This study examined the removal of ammonia nitrogen from the leachate of a landfill site using the chemical precipitation of struvite (MgNH4PO4.6H2O). This procedure achieved a reduction in the ammonia concentration that was higher than 99% when the molar ratio of 1.8:1.0:1.4 for Mg2+:NH4+:PO43− was adopted. The metal concentration found in the precipitate formed was lower than the limits set by Brazilian and American regulations (CONAMA 375/2006 and US EPA, 40 CFR 503.1993). This demonstrates the potential use for this practice in agriculture. However, the effluent obtained from the tests presented a phosphorus concentration higher than the one in the raw leachate. This shows that removing this compound from the effluent must be further studied. Otherwise, adopting the chemical precipitation of ammonia by the formation of struvite may become unfeasible.

INTRODUCTION

The disposal of urban solid waste at landfill sites causes greenhouse gases to be emitted and the generation of leachate. Clément et al. (1997) and Pablos et al. (2011) suggest that the toxicity of the leachate is strongly related to ammonia concentration, alkalinity and organic matter. Thus, in order to biologically treat the leachate, its chemical characteristics must be altered, so as to reduce this toxicity.

Ammonia can be chemically removed by precipitation and through the formation of struvite (MgNH4PO4.6H2O). This compound is an inorganic mineral in the form of a white crystal. It may be formed from the ammonia present in the leachate after the addition of phosphorus and magnesium source. The reaction formation is quick and can occur in less than 1 minute (Turker & Çelen 2007).

The solubility of the formed crystal is low (Ksp = 12.6), which facilitates its sedimentation (Di Iaconi et al. 2010). Struvite formation is also a convenient source of phosphorus from wastewater, and, with the rapid depletion of phosphate rock resources, it could be applied as an eco-friendly fertilizer for crop production (Rahman et al. 2014; Yan & Shih 2016). Since it is a hardly soluble salt, it may function as a buffer of the soil components, releasing nutrients in accordance with solubility and the needs of the plants.

Phosphorus is a critical ingredient in food production and its natural reserves have been quickly depleted (Cordell et al. 2009; Lu et al. 2016). Some studies suggest that by the 2060–2070 period, about half the world's current economic phosphate resources will have been used up (Driver et al. 1999; Lizarralde et al. 2015).

Because raw leachates from landfill sites contain nitrogen and phosphorus, a large number of studies have been conducted to examine the precipitation potential of struvite under different operational conditions (Zhang & Chen 2009; Lew et al. 2010; Etter et al. 2011; Lu et al. 2016). They show that the precipitation process is technically feasible and economically beneficial (Shu et al. 2006; Pastor et al. 2008; Lizarralde et al. 2015).

Struvite also contains some heavy metals, because wastewater contains a significant amount of heavy metals (Rahman et al. 2014). Heavy-metal ions can be incorporated into the struvite crystalline network not only by nucleation, but also during the crystal growth process (Rahman et al. 2014). Although the struvite contains heavy metals, the legal limits for fertilizers are perfectly maintained (Rahman et al. 2014). Di Iaconi et al. (2010) also found that the concentration of potentially toxic metals in the precipitate formed by struvite was lower than the one existing in natural soil. Similarly, Uysal et al. (2010) analyzed the metal concentrations obtained in the struvite from the effluent of an anaerobic digester of sewage sludge, and concluded that its application was possible in agriculture.

However, these studies show little concern with the supernatant generated, principally, as phosphorus concentration. Thus, the aim of this study was to analyze the reduction of the ammonia nitrogen concentration of landfill leachate by the formation of struvite. The purity of the precipitate and supernatant formed, particularly regarding the presence of potentially toxic metals, total organic carbon and phosphorus, was also evaluated.

METHODOLOGY

The leachate used in this study came from the landfill site in the city of Campinas, Brazil. The city has a population of about one million inhabitants and, since 1992, all of its urban solid waste has been sent to this landfill. It occupies an area of almost 400,000 m2 and produces approximately 250 m3 of leachate per day (Camargo et al. 2014).

The reagents used for the formation of struvite were chosen based on the study by Li et al. (1999). These authors concluded that the combination which most effectively removed the ammonia was composed by MgCl2.6H2O + Na2HPO4.12H2O. Before the precipitation assays (Table 1), a sample of raw leachate was collected to determine the ammonia nitrogen concentration. This value was used to calculate the amount of reagents (MgCl2.6H2O and Na2HPO4.12H2O) to be used in each test. The reagents were subsequently added to the reaction jars.

Table 1

Summary of the performed assays

SteppHAssayAim of the assay
6.67 (J1)
8.00 (J2)
8.50 (J3)
8.75 (J4)
9.00 (J5)
9.50 (J6) 
Ratio equal to 1.0:1.0:1.0 Find the ideal pH of the reaction 
8.75 (J1 to J6) Ratio equal to: 1.0:1.0:1.0 (J1), 1.0:1.0:1.2 (J2), 1.0:1.0:1.4 (J3), 1.0:1.0:1.6 (J4), 1.0:1.0:1.8 (J5) e 1.0:1.0:2.0 (J6) Find the best proportion of phosphorus 
8.75 (J1 to J6) Ratio equal to: 1.4:1.0:1.4 (J1); 1.6:1.0:1.4 (J2); 1.7:1.0:1.4 (J3); 1.8:1.0:1.4 (J4); 2.0:1.0:1.4 (J5) e 2.2:1.0:1.4 (J6) Find the best proportion of magnesium 
8.75 (J1 to J6) Ratio was kept constant 1.8:1.0:1.4 in all jars Evaluate the metal concentration in the precipitate 
SteppHAssayAim of the assay
6.67 (J1)
8.00 (J2)
8.50 (J3)
8.75 (J4)
9.00 (J5)
9.50 (J6) 
Ratio equal to 1.0:1.0:1.0 Find the ideal pH of the reaction 
8.75 (J1 to J6) Ratio equal to: 1.0:1.0:1.0 (J1), 1.0:1.0:1.2 (J2), 1.0:1.0:1.4 (J3), 1.0:1.0:1.6 (J4), 1.0:1.0:1.8 (J5) e 1.0:1.0:2.0 (J6) Find the best proportion of phosphorus 
8.75 (J1 to J6) Ratio equal to: 1.4:1.0:1.4 (J1); 1.6:1.0:1.4 (J2); 1.7:1.0:1.4 (J3); 1.8:1.0:1.4 (J4); 2.0:1.0:1.4 (J5) e 2.2:1.0:1.4 (J6) Find the best proportion of magnesium 
8.75 (J1 to J6) Ratio was kept constant 1.8:1.0:1.4 in all jars Evaluate the metal concentration in the precipitate 

For the experiment, a 20 L sample of raw leachate was collected and stored in a refrigerator for the entire experimental period. However, their characterization was performed for each test.

The assays were performed in a piece of equipment composed of six jars (20.0 × 11.5 × 11.5 cm), identified as J1, J2, J3, J4, J5 and J6. Each jar had a capacity to hold 2 L of the sample. The volume of leachate stored resulted in a water depth of 0.15 m in the jar. The tube for sample collection was placed halfway in the jar (Figure 1). Each assay was also performed in triplicate.
Figure 1

Jars used in the assays.

Figure 1

Jars used in the assays.

During the performance of each test, the pH adjustment was done with a solution of NaOH 15 mol L−1 and H2SO4 6 mol L−1. For each assay, the reaction period was 30 min (Figure 2). The stirring was maintained at 100 rpm, with an equal velocity gradient of 64 s−1. Following the reaction, there was a period of 30 min for decantation. At the end, 500 mL of sample were collected from a hole in the central area of the jar (Figure 1).
Figure 2

Flowchart of the tests.

Figure 2

Flowchart of the tests.

The physicochemical analyzes performed were: pH, ammonia nitrogen, total Kjeldahl nitrogen, phosphorus, chemical oxygen demand (COD) and alkalinity. All analyses were based on Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF 2012). The total organic carbon (TOC) was determined according to the methodology described by APHA/AWWA/WEF (2012), by a high-temperature combustion method, using the Shimadzu 5000A/SSM-5000A analyzer.

Assays performed

During step 1 (Table 1), the molar ratio of used was 1.0:1.0:1.0 and the pH ranged between 8.0 and 9.5 (J2 to J6). The chosen molar ratio (1.0:1.0:1.0) was defined based on the composition of the struvite molecule (MgNH4PO4.6H2O). Regarding the pH, the scientific literature reports that the minimum solubility of struvite occurred in the range of 8.0 to 9.5 (Li et al. 1999; Munch & Barr 2001; Kim et al. 2007). At all stages (Table 1), the pH correction only occurred after the reagent was added (Figure 2). During step 1, there was no adjustment of pH in the jar 1 (J1) so that evaluating the influence of adding reagents in the alkalinity of the leachate was possible.

During step 2, we studied the best ratio of phosphorus and, during step 3, the best ratio of magnesium was analyzed. During step 4, the molar ratio of the reactants was determined that defined as ideal (1.4 mol ) and the pH was adjusted to 8.75 following the addition of the reagents. The purpose of this step was to evaluate the presence of potentially toxic metals (chromium, cadmium, nickel, lead, zinc and copper) and TOC concentration in the precipitate. Statistical comparison among the treatments was performed using analysis of variance (ANOVA) and the post hoc Tukey's test. A p value of <0.05 was considered statistically significant.

RESULTS AND DISCUSSION

Table 2 shows the results obtained from the characterization of the raw leachate. These data are the same that were used by Camargo et al. (2014). In this case, these authors evaluated the formation of struvite with the pH correction performed before the reagents were added.

Table 2

Characterization of the raw leachate

ParametersValue
pH 8.14 ± 0.15 
BOD (mgL−1173 ± 38 
COD (mgL−12,386 ± 485 
BOD/COD 0.07 
N-NH4+ (mgL−11,716.1 ± 293.8 
Phosphorus (mgL−122.9 ± 3.8 
Magnesium (mgL−1152 ± 28 
Total solids (mgL−110.416 
Total fixed solids (mgL−17.770 
Total volatile solids (mgL−12.646 
ParametersValue
pH 8.14 ± 0.15 
BOD (mgL−1173 ± 38 
COD (mgL−12,386 ± 485 
BOD/COD 0.07 
N-NH4+ (mgL−11,716.1 ± 293.8 
Phosphorus (mgL−122.9 ± 3.8 
Magnesium (mgL−1152 ± 28 
Total solids (mgL−110.416 
Total fixed solids (mgL−17.770 
Total volatile solids (mgL−12.646 

During step 1, it was possible to observe that the best ammonia removal (85.1%) was achieved at a pH equal to 8.75 (Table 3). This finding is consistent with that observed during the study by Li et al. (1999). These authors also found that the pH of the minimum solubility of struvite was between 8.5 and 9.0. However, in this study, there was no significant difference in the pH range between 8.00 and 9.50. Struvite precipitation is controlled by pH, supersaturation, temperature, and impurities such as calcium. Struvite solubility decreases with increasing pH, while above a pH of 9 its solubility it begins to increase (Michalowski & Pietrzyk 2006). Beyond the optimum pH, the solubility of precipitated magnesium ammonium phosphate hexahydrate (MgNH4PO4.6H2O, MAP) will increase and the precipitated MAP can be redissolved (Li et al. 1999).

Table 3

Determination of the ideal pH (Step 1 – Table 1)*

Assay
SampleRaw leachateJ1J2J3J4J5J6
pH** 8.06 (0.03) 6.67 (0.09) 8.04 (0.04) 8.53 (0.02) 8.75 (0.02) 9.05 (0.01) 9.53 (0.05) 
N-NH4+ (mg L−11,861.9 (0.0)a 464.3 (12.2)b 326.0 (24.7)c 311.9 (29.3)c 276.7 (31.7)c 295.5 (25.3)c 314.2 (14.6)c 
Ammonia removal N-NH4+ (%) – 75.1 (0.7) 82.5 (1.3) 83.3 (1.6) 85.1 (1.7) 84.1 (1.4) 83.1 (0.8) 
COD (mg L−12,713 (14)a 2,498 (28)b 2,469 (33)b 2,440 (50)b 2,448 (36)b 2,473 (31)b 2,457 (65)b 
Phosphorus (mg L−122.6 (0.3)a 384.3 (70.0)b 69.6 (45.9)c 34.2 (17.7)ac 22.1 (5.6)ac 14.0 (1.7)d 10.9 (3.7)d 
Assay
SampleRaw leachateJ1J2J3J4J5J6
pH** 8.06 (0.03) 6.67 (0.09) 8.04 (0.04) 8.53 (0.02) 8.75 (0.02) 9.05 (0.01) 9.53 (0.05) 
N-NH4+ (mg L−11,861.9 (0.0)a 464.3 (12.2)b 326.0 (24.7)c 311.9 (29.3)c 276.7 (31.7)c 295.5 (25.3)c 314.2 (14.6)c 
Ammonia removal N-NH4+ (%) – 75.1 (0.7) 82.5 (1.3) 83.3 (1.6) 85.1 (1.7) 84.1 (1.4) 83.1 (0.8) 
COD (mg L−12,713 (14)a 2,498 (28)b 2,469 (33)b 2,440 (50)b 2,448 (36)b 2,473 (31)b 2,457 (65)b 
Phosphorus (mg L−122.6 (0.3)a 384.3 (70.0)b 69.6 (45.9)c 34.2 (17.7)ac 22.1 (5.6)ac 14.0 (1.7)d 10.9 (3.7)d 

*The different superscript letters in each line indicate significant difference using ANOVA and post hoc Tukey's test at p < 0.05 and n = 3.

The number in parentheses indicates the standard deviation.

**It was not possible to obtain at all the essays performed the exact pH established at Table 1.

Generally, struvite solubility decreases with increasing pH. However, as pH continues to rise above a pH of 9, the solubility of struvite begins to increase since the ammonium ion concentration will decrease and the phosphate ion concentration will increase (Doyle & Parsons 2002). The fundamental problem of predicting struvite precipitation is calculating how much of the total concentrations of magnesium, ammonium and phosphate will be available, at a given pH, in a given solution to subsequently form struvite. In fact, many different sources of wastewater are likely to have a distinct conditional solubility product (Kso) value at a specific pH with regard to its struvite precipitation potential, which is because wastewater composition will vary from one water treatment works to another. Any variation in water chemistry will result in differences in ionic strength and ion activity, thereby changing the struvite precipitation potential of the wastewater.

Adding phosphorus did not cause an increase in its concentration in the final effluent after the precipitation of the struvite at a pH of 8.75. There was no significant difference from the average phosphorus concentration in the raw leachate. Therefore, when this operating procedure is adopted, the struvite precipitation does not cause an unnecessary phosphorus disposal with the effluent.

In all the tests, at a pH of 8.75, there was a significant decrease in the values of total alkalinity of the raw leachate (8,800 to 4,100 mg L−1). This indicates that adding reagents strongly contributes to reducing the leachate alkalinity. According to Li et al. (1999) and Camargo et al. (2014), this can be explained by analyzing Equation (1). The reaction between magnesium cations and phosphate anions releases acid H3O+ ions in the solution, causing a decrease in pH values.
formula
1
In fact, the reduction of the alkalinity after the addition of the reagents would mean a significant decrease in the amount of acid required to correct the solution pH in the next step of the process. This would imply lower operational costs to treat the leachate. If this correction of pH was sought before the addition of the reagents, the highest alkalinity present in the raw leachate could lead to a higher consumption of acid until the desired pH value is reached (Camargo et al. 2014).

In turn, regardless of the adopted pH, there was no significant difference from the COD value of the effluent produced (Table 3). At the point when the lowest value for this parameter (pH = 8.50) was obtained, the COD removal was only 10%. In this case, it ranged from 2,713 mg L−1 in the raw leachate to 2,440 mg L−1 in the treated effluent. As will be discussed in the section to evaluate the quality of the precipitate formed, this result points to a large selectivity in struvite precipitation.

With the aim of improving the efficiency of the ammonia removal process, during step 2 (Table 1), the pH was set at 8.75. This value was defined based on the results obtained in previous assays (Table 3). However, higher phosphorus doses were added (Table 4). As a result, a 98.8% reduction was obtained in the ammonia concentration when the molar ratio of equal to 1.0:1.0:1.4 was adopted (Table 4).

Table 4

Determination of the ideal concentration of phosphorus (Step 2 – Table 1)*

Assays
SampleRaw leachateJ1J2J3J4J5J6
 – 1.0:1.0:1.0 1.0:1.0:1.2 1.0:1.0:1.4 1.0:1.0:1.6 1.0:1.0:1.8 1.0:1.0:2.0 
pH 8.10 (0.11) 8.77 (0.02) 8.76 (0.01) 8.75 (0.01) 8.76 (0.02) 8.76 (0.01) 8.77 (0.01) 
(mgL−11,836.1 (0.0) 308.4 (20.0)a 70.3 (13.6)b 21.9 (1.1)c 25.8 (2.6)d 25.5 (1.5)d 24.1 (3.6)d 
Removal of N-NH4+ (%) – 83.2 (1.1) 96.2 (0.7) 98.8 (0.1) 98.6 (0.1) 98.6 (0.1) 98.7 (0.2) 
COD (mgL−12,631 (162)a 2,304 (125)b 2,312 (168)b 2,225 (124)b 2,283 (81)b 2,254 (96)b 2,327 (44)b 
Phosphorus (mgL−122.1 (1.5)a 19.0 (1.0)b 49.1 (9.9)c 98.4 (13.4)d 117.4 (14.7)de 136.1 (25.2)de 155.6 (26.4)e 
Assays
SampleRaw leachateJ1J2J3J4J5J6
 – 1.0:1.0:1.0 1.0:1.0:1.2 1.0:1.0:1.4 1.0:1.0:1.6 1.0:1.0:1.8 1.0:1.0:2.0 
pH 8.10 (0.11) 8.77 (0.02) 8.76 (0.01) 8.75 (0.01) 8.76 (0.02) 8.76 (0.01) 8.77 (0.01) 
(mgL−11,836.1 (0.0) 308.4 (20.0)a 70.3 (13.6)b 21.9 (1.1)c 25.8 (2.6)d 25.5 (1.5)d 24.1 (3.6)d 
Removal of N-NH4+ (%) – 83.2 (1.1) 96.2 (0.7) 98.8 (0.1) 98.6 (0.1) 98.6 (0.1) 98.7 (0.2) 
COD (mgL−12,631 (162)a 2,304 (125)b 2,312 (168)b 2,225 (124)b 2,283 (81)b 2,254 (96)b 2,327 (44)b 
Phosphorus (mgL−122.1 (1.5)a 19.0 (1.0)b 49.1 (9.9)c 98.4 (13.4)d 117.4 (14.7)de 136.1 (25.2)de 155.6 (26.4)e 

The number in parentheses indicates the standard deviation.

*The different superscript letters in each line indicate significant difference using ANOVA and post hoc Tukey's test at p < 0.05 and n = 3.

Comparatively, when the ratio 1.0:1.0:1.0 at pH 8.75 was used, the concentration of reached 276.7 mg L−1 (Table 3). By increasing the amount of phosphorus and adopting the ratio of 1.0:1.0:1.4, the ammonia nitrogen concentration dropped to 21.9 mg L−1 (Table 4). However, the effect of the phosphate concentration on struvite precipitation has not been adequately investigated so far. Phosphate salts are necessary reactants for struvite production, and overdosing phosphate salts can increase the NH4-N removal rate. These facts indicate that phosphorus salts can be the main limiting factor involved in the precipitation process.

The improvement in the performance of the treatment determined by the higher amounts of phosphorus, in terms of the ammonia molar concentration, has also been demonstrated in other studies (Turker & Çelen 2007; Di Iaconi et al. 2010; Siciliano et al. 2013). A greater availability of phosphorus than the theoretical stoichiometric requirement is attributable to the possible formation of magnesium complexes and insoluble compounds of phosphorus that can interfere with struvite formation (Siciliano et al. 2013). Furthermore, overdosing phosphorus favors the displacement of the equilibrium of Equation (1) for the formation of struvite, thereby causing a decrease in ammonia concentration.

Despite there having been a good result in regards to the removal of ammonia, phosphorus residual concentration was considered high. In this case, since the use of a molar ratio between was higher than 1.0:1.2, the phosphorus concentration would exceed 49.1 mg L−1. Thus, we decided to change the magnesium molar ratios in order to provide the removal of both the ammonia nitrogen and the phosphorus in the final effluent. For this, the ratio between in 1.0:1.4 was studied. In turn, the magnesium ratio ranged from 1.4 to 2.2 (Table 5).

Table 5

Determination of the ideal concentration of magnesium (Step 3 – Table 1)*

Assays
SampleRaw leachateJ1J2J3J4J5J6
 – 1.4:1.0:1.4 1.6:1.0:1.4 1.7:1.0:1.4 1.8:1.0:1.4 2.0:1.0:1.4 2.2:1.0:1.4 
pH 8.05 (0.04) 8.76 (0.02) 8.76 (0.01) 8.75 (0.01) 8.75 (0.01) 8.75 (0.01) 8.76 (0.01) 
(mgL−11,836.1 (0.0) 26.9 (4.1)a 22.3 (2.1)a 16.4 (2.8)b 15.6 (5.8)bc 11.9 (2.1)c 17.3 (0.8)b 
Removal of (%)  98.5 (0.2)a 98.8 (0.1)a 99.1 (0.2)b 99.2 (0.3)bc 99.4 (0.1)c 99.1 (0.0)b 
COD (mgL−12,688 (126)a 2,730 (849)a 2,668 (825)a 2,713 (616)a 2,494 (441)a 2,573 (645)a 2,531 (492)a 
Phosphorus (mgL−123.1 (1.1)a 123.8 (13.1)b 42.3 (3.6)c 37.4 (3.6)c 34.9 (1.7)dc 35.4 (4.4)c 28.7 (2.0)e 
Assays
SampleRaw leachateJ1J2J3J4J5J6
 – 1.4:1.0:1.4 1.6:1.0:1.4 1.7:1.0:1.4 1.8:1.0:1.4 2.0:1.0:1.4 2.2:1.0:1.4 
pH 8.05 (0.04) 8.76 (0.02) 8.76 (0.01) 8.75 (0.01) 8.75 (0.01) 8.75 (0.01) 8.76 (0.01) 
(mgL−11,836.1 (0.0) 26.9 (4.1)a 22.3 (2.1)a 16.4 (2.8)b 15.6 (5.8)bc 11.9 (2.1)c 17.3 (0.8)b 
Removal of (%)  98.5 (0.2)a 98.8 (0.1)a 99.1 (0.2)b 99.2 (0.3)bc 99.4 (0.1)c 99.1 (0.0)b 
COD (mgL−12,688 (126)a 2,730 (849)a 2,668 (825)a 2,713 (616)a 2,494 (441)a 2,573 (645)a 2,531 (492)a 
Phosphorus (mgL−123.1 (1.1)a 123.8 (13.1)b 42.3 (3.6)c 37.4 (3.6)c 34.9 (1.7)dc 35.4 (4.4)c 28.7 (2.0)e 

*The different superscript letters in each line indicate significant difference using ANOVA and post hoc Tukey's test at p < 0.05 and n = 3.

The number in parentheses indicates the standard deviation.

It was possible to observe that adding the extra magnesium was not significantly effective to decrease the phosphorus concentration in the final effluent. Based on the ratio of 1.6:1.0:1.4 , the residual phosphorus concentration does not significantly decrease when compared to the reduction achieved between the ratio of 1.4:1.0:1.4 and 1.6:1.0:1.4. In all the evaluated ratios, the phosphorus concentration in the effluent was higher than the one present in the raw leachate. It was also observed that, at the ratio of 2.0:1.0:1.4, there was a loss in efficiency when the ammonia nitrogen was removed.

Therefore, there is an indication that adding extra magnesium leads to losses in the precipitation of this compound. This result is consistent with the one described by Korchef et al. (2010). These authors concluded that adding extra magnesium contributes towards enhancing phosphorus removal efficiency, but it also leads to the precipitation of other compounds of magnesium and phosphorus. Consequently, there is not only the formation of struvite. A significant portion of the added phosphorus ends up staying in some soluble form. Thus, it is important to highlight that, by using this ideal molar ratio of 1.8:1.0:1.4 , there will be a significant removal of ammonia nitrogen and production of struvite.

However, the need to add phosphorus will be so high that a portion of this compound will be kept dissolved in the final effluent. The scientific literature that researches struvite precipitation for removing ammonia nitrogen from the leachates did not present any discussion concerning this increase in the phosphorus concentration in the final effluent. Thus, it is important to highlight this problem. The removal of the ammonia nitrogen by chemical precipitation can facilitate a biological treatment, which would enable the removal of the added phosphorus. However, further investigation must be done in order to determine whether the removal of the ammonia nitrogen actually decreases the toxicity of the final effluent to the point of providing a biological treatment. Otherwise, ammonia nitrogen would be removed along with the phosphorus consumption.

There is currently a vigorous debate progressing regarding the lifetime of the phosphate rock reserves or the timeline of the phosphorus peak, and what is clear is that the remaining rock is lower in the phosphorus concentration, higher in contaminants, and more difficult to access in environmentally or culturally sensitive areas; it will require more effort to extract and produce, and the costs will be higher to refine and ship (Cordell et al. 2009; Ashley et al. 2011). Thus, this component should be used very sparingly.

Since the main goal of this study was to remove the ammonia nitrogen from the leachate with the formation of struvite, the best molar ratio of the three ions was equal to 1.8:1.0:1.4 . However, it is worth highlighting that in this situation there was only a slight increase in the removal of the ammonia nitrogen. It went from 98.8% (Table 4, ratio of 1.0:1.0:1.4) to a maximum of 99.2% when adopting the ratio of 1.8:1.0:1.4 (Table 5).

Although the ratio of 2.0:1.0:1.4 showed higher N removal values, we decided to use the ratio of 1.8:1.0:1.4. This occurred because there was no significant difference among the averages. Therefore, this was the adopted value, which caused a lower reagent consumption rate.

In order to proceed with the assays, during step 4 (Table 1), the pH of the solution was fixed at 8.75 and the molar ratio of was set at 1.8:1.0:1.4 in order to evaluate the quality of the precipitate, regarding the presence of chromium, nickel, cadmium, lead, zinc and copper (Table 6). The metal concentrations in the raw and treated leachate and precipitate (Table 6) were much lower than the limits permitted by legislation in Brazil (CONAMA 375 2006) and the USA (USEPA, 40 CFR 503 1993). These results indicate that there is great potential for the use of the precipitate directly as a fertilizer, allowing the recycling of nutrients. Interestingly, the concentrations of nickel and lead in the treated leachate were higher than in the raw leachate ones. This demonstrates that the addition of reagents seem to have contributed to the concentration of metals in the effluent.

Table 6

Results of the metal analyses in the raw, treated and precipitated leachate

 Cr
Ni
Cd
SamplesValueLimit*ValueLimit*ValorLimit*
Raw leachate 0.1 – 0.37 – <0.13 – 
Treated leachate <0.088 0.1 mg L−1 0.74 (0.025) 2.0 mg L−1 <0.13 0.2 mgL−1 
Precipitate <0.088 1,000 mg kg−1 0.22 (0.014) 420 mg kg−1 <0.13 39 mg kg−1 
 Pb Zn Cu 
Samples Value Limit* Value Limit* Value Limit* 
Raw leachate 0.08 – <0.2 – <0.5 – 
Treated leachate 0.23 (0.03) 0.5 mg L−1 <0.2 5 mg L−1 <0.5 1.0 mg L−1 
Precipitate 0.225 (0.007) 300 mg kg−1 <0.2 2,800 mg kg−1 <0.5 1,500 mg kg−1 
 Cr
Ni
Cd
SamplesValueLimit*ValueLimit*ValorLimit*
Raw leachate 0.1 – 0.37 – <0.13 – 
Treated leachate <0.088 0.1 mg L−1 0.74 (0.025) 2.0 mg L−1 <0.13 0.2 mgL−1 
Precipitate <0.088 1,000 mg kg−1 0.22 (0.014) 420 mg kg−1 <0.13 39 mg kg−1 
 Pb Zn Cu 
Samples Value Limit* Value Limit* Value Limit* 
Raw leachate 0.08 – <0.2 – <0.5 – 
Treated leachate 0.23 (0.03) 0.5 mg L−1 <0.2 5 mg L−1 <0.5 1.0 mg L−1 
Precipitate 0.225 (0.007) 300 mg kg−1 <0.2 2,800 mg kg−1 <0.5 1,500 mg kg−1 

*Limits according the Brazilian legislation (CONAMA 375 2006) and USEPA (40CFR 503 1993).

Besides the metals, a TOC analysis was also performed in the precipitate. The concentration found was very low (0.6%, percentage by mass), which highlights the highly inorganic nature of the precipitate formed and strengthens the selectivity of the precipitation of struvite. TOC in the raw leachate was 882.6 mg L−1 and the treatment reached 854.3 mg L−1, confirming the great selectivity of the precipitation.

CONCLUSIONS

The chemical precipitation by struvite formation was effective in the removal of ammonia nitrogen of leachate from a landfill site. The molar ratio of , considered the most effective for removing this compound, was equal to 1.8:1.0:1.4. Through this ratio, it was possible to achieve removal levels higher than 99%. The precipitate formed had low concentrations of organic matter (0.6%) and metals. This highlights the great selectivity of the formation of struvite and its potential to be applied in agriculture instead of commercial fertilizers. However, the ratio considered ideal led to the production of an effluent with phosphorus concentrations higher than those present in the raw leachate. This shows that there is a need for more studies aimed at recovering this compound in the sequence stages of the treatment. Otherwise, adopting the chemical precipitation of ammonia by the formation of struvite will become unfeasible.

ACKNOWLEDGEMENTS

The authors would like to thank CNPq (the Brazilian National Council for Scientific and Technological Development) for the scholarships granted, in addition to FAPESP (São Paulo Research Foundation) (Processo 2011/21919-7) and FINEP (Studies and Projects Financing Agency) for financing this study. The authors would also like to acknowledge the service provided by Espaço da Escrita/General Coordination of UNICAMP for their help translating the original manuscript.

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