global warming (Nevison, 2000). One kg of volatilized NH₃-N contributes approximately 0.01 kg of N₂O (Mkhabela et al., 2008). A large amount of N₂O is emitted every year through urea fertilization throughout the world. From this perspective, struvite would be a useful alternative to traditional N fertilizers because reducing the greenhouse-gas emissions might be possible due to its slow releasing pattern. Mao et al. (2005) stated that slow releasing N fertilizers would be the best alternative to traditional N fertilizer as they reduce N loss through leaching and volatilization. Not only this, production of struvite simultaneously reduced the P and N from wastewaters that help to reduce the risk of eutrophication in water bodies. Liu et al. (2011a) stated that MAP might be the ideal eco-friendly fertilizer for crop production. They conducted a comparative experiment with MAP and commercial P and N fertilizer for maize forage growth. They used MAP as P and N source; on the other hand, equivalent weight of P was used from FSP, and N was used from urea fertilizer. The results revealed that biomass yield and crude protein percentage were significantly higher in MAP treated groups compared to FSP-urea treated groups. At the same time, N₂O emission was 3-fold higher in FSP-urea treated group than in MAP treated group. A similar study (Rahman et al., 2011) also found that N leaching was also 3-fold lower in MAP than in commercial N fertilizer. Slow solubility rate might be the cause of a lower leaching tendency in MAP treated groups. Lower leaching rate of N and lower emission of N₂O made the struvite as an eco-friendly fertilizer (Ueno and Yamamuro, 1996). Finally, it could be said that struvite was an ideal slow-release fertilizer, and thus, application of it as a fertilizer could create an eco-friendly environment by reducing the greenhouse-gas emissions.