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Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing by-product
Water Research 37 (2003) 1601–1607
Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing by-product J.M. Chimenosa, A.I. Ferna! ndeza, G. Villalbaa, M. Segarraa, A. Urruticoecheab, B. Artazab, F. Espiella,* a
Department of Chemical Engineering and Metallurgy, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain b ! Asistencia Tecnologica Medioambiental, S.A., Epele Bailara, 29, 20120 Hernani, Spain Received 24 April 2001; received in revised form 29 March 2002; accepted 30 September 2002
Abstract The wastewater produced by the cochineal extract process to obtain the carminic acid colouring pigment (carmin red E120) has high concentrations of phosphates and ammonium. It is known that both ions can be precipitated with magnesium in the form of struvite, MgNH4PO4, or ammonium magnesium phosphate (MAP) compounds. In this study, the use of an alternative MgO-containing by-product is investigated. The optimal pH, reaction time and solid/ liquid ratio have been studied. It has been found that the low-grade MgO needed is greater than the stoichiometric value for the full removal of ammonium and phosphate as MAP compounds. Although the low-grade MgO (LG-MgO) reacts slower than pure MgO, it has considerable economic advantages. A batch process has been proposed for the removal of ammonium and phosphates from wastewater obtained in cochineal extracts processing, previously to biological treatment to diminish the COD. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Struvite; Map; Cochineal extracts; Ammonium removal; Phosphate removal
1. Introduction The cochineal insect, dactylopius coccus, is the raw material in the production of carmine lake, a natural red dyestuff (E120) obtained from the carminic acid. It is used principally as a colouring agent in cosmetics, beverages and products with low pH. The colour hue ranges from orange to red as a result of the different modes of cochineal extraction. CHR Hansen is among the world’s largest producers and suppliers of natural carmine lake, mainly produced in its factory located in Spain. The wastewater obtained at the end of the process has high contents of *Corresponding author. Tel.: +34-3-4021316; fax: +34-34021291. E-mail address: [email protected] (F. Espiell).
ammonium and phosphates as well as high concentration of soluble chemical oxygen demand (COD). Nowadays, the wastewater is being treated in a subcontracted water treatment plant where the legal requirements for its discharge are met. However, the company, considers that having its own treatment plant will make the whole process more environmentally friendly and will have important economic savings. In this wastewater treatment plant the ammonium and the phosphates must be removed in a physico-chemical step prior to a conventional biological treatment. The removal of phosphorus has been largely studied, and at present, there are two effective and reliable methods established, chemical precipitation and biological removal [1]. In most chemical treatments, the phosphorus can be removed from sewage by precipitating via a metal salt, i.e. iron, aluminium and mainly
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J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607
calcium salts [2–5]. The phosphorus removal through biological treatment has been developed during the last twenty years and is now beginning to compete with the more conventional physico-chemical approach of precipitation with metal salts, mainly in municipal wastewater and animal manure treatments [6,7]. Chemical and biological removal methods both allow phosphorus to be recycled as a sustainable product for use as raw material in industrial or agricultural applications [8,9]. Ammonium is a common parameter in industrial sewage, and a biological treatment plant can only remove N–NH3 concentration of down to 200 mg/L. Higher concentrations of ammonium must be diminished prior to a biological treatment. This may be accomplished through a stripping step of ammonia, generated at a pH higher than 9.2, with an airflow which must be rinsed later to remove ammonia contents. Nevertheless, it is well known that ammonium and phosphates can be precipitated together with magnesium in the form of struvite, MgNH4PO4, or ammonium magnesium phosphate (MAP) compounds. [10–13]. Struvite or MAP precipitation has been mainly used prior the biological treatment of animal manures or municipal wastewater. Moreover, phosphorous and ammonium recovered as MAP compounds may find an application in the fertiliser sector as a slow release fertiliser. There are many advantages in using MgO as raw material, namely, magnesium oxide has minimal environmental impact, has a low solubility, and is essential for plant, animal, and human growth; also, it has a high alkalinity, more than other alkalis, which helps to neutralise acids and precipitate metals requiring less Mg(OH)2 to neutralise the same amount of acid; Mg(OH)2 is a weak base and its dissolution is not exothermic, reaching a maximum pH of 10 which is in order with the Clean Water Act basic limits; the sludge formed by the reaction is conducive to crystal growth and is not light, fragile, or gelatinous like that formed by other alkalis [14]. While the use of struvite for the removal and recovery of phosphates and ammonium is technically feasible for the treatment of high strength wastewater, it is not adopted economically since the high cost of magnesium compounds [15], i.e. magnesium hydroxide chemical reactive is eight to ten times more expensive than similar quality of calcium hydroxide. However, to obtain struvite, it is possible to use other sources of magnesium that are more economically feasible. In this way, lowgrade magnesium oxide (LG-MgO) may be used for the removal of phosphates and ammonium as MAP compounds. Nevertheless, if the struvite formed is used later as slow release fertiliser, the other compounds contained in LG-MgO must be natural, insoluble or stable substances in the working media, i.e. having very low concentrations of heavy metals.
In this study, experiments to remove phosphates and ammonium from cochineal insect processing wastewater are performed using an LG-MgO. The aim of this study has been to determine the optimum parameters at laboratory scale, needed for a further design of a physico-chemical pilot plant installed prior to conventional biological treatment plant. A reaction mechanism involving LG-MgO to form MAP compounds is proposed in accordance with the obtained results.
2. Methods and materials This study was carried out with the wastewater from the cochineal insects processing, developed by CHR Hansen, to produce natural carmine (E120). The factory, located in Navarra (Spain), processes 800–1000 kg of cochineal insects per day to obtain 135– 165 kg of carminic acid as the precursor of the different carmine lakes commercialised. The extraction process from the scale of cochineal insects to obtain carmine lake uses ammonium hydroxide as extracting agent and phosphoric acid as acidifying agent. At the end of the process, 35,000 L of wastewater per day are produced with a high content of phosphates and ammonium as well as the high COD. A sample of 50 L of wastewater obtained from the optimum and representative conditions of the process was used to perform laboratory trials. The chemical bulk analysis of an aliquot from initial wastewater is shown in Table 1. LG-MgO used as source of magnesium is produced and sold by Magnesitas Navarras, S.A. located in
Table 1 Composition of initial wastewater from the cochineal insects processing to produce natural carmine (CHR Hansen) and lowgrade magnesium oxide (Magnesitas Navarras) Wastewater
Low-grade MgO
pH
2.1
N–NH3 (mg/L)
2320
P–PO34 (mg/L)
3490
COD (mg O2/L)
10205
Ca2+ (mg/L)
42
SO24 (mg/L)
3458
TSS (mg/L)a
160
a
MgO (%) CaO (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) SO3 (%) d100 (mm) d50 (mm) d10 (mm) Bulk density (g/cm3) LOI (11001C) (%)b BET (m2/g)
70.0 9.7 4.2 2.7 0.6 4.9 100 8 3 3.0 8.9 8.3
TSS: Total suspended solids. LOI: Loss of ignition. dx : Accumulated fraction lower than particle size. BET: Specific surface area measured by single point BET. b
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Navarra (Spain). The initial price of this low-grade product (approximately 100 per ton) is close to calcium hydroxide price commonly used in the wastewater treatment plants. It comes from the calcination in rotary kiln at 11001C of natural magnesite. The product is collected as cyclone dusts in the fabric filters from the air pollution control system. The bulk composition and other physical parameters are shown in Table 1. The content of calcium oxide is due to the presence of small amounts of dolomite—MgCa(CO3)2—in natural magnesite, and may contribute to diminish phosphorus as insoluble calcium phosphate. The high loss of ignition (LOI) value obtained means that the product still contains unburned magnesite and dolomite. The presence of iron and aluminium may also contribute to decrease phosphorus concentration forming the insoluble iron/aluminium phosphates. The presence of silica, from natural origin, does not interfere with the physicochemical wastewater treatment and remains inert in the precipitated compounds. Finally, it is possible to establish a relation between the reactivity of the LGMgO and its specific surface area (BET) [16]. In this case, the low BET value obtained (8.3 m2/g) means that kinetics of precipitation of MAP compounds will be slower than using pure MgO with a BET value of 115 m2/g, which is a consequence of the mean particle size determined (d50) lower than 1 mm and the high porosity of the particles. The experiment trials were performed using a flocculation tester that consists of six 400 mL beakers that are agitated simultaneously at 200 rpm. All experiments were carried out at a room temperature of 251C. Different solid/liquid (S/L) ratios (12, 16, 20, 24, 30 and 34 g/L) and ten reaction times (0.5, 1, 2, 4, 6, 8, 12, 16, 20 and 24 h) were studied. The resulting suspensions were filtered through 45 mm membrane filters and the pH was determined from the clear filtrates. The resulting clear solution was acidified with concentrated HNO3 and was used for the analysis of phosphorus and ammonium. The phosphorus as phosphate was analysed by Inductive Coupled Plasma -Atomic Emission Spectrometry (ICP-AES) and ammonium by injection flow analysis (IFA). An evaluation of decantation rate was performed using volumetric laboratory equipment to determine the LG-MgO at different S/L ratio. The progression of solid sedimentation in a batch process was followed using a 2 L graduated cylinder. The results have been compared to the results obtained using commercial pure MgO obtained by electrofused process. The precipitates obtained were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) analyser to determine the different compound formed and help to elucidate a possible reaction mechanism.
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3. Results and discussion The initial wastewater has an ammonium/phosphate molar ratio of 1.5:1 Considering that the molar ratio 2+ NH+ :PO34 to form struvite is 1:1:1, the stoichio4 :Mg metric amount of magnesium oxide necessary to remove all phosphorus contained in wastewater is 4.6 g/L of pure MgO or 6.6 g/L of LG-MgO, according with the composition described in Table 1. Pure MgO reacted two to three times faster than the low-grade MgO for pH range lower than 7, as a consequence of the lower particle size and the greater BET (m2/g). Nevertheless, in the pH range greater than 7, both sources of MgO have similar reactivity to form struvite. As a result of that, the needed time to obtain the theoretical optimal pH [12] is lower (approximately 3 times) when using pure MgO. Regarding the decantation rate, pure MgO showed worse results than LG-MgO. The sedimentation rate for pure MgO was 15–20 times lower than LG-MgO as a function of solid/liquid ratio studied. These differences in the settling velocity may be explained mainly by the physical characteristic of both MgO particles. While the mean particle size (d50) for LG-MgO is lower than 8 mm, see Table 1, for the pure MgO is lower than 1 mm. Using LG-MgO, the effects of S/L ratio and time on the concentration of nitrogen and phosphorous and pH is depicted in Figs. 1–3. Figs. 1 and 2 show respectively the phosphate and ammonium concentration in wastewater versus reaction time as a function of LG-MgO added. In both experimental trials the initial amount of LG-MgO added ranged from 12 to 34 g/L. That means from 1.8 to 5.1 times the stoichiometric MgO needed. For all reaction times studied, it can be observed in Fig. 1 that the phosphate concentration decreases with the increase of the S/L ratio. Moreover, all phosphorus has been removed, excluding the batch experiment performed with 12 g/L, for a reaction time of 24 h. Nevertheless, for S/L ratios greater than 20 g/L the phosphorus concentration plunges in short reaction times. For example, phosphorus concentrations under 50 mg/L were obtained in less than 6 h by adding LGMgO amounts over 20 g/L. Regarding the nitrogen removal (stated as ammonium) similar results were obtained while increasing the S/L ratio. For a reaction time of 4 h, the nitrogen (N– NH3) concentration slightly decreases for high LG-MgO slurries, or remains steady for LG-MgO slurries lower than 24 g/L. Nitrogen concentrations under 200 mg/L only were obtained with a S/L ratio over 30 g/L and more than 16 h, or 24 g/L and reaction time of 24 h respectively. In Fig. 3 the pH values as a function of LG-MgO added and reaction time are depicted. It can be observed that pH values greater than 8 cannot be achieved with S/ L ratios lower than 20 g/L. The lowest values for the
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10 12 g/L 16 g/L 20 g/L 24 g/L 30 g/L 34 g/L
3000 2500 2000
12 g/l 16 g/l 20 g/l 24 g/l 30 g/l 34 g/l
9
8 pH
Phosphorus concentration (mg/L)
3500
1500
7 1000
6
500 0 0
5
10
15
20
5
25
0
Reaction time (h)
Fig. 1. Variation of the phosphorus (P–PO34 ) concentration in the natural carmine lake process wastewater as a function of low-grade MgO slurries versus reaction time.
12 g/L 16 g/L 20 g/L 24 g/L 30 g/L 34 g/L
2000
1500
10
15
20
25
30
Reaction time (h)
Fig. 3. pH recorder in the natural carmine lake process wastewater treatment as a function of low-grade MgO slurries versus reaction time.
3500 Experimetnal values
Phosphorus concentration (mg/L)
Nitrogen concentration (mg/L)
2500
5
1000
500
3000
Theoretical values
2500 2000 1500 1000 500
0 0
5
10 15 Reaction time (h)
20
25
Fig. 2. Variation of the nitrogen (N–NH3) concentration in the natural carmine lake process wastewater as a function of lowgrade MgO slurries versus reaction time.
solubility of MAP compounds were reported in the pH range of 8–10 [11,12,17]. Nevertheless, pH greater than 9.2 may be attributed to equilibrium solubility of Mg(OH)2 (pKsp=11.1). That means that an excess of LG-MgO has been added. The variation of phosphorus (P–PO34 ) concentration in the wastewater as a function of pH is shown in Fig. 4. In the same figure, calculated concentrations from equilibrium solubility of struvite have been depicted, using the solubility product (pKsp=12.6) cited in the literature [18,19], as well as other parasite reactions as the formation of acid species (NH3, H3PO4, H2PO4 , HPO24 and PO34 ) and the formation of magnesium phosphate. However, to obtain this theoretical curves the presence of other ions capable to forme insoluble
0 4
5
6
7
8
9
10
pH
Fig. 4. Comparison of phosphorus concentration experimental values and phosphorus concentration from struvite solubility product versus pH.
phosphate compounds, as iron or calcium, has not been considered. The experimental values analysed at pH lower than 6.2 indicate that MAP precipitated compounds exceed the solubility product and the solution remains oversaturated, while all phosphorus concentrations obtained at pH greater than 6.2 lie under the calculated solubilities. Iron, aluminium, calcium and carbonate ions contained in LG-MgO, and the calcium contained in the wastewater resulting from the production of the natural dyestuff, as well as the low magnesium activity, may contribute to phosphates removal thus varying the equilibrium conditions of the struvite. Under these conditions, experimental values for pKsp of struvite should be determined. Nevertheless, it seems that a struvite solubility control is achieved after
J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607
1605
2000
1600 a
1400 1200 1000 800
b
a
Intensity (a.u)
Nitrogen concentration (mg/L)
a
Experimental values Theoretical values
1800
600 a
a
a a c d a a a e a c
b
c ba d a
400
a: Struvite MgNH4 PO4 .6H2O b: Periclase MgO c: Magnesite MgCO3 ; d: Dolomite MgCa(CO3)2 e: Quartz SiO2
a
a a d e
b
45
50
aa
a
0
200
5
0 4
5
6
7
8
9
10
pH
Fig. 5. Comparison of nitrogen concentration experimental values and nitrogen concentration from struvite solubility product versus pH.
these impurities have reacted forming insoluble phosphates. Fig. 5 shows the wastewater nitrogen (N–NH3) concentration, obtained from all experimental trials performed, as a function of pH. It is always over the theoretical struvite solubility curve. This fact agrees with the initial wastewater ammonium/phosphate ratio. Nevertheless, the removed ammonium/phosphate ratio, calculated from concentrations analysed at pH values from 5.5 to 9, is in accordance with the theoretical molar ratio 1:1. So, the removal of ammonium is only due to struvite-MAP compounds formation, which needs a minimum phosphate concentration to precipitate. Finally, at a pH greater than 9.2 the nitrogen concentration slumps. This fact may be explained by the ammonia gas formation (pKa=9.2), which is stripped due the vigorous reactor agitation. The sludge obtained at the pH considered as optimum (pH 8) was used for the analysis of crystalline mineral phases. X-ray diffractogram of a sample obtained by adding 24 g/L of LG-MgO to wastewater resulting from the production of carmin red dye is shown in Fig. 6. Peaks of struvite were identified as the main phase, as well as MgO periclase. Magnesite, dolomite and quartz have been also identified as minor phases present in the sludge. The identity of other phases present in small quantities was very difficult to establish, because the pattern is characterised by a large number of small overlapping peaks. The presence of magnesium as MgO periclase as well as other non-reacted mineral phases from initial LG-MgO, suggests that the initial particles have not totally dissolved and struvite may be formed on the particle surface. This is in accordance with the absence of brucite Mg (OH) 2. This fact has been corroborated by SEM/EDS microanalysis performed on microtomed thin-sections of sludge particles. It can be observed in the micrograph
10
15
20
25
30
35
40
55
60
65
2θ
Fig. 6. X-ray diffractogram of a sludge obtained in the wastewater treatment with low-grade MgO (24 g/L and 20 h).
Fig. 7. Scanning electron micrograph of a sludge particle obtained in the wastewater treatment with low-grade MgO: inside an MgO periclase zone (a) and struvite growth on the low-grade MgO particle surface (b).
showed in Fig. 7 that there are two different morphologies, corresponding to inside (a) and outside (b) of the particle. EDS microanalysis from part (a) indicates than magnesium is the main element contained. Furthermore, small peaks corresponding to calcium, silicon, iron and aluminium, as well as oxygen, sulphur and carbon have also been identified. Nevertheless phosphorus and nitrogen have not been identified. These results corroborate that the inside of the particle had not reacted with the aqueous medium and remain unchanged. On the other hand, EDS microanalysis from part (b) reveals the important presence of phosphorus and nitrogen together with magnesium. Furthermore, small peaks corresponding to iron and calcium, both from the LG-MgO, have also been identified in the outside of the particle. That means that struvite crystals growth on the LG-MgO particle surface may be blocking the phosphate and ammonium diffusion.
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According to these results, a reaction mechanism is proposed to illustrate the removal of phosphates and ammonium using LG-MgO as source of magnesium. At the beginning, the Nernst boundary layer interface involving solid particle is formed and hydrolysis of MgO surface takes place. The solubility equilibrium of magnesium hydroxide is achieved and hydroxyl and magnesium concentration increase in the interface. Next, the diffusion of hydroxyl and magnesium ions to the bulk solution takes place. Generally, the rate of these steps is faster when pure MgO is used. However, when using LG-MgO less reactive, these reactions may become the rate controlling steps. Simultaneously, phosphoric acid and ammonium diffusion occurs from the bulk solution to the interface. According to experimental results obtained, the diffusion rate of phosphoric acid and ammonium to the interface is faster than the diffusion rate of magnesium and hydroxyls to bulk solution. Consequently, the struvite formation takes place in the interface instead of the bulk solution. In this proposed mechanism only the formation of struvite has been taken into account. However other magnesium and phosphate compounds such as formulas newberyte and bobierrite are also formed [20,21], and calcium and iron phosphates as well as gypsum may be formed too. The formation and growth of the MAP compounds forms a layer on the surface particle that does not allow the diffusion of magnesium and hydroxyls thereby stopping the reaction.
4. Conclusions It is possible to remove phosphate and ammonium from wastewater by precipitation of MAP compounds using low-grade MgO as source of magnesium. Due to the price of different grades of MgO, the use of lowgrade instead of pure MgO diminishes the costs of wastewater treatment in a physico-chemical plant. According to the results above described, 24 g/L of LG-MgO and a reaction time of 5 h is required to remove all the phosphate as MAP precipitated compounds. In this period of time the pH reaches values around 8.5–9, the value considered as optimum. There is a remainder of ammonium that does not precipitate after this point; to remove it is necessary increase the pH above 9.2 so the ammonium is eliminated as ammonia gas. This increase of pH takes place after 15 h due to the poor reactivity of the low grade MgO. Nevertheless, reaction time is not a critical parameter in this wastewater treatment, mainly due to the volume of wastewater daily produced. Thus, a reaction time of 20 h is the time considered as optimum to remove the ammonium
and phosphate from wastewater resulting from the production of carmin red dye from cochineal. The pH measurement may be used as a control parameter in a physico-chemical plant to establish the end of the reaction and the amount of LG-MgO slurry added. The use of the optimum conditions allows values lower than 35 mg/L of phosphorus (P–PO34 ) and 230 mg/L of nitrogen (N–NH3) in the final water, removing the 99 and 90 percent of initial concentrations, respectively. These minimum ammonium and phosphate concentrations may be removed in a biological plant, which is necessary for the removal of COD. Because of low activity of the LG-MgO used in this work, the formation of MAP compounds takes place on LG-MgO particle surface. This MAP layer blocks the diffusion of magnesium from inside of the particle to the boundary layer stopping the growth of struvite. As a result of this, it is necessary to add 3.5–4 times more MgO than is stoichiometrically needed, and extra LGMgO remains in the product obtained. Consequently, about 45 g/L of dry sludge are obtained. However, because of the composition of LG-MgO and the natural source of the red dyestuff, both without harmful substances, the sludge may be used as slow-release fertiliser. On the other hand, due to the particle size and specific gravity of LG-MgO, the decantation rate is greater than that obtained by using pure MgO and less volume of sludge is obtained. This dense sludge can be quickly and easily dewatered. As a result of these findings, a batch physico-chemical plant will be installed at CHR Hansen, S.A. (Spain) to treat 35,000 L of wastewater per day. Low-grade MgO used in this experimental work, may be also used to treat another sort of wastewater containing phosphates and ammonium, i.e. animal manures or municipal wastewater.
Acknowledgements The authors would like to thank Magnesitas Navarras S.A. for its cooperation in financing and supporting the work and CHR Hansen S.A. who kindly provided logistical support and samples. The authors also acknowledge to Scientific and Technical Services of the University of Barcelona for its analytical assistance.
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J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607 [2] Strickland J. The development and application of phosphorus removal from wastewater using biological and metal precipitation techniques. J CIWEM 1998;12: 30–7. [3] Donnert D, Salecker M. Elimination of phosphorus waste water by crystallization. Environ Technol 1999;20:735–42. [4] Angel R. Removal of phosphate from sewage as amorphous calcium phosphate. Environ Technol 1999;20:709– 20. [5] House WA. The physico-chemical conditions for the precipitation of phosphate with calcium. Environ Technol 1999;20:727–33. [6] Woods NC, Sock SM, Daigger GT. Phosphorus recovery technology modeling and feasibility evaluation for municipal wastewater treatment plants. Environ Technol 1999;20:663–79. [7] Greaves J, Hobbs P, Chadwich D, Haygarth P. Prospects for the recovery of phosphorus from animal manures: a review. Environ Technol 1999;20:697–708. [8] Durrant AE, Scrimshaw MD, Stratful I, Lester JN. Review of the feasibility of recovering phosphate from wastewater for use as a raw material by the phosphate industry. Environ Technol 1999;20:749–58. [9] Stratful I, Bret S, Scrimshaw MB, Lester JN. Biological phosphorus removal, its role in phosphorus recycling. Environ technol 1999;20:681–95. [10] Maqueda C, P!erez Rodr!ıguez JL, Lebrato J. Study of struvite precipitation in anaerobic digesters. Water Res. 1994;28:411–6. [11] Schulze-Rettmer R. The simultaneous chemical precipitation of ammonium and phosphate in the form of magnesium-ammonium-phosphate. Water Sci Technol 1991;23:659–67.
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[12] Tunay . O, Kabdasli I, Orhon D, Kolc-ak S. Ammonia removal by Magnesium Ammonium Phosphate Precipitation in Industrial Wastewater. Water Sci Technol 1997;36(2-3):225–8. [13] Pettlicka-Raj E. Removal of ammonia nitrogen from wastewater through precipitation of magnesium ammonia phosphate. Environ Prot Eng 1998;14:5–13. [14] Teringo III J. Magnesium Hydroxide Reduces Sludge/ Improves Filtering. Pollut Eng 1987;19:78–83. [15] Giesen A. Crystallisation Process Enables Environmental Friendly Phosphate Removal at Low Costs. Environ Technol 1999;20:769–75. [16] Fern!andez AI, Chimenos JM, Segarra M, Fern!andez MA, Espiell F. Procedure to obtain Hydromagnesite from a MgO-Containing Residue. Kinetic Study. Ind. Eng. Chem. Res. 2000;39:3653–8. [17] Abbona F, Boistelle R, Lundager HE. Crystallization of two magnesium phosphates, struvite and newberyte: effect of ph and concentration. J Cryst Growth 1982;57:6–14. [18] Borgerding J. Phosphate deposits in digestion system. J. Water Pollut 1972;44:813–9. [19] Loewentahal RE, Kornmuller URC, van Heerden EP. Modelling struvite precipitation in anaerobic treatment systems. Water. Sci. Technol. 1994;30:107–16. [20] Frazier AW, Waestad KJ. The phase system MgO(NH4)2O-P2O5-H2O at 251C. Ind. Eng. Chem Res. 1992;31:2065–8. [21] Miles A, Ellis TG. Recovery of nitrogen and phosphorus from anaerobically treated waste using struvite precipitation. Water Resour. Urban Environ.’98. Proceedings of the National Conferance Environmental Engineering In: Wilson, TE, editor. New York: American Society of Civil Engineer, pp. 161–6.
Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing by-product J.M. Chimenosa, A.I. Ferna! ndeza, G. Villalbaa, M. Segarraa, A. Urruticoecheab, B. Artazab, F. Espiella,* a
Department of Chemical Engineering and Metallurgy, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain b ! Asistencia Tecnologica Medioambiental, S.A., Epele Bailara, 29, 20120 Hernani, Spain Received 24 April 2001; received in revised form 29 March 2002; accepted 30 September 2002
Abstract The wastewater produced by the cochineal extract process to obtain the carminic acid colouring pigment (carmin red E120) has high concentrations of phosphates and ammonium. It is known that both ions can be precipitated with magnesium in the form of struvite, MgNH4PO4, or ammonium magnesium phosphate (MAP) compounds. In this study, the use of an alternative MgO-containing by-product is investigated. The optimal pH, reaction time and solid/ liquid ratio have been studied. It has been found that the low-grade MgO needed is greater than the stoichiometric value for the full removal of ammonium and phosphate as MAP compounds. Although the low-grade MgO (LG-MgO) reacts slower than pure MgO, it has considerable economic advantages. A batch process has been proposed for the removal of ammonium and phosphates from wastewater obtained in cochineal extracts processing, previously to biological treatment to diminish the COD. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Struvite; Map; Cochineal extracts; Ammonium removal; Phosphate removal
1. Introduction The cochineal insect, dactylopius coccus, is the raw material in the production of carmine lake, a natural red dyestuff (E120) obtained from the carminic acid. It is used principally as a colouring agent in cosmetics, beverages and products with low pH. The colour hue ranges from orange to red as a result of the different modes of cochineal extraction. CHR Hansen is among the world’s largest producers and suppliers of natural carmine lake, mainly produced in its factory located in Spain. The wastewater obtained at the end of the process has high contents of *Corresponding author. Tel.: +34-3-4021316; fax: +34-34021291. E-mail address: [email protected] (F. Espiell).
ammonium and phosphates as well as high concentration of soluble chemical oxygen demand (COD). Nowadays, the wastewater is being treated in a subcontracted water treatment plant where the legal requirements for its discharge are met. However, the company, considers that having its own treatment plant will make the whole process more environmentally friendly and will have important economic savings. In this wastewater treatment plant the ammonium and the phosphates must be removed in a physico-chemical step prior to a conventional biological treatment. The removal of phosphorus has been largely studied, and at present, there are two effective and reliable methods established, chemical precipitation and biological removal [1]. In most chemical treatments, the phosphorus can be removed from sewage by precipitating via a metal salt, i.e. iron, aluminium and mainly
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calcium salts [2–5]. The phosphorus removal through biological treatment has been developed during the last twenty years and is now beginning to compete with the more conventional physico-chemical approach of precipitation with metal salts, mainly in municipal wastewater and animal manure treatments [6,7]. Chemical and biological removal methods both allow phosphorus to be recycled as a sustainable product for use as raw material in industrial or agricultural applications [8,9]. Ammonium is a common parameter in industrial sewage, and a biological treatment plant can only remove N–NH3 concentration of down to 200 mg/L. Higher concentrations of ammonium must be diminished prior to a biological treatment. This may be accomplished through a stripping step of ammonia, generated at a pH higher than 9.2, with an airflow which must be rinsed later to remove ammonia contents. Nevertheless, it is well known that ammonium and phosphates can be precipitated together with magnesium in the form of struvite, MgNH4PO4, or ammonium magnesium phosphate (MAP) compounds. [10–13]. Struvite or MAP precipitation has been mainly used prior the biological treatment of animal manures or municipal wastewater. Moreover, phosphorous and ammonium recovered as MAP compounds may find an application in the fertiliser sector as a slow release fertiliser. There are many advantages in using MgO as raw material, namely, magnesium oxide has minimal environmental impact, has a low solubility, and is essential for plant, animal, and human growth; also, it has a high alkalinity, more than other alkalis, which helps to neutralise acids and precipitate metals requiring less Mg(OH)2 to neutralise the same amount of acid; Mg(OH)2 is a weak base and its dissolution is not exothermic, reaching a maximum pH of 10 which is in order with the Clean Water Act basic limits; the sludge formed by the reaction is conducive to crystal growth and is not light, fragile, or gelatinous like that formed by other alkalis [14]. While the use of struvite for the removal and recovery of phosphates and ammonium is technically feasible for the treatment of high strength wastewater, it is not adopted economically since the high cost of magnesium compounds [15], i.e. magnesium hydroxide chemical reactive is eight to ten times more expensive than similar quality of calcium hydroxide. However, to obtain struvite, it is possible to use other sources of magnesium that are more economically feasible. In this way, lowgrade magnesium oxide (LG-MgO) may be used for the removal of phosphates and ammonium as MAP compounds. Nevertheless, if the struvite formed is used later as slow release fertiliser, the other compounds contained in LG-MgO must be natural, insoluble or stable substances in the working media, i.e. having very low concentrations of heavy metals.
In this study, experiments to remove phosphates and ammonium from cochineal insect processing wastewater are performed using an LG-MgO. The aim of this study has been to determine the optimum parameters at laboratory scale, needed for a further design of a physico-chemical pilot plant installed prior to conventional biological treatment plant. A reaction mechanism involving LG-MgO to form MAP compounds is proposed in accordance with the obtained results.
2. Methods and materials This study was carried out with the wastewater from the cochineal insects processing, developed by CHR Hansen, to produce natural carmine (E120). The factory, located in Navarra (Spain), processes 800–1000 kg of cochineal insects per day to obtain 135– 165 kg of carminic acid as the precursor of the different carmine lakes commercialised. The extraction process from the scale of cochineal insects to obtain carmine lake uses ammonium hydroxide as extracting agent and phosphoric acid as acidifying agent. At the end of the process, 35,000 L of wastewater per day are produced with a high content of phosphates and ammonium as well as the high COD. A sample of 50 L of wastewater obtained from the optimum and representative conditions of the process was used to perform laboratory trials. The chemical bulk analysis of an aliquot from initial wastewater is shown in Table 1. LG-MgO used as source of magnesium is produced and sold by Magnesitas Navarras, S.A. located in
Table 1 Composition of initial wastewater from the cochineal insects processing to produce natural carmine (CHR Hansen) and lowgrade magnesium oxide (Magnesitas Navarras) Wastewater
Low-grade MgO
pH
2.1
N–NH3 (mg/L)
2320
P–PO34 (mg/L)
3490
COD (mg O2/L)
10205
Ca2+ (mg/L)
42
SO24 (mg/L)
3458
TSS (mg/L)a
160
a
MgO (%) CaO (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) SO3 (%) d100 (mm) d50 (mm) d10 (mm) Bulk density (g/cm3) LOI (11001C) (%)b BET (m2/g)
70.0 9.7 4.2 2.7 0.6 4.9 100 8 3 3.0 8.9 8.3
TSS: Total suspended solids. LOI: Loss of ignition. dx : Accumulated fraction lower than particle size. BET: Specific surface area measured by single point BET. b
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Navarra (Spain). The initial price of this low-grade product (approximately 100 per ton) is close to calcium hydroxide price commonly used in the wastewater treatment plants. It comes from the calcination in rotary kiln at 11001C of natural magnesite. The product is collected as cyclone dusts in the fabric filters from the air pollution control system. The bulk composition and other physical parameters are shown in Table 1. The content of calcium oxide is due to the presence of small amounts of dolomite—MgCa(CO3)2—in natural magnesite, and may contribute to diminish phosphorus as insoluble calcium phosphate. The high loss of ignition (LOI) value obtained means that the product still contains unburned magnesite and dolomite. The presence of iron and aluminium may also contribute to decrease phosphorus concentration forming the insoluble iron/aluminium phosphates. The presence of silica, from natural origin, does not interfere with the physicochemical wastewater treatment and remains inert in the precipitated compounds. Finally, it is possible to establish a relation between the reactivity of the LGMgO and its specific surface area (BET) [16]. In this case, the low BET value obtained (8.3 m2/g) means that kinetics of precipitation of MAP compounds will be slower than using pure MgO with a BET value of 115 m2/g, which is a consequence of the mean particle size determined (d50) lower than 1 mm and the high porosity of the particles. The experiment trials were performed using a flocculation tester that consists of six 400 mL beakers that are agitated simultaneously at 200 rpm. All experiments were carried out at a room temperature of 251C. Different solid/liquid (S/L) ratios (12, 16, 20, 24, 30 and 34 g/L) and ten reaction times (0.5, 1, 2, 4, 6, 8, 12, 16, 20 and 24 h) were studied. The resulting suspensions were filtered through 45 mm membrane filters and the pH was determined from the clear filtrates. The resulting clear solution was acidified with concentrated HNO3 and was used for the analysis of phosphorus and ammonium. The phosphorus as phosphate was analysed by Inductive Coupled Plasma -Atomic Emission Spectrometry (ICP-AES) and ammonium by injection flow analysis (IFA). An evaluation of decantation rate was performed using volumetric laboratory equipment to determine the LG-MgO at different S/L ratio. The progression of solid sedimentation in a batch process was followed using a 2 L graduated cylinder. The results have been compared to the results obtained using commercial pure MgO obtained by electrofused process. The precipitates obtained were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) analyser to determine the different compound formed and help to elucidate a possible reaction mechanism.
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3. Results and discussion The initial wastewater has an ammonium/phosphate molar ratio of 1.5:1 Considering that the molar ratio 2+ NH+ :PO34 to form struvite is 1:1:1, the stoichio4 :Mg metric amount of magnesium oxide necessary to remove all phosphorus contained in wastewater is 4.6 g/L of pure MgO or 6.6 g/L of LG-MgO, according with the composition described in Table 1. Pure MgO reacted two to three times faster than the low-grade MgO for pH range lower than 7, as a consequence of the lower particle size and the greater BET (m2/g). Nevertheless, in the pH range greater than 7, both sources of MgO have similar reactivity to form struvite. As a result of that, the needed time to obtain the theoretical optimal pH [12] is lower (approximately 3 times) when using pure MgO. Regarding the decantation rate, pure MgO showed worse results than LG-MgO. The sedimentation rate for pure MgO was 15–20 times lower than LG-MgO as a function of solid/liquid ratio studied. These differences in the settling velocity may be explained mainly by the physical characteristic of both MgO particles. While the mean particle size (d50) for LG-MgO is lower than 8 mm, see Table 1, for the pure MgO is lower than 1 mm. Using LG-MgO, the effects of S/L ratio and time on the concentration of nitrogen and phosphorous and pH is depicted in Figs. 1–3. Figs. 1 and 2 show respectively the phosphate and ammonium concentration in wastewater versus reaction time as a function of LG-MgO added. In both experimental trials the initial amount of LG-MgO added ranged from 12 to 34 g/L. That means from 1.8 to 5.1 times the stoichiometric MgO needed. For all reaction times studied, it can be observed in Fig. 1 that the phosphate concentration decreases with the increase of the S/L ratio. Moreover, all phosphorus has been removed, excluding the batch experiment performed with 12 g/L, for a reaction time of 24 h. Nevertheless, for S/L ratios greater than 20 g/L the phosphorus concentration plunges in short reaction times. For example, phosphorus concentrations under 50 mg/L were obtained in less than 6 h by adding LGMgO amounts over 20 g/L. Regarding the nitrogen removal (stated as ammonium) similar results were obtained while increasing the S/L ratio. For a reaction time of 4 h, the nitrogen (N– NH3) concentration slightly decreases for high LG-MgO slurries, or remains steady for LG-MgO slurries lower than 24 g/L. Nitrogen concentrations under 200 mg/L only were obtained with a S/L ratio over 30 g/L and more than 16 h, or 24 g/L and reaction time of 24 h respectively. In Fig. 3 the pH values as a function of LG-MgO added and reaction time are depicted. It can be observed that pH values greater than 8 cannot be achieved with S/ L ratios lower than 20 g/L. The lowest values for the
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10 12 g/L 16 g/L 20 g/L 24 g/L 30 g/L 34 g/L
3000 2500 2000
12 g/l 16 g/l 20 g/l 24 g/l 30 g/l 34 g/l
9
8 pH
Phosphorus concentration (mg/L)
3500
1500
7 1000
6
500 0 0
5
10
15
20
5
25
0
Reaction time (h)
Fig. 1. Variation of the phosphorus (P–PO34 ) concentration in the natural carmine lake process wastewater as a function of low-grade MgO slurries versus reaction time.
12 g/L 16 g/L 20 g/L 24 g/L 30 g/L 34 g/L
2000
1500
10
15
20
25
30
Reaction time (h)
Fig. 3. pH recorder in the natural carmine lake process wastewater treatment as a function of low-grade MgO slurries versus reaction time.
3500 Experimetnal values
Phosphorus concentration (mg/L)
Nitrogen concentration (mg/L)
2500
5
1000
500
3000
Theoretical values
2500 2000 1500 1000 500
0 0
5
10 15 Reaction time (h)
20
25
Fig. 2. Variation of the nitrogen (N–NH3) concentration in the natural carmine lake process wastewater as a function of lowgrade MgO slurries versus reaction time.
solubility of MAP compounds were reported in the pH range of 8–10 [11,12,17]. Nevertheless, pH greater than 9.2 may be attributed to equilibrium solubility of Mg(OH)2 (pKsp=11.1). That means that an excess of LG-MgO has been added. The variation of phosphorus (P–PO34 ) concentration in the wastewater as a function of pH is shown in Fig. 4. In the same figure, calculated concentrations from equilibrium solubility of struvite have been depicted, using the solubility product (pKsp=12.6) cited in the literature [18,19], as well as other parasite reactions as the formation of acid species (NH3, H3PO4, H2PO4 , HPO24 and PO34 ) and the formation of magnesium phosphate. However, to obtain this theoretical curves the presence of other ions capable to forme insoluble
0 4
5
6
7
8
9
10
pH
Fig. 4. Comparison of phosphorus concentration experimental values and phosphorus concentration from struvite solubility product versus pH.
phosphate compounds, as iron or calcium, has not been considered. The experimental values analysed at pH lower than 6.2 indicate that MAP precipitated compounds exceed the solubility product and the solution remains oversaturated, while all phosphorus concentrations obtained at pH greater than 6.2 lie under the calculated solubilities. Iron, aluminium, calcium and carbonate ions contained in LG-MgO, and the calcium contained in the wastewater resulting from the production of the natural dyestuff, as well as the low magnesium activity, may contribute to phosphates removal thus varying the equilibrium conditions of the struvite. Under these conditions, experimental values for pKsp of struvite should be determined. Nevertheless, it seems that a struvite solubility control is achieved after
J.M. Chimenos et al. / Water Research 37 (2003) 1601–1607
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2000
1600 a
1400 1200 1000 800
b
a
Intensity (a.u)
Nitrogen concentration (mg/L)
a
Experimental values Theoretical values
1800
600 a
a
a a c d a a a e a c
b
c ba d a
400
a: Struvite MgNH4 PO4 .6H2O b: Periclase MgO c: Magnesite MgCO3 ; d: Dolomite MgCa(CO3)2 e: Quartz SiO2
a
a a d e
b
45
50
aa
a
0
200
5
0 4
5
6
7
8
9
10
pH
Fig. 5. Comparison of nitrogen concentration experimental values and nitrogen concentration from struvite solubility product versus pH.
these impurities have reacted forming insoluble phosphates. Fig. 5 shows the wastewater nitrogen (N–NH3) concentration, obtained from all experimental trials performed, as a function of pH. It is always over the theoretical struvite solubility curve. This fact agrees with the initial wastewater ammonium/phosphate ratio. Nevertheless, the removed ammonium/phosphate ratio, calculated from concentrations analysed at pH values from 5.5 to 9, is in accordance with the theoretical molar ratio 1:1. So, the removal of ammonium is only due to struvite-MAP compounds formation, which needs a minimum phosphate concentration to precipitate. Finally, at a pH greater than 9.2 the nitrogen concentration slumps. This fact may be explained by the ammonia gas formation (pKa=9.2), which is stripped due the vigorous reactor agitation. The sludge obtained at the pH considered as optimum (pH 8) was used for the analysis of crystalline mineral phases. X-ray diffractogram of a sample obtained by adding 24 g/L of LG-MgO to wastewater resulting from the production of carmin red dye is shown in Fig. 6. Peaks of struvite were identified as the main phase, as well as MgO periclase. Magnesite, dolomite and quartz have been also identified as minor phases present in the sludge. The identity of other phases present in small quantities was very difficult to establish, because the pattern is characterised by a large number of small overlapping peaks. The presence of magnesium as MgO periclase as well as other non-reacted mineral phases from initial LG-MgO, suggests that the initial particles have not totally dissolved and struvite may be formed on the particle surface. This is in accordance with the absence of brucite Mg (OH) 2. This fact has been corroborated by SEM/EDS microanalysis performed on microtomed thin-sections of sludge particles. It can be observed in the micrograph
10
15
20
25
30
35
40
55
60
65
2θ
Fig. 6. X-ray diffractogram of a sludge obtained in the wastewater treatment with low-grade MgO (24 g/L and 20 h).
Fig. 7. Scanning electron micrograph of a sludge particle obtained in the wastewater treatment with low-grade MgO: inside an MgO periclase zone (a) and struvite growth on the low-grade MgO particle surface (b).
showed in Fig. 7 that there are two different morphologies, corresponding to inside (a) and outside (b) of the particle. EDS microanalysis from part (a) indicates than magnesium is the main element contained. Furthermore, small peaks corresponding to calcium, silicon, iron and aluminium, as well as oxygen, sulphur and carbon have also been identified. Nevertheless phosphorus and nitrogen have not been identified. These results corroborate that the inside of the particle had not reacted with the aqueous medium and remain unchanged. On the other hand, EDS microanalysis from part (b) reveals the important presence of phosphorus and nitrogen together with magnesium. Furthermore, small peaks corresponding to iron and calcium, both from the LG-MgO, have also been identified in the outside of the particle. That means that struvite crystals growth on the LG-MgO particle surface may be blocking the phosphate and ammonium diffusion.
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According to these results, a reaction mechanism is proposed to illustrate the removal of phosphates and ammonium using LG-MgO as source of magnesium. At the beginning, the Nernst boundary layer interface involving solid particle is formed and hydrolysis of MgO surface takes place. The solubility equilibrium of magnesium hydroxide is achieved and hydroxyl and magnesium concentration increase in the interface. Next, the diffusion of hydroxyl and magnesium ions to the bulk solution takes place. Generally, the rate of these steps is faster when pure MgO is used. However, when using LG-MgO less reactive, these reactions may become the rate controlling steps. Simultaneously, phosphoric acid and ammonium diffusion occurs from the bulk solution to the interface. According to experimental results obtained, the diffusion rate of phosphoric acid and ammonium to the interface is faster than the diffusion rate of magnesium and hydroxyls to bulk solution. Consequently, the struvite formation takes place in the interface instead of the bulk solution. In this proposed mechanism only the formation of struvite has been taken into account. However other magnesium and phosphate compounds such as formulas newberyte and bobierrite are also formed [20,21], and calcium and iron phosphates as well as gypsum may be formed too. The formation and growth of the MAP compounds forms a layer on the surface particle that does not allow the diffusion of magnesium and hydroxyls thereby stopping the reaction.
4. Conclusions It is possible to remove phosphate and ammonium from wastewater by precipitation of MAP compounds using low-grade MgO as source of magnesium. Due to the price of different grades of MgO, the use of lowgrade instead of pure MgO diminishes the costs of wastewater treatment in a physico-chemical plant. According to the results above described, 24 g/L of LG-MgO and a reaction time of 5 h is required to remove all the phosphate as MAP precipitated compounds. In this period of time the pH reaches values around 8.5–9, the value considered as optimum. There is a remainder of ammonium that does not precipitate after this point; to remove it is necessary increase the pH above 9.2 so the ammonium is eliminated as ammonia gas. This increase of pH takes place after 15 h due to the poor reactivity of the low grade MgO. Nevertheless, reaction time is not a critical parameter in this wastewater treatment, mainly due to the volume of wastewater daily produced. Thus, a reaction time of 20 h is the time considered as optimum to remove the ammonium
and phosphate from wastewater resulting from the production of carmin red dye from cochineal. The pH measurement may be used as a control parameter in a physico-chemical plant to establish the end of the reaction and the amount of LG-MgO slurry added. The use of the optimum conditions allows values lower than 35 mg/L of phosphorus (P–PO34 ) and 230 mg/L of nitrogen (N–NH3) in the final water, removing the 99 and 90 percent of initial concentrations, respectively. These minimum ammonium and phosphate concentrations may be removed in a biological plant, which is necessary for the removal of COD. Because of low activity of the LG-MgO used in this work, the formation of MAP compounds takes place on LG-MgO particle surface. This MAP layer blocks the diffusion of magnesium from inside of the particle to the boundary layer stopping the growth of struvite. As a result of this, it is necessary to add 3.5–4 times more MgO than is stoichiometrically needed, and extra LGMgO remains in the product obtained. Consequently, about 45 g/L of dry sludge are obtained. However, because of the composition of LG-MgO and the natural source of the red dyestuff, both without harmful substances, the sludge may be used as slow-release fertiliser. On the other hand, due to the particle size and specific gravity of LG-MgO, the decantation rate is greater than that obtained by using pure MgO and less volume of sludge is obtained. This dense sludge can be quickly and easily dewatered. As a result of these findings, a batch physico-chemical plant will be installed at CHR Hansen, S.A. (Spain) to treat 35,000 L of wastewater per day. Low-grade MgO used in this experimental work, may be also used to treat another sort of wastewater containing phosphates and ammonium, i.e. animal manures or municipal wastewater.
Acknowledgements The authors would like to thank Magnesitas Navarras S.A. for its cooperation in financing and supporting the work and CHR Hansen S.A. who kindly provided logistical support and samples. The authors also acknowledge to Scientific and Technical Services of the University of Barcelona for its analytical assistance.
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