Formation of Struvite Crystals in a Simulated Food Waste Aerobic Composting Process

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1 CHEM. RES. CHINESE UNIVERSITIES 2010, 26(2), Formation of Struvite Crystals in a Simulated Food Waste Aerobic Composting Process DU Xian-yuan 1, LIU Jian-lin 1, HUANG Guo-he 3 and LI Yu 1,2* 1. Energy and Environmental Research Centre, North China Electric Power University, Beijing , P. R. China; 2. College of Environment and Resources, Jilin University, Changchun , P. R. China; 3. Environmental Systems Engineering Program, Faculty of Engineering, University of Regina, Regina, SK S4S 0A2, Canada Abstract Bench-scale treatments with three mixtures of Mg and P salts, including K 3 PO 4 +MgSO 4, K 2 HPO 4 +MgSO 4, and KH 2 PO 4 +MgSO 4 as additives in a simulated food waste aerobic composting process, were conducted to test the magnesium ammonium phosphate(map) formation, and the compost products were analyzed by X-ray diffraction(xrd), Scanning electron microscopy(sem), and Energy dispersive X-ray spectroscopy(eds) analyses. The comparison results between XRD, SEM, and EDS analyses of MAPs in the dried compost and synthesized MAPs confirm the formation of MAP crystals in the simulated food waste aerobic composting process. The analysis of the compost also indicates that the addition of all the three mixtures of Mg and P salts in the aerobic composting process can increase nitrogen conservation and decrease nitrogen loss because of the formation of MAPs. The mechanism of MAP formation was verified as the reaction of ammonium(nh 4 + ) with magnesium(mg 2+ ) and phosphate[h n PO 4 (3 n), n=0, 1, and 2). Keywords Phosphate; Magnesium sulphate; Magnesium ammonium phosphate; Food waste treatment; Aerobic composting Article ID (2010) Introduction Food waste(fw) accounts for 12.5%(mass fraction) of municipal solid waste(msw) in the United States, and 20% 45% in Asian and European nations [1]. Food waste is usually collected with other MSW and sent to landfills or incinerators for final disposal. As landfills have been closed at an alarming rate and fewer incinerators have been under construction in recent years, waste disposal has become a serious problem in many cities [2,3]. Since food waste has a high moisture content, a high organics-to-ash ratio, and a loose physical structure, composting seems to be an ideal disposal method [3]. Aerobic composting, which is a biochemical process converting various components in organic wastes into relatively stable humus-like substances that can be used as a soil amendment or organic fertilizer for sustainable complete recycling [4], has been applied to the spots extensively with the advantage of mitigating pollution and recovering nutrients compared with incineration and landfill. However, a key issue in aerobic composting process is the higher nitrogen loss occurring mainly in *Corresponding author. liyuxx@jlu.edu.cn Received June 18, 2009; accepted September 7, Supported by the Scholarship from China Scholarship Council(No ). the forms of ammonia, nitrogen, and NO [5] x, and the ammonia emission reduces the ambient atmosphere quality for terrible odor [6]. Food waste composting in a variety of composting systems has gained considerable attention in last decade, some composting systems were in multiple scales [7] and used various synthetic food waste with different compositions [8], some composting systems run successfully with the aid of diverse bulking agents [9] and a variety of additives [10]. Jeong and Kim [11] reported that struvite(magnesium ammonium phosphate, MAP) crystals could be formed in aerobic composting when sufficient Mg and P salts were added; the crystallization process resulted in a substantial reduction of ammonia loss. MAP in the wastewater treatment plant was identified as early as 1939 by Rawn et al. [12]. And MAP is a white crystalline substance consisting of magnesium, ammonium and phosphorus in equal molar fractions (MgNH 4 PO 4 6H 2 O), it is a common phenomenon in anaerobic treatment facilities for MAP to form to foul and/or reduce the capacity of the system because the

2 No.2 DU Xian-yuan et al. 211 crystals often adhere to the surfaces of pipes and mechanical equipment [13]. However, this process has also been used to reserve some nutrients such as nitrogen and phosphorus in wastewater treatment facilities [14], and recover them from wastewater [7,15]. In terms of nutrient recovery, MAP gained from the stream can be taken for its agricultural reuse [11], and is a preferred fertilizer because of the slow release rate of nutrients, low frequency of application needed, and a lower content of impurities [16]. Full-scale MAP recovery from wastewater has been operated successfully in Japan [15,17] which shows very promising perspectives. In this study, the MAP crystallization reaction was applied to the simulated food waste aerobic composting process. Bench-scale treatments with mixtures of different Mg and P salts, including K 3 PO 4 +MgSO 4, K 2 HPO 4 +MgSO 4, and KH 2 PO 4 +MgSO 4 as additives, were conducted to test the formation of MAP, and the compost products were analyzed by XRD(X-ray diffraction), SEM(Scanning electron microscopy), and EDS(Energy dispersive X-ray spectroscopy) analyses in the food waste aerobic composting process. Furthermore, the formation mechanism of MAPs with three different additives, as well as the advantages and disadvantages, were evaluated and discussed in an aerobic composting process. 2 Materials and Methods 2.1 Reactor Set-up and Operation The reactors used and the detailed description in this study were similar to those reported by Lin et al. [8]. Four reactors(d=300 mm, H=450 mm) with an effective volume of approximately 30 L and a wall thickness of 6 mm were employed to simulate the food waste aerobic composting process. Three layers of heat-insulating materials were used to prevent the loss of heat. Two thermometers(traceable E, Control Company, Friendswood, TX, USA) were placed at the center(lower and upper levels) of the reactor to monitor the temperatures; the lower one is set at 2 inch above the segmentation plate and fixed. Air was driven to the bottom of each reactor by a vacuum pump(moa-p101-aa, GAST Manufacturing Inc., Benton Harbor, MI, USA) and evenly distributed to the food waste mixture through a perforated plate at a flow rate of 3.0 L/min. The exhaust gas from the reactor was passed through a condensate trap(with a buffer flask) and 100 ml of a 1 mol/l H 2 SO 4 solution in series to capture water condensate and ammonia, respectively. Finally, the gas was discharged to the laboratory ventilation system. The schematic diagram of composting system is shown in Fig.1. Fig.1 Schematic diagram of composting system The reactors were installed in a temperaturecontrolled room at (22±1) C. When the simulated mixture was loaded into the reactor, the original sample was taken immediately and the composting reaction was started, and the reaction time was remarked as zero simultaneously. The mixture in the reactor was stirred well with a shovel to homogenize the mixture, expose new surfaces, and keep the mixture permeable to airflow [8]. The reactor was opened once every three hours for weighing, turning, observing and sampling. Samples were taken from three locations(top, middle and bottom) of the central reactor. After mixing, sub-samples of approximate 5 g were drawn randomly for the analysis of organic carbon, and ammonium nitrogen. 2.2 Raw Materials The simulated FW employed was composed of potato, rice, carrot, leaves, pork, soybean, and seed soil, which were minced into pieces of less than 5 mm in diameter with a food processor(philips International, The Netherlands) and mixed well before the composting process. Three mixtures of Mg and P salts, including K 3 PO 4 +MgSO 4, K 2 HPO 4 +MgSO 4, and KH 2 PO 4 +MgSO 4, were respectively dissolved in 700 g of water, and then mixed with approximate 10 kg of wet simulated FW mixture in a 1:2:5 molar ratio of Mg, P, and N according to previous experiments. Finally, the simulated FW mixtures were loaded in there composting reactors(r 1 =K 3 PO 4 +MgSO 4, R 2 = K 2 HPO 4 +MgSO 4, and R 3 =KH 2 PO 4 +MgSO 4 ) and another control reactor R 0 with only simulated FW without any Mg and P salts added. The initial characteristics for both the runs were as follows: water content, about 60%; ph=6.0; ash content, 7.2%;

3 212 CHEM. RES. CHINESE UNIVERSITIES Vol.26 NH 4 + -N 11.2% dry basis; C:N molar ratio, 17:1; and organic content, 92.8% dry basis. 2.3 Analytical Methods The thermometers were adjusted everyday alone with the decrease of material height. The reading of the upper thermometer was used as the temperature of the compost. Ammonium nitrogen in the solid samples and ammonia in the exhaust gas were analyzed on the FIAstar 5000 Analyzer(Foss Analytic AB, Hoeganaes, Sweden). Volatile matter content was measured by heating duplicate samples at 550 ºC for 4 h [5]. The solid sample was acidified by pouring 3 ml of 0.1 mol/l H 2 SO 4 over the entire sample before drying to avoid NH 4 + -N loss, and the dry sample was then ground to homogeneous powder for analysis. A LECO TruSpec CN determinator(leco Corporation, St. Joseph, MI, USA) was employed to measure the total amount of carbon and nitrogen in the compost. Finally, the dried compost was analyzed by X-ray diffraction(xrd, Bruker Axs Inc., USA), Scanning electron microscope(sem, Jeol USA, Inc., USA), Energy dispersive X-ray spectroscopy(eds, EDAX Inc., USA) to confirm the crystals identification, and to determine the chemical composition, the surface characterization, as well as its morphology and structure. 3 Results and Discussion 3.1 Simulated Food Waste Aerobic Composting Process Heat was generated as the microbial degradation began and the temperature was elevated due to the conservation of metabolically generated heat [18]. The temperatures of the four runs began to rise soon after the establishment of composting conditions and maintained relatively high values( 50 ºC) for several days (thermopiles stage), and finally, decreased slowly to the ambient temperatures(maturing stage). Temperature variation followed a typical pattern which is often observed during decomposition in composting. The NH + 4 -N levels all increased gradually at the initial stage and then reached to the peak values of g on days 11, 12, 18, and 21, in R 0, R 1, R 2 and R 3, respectively. Subsequently, the NH + 4 -N decreased in four runs as a consequence of the addition of Mg and P salts mixtures. In the end, the NH + 4 -N values in the manures reached to 29.04, 13.11, and g apart from the NH 4 + -N which is conserved in microbial biomass, stable molecules and MAP [11] and so on in R 0, R 1, R 2 and R 3, respectively. Ammonium(NH 4 + ) can be transformed into ammonia (NH 3 ) as a volatility gas at ph above 7, and the odorous ammonia(nh 3 ) emissions impact the ambient atmosphere, so the higher the nitrogen conservation the lower ammonia release. Therefore, the addition of the mixtures of Mg and P salts could improve nitrogen conservation and maybe reduce the nitrogen losses during the aerobic food waste composting process. The nitrogen losses for four runs showed significantly different in terms of the nitrogen loss peaks and total loss amounts. On days 11 and 12, the amount of nitrogen loss reached to the maxima of 6.14 and 4.05 g respectively in R 1 and R 0 and coincided with the temperatures change, which were at least 2 3 times greater than those in R 2 and R 3, respectively. The total amount of nitrogen loss was approximately g in R 1, greater than those in other three runs. Finally, gaseous NH 3 -N losses, about 21%, 32%, 12% and 4% of initial total nitrogen quantities in R 0, R 1, R 2 and R 3 were, respectively, observed, which were much lower than the corresponding data reported, especially those for runs R 2 and R 3. The lower nitrogen loss means the higher nitrogen conservation. Mg and P salts mixtures in the aerobic food waste composting process could increase nitrogen conservation and decrease nitrogen loss by generating MAPs with NH 4 +. The ph firstly dropped down as the result of the acidification of compost caused by the microbial fatty formation of the degradation of the organic materials and gradually increased due to the presence of ammonia formed during mineralization of the organic materials, and finally reached to a steady state. During the composting process, the ph value of run R 1 was the highest, and then that of the run R 0, finally both of them reached to about 8.6 and 9.0, respectively. But in runs R 2 and R 3 the ph values were basically the same in the first two weeks and then a ph of 8.5 of run R 2 was observed which was higher than a ph of 7.6 of R 3 in the steady state. So this can be used to explain the varying NH 3 -N loss in runs R 0 and R Formation Mechanisms of MAPs in an Aerobic Composting Process Aerobic composting is a biochemical process converting various components in organic wastes into

4 No.2 DU Xian-yuan et al. 213 relatively stable humus-like substances such as ammonium that can be used as a soil amendment or organic fertilizer for sustainable complete recycling [4]. Nitrogen reactions in the composting process are very complex. The principal processes governing the forms of nitrogen species are mineralization[also known as ammonification, the conversion of nitrogen contained in organic compounds to the inorganic form ammonia(nh 3 ), which exists primarily as ammonium(nh + 4 ) at a ph of 7 or below], volatilization[release of ammonia(nh 3 ) into the atmosphere], nitrification[the conversion of ammonium(nh + 4 -N) to nitrite and then to nitrate], immobilization(assimilation by microbial population), and denitrification[the reduction of the oxides of nitrogen(no x -N) to N 2 gas and/or nitrous oxide(n 2 O) followed by the release of the gases to the atmosphere] [5]. Ammonium(NH + 4 ) can react with magnesium (Mg 2+ ) and phosphate[h n PO (3 n) 4, n=0, 1, and 2] to produce MAPs. MAP usually precipitates as stable white orthorhombic crystals at the stoichiometric ratio according to Eq.(1), with n=0, 1, and 2 [19] : Mg NH 4 +H n PO (3 n) 4 +6H 2 O MgNH 4 PO 4 6H 2 O + nh + (1) This reaction has been used to control nitrogen and phosphorus in wastewater treatment [14,15]. Jeong and Kim [11] confirmed that MAP crystals could be formed in an aerobic composting process with KH 2 PO 4 as a source of phosphate. The molar ratio of component ions([mg 2+ ]: [NH + 4 ]:[H n PO (3 n) 4 ]) can significantly affect the formation of MAPs according to Eq.(1). More mixtures of Mg and P salts added in the aerobic food waste composting process could generate more MAPs in the presence of NH 4 + ions. And ph is another key factor in the MAPs formation process, the optimum ph for the formation of MAPs was in a range of 8 9 [15]. The three mixtures of Mg and P salts could make the compost in different ph values with varying the number of H +. The salt together with K 3 PO 4 made the highest ph and caused a large amount of NH 3 -N loss in run R 1. The KH 2 PO 4 reduced the ph value to 7.6 which is lower than the optimum ph for the formation of MAPs so the manure NH 4 + -N value in run R 3 is higher than those in runs R 1 and R 2. And in run R 2 with the K 2 HPO 4 and MgSO 4 as additives the ph is below 7 during the first two weeks of the process and then reached to about 8.5 in the steady state, which are favorable for the formation of MAPs. 3.3 Syntheses and Characterizations of MAPs in Lab Synthetic MAPs were prepared with three different mixtures of Mg and P salts(k 3 PO 4 +MgSO 4, K 2 HPO 4 +MgSO 4, and KH 2 PO 4 +MgSO 4 ) in the presence of ammonium salt in a 1:1:1 molar ratio of Mg:P:N in the lab. MAP can be identified via X-ray diffraction by matching the intensity and position of the peaks with those searched out from the crystal structure database [20]. Therefore these crystal were analyzed by XRD to confirm the MAP identification. The XRD patterns generated from the MAPs synthesized from three mixtures of Mg and P salts in the presence of ammonium salt were matched with the database model for MAP(i.e., position and intensity of the peaks, Fig.2), the precipitate is identified as MAP [Bruker database No (C), MgNH 4 PO 4 6H 2 O]. Fig.2 XRD analyses of synthetic MAPs (A) NH 4 + +PO 4 3 +Mg 2+ MgNH 4 PO 4 6H 2 O; (B) NH 4 + +HPO 4 2 +Mg 2+ MgNH 4 PO 4 6H 2 O; (C) NH 4 + +H 2 PO 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (D) blank; (E) Bruker database No.: (C) MgNH 4 PO 4 6H 2 O.

5 214 CHEM. RES. CHINESE UNIVERSITIES Vol.26 SEM and EDS analyses were carried out to describe the surface characterization and crystal system. SEM confirmed the results of the XRD analysis, the synthetic MAPs compounds from three mixtures of Mg and P salts in the presence of ammonium salt are orthorhombic crystal systems with μm in length(fig.3), especially the MAP prepared with H 2 PO 4, Mg 2+, and NH 4 + [Fig.3(C)]. The orthorhombic shape of the MAPs particles can still be distinguished but covered by another crystal, which may be some unreacted Mg and P salts or by-product[fig.3(a) and (B)]. To identify the elemental composition of the precipitation, energy dispersive spectrometry was applied to a specific area localized on the SEM pictures. The EDS analyses of the synthetic MAPs are shown in Fig.4, and the compositions of the synthetic MAPs contained a great deal of C, Mg, O, and P, and trace of K, Na, Al, and Fe. Fig.3 SEM analyses of synthetic MAPs (A) NH + 4 +PO 3 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (B) NH + 4 +HPO 2 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (C) NH + 4 +H 2 PO 4 +Mg 2+ MgNH 4 PO 4 6H 2 O. Fig.4 EDS analyses of synthetic MAPs (A) NH + 4 +PO 3 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (B) NH + 4 +HPO 2 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (C) NH + 4 +H 2 PO 4 +Mg 2+ MgNH 4 PO 4 6H 2 O; (D) carbon tape(background). solid in R 0 as a control run were observed compared 3.4 Characterizations of MAPs Formed in Aerobic Composting Process to those of the blank. The results indicate that MAPs only formed in R 1, R 2, and R 3 due to the addition of MAPs have a distinctive orthorhombic crystal the mixtures of Mg and P salts, but MAP formation structure and can be identified via X-ray diffraction by did not take place in R 0. matching the intensity and position of the peaks of To confirm the XRD results, SEM and EDS analyses were carried out for the surface characterizations, them with those stored in a database for the crystal structure [20]. Therefore, the dried deposited samples the chemical composition, as well as the morphology were analyzed by XRD to validate the presence of the and structure of the MAPs crystals formed in the MAP crystals. The diffraction peaks of the solids aerobic composting process. The SEM images and formed in R 1, R 2 and R 3 coincide with those of EDS analysis of the MAPs in the compost are illustrated in Figs.6 and 7. Though there are some other MgNH 4 PO 4 6H 2 O at around 31º in Bruker database No (C)(Fig.5), and no available peaks of the components, the orthorhombic shape crystals can still

6 No.2 DU Xian-yuan et al. 215 Fig.5 XRD patterns of MAPs formed in the aerobic composting process (A) R 1 (K 3 PO 4 +MgSO 4 as additives); (B) R 2 (K 2 HPO 4 +MgSO 4 as additives); (C) R 3 (KH 2 PO 4 +MgSO 4 as additives); (D) R 0 (control); (E) blank; (F) Bruker database No (C), MgNH 4 PO 4 6H 2 O. Fig.6 SEM analyses of MAPs formed in aerobic composting process (A) R 1 (K 3 PO 4 +MgSO 4 as additives); (B) R 2 (K 2 HPO 4 +MgSO 4 as additives); (C) R 3 (KH 2 PO 4 +MgSO 4 as additives); (D) R 0 (control). Fig.7 EDS analyses of MAPs formed in aerobic composting process (A) R 1 (K 3 PO 4 +MgSO 4 as additives); (B) R 2 (K 2 HPO 4 +MgSO 4 as additives); (C) R 3 (KH 2 PO 4 +MgSO 4 as additives); (D) R 0 (control); (E) background. be distinguished in the samples in R 1, R 2 and R 3 with Mg and P salts added[fig.6(a) (C)], and the length of these crystals is about 25 μm. On the contrary, there are few crystals in R 0 as a control experiment without these additives. To identify the elemental composition of the crystals formed in the aerobic composting process, EDS was applied to a specific area localized on the SEM pictures, and Fig.7 shows the EDS analysis of the elemental composition in the dried deposited samples of the aerobic composting process. Analysis of EDS demonstrates that the surface composition of the solids in runs R 1, R 2 and R 3 contains a great amount of Na, Mg, O, and P, but less of these elements are observed in the control reactor R 0 without the addition of Mg and P salts. The chemical compositions of the crystals are almost the same as the MAPs synthesized with pure chemicals(fig.4), which reveals that the crystals formed in the aerobic composting process with the addition of Mg and P salts are MAP crystals. MAP is a white crystalline substance consisting of magnesium, ammonium and phosphorus in equal molar concentrations(mgnh 4 PO 4 6H 2 O), and can be formed by adding ammonium, magnesium salts and phosphate. The XRD, SEM and EDS analyses prove that MAPs could be formed in the food waste

7 216 CHEM. RES. CHINESE UNIVERSITIES Vol.26 aerobic composting process with different mixtures of Mg and Psalts(K 3 PO 4 +MgSO 4, K 2 HPO 4 +MgSO 4, and KH 2 PO 4 +MgSO 4 ) added. The formation of MAP crystals in the aerobic composting process with three different species of phosphate as additives has been verified with XRD, SEM, and EDS analyses of MAPs in the dried composts and synthesized MAPs. In addition, the formation mechanism of MAP can also been confirmed by XRD, SEM, and EDS. The addition of the mixtures of Mg and P salts during the aerobic food waste composting process could increase nitrogen conservation and decrease nitrogen losses by generating MAPs, and MAP is also a preferred fertilizer because of a slow release rate of nutrients and a low frequency of application needed [16], and could enhance the agronomic value if the compost was used as a soil amendment and organic fertilizer as well. XRD, SEM, and EDS analyses verify the formation of MAP crystals in the aerobic composting process with Mg and P salts as additives, and confirm the formation mechanism of MAP as follow: Mg 2+ + NH 4 + +H n PO 4 (3 n) +6H 2 O MgNH 4 PO 4 6H 2 O+nH + (n=0, 1, and 2). References [1] Pipatti R., Sharma C., Yamada M., Intergovernmental Panel on Climate Change, WMO/UNEP, 2006 [2] Adhikari B. K., Barrington S., Martinez J., et al., Waste Manage., 2008, 28(5), 795 [3] Chang J. I., Hsu T., Bioresource Technol., 2008, 99(17), 8068 [4] Moldes A., Cendón Y., Barral M. T., Bioresource Technol., 2007, 98(16), 3069 [5] Hong J. H., Park K. J., Bioresource Technol., 2005, 96(6), 741 [6] Liang Y., Leonard J. J., Feddes J., et al., Bioresource Technol., 2006, 97(5), 748 [7] Pastor L., Mangin D., Barat R., et al., Bioresource Technol., 2008, 99(14), 6285 [8] Lin Y. P., Huang G. H., Lu H. W., et al., Waste Manage., 2008, 28(8), 1375 [9] Li Y., Liu J. L., Huang G. H., et al., Chem. Biochem. Eng. Q., 2009, 23(3), 359 [10] Yu H., Huang G. H., Bioresource Technol., 2009, 100(6), 2005 [11] Jeong Y., Kim J., Bioresource Technol., 2001, 79(2), 129 [12] Rawn A. M., Banta A. P., Pomeroy R., Trans. Am. Soc. Agric. Eng., 1939, 105, 93 [13] Barat R., Bouzas A., Mart N., et al., J. Environ. Manage., 2009, 90(2), 850 [14] Saidou H., Korchef A., Ben Moussa S., et al., Chemosphere, 2009, 74(2), 338 [15] Warmadewanthi, Liu J. C., Sep. Purif. Technol., 2009, 64(3), 368 [16] Wang J., Burken J. G., Zhang X. Q., et al., J. Environ. Eng. Asce., 2005, 131(10), 1433 [17] Ueno Y., Fujii M., Environ. Technol., 2001, 22(11), 1373 [18] Yamada Y., Kawase Y., Waste Manage., 2006, 26(1), 49 [19] Le Corre K. S., Valsami-Jones E., Hobbs P., et al., J. Cryst. Growth, 2005, 283(3/4), 514 [20] Parsons S. A., Wall F., Doyle J., et al., Environ. Technol., 2001, 22(11), 1279

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