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Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents a
A. Siciliano & S. De Rosa
a
a
Department of Environmental and Chemical Engineering, University of Calabria, via P. Bucci, Cubo 44B. 87046, Rende, CS, Italy. Published online: 05 Nov 2013.
To cite this article: A. Siciliano & S. De Rosa (2014) Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents, Environmental Technology, 35:7, 841-850, DOI: 10.1080/09593330.2013.853088 To link to this article: http://dx.doi.org/10.1080/09593330.2013.853088
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 7, 841–850, http://dx.doi.org/10.1080/09593330.2013.853088
Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents A. Siciliano∗ and S. De Rosa Department of Environmental and Chemical Engineering, University of Calabria, via P. Bucci, Cubo 44B, Rende (CS) 87046, Italy
Downloaded by [University of Boras] at 03:07 05 October 2014
(Received 3 January 2013; final version received 26 September 2013 ) Land spreading of digestates causes the discharge of large quantities of nutrients into the environment, which contributes to eutrophication and depletion of dissolved oxygen in water bodies. For the removal of ammonia nitrogen, there is increasing interest in the chemical precipitation of struvite, which is a mineral that can be reused as a slow-release fertilizer. However, this process is an expensive treatment of digestate because large amounts of magnesium and phosphorus reagents are required. In this paper, a struvite precipitation-based process is proposed for an efficient recovery of digestate nutrients using low-cost reagents. In particular, seawater bittern, a by-product of marine salt manufacturing and bone meal, a by-product of the thermal treatment of meat waste, have been used as low-cost sources of magnesium and phosphorus, respectively. Once the operating conditions are defined, the process enables the removal of more than 90% ammonia load, the almost complete recovery of magnesium and phosphorus and the production of a potentially valuable precipitate containing struvite crystals. Keywords: struvite; bone meal; seawater bittern; digestate; ammonia removal
Introduction The anaerobic digestion of animal manure has rapidly expanded within the last few decades. This treatment not only produces biogas and reduces the quantities of waste, but also increases land spreading of digestates, causing the release of large amounts of nutrients into the environment. Indeed, this wastewater is typically very rich in inorganic nitrogen because ammonia is formed from decomposition of proteins under anaerobic conditions.[1] Some recent studies have demonstrated that the use of digestates as fertilizers, instead of cattle manure, releases comparatively more nitrogen that can be easily drained to surface waters and leached to groundwaters. [2] This phenomenon, which results in the accumulation of nutrients in water bodies, is directly linked to eutrophication and to the subsequent depletion of dissolved oxygen. Moreover, nitrogen compounds can be toxic to aquatic life or lead to diseases caused by the consumption of contaminated drinking water. The most common and economical method for removing nitrogen from wastewater is the process of biological nitrification and denitrification. However, high contents of ammonia have a toxic effect on micro-organisms,[3] which may lead to a decrease in the treatment effectiveness. Autotrophic processes (SHARON-ANAMMOX) are suitable for the treatment of high concentrated wastewater, but their control and management are complex and difficult to operate. Moreover, conventional processes do not recycle nitrogen compounds as truly sustainable products. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
The recovery of nitrogen and phosphorus from wastewater in the form of crystalline struvite (MgNH4 PO4 · 6H2 O) is found to be a sustainable option because struvite is considered a potential fertilizer.[3–13] Struvite precipitation +2 occurs when the combination of NH+ and PO−3 4 , Mg 4 concentration exceeds the struvite solubility product under alkaline conditions. Because of the low amounts of magnesium and phosphorus relative to ammonia concentrations, large amounts of the reagents are required for the treatment of digestates, which results in an expensive process. Different research works, reported in the literature, have evaluated the feasibility of using alternative magnesium compounds, such as, for instance, the by-products generated in the production of magnesium oxide,[14–16] pyrolysate of magnesite,[17] magnesite mineral [18] and seawater bittern,[19] to reduce the process costs. However, there is a lack of research about the identification of alternative sources of phosphorus, which is rarer, more expensive and whose consumption generally represents the primary cost of the whole treatment. To overcome this issue, the use of bone meal, a by-product of the thermal treatment of meat waste, is proposed in this paper as a low-cost source of phosphorus for struvite precipitation. In Europe, the use of this by-product as a fertilizer is restricted [20] and, since it is only suitable for limited alternative utilizations, it is usually disposed in landfills.[21,22] Thus, the exploitation of bone meal for the struvite precipitation process is particularly meaningful because it would allow
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the recovery of its phosphorus content with the production of a more valuable fertilizer containing both phosphorus and nitrogen. Together with bone meal, seawater bittern, a by-product of marine salt manufacturing, was used: its effectiveness has been already demonstrated as a low-cost source of magnesium.[19] Using the aforementioned byproducts, several experiments were conducted to identify the optimal operating parameters for the efficient application of struvite precipitation in the treatment of calf manure digestates. Materials and methods Materials The experiments were conducted on a digestate that was collected from an anaerobic digester, which operates with a solid retention time of approximately 20 days and treats mainly cattle manure with some addition of food waste (corn silage and olive oil waste). The sample was withdrawn from the thickener of the treatment plant and stored in a 25L container at 4◦ C. The characteristics of the wastewater are summarized in Table 1. The struvite precipitation process was developed using seawater bittern and bone meal as sources of magnesium and phosphorus, respectively. The bittern contained more than 97% of magnesium chloride with traces of other seawater compounds, and the bone meal contained 61.6% of phosphate and 35.7% calcium, with only small amounts of sodium and magnesium. The main elements of bone meal and seawater bittern are also reported in Table 2. Magnesium and phosphate were properly solubilized before carrying out the experimental runs. The bittern was simply dissolved by mixing 50 g into 50 mL of tap water to achieve a concentration of roughly 69 g Mg+2 /L. To dissolve the phosphorus content of bone meal, 38 g of powder was mixed with 100 mL of 3 N sulphuric acid, which resulted in a phosphorous concentration of approximately 80 g PO−3 4 - P/L. Sulphuric acid was specifically used to prevent dissolving of calcium in the solution; in this manner, it remained mainly in the insoluble form as calcium sulphate and only Table 1.
Digestate characteristics.
Parameter pH Salinity TKN NH+ 4 -N TP PO−3 4 -P Mg+2 Ca+2 TSS TDS VSS NH4 /PO4 /Mg/Ca
Unit
Value
– mS/cm gN/L gN/L gP/L gP/L gMg/L gCa/L g/L g/L g/L –
8.3 ± 0.3 16.5 ± 0.8 1.55 ± 0.2 1.06 ± 0.2 0.46 ± 0.08 0.45 ± 0.06 0.15 ± 0.02 0.57 ± 0.02 60.3 ± 2.3 29.6 ± 3.4 38.5 ± 2.8 1:0.19:0.08:0.19
Table 2.
Main elements of bone meal and seawater bittern.
Parameter
Unit
Bone meal
Seawater bittern
P Ca Mg Cl Na K
mg/g mg/g mg/g mg/g mg/g mg/g
200.1 357.0 2.5 2.2 2.3 1.3
– 1.1 112 335 26 1.8
a solubilized amount of 0.12 gCa+2 /L was detected. The magnesium and phosphorus-rich solutions were characterized by pH values of 5.7 and 2.1, respectively. In the experiments, the sample pH was adjusted by means of 10 N caustic soda or air insufflation.
Description of experiments The first set of experiments was designed to investigate the influence of pH setting on the effectiveness of the process. To minimize the addition of reagents, in some tests the pH correction was carried out by means of an injection of atmospheric air leading to CO2 degasification. In addition, many other experiments were performed by adding alkaline chemicals. Different operating modes have been tested for both pH correction techniques (Figure 1). All these tests were conducted for 30 min, using stoichiometric ratios (RPN = 1 and RMN = 1). Moreover, the tests conducted by chemical pH adjustments were planned to a value of 10, considered by many authors as preferable for struvite formation.[23] After identifying the operating mode and holding the theoretical dosages, to optimize the process conditions, other experiments were carried out first by testing the pH values in the range 8–11, where struvite precipitation could efficiently occur,[3,10,24] and afterwards by testing different reaction times (1, 15 and 30 min; Figure 1). Finally, further experiments were performed to establish the required dosages of reagents for maximizing the ammonia removal by struvite precipitation. In some cases, the RMN and RPN were increased separately from the theoretical value of 1 to 1.1, 1.3 and 1.5; in other cases, the dosages of reagents were changed simultaneously (Figure 1). To evaluate the possible ammonia loss by stripping and the precipitate composition, a mass balance was established. In this regard, a known quantity of precipitate was dissolved into 100 mL of 3 N sulphuric acid, so as to measure the solubilized amounts of ammonia, magnesium, phosphate and calcium.
Test procedure The experiments were conducted in batch mode, at room temperature (20–22◦ C), and in a 0.5-L glass flask equipped with a thermometer and a pH meter. The reaction mixture was prepared by adding volumes of concentrated
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representative of the actual removal of pollutant amounts; thus, the values of the reported efficiencies were not affected by dilution due to reactant addition in the various processes.
Analytical methods Salinity and pH were measured by a multiparametric analyser; TSS, TDS and VSS were measured by drying the sample at 105◦ C and 550◦ C, respectively; total Kjeldahl nitrogen and ammonium nitrogen were measured by Kjeldahl procedures; phosphorus species were measured by colorimetric methods, using an UV spectrophotomer; magnesium and calcium were measured by atomic adsorption spectrophotometry.[25] The analysis of precipitate was performed on samples washed with deionized water, filtered at 0.45 μm and dried at 40◦ C. This procedure prevented the decomposition of struvite during the drying phase.[26]
Results and discussion
Figure 1.
Plan of experiments carried out.
solutions of magnesium and phosphorus to the digestate samples to obtain the stoichiometric ratios established for the experiment. To correct the pH with the CO2 degasification technique, an airflow of 10 L/min was supplied to the reaction mixture for 30 min. In the experiments with the chemical pH adjustment, 10 N caustic soda was used. Each test was performed on a total volume of 0.25 L magnetically stirred at 300 rpm over the defined reaction time. After this period, the mixture was left to settle for 30 min to allow precipitation of the insoluble compounds. The supernatant was withdrawn and filtered through a 0.45-μm filter before its chemical characterization. After optimization of the treatment, the precipitate that was collected from the test performed under the best operating conditions was analysed by X-ray diffraction and scanning electron microscopy (SEM) to analyse crystal morphology. Presentation of results The experiments were repeated four times and the results expressed as mean ± standard deviations. The results were
Tests conducted with air insufflation The results of the first set of tests showed that an efficient removal of ammonia load in real digestates was hardly achievable by using CO2 degasification to correct pH. By aerating the reaction mixture after the addition of reagents, only 37.8% NH+ 4 removal was achieved, with a phosphorus and magnesium recovery of 44.5% and 27.7%, respectively (Table 3). This limited efficiency was due to the pH trend, which, mainly because of the supply of phosphorus and magnesium acid solutions, rapidly decreased from the natural value of 8.3 to 5.7, and, subsequently, due to the stripping of carbon dioxide slightly increased up to a plateau value of only 6.3 (Figure 2). The initial pH drop limited struvite formation, whose solubility decreases in a steady basic environment but did not completely arrest ammonia removal. This result suggested that struvite nucleation, which began immediately after the addition of Mg+2 and PO−3 4 facilitated a partial abatement of ammonia, whereas the occurrence of an acid environment stopped the growth of crystals. Indeed, small crystals of size lower than 10 μm were found (Figure 4). The experiments also showed that air insufflations after the addition of reagents did not lead pH to basic values, which are necessary to promote further struvite production. These findings are in agreement with the work of Saidou et al.,[27] who did not observe struvite formation when aeration was applied to synthetic solutions characterized by pH lower than 6.5. Slightly higher efficiencies of 44.8%, 45.7% and 29.2% for ammonia, phosphorus and magnesium, respectively, were reached by aerating the samples prior to the addition of reagents (Table 3). The improvement in performance with respect to the previous tests is a consequence of the higher pH level at which the process occurred. Indeed, Mg+2 and PO−3 4 solutions were added to
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A. Siciliano and S. De Rosa
Figure 2. pH trends during tests conducted with air insufflation (× start of aeration before dosage of reagents; ♦ start of aeration after dosage of reagents). Table 3.
Results of experiments conducted using air insufflation to correct pH, reaction time = 30 min. Molar abatements detected on treated digestate (%)
Operating conditions Start of aeration after dosage of reagents Start of aeration before dosage of reagents
Stoichiometric ratios in precipitate
NH+ 4
PO−3 4
Mg+2
N/P/Mg/Ca
NH+ 4 loss via stripping (%)
37.8 ± 1.6
44.5 ± 2.6
27.7 ± 4.5
1:1.22:0.75:0.25
1.54 ± 0.4
44.8 ± 2.1
45.7 ± 1.9
29.2 ± 2.7
1:1.17:0.75:0.30
5.75 ± 0.8
Values represent the mean ± SD, n = 4.
a sample whose pH had increased to 9.3 as a result of the initial air supply (Figure 2). Although this improvement in initial basic conditions supports struvite formation, the subsequent occurrence of an acid environment did not permit the achievement of high efficiencies. The moderate alkaline conditions also caused a slight increase in ammonia volatilization. Indeed, the results of analysis on precipitate showed a loss of ammonia by stripping equal to 5.75% and 1.54%, respectively, starting with the air insufflations before and after the reagents dosage (Table 3). Thus, about 3% gain in ammonia recovery by precipitation could be observed by operating according to the second procedure. Moreover, in both cases, despite the low Ca/NH4 ratio in the digestate, the precipitate was characterized by a significant amount of Ca+2 , which suggests the presence of other compounds, probably calcium phosphates. Anyhow, while aeration is a valid method, as demonstrated in many works,[28,29] for pH correction in the struvite precipitation process for the recovery of the phosphorus content of digestate, it is less effective when the NH+ 4 load must be removed. A viable option of this technique to effectively raise the pH in the production of struvite is the use of the cascade stripper.[30] This system allows a remarkable reduction of chemical consumption but does not significantly improve the performance of treatment,[30] especially with regard to ammonia removal.
Indeed, the addition of reagents necessary to set the RPN and RMN stoichiometric ratios causes a remarkable solution acidification that could not be completely counterbalanced by CO2 stripping. Tests conducted with chemical pH adjustment As compared with tests performed with air insufflation, significantly higher efficiencies were obtained in the experiments conducted by chemical adjustment of pH samples. The results showed that larger ammonia removals were achieved during the tests in which the solutions of magnesium and phosphorus were added prior to pH correction. Indeed, in comparison with the tests conducted using a preliminary pH correction, approximate increases of 11% in ammonia and phosphorus abatements, resulting in maximum efficiencies of approximately 75% and 95%, respectively, were obtained (Table 4). These findings are in agreement with the results observed by Kim et al.,[3] who stated that optimal conditions for struvite precipitation are created by the addition of magnesium and phosphorus sources, followed by pH adjustments. This operating mode also reduces to half the amount of Ca+2 transferred into precipitate (Table 4). Indeed, the initial pH increase to a value of 10 promotes the formation of calcite. This explains the higher precipitated amount of calcium detected performing
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Environmental Technology the pH correction before the phosphorus and magnesium dosage. Our experiments, moreover, demonstrated that the addition mode of caustic soda to set the pH did not significantly affect the process performance. In fact, similar abatement of ammonia, phosphorus and magnesium was observed with a gradual or instantaneous dosage of NaOH solution after the reagents dosage. The observed ammonia abatement was much higher than that reported in studies where seawater and seawater bittern were used as the sources of magnesium and analytical-grade reagents as the sources of phosphorus. Lee et al. [19] dosed these compounds to synthetic solutions prior to pH correction in the range of 9.5–10 and observed maximum abatements of ammonia between 39% and 54%. As previously stated, the greater efficiencies reached in our tests can most likely be attributed to the improved feeding sequence of added reagents. On the basis of all these considerations, the correction of the pH in a single step after the reagent dosage was considered preferable. With this operating mode, further investigations were conducted with RMN and RPN ratios held to theoretical values and pH values ranging between 8 and 11. The results showed that an increase in pH up to 9 yielded ammonia removal efficiencies of about 76%; this value later did not significantly change for higher pH values (Table 4). This observation confirmed that the positive effects of alkaline conditions are not significant at pH values higher than 9. Moreover, above this value a small ammonia volatilization, about 4%, could be observed (Table 4); in agreement with Uysal et al.,[31] it slightly reduces the nitrogen recovery as struvite crystals. This effect, instead, is completely negligible for the lower pH tested (Table 4). Thus, the results obtained are in agreement with many works [5,19,31,32]
Table 4.
845
that report the most favourable pH value for struvite precipitation around 9. Furthermore, the pH 9 is sufficient for achieving the maximum recovery of magnesium and phosphorus (Table 4); thus, this value was used for the execution of the following tests, aimed to determining the minimum required reaction time for struvite precipitation. In these tests, a 70% ammonia removal was observed only after 1 min of reaction time and a 5% efficiency improvement when the reaction time was extended to 15 min. Prolonging, however, the treatment up to 30 min gave no further improvement to process efficiency, which remained at roughly 75% (Table 4). A moderate growth of approximately 3% in phosphorus removal corresponded to an increase in reaction time up to 15 min, whereas magnesium abatement was constantly around 99% (Table 4). In all experiments, the ammonia volatilization was negligible and the molar Ca/NH4 ratio in the precipitate was
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents a
A. Siciliano & S. De Rosa
a
a
Department of Environmental and Chemical Engineering, University of Calabria, via P. Bucci, Cubo 44B. 87046, Rende, CS, Italy. Published online: 05 Nov 2013.
To cite this article: A. Siciliano & S. De Rosa (2014) Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents, Environmental Technology, 35:7, 841-850, DOI: 10.1080/09593330.2013.853088 To link to this article: http://dx.doi.org/10.1080/09593330.2013.853088
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2014 Vol. 35, No. 7, 841–850, http://dx.doi.org/10.1080/09593330.2013.853088
Recovery of ammonia in digestates of calf manure through a struvite precipitation process using unconventional reagents A. Siciliano∗ and S. De Rosa Department of Environmental and Chemical Engineering, University of Calabria, via P. Bucci, Cubo 44B, Rende (CS) 87046, Italy
Downloaded by [University of Boras] at 03:07 05 October 2014
(Received 3 January 2013; final version received 26 September 2013 ) Land spreading of digestates causes the discharge of large quantities of nutrients into the environment, which contributes to eutrophication and depletion of dissolved oxygen in water bodies. For the removal of ammonia nitrogen, there is increasing interest in the chemical precipitation of struvite, which is a mineral that can be reused as a slow-release fertilizer. However, this process is an expensive treatment of digestate because large amounts of magnesium and phosphorus reagents are required. In this paper, a struvite precipitation-based process is proposed for an efficient recovery of digestate nutrients using low-cost reagents. In particular, seawater bittern, a by-product of marine salt manufacturing and bone meal, a by-product of the thermal treatment of meat waste, have been used as low-cost sources of magnesium and phosphorus, respectively. Once the operating conditions are defined, the process enables the removal of more than 90% ammonia load, the almost complete recovery of magnesium and phosphorus and the production of a potentially valuable precipitate containing struvite crystals. Keywords: struvite; bone meal; seawater bittern; digestate; ammonia removal
Introduction The anaerobic digestion of animal manure has rapidly expanded within the last few decades. This treatment not only produces biogas and reduces the quantities of waste, but also increases land spreading of digestates, causing the release of large amounts of nutrients into the environment. Indeed, this wastewater is typically very rich in inorganic nitrogen because ammonia is formed from decomposition of proteins under anaerobic conditions.[1] Some recent studies have demonstrated that the use of digestates as fertilizers, instead of cattle manure, releases comparatively more nitrogen that can be easily drained to surface waters and leached to groundwaters. [2] This phenomenon, which results in the accumulation of nutrients in water bodies, is directly linked to eutrophication and to the subsequent depletion of dissolved oxygen. Moreover, nitrogen compounds can be toxic to aquatic life or lead to diseases caused by the consumption of contaminated drinking water. The most common and economical method for removing nitrogen from wastewater is the process of biological nitrification and denitrification. However, high contents of ammonia have a toxic effect on micro-organisms,[3] which may lead to a decrease in the treatment effectiveness. Autotrophic processes (SHARON-ANAMMOX) are suitable for the treatment of high concentrated wastewater, but their control and management are complex and difficult to operate. Moreover, conventional processes do not recycle nitrogen compounds as truly sustainable products. ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
The recovery of nitrogen and phosphorus from wastewater in the form of crystalline struvite (MgNH4 PO4 · 6H2 O) is found to be a sustainable option because struvite is considered a potential fertilizer.[3–13] Struvite precipitation +2 occurs when the combination of NH+ and PO−3 4 , Mg 4 concentration exceeds the struvite solubility product under alkaline conditions. Because of the low amounts of magnesium and phosphorus relative to ammonia concentrations, large amounts of the reagents are required for the treatment of digestates, which results in an expensive process. Different research works, reported in the literature, have evaluated the feasibility of using alternative magnesium compounds, such as, for instance, the by-products generated in the production of magnesium oxide,[14–16] pyrolysate of magnesite,[17] magnesite mineral [18] and seawater bittern,[19] to reduce the process costs. However, there is a lack of research about the identification of alternative sources of phosphorus, which is rarer, more expensive and whose consumption generally represents the primary cost of the whole treatment. To overcome this issue, the use of bone meal, a by-product of the thermal treatment of meat waste, is proposed in this paper as a low-cost source of phosphorus for struvite precipitation. In Europe, the use of this by-product as a fertilizer is restricted [20] and, since it is only suitable for limited alternative utilizations, it is usually disposed in landfills.[21,22] Thus, the exploitation of bone meal for the struvite precipitation process is particularly meaningful because it would allow
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the recovery of its phosphorus content with the production of a more valuable fertilizer containing both phosphorus and nitrogen. Together with bone meal, seawater bittern, a by-product of marine salt manufacturing, was used: its effectiveness has been already demonstrated as a low-cost source of magnesium.[19] Using the aforementioned byproducts, several experiments were conducted to identify the optimal operating parameters for the efficient application of struvite precipitation in the treatment of calf manure digestates. Materials and methods Materials The experiments were conducted on a digestate that was collected from an anaerobic digester, which operates with a solid retention time of approximately 20 days and treats mainly cattle manure with some addition of food waste (corn silage and olive oil waste). The sample was withdrawn from the thickener of the treatment plant and stored in a 25L container at 4◦ C. The characteristics of the wastewater are summarized in Table 1. The struvite precipitation process was developed using seawater bittern and bone meal as sources of magnesium and phosphorus, respectively. The bittern contained more than 97% of magnesium chloride with traces of other seawater compounds, and the bone meal contained 61.6% of phosphate and 35.7% calcium, with only small amounts of sodium and magnesium. The main elements of bone meal and seawater bittern are also reported in Table 2. Magnesium and phosphate were properly solubilized before carrying out the experimental runs. The bittern was simply dissolved by mixing 50 g into 50 mL of tap water to achieve a concentration of roughly 69 g Mg+2 /L. To dissolve the phosphorus content of bone meal, 38 g of powder was mixed with 100 mL of 3 N sulphuric acid, which resulted in a phosphorous concentration of approximately 80 g PO−3 4 - P/L. Sulphuric acid was specifically used to prevent dissolving of calcium in the solution; in this manner, it remained mainly in the insoluble form as calcium sulphate and only Table 1.
Digestate characteristics.
Parameter pH Salinity TKN NH+ 4 -N TP PO−3 4 -P Mg+2 Ca+2 TSS TDS VSS NH4 /PO4 /Mg/Ca
Unit
Value
– mS/cm gN/L gN/L gP/L gP/L gMg/L gCa/L g/L g/L g/L –
8.3 ± 0.3 16.5 ± 0.8 1.55 ± 0.2 1.06 ± 0.2 0.46 ± 0.08 0.45 ± 0.06 0.15 ± 0.02 0.57 ± 0.02 60.3 ± 2.3 29.6 ± 3.4 38.5 ± 2.8 1:0.19:0.08:0.19
Table 2.
Main elements of bone meal and seawater bittern.
Parameter
Unit
Bone meal
Seawater bittern
P Ca Mg Cl Na K
mg/g mg/g mg/g mg/g mg/g mg/g
200.1 357.0 2.5 2.2 2.3 1.3
– 1.1 112 335 26 1.8
a solubilized amount of 0.12 gCa+2 /L was detected. The magnesium and phosphorus-rich solutions were characterized by pH values of 5.7 and 2.1, respectively. In the experiments, the sample pH was adjusted by means of 10 N caustic soda or air insufflation.
Description of experiments The first set of experiments was designed to investigate the influence of pH setting on the effectiveness of the process. To minimize the addition of reagents, in some tests the pH correction was carried out by means of an injection of atmospheric air leading to CO2 degasification. In addition, many other experiments were performed by adding alkaline chemicals. Different operating modes have been tested for both pH correction techniques (Figure 1). All these tests were conducted for 30 min, using stoichiometric ratios (RPN = 1 and RMN = 1). Moreover, the tests conducted by chemical pH adjustments were planned to a value of 10, considered by many authors as preferable for struvite formation.[23] After identifying the operating mode and holding the theoretical dosages, to optimize the process conditions, other experiments were carried out first by testing the pH values in the range 8–11, where struvite precipitation could efficiently occur,[3,10,24] and afterwards by testing different reaction times (1, 15 and 30 min; Figure 1). Finally, further experiments were performed to establish the required dosages of reagents for maximizing the ammonia removal by struvite precipitation. In some cases, the RMN and RPN were increased separately from the theoretical value of 1 to 1.1, 1.3 and 1.5; in other cases, the dosages of reagents were changed simultaneously (Figure 1). To evaluate the possible ammonia loss by stripping and the precipitate composition, a mass balance was established. In this regard, a known quantity of precipitate was dissolved into 100 mL of 3 N sulphuric acid, so as to measure the solubilized amounts of ammonia, magnesium, phosphate and calcium.
Test procedure The experiments were conducted in batch mode, at room temperature (20–22◦ C), and in a 0.5-L glass flask equipped with a thermometer and a pH meter. The reaction mixture was prepared by adding volumes of concentrated
Environmental Technology
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Downloaded by [University of Boras] at 03:07 05 October 2014
representative of the actual removal of pollutant amounts; thus, the values of the reported efficiencies were not affected by dilution due to reactant addition in the various processes.
Analytical methods Salinity and pH were measured by a multiparametric analyser; TSS, TDS and VSS were measured by drying the sample at 105◦ C and 550◦ C, respectively; total Kjeldahl nitrogen and ammonium nitrogen were measured by Kjeldahl procedures; phosphorus species were measured by colorimetric methods, using an UV spectrophotomer; magnesium and calcium were measured by atomic adsorption spectrophotometry.[25] The analysis of precipitate was performed on samples washed with deionized water, filtered at 0.45 μm and dried at 40◦ C. This procedure prevented the decomposition of struvite during the drying phase.[26]
Results and discussion
Figure 1.
Plan of experiments carried out.
solutions of magnesium and phosphorus to the digestate samples to obtain the stoichiometric ratios established for the experiment. To correct the pH with the CO2 degasification technique, an airflow of 10 L/min was supplied to the reaction mixture for 30 min. In the experiments with the chemical pH adjustment, 10 N caustic soda was used. Each test was performed on a total volume of 0.25 L magnetically stirred at 300 rpm over the defined reaction time. After this period, the mixture was left to settle for 30 min to allow precipitation of the insoluble compounds. The supernatant was withdrawn and filtered through a 0.45-μm filter before its chemical characterization. After optimization of the treatment, the precipitate that was collected from the test performed under the best operating conditions was analysed by X-ray diffraction and scanning electron microscopy (SEM) to analyse crystal morphology. Presentation of results The experiments were repeated four times and the results expressed as mean ± standard deviations. The results were
Tests conducted with air insufflation The results of the first set of tests showed that an efficient removal of ammonia load in real digestates was hardly achievable by using CO2 degasification to correct pH. By aerating the reaction mixture after the addition of reagents, only 37.8% NH+ 4 removal was achieved, with a phosphorus and magnesium recovery of 44.5% and 27.7%, respectively (Table 3). This limited efficiency was due to the pH trend, which, mainly because of the supply of phosphorus and magnesium acid solutions, rapidly decreased from the natural value of 8.3 to 5.7, and, subsequently, due to the stripping of carbon dioxide slightly increased up to a plateau value of only 6.3 (Figure 2). The initial pH drop limited struvite formation, whose solubility decreases in a steady basic environment but did not completely arrest ammonia removal. This result suggested that struvite nucleation, which began immediately after the addition of Mg+2 and PO−3 4 facilitated a partial abatement of ammonia, whereas the occurrence of an acid environment stopped the growth of crystals. Indeed, small crystals of size lower than 10 μm were found (Figure 4). The experiments also showed that air insufflations after the addition of reagents did not lead pH to basic values, which are necessary to promote further struvite production. These findings are in agreement with the work of Saidou et al.,[27] who did not observe struvite formation when aeration was applied to synthetic solutions characterized by pH lower than 6.5. Slightly higher efficiencies of 44.8%, 45.7% and 29.2% for ammonia, phosphorus and magnesium, respectively, were reached by aerating the samples prior to the addition of reagents (Table 3). The improvement in performance with respect to the previous tests is a consequence of the higher pH level at which the process occurred. Indeed, Mg+2 and PO−3 4 solutions were added to
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A. Siciliano and S. De Rosa
Figure 2. pH trends during tests conducted with air insufflation (× start of aeration before dosage of reagents; ♦ start of aeration after dosage of reagents). Table 3.
Results of experiments conducted using air insufflation to correct pH, reaction time = 30 min. Molar abatements detected on treated digestate (%)
Operating conditions Start of aeration after dosage of reagents Start of aeration before dosage of reagents
Stoichiometric ratios in precipitate
NH+ 4
PO−3 4
Mg+2
N/P/Mg/Ca
NH+ 4 loss via stripping (%)
37.8 ± 1.6
44.5 ± 2.6
27.7 ± 4.5
1:1.22:0.75:0.25
1.54 ± 0.4
44.8 ± 2.1
45.7 ± 1.9
29.2 ± 2.7
1:1.17:0.75:0.30
5.75 ± 0.8
Values represent the mean ± SD, n = 4.
a sample whose pH had increased to 9.3 as a result of the initial air supply (Figure 2). Although this improvement in initial basic conditions supports struvite formation, the subsequent occurrence of an acid environment did not permit the achievement of high efficiencies. The moderate alkaline conditions also caused a slight increase in ammonia volatilization. Indeed, the results of analysis on precipitate showed a loss of ammonia by stripping equal to 5.75% and 1.54%, respectively, starting with the air insufflations before and after the reagents dosage (Table 3). Thus, about 3% gain in ammonia recovery by precipitation could be observed by operating according to the second procedure. Moreover, in both cases, despite the low Ca/NH4 ratio in the digestate, the precipitate was characterized by a significant amount of Ca+2 , which suggests the presence of other compounds, probably calcium phosphates. Anyhow, while aeration is a valid method, as demonstrated in many works,[28,29] for pH correction in the struvite precipitation process for the recovery of the phosphorus content of digestate, it is less effective when the NH+ 4 load must be removed. A viable option of this technique to effectively raise the pH in the production of struvite is the use of the cascade stripper.[30] This system allows a remarkable reduction of chemical consumption but does not significantly improve the performance of treatment,[30] especially with regard to ammonia removal.
Indeed, the addition of reagents necessary to set the RPN and RMN stoichiometric ratios causes a remarkable solution acidification that could not be completely counterbalanced by CO2 stripping. Tests conducted with chemical pH adjustment As compared with tests performed with air insufflation, significantly higher efficiencies were obtained in the experiments conducted by chemical adjustment of pH samples. The results showed that larger ammonia removals were achieved during the tests in which the solutions of magnesium and phosphorus were added prior to pH correction. Indeed, in comparison with the tests conducted using a preliminary pH correction, approximate increases of 11% in ammonia and phosphorus abatements, resulting in maximum efficiencies of approximately 75% and 95%, respectively, were obtained (Table 4). These findings are in agreement with the results observed by Kim et al.,[3] who stated that optimal conditions for struvite precipitation are created by the addition of magnesium and phosphorus sources, followed by pH adjustments. This operating mode also reduces to half the amount of Ca+2 transferred into precipitate (Table 4). Indeed, the initial pH increase to a value of 10 promotes the formation of calcite. This explains the higher precipitated amount of calcium detected performing
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Environmental Technology the pH correction before the phosphorus and magnesium dosage. Our experiments, moreover, demonstrated that the addition mode of caustic soda to set the pH did not significantly affect the process performance. In fact, similar abatement of ammonia, phosphorus and magnesium was observed with a gradual or instantaneous dosage of NaOH solution after the reagents dosage. The observed ammonia abatement was much higher than that reported in studies where seawater and seawater bittern were used as the sources of magnesium and analytical-grade reagents as the sources of phosphorus. Lee et al. [19] dosed these compounds to synthetic solutions prior to pH correction in the range of 9.5–10 and observed maximum abatements of ammonia between 39% and 54%. As previously stated, the greater efficiencies reached in our tests can most likely be attributed to the improved feeding sequence of added reagents. On the basis of all these considerations, the correction of the pH in a single step after the reagent dosage was considered preferable. With this operating mode, further investigations were conducted with RMN and RPN ratios held to theoretical values and pH values ranging between 8 and 11. The results showed that an increase in pH up to 9 yielded ammonia removal efficiencies of about 76%; this value later did not significantly change for higher pH values (Table 4). This observation confirmed that the positive effects of alkaline conditions are not significant at pH values higher than 9. Moreover, above this value a small ammonia volatilization, about 4%, could be observed (Table 4); in agreement with Uysal et al.,[31] it slightly reduces the nitrogen recovery as struvite crystals. This effect, instead, is completely negligible for the lower pH tested (Table 4). Thus, the results obtained are in agreement with many works [5,19,31,32]
Table 4.
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that report the most favourable pH value for struvite precipitation around 9. Furthermore, the pH 9 is sufficient for achieving the maximum recovery of magnesium and phosphorus (Table 4); thus, this value was used for the execution of the following tests, aimed to determining the minimum required reaction time for struvite precipitation. In these tests, a 70% ammonia removal was observed only after 1 min of reaction time and a 5% efficiency improvement when the reaction time was extended to 15 min. Prolonging, however, the treatment up to 30 min gave no further improvement to process efficiency, which remained at roughly 75% (Table 4). A moderate growth of approximately 3% in phosphorus removal corresponded to an increase in reaction time up to 15 min, whereas magnesium abatement was constantly around 99% (Table 4). In all experiments, the ammonia volatilization was negligible and the molar Ca/NH4 ratio in the precipitate was
