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A new method for conservation of nitrogen in aerobic composting processes
Bioresource Technology 79 (2001) 129±133
A new method for conservation of nitrogen in aerobic composting processes Yeon-Koo Jeong *, Jin-Soo Kim School of Civil, Environmental and Architectural Engineering, Kumoh National University of Technology, 188 Shinpyung-dong, Kumi, Kyungbuk 730-701, Republic of Korea Received 29 January 2001; received in revised form 6 March 2001; accepted 12 March 2001
Abstract Several factors, such as pH, C/N ratio, temperature, mixing and turning, and aeration rate, could aect the loss of ammonia in composting reactions. Substantial loss of ammonia can reduce the nutrient value of the compost product and may lead to a severe odor problem in the composting facility. A new method for conservation of ammonia in composting was proposed and tested in this study. The ammonia being produced during the composting was precipitated into struvite crystals by addition of Mg and P salts. Ammonia volatilization was greatly reduced by this method and it also contributed to a remarkable increase in total ammoniacal-N (TAN) content in the compost, reaching up to 1.4% of dry mass. This value of TAN content was 3±5 times higher than that in normal compost. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses con®rmed the formation of struvite crystals in the aerobic composting process. Ó 2001 Published by Elsevier Science Ltd. Keywords: Ammonia; Aaerobic composting; Struvite crystals; Oxygen consumption rate
1. Introduction Composting 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. Even though composting is a proven-technology that can be applied on the spot, there are many aspects that should be improved in the performance of current composting facilities. One of these areas is the conservation and enhancement of the nutrient value of the product by reducing the loss of nitrogen. The decreased ammonia loss may lead to an alleviation of the odor problem that is usually encountered in full-scale composting facilities (Switzenbaum et al., 1994). Several factors such as C/N ratio, temperature, mixing and turning, and aeration rate can in¯uence the volatilization of ammonia during composting (Morisaki et al., 1989). Gaseous nitrogen losses during composting occur mainly as ammonia, but may also occur as nitrogen and NOx (Eklind and Kirchmann, 2000). Witter and Lopez-Real (1988) reported that total nitrogen loss
*
Corresponding author. Fax: +82-54-467-4050. E-mail address: [email protected] (Y.-K. Jeong).
could amount to 50% of the initial nitrogen in composting of sewage and straw mixtures. Nitrogen losses increased up to 33% of the initial nitrogen during composting of poultry manure (Hansen et al., 1989). Some amendments such as peat, zeolite and basalts have been used to adsorb ammonia in composting (Bernal et al., 1993; Witter and Kirchmann, 1989a). Calcium and magnesium salts have also been added to precipitate ammonia with carbonate and to remove the alkalinity that could prevent a rise in pH (Witter and Kirchmann, 1989b). Peat moss and vermiculite were found to be good amendments in management of ammonia during the composting process (Liao et al., 1997). Another possible way to reduce ammonia losses is to add a carbon-rich material, increasing the C/N ratio of the compost mixture. Eklind and Kirchmann (2000), however, insisted that there is no obvious way to eciently decrease nitrogen loss during composting and compost maturation through the addition of carbonrich litter materials. Precipitation of struvite MgNH4 PO4 6H2 O is a common phenomenon in anaerobic treatment facilities. The crystals adhere to surfaces of pipes and mechanical equipment, fouling and/or restricting the capacity of the system (Ohlinger et al., 1999). The mass of struvite formed can be so extensive that it may lead to
0960-8524/01/$ - see front matter Ó 2001 Published by Elsevier Science Ltd. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 0 6 2 - 1
130
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
operational failure. This phenomenon has also been used to control some nutrients such as nitrogen and phosphorus in wastewater treatment facilities (SchulzeRettmer, 1991). A major advantage of this application is that the struvite produced can be utilized as a valuable slow-release fertilizer. In this study, the struvite crystallization reaction was applied to the aerobic composting process. Bench-scale treatments were conducted to test that the struvite can be formed in the aerobic composting process. It was found that the struvite crystallization reaction had a bene®cial eect on the conservation of nitrogen in composting. The possible adverse eects that result from the supplementation of Mg and P salts to the waste mixture were also determined by measuring the change in temperature, pH, and the degree of degradation of organic materials.
2. Methods 2.1. Composting The cylindrical composting reactor was formed from an acrylic column (250 mm in diameter, 450 mm in depth) with 20 l working volume. Two sensors for temperature check and control were placed at the center of the reactor. The reaction temperature was controlled so as not to exceed 60°C by regulating the air ¯ow rate in the range 0.4±6.0 l/min. Compressed air was introduced to the bottom of each reactor and evenly distributed to the waste mixture through a perforated plate. To minimize the conductive heat loss along the reactor wall, it was insulated with ®berglass. Food waste, collected from a cafeteria, was dried, and mixed to get uniform feed material. About 3.0 kg mixture including 1.8 kg food waste, 0.5 kg wood chips and 0.8 kg seed compost (on dry weight basis) was placed in each reactor. The seed compost was obtained from the full-scale composting plant treating the source separated food waste. In Run B, water-soluble salts of Mg and P were added to the compost mixture to induce the precipitation of ammonia into struvite crystals. Mg and P were supplemented in a molar basis equivalent to 20% of total nitrogen in the compost mixture, respectively. Run A was operated as a control experiment without addition of Mg and P. Initial C:N ratio was about 18.5 in both runs. Exhaust gas from the reactor passed through a condensate trap and 500 ml of 1 N H2 SO4 solution in series to capture water condensate and ammonia, respectively. The compost was mixed manually at 2- or 4-day interval throughout the composting reaction. About 50 g of composite sample was taken from the reactor after every complete mixing and analyzed immediately.
2.2. Analytical methods The reaction temperature was monitored with a platinum resistance temperature detector (RTD) and a recorder. The pH was determined in a water extract (1:10 by weight) from the compost sample after shaking for 30 min. The dry and volatile matters were determined on triplicate samples at 105°C for 24 h and 550°C for 4 h, respectively. The degree of degradation of organic materials was calculated from the volatile solid contents assuming that the total amount of ®xed solids remained unchanged throughout the composting reaction. The ammonia captured in sulfuric acid and condensate was determined by the distillation method (Standard Methods, 1995). The total ammoniacal-N (TAN) in the compost mixture was also determined using the same procedure. After the compost had been dried at 105°C for 1 day, it was used in scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis (Hitachi S-4200). 2.3. Determination of oxygen consumption rate To assess the microbial activity in compost, oxygen consumption rate (OCR) was measured by the modi®ed constant volume respirometer (Haug, 1993). All experiments were conducted in an incubator where temperature was maintained at 30°C. Glucose was added at a rate of 0.1 g/g dry compost as an additional substrate. Reactor without glucose addition was also operated as a control. OCR was calculated from the cumulative consumption of oxygen at the initial stage of reaction. 3. Results The well-balanced composting reaction proceeded from the start of reaction in both Runs, as shown in temperature pro®le in Fig. 1. The thermophilic phase (more than 45°C) lasted for about 10 days in both runs. The pH in both runs showed the typical pattern of change observed in conventional composting reactions. The pH dropped down to 5.5 during the ®rst 4 days and rose over 8.5 right after that. The degree of degradation of organic materials in Run A was about 45% after 20-day composting, while it reached only 40% in Run B. This dierence could be regarded as insigni®cant considering that maturation of several months is required after the high-rate fermentation stage in composting. This result, however, indicated that the large addition of Mg and P salts could retard the degradation of organic materials due to an increase in salinity in the compost mixture. The behavior of ammonia showed remarkable differences between the two composting reactions, as shown in Fig. 2. In Run A, ammonia loss was observed from the 7th day, and reached a maximum value on the
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
Fig. 1. Temperature pro®les in both runs.
10th day. The ammonia loss in Run B, however, lagged behind, by about 4 days, that of Run A and the maximum value was much lower than that of Run A. Consequently, the total amount of ammonia lost in Run B was 3.7 g, while it was 16.9 g in Run A. The losses of ammonia in Runs A and B were equivalent to 22.0% and 4.8% of initial nitrogen, respectively. The TAN content in the compost increased gradually and stabilized to a constant value with the progress of the composting reaction. The ®nal content of TAN in Run A reached to 5.6 g/kg TS. This level of TAN in compost is relatively high, considering the usual TAN level is in the range from 2.0 to 6.0 g/kg TS depending on the initial C:N ratio (Switzenbaum et al., 1994; Morisaki et al., 1989). A great amount of TAN was accumulated in Run B, up to 14.3 g/kg TS. This indicated that Mg and P addition had contributed to the conservation of nitrogen during the composting. It can be postulated that the added Mg and P reacted with
131
Fig. 3. OCR after 21 days reaction in both runs.
ammonia that was produced by the mineralization of organic nitrogen during the composting to form the struvite crystals. The TAN content observed in Run B was higher than that reported elsewhere. Since a substantial increase in TAN content could inhibit the microbial activity responsible for the degradation of organic materials, OCR was determined at the end of 20 days of composting. The OCRs in both runs were similar when glucose was used as a substrate, as shown in Fig. 3. The OCR values in control experiments, however, showed slight dierences between Run A and B. These results implied that the elevated TAN content observed in Run B had no signi®cant eects on the microbial activity when substrate provided was sucient. It also meant that most of the ammonia in Run B existed in the form of solid struvite crystals that were not easily dissolved in water at pH over 8.0. The formation of struvite crystals in Run B was veri®ed with SEM±EDS analysis, as shown in Fig. 4. Crystals were observed without magni®cation on the surface of oven-dried compost in Run B. The chemical composition of the crystals was almost the same as the struvite synthesized with pure chemicals, con®rming that the crystals formed in Run B were struvite crystals. Potassium was detected in both samples, because KH2 PO4 was used as a source of phosphate in both cases. Struvite crystals were not observed in compost from Run A. Crystallization of struvite is not restricted to a supersaturated liquid medium. Gonzalez-Munoz et al. (1996) reported that the crystallization of struvite was quickly achieved in the 1.6% solid agar media.
4. Discussion Fig. 2. Change in ammonia loss rate and TAN content in compost during composting reaction.
It is well known that aerobic composting reduces the agronomic quality of the composted product due to
132
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
Fig. 4. Struvite crystals observed with a SEM and results of energy dispersive X-ray analyses of struvite crystals. (a), (c): struvite crystals formed in compost in Run B. (b), (d): struvite standards precipitated with pure chemicals.
signi®cant loss of nitrogen (Tiquia and Tam, 2000). The formation of struvite crystals resulted in a remarkable reduction of ammonia loss and a substantial increase in TAN content of the compost product. The phosphorus that was added to induce the crystallization of struvite is also a major plant nutrient. It has been found that struvite improved plant growth compared with conventional fertilizers for grasses, fruit and high-value crops such as ornamentals and strawberries (Wrigley et al., 1992). The increase in nutrient value of compost via struvite crystallization may help to enlarge the market of compost as an organic fertilizer, because it can oer some additional nutrients that normal compost does not have. Investigation of the eects of the compost reinforced with struvite crystals on the plant growth is needed prior to ®eld application. The addition of magnesium and phosphate salts to compost may increase the total salinity of the compost product. High salinity could reduce the recycling potential of compost. Using such compounds as KH2 PO4 can alleviate this problem, because potassium and phosphorus supplied can also be used as nutrients for plant growth. Schulze-Rettmer (1991) recommended the use of phosphoric acid H3 PO4 and magnesium oxide (MgO) for the formation of struvite to avoid this problem of high salinity in wastewater treatment facili-
ties. Since the addition of extra chemicals such as magnesium and phosphorus to a compost mixture can increase cost, tests are needed to see if Mg and P salts can be replaced with some waste materials containing Mg and/or P in crystallization of struvite. The ammonia that remains in compost is usually transformed into nitrate and escapes as nitrogen gas via nitri®cation and denitri®cation reactions during the maturation stage (Tiquia and Tam, 2000). As a result, inorganic nitrogen content in the matured compost is usually very low. Struvite compost may have a great potential for nitrogen conservation during the maturation stage, because the nitrogen in the form of struvite crystals must be dissolved into water prior to nitri®cation and denitri®cation reactions. The dissolution of struvite crystals may be limited due to the low level of moisture in compost. Further investigation on the behavior of inorganic nitrogen at the maturation stage, however, should be performed to con®rm the ecacy of struvite crystals in conservation of nitrogenous materials. Increased dosing of magnesium and phosphorus salts could suppress the microbial activity responsible for the composting reaction. Therefore optimal doses for magnesium and phosphorus salts should be studied in context with microbial activity and the conservation of nitrogen.
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
5. Conclusions It was demonstrated that struvite crystals could be formed in aerobic composting, when sucient Mg and P were added. This crystallization process resulted in a substantial reduction of ammonia loss. Consequently the ammoniacal-N content in compost was as much as 1.4% of dry compost, which is seldom attained in normal composting. It is expected that the struvite crystals in compost could enlarge the use of compost product as an organic fertilizer. The formation of struvite crystals in the aerobic composting process was con®rmed by the SEM±EDS analysis. Acknowledgements This work was supported by the grant from Kumoh National University of Technology in 1999. The authors thank the Taegu Branch of KBSI (Korea Basic Science Institutes) for their assistance in SEM±EDS analysis of struvite crystals. References APHA, AWWA, WEF, 1995. Standard methods for the examination of water and wastewater, 19th ed. Publication Oce APHA, Washington, DC. Bernal, M.P., Lopez-Real, J.M., Scott, K.M., 1993. Application of natural zeolites for the reduction of ammonia emission during the composting of organic wastes in a laboratory composting simulators. Bioresource Technology 43, 35±39. Eklind, Y., Kirchmann, H., 2000. Composting and storage of organic household waste with dierent litter amendments II: nitrogen turnover and losses. Bioresource Technology 74, 125±133.
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Gonzalez-Munoz, M.T., Omar, N.B., Martinez-Canamero, M., Rodriguez-Gallego, M., Galindo, A.L., Arias, J.M., 1996. Struvite and calcite crystallization induced by cellular membranes of Myxococcus xanthus. Journal of Crystal Growth 163, 434±439. Hansen, R.C., Keener, H.M., Hoitink, H.A.J., 1989. Poultry manure composting: an exploratory study. Transactions of the ASAE 36, 2151±2157. Haug, R.T., 1993. The Practical Handbook of Compost Engineering. Lewis Publishers, Boca Raton. Liao, P.H., Jones, L., Lau, A.K., Walkemeyer, S., Eagan, B., Holbek, N., 1997. Composting of ®sh wastes in a full-scale in-vessel system. Bioresource Technology 59, 163±169. Morisaki, N., Phae, C.G., Nakasaki, K., Shoda, M., Kubota, H., 1989. Nitrogen transformation during thermophilic composting. Journal of Fermentation & Bioengineering 67, 57±61. Ohlinger, K.N., Young, T.M., Schroeder, E.D., 1999. Kinetics eects on preferential struvite accumulation in wastewater. Journal of Environmental Engineering ASCE 125 (8), 730±737. Schulze-Rettmer, R., 1991. The simultaneous chemical precipitation of ammonium and phosphate in the form of magnesium-ammoniumphosphate. Water Science Technology 23, 659±667. Switzenbaum, M.S., Holden, C., Soares, H., Kuter, G.A., 1994. Eect of amendments on nitrogen conservation in wastewater biosolids composting. Compost Science & Utilization, Spring, 35±43. Tiquia, S.M., Tam, N.F.Y., 2000. Fate of nitrogen during composting of chicken litter. Environmental Pollution 110, 535±541. Witter, E., Lopez-Real, J.M., 1988. Nitrogen losses during the composting of sewage sludge and the eectiveness of clay soil, zeolite, and compost in adsorbing the volatilized ammonia. Biological Wastes 23, 279±294. Witter, E., Kirchmann, H., 1989a. Peat, zeolite, and basalt as adsorbent of ammonical nitrogen during manure decomposition. Plant and Soil 115, 43±52. Witter, E., Kirchmann, H., 1989b. Eects of addition of calcium and magnesium salts on ammonia volatilization during manure composting. Plant and Soil 115, 53±58. Wrigley, T.J., Webb, K.M., Venkitachalm, H., 1992. A laboratory study of struvite precipitation after anaerobic digestion of piggery wastes. Bioresouce Technology 41, 117±121.
A new method for conservation of nitrogen in aerobic composting processes Yeon-Koo Jeong *, Jin-Soo Kim School of Civil, Environmental and Architectural Engineering, Kumoh National University of Technology, 188 Shinpyung-dong, Kumi, Kyungbuk 730-701, Republic of Korea Received 29 January 2001; received in revised form 6 March 2001; accepted 12 March 2001
Abstract Several factors, such as pH, C/N ratio, temperature, mixing and turning, and aeration rate, could aect the loss of ammonia in composting reactions. Substantial loss of ammonia can reduce the nutrient value of the compost product and may lead to a severe odor problem in the composting facility. A new method for conservation of ammonia in composting was proposed and tested in this study. The ammonia being produced during the composting was precipitated into struvite crystals by addition of Mg and P salts. Ammonia volatilization was greatly reduced by this method and it also contributed to a remarkable increase in total ammoniacal-N (TAN) content in the compost, reaching up to 1.4% of dry mass. This value of TAN content was 3±5 times higher than that in normal compost. The scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analyses con®rmed the formation of struvite crystals in the aerobic composting process. Ó 2001 Published by Elsevier Science Ltd. Keywords: Ammonia; Aaerobic composting; Struvite crystals; Oxygen consumption rate
1. Introduction Composting 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. Even though composting is a proven-technology that can be applied on the spot, there are many aspects that should be improved in the performance of current composting facilities. One of these areas is the conservation and enhancement of the nutrient value of the product by reducing the loss of nitrogen. The decreased ammonia loss may lead to an alleviation of the odor problem that is usually encountered in full-scale composting facilities (Switzenbaum et al., 1994). Several factors such as C/N ratio, temperature, mixing and turning, and aeration rate can in¯uence the volatilization of ammonia during composting (Morisaki et al., 1989). Gaseous nitrogen losses during composting occur mainly as ammonia, but may also occur as nitrogen and NOx (Eklind and Kirchmann, 2000). Witter and Lopez-Real (1988) reported that total nitrogen loss
*
Corresponding author. Fax: +82-54-467-4050. E-mail address: [email protected] (Y.-K. Jeong).
could amount to 50% of the initial nitrogen in composting of sewage and straw mixtures. Nitrogen losses increased up to 33% of the initial nitrogen during composting of poultry manure (Hansen et al., 1989). Some amendments such as peat, zeolite and basalts have been used to adsorb ammonia in composting (Bernal et al., 1993; Witter and Kirchmann, 1989a). Calcium and magnesium salts have also been added to precipitate ammonia with carbonate and to remove the alkalinity that could prevent a rise in pH (Witter and Kirchmann, 1989b). Peat moss and vermiculite were found to be good amendments in management of ammonia during the composting process (Liao et al., 1997). Another possible way to reduce ammonia losses is to add a carbon-rich material, increasing the C/N ratio of the compost mixture. Eklind and Kirchmann (2000), however, insisted that there is no obvious way to eciently decrease nitrogen loss during composting and compost maturation through the addition of carbonrich litter materials. Precipitation of struvite MgNH4 PO4 6H2 O is a common phenomenon in anaerobic treatment facilities. The crystals adhere to surfaces of pipes and mechanical equipment, fouling and/or restricting the capacity of the system (Ohlinger et al., 1999). The mass of struvite formed can be so extensive that it may lead to
0960-8524/01/$ - see front matter Ó 2001 Published by Elsevier Science Ltd. PII: S 0 9 6 0 - 8 5 2 4 ( 0 1 ) 0 0 0 6 2 - 1
130
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
operational failure. This phenomenon has also been used to control some nutrients such as nitrogen and phosphorus in wastewater treatment facilities (SchulzeRettmer, 1991). A major advantage of this application is that the struvite produced can be utilized as a valuable slow-release fertilizer. In this study, the struvite crystallization reaction was applied to the aerobic composting process. Bench-scale treatments were conducted to test that the struvite can be formed in the aerobic composting process. It was found that the struvite crystallization reaction had a bene®cial eect on the conservation of nitrogen in composting. The possible adverse eects that result from the supplementation of Mg and P salts to the waste mixture were also determined by measuring the change in temperature, pH, and the degree of degradation of organic materials.
2. Methods 2.1. Composting The cylindrical composting reactor was formed from an acrylic column (250 mm in diameter, 450 mm in depth) with 20 l working volume. Two sensors for temperature check and control were placed at the center of the reactor. The reaction temperature was controlled so as not to exceed 60°C by regulating the air ¯ow rate in the range 0.4±6.0 l/min. Compressed air was introduced to the bottom of each reactor and evenly distributed to the waste mixture through a perforated plate. To minimize the conductive heat loss along the reactor wall, it was insulated with ®berglass. Food waste, collected from a cafeteria, was dried, and mixed to get uniform feed material. About 3.0 kg mixture including 1.8 kg food waste, 0.5 kg wood chips and 0.8 kg seed compost (on dry weight basis) was placed in each reactor. The seed compost was obtained from the full-scale composting plant treating the source separated food waste. In Run B, water-soluble salts of Mg and P were added to the compost mixture to induce the precipitation of ammonia into struvite crystals. Mg and P were supplemented in a molar basis equivalent to 20% of total nitrogen in the compost mixture, respectively. Run A was operated as a control experiment without addition of Mg and P. Initial C:N ratio was about 18.5 in both runs. Exhaust gas from the reactor passed through a condensate trap and 500 ml of 1 N H2 SO4 solution in series to capture water condensate and ammonia, respectively. The compost was mixed manually at 2- or 4-day interval throughout the composting reaction. About 50 g of composite sample was taken from the reactor after every complete mixing and analyzed immediately.
2.2. Analytical methods The reaction temperature was monitored with a platinum resistance temperature detector (RTD) and a recorder. The pH was determined in a water extract (1:10 by weight) from the compost sample after shaking for 30 min. The dry and volatile matters were determined on triplicate samples at 105°C for 24 h and 550°C for 4 h, respectively. The degree of degradation of organic materials was calculated from the volatile solid contents assuming that the total amount of ®xed solids remained unchanged throughout the composting reaction. The ammonia captured in sulfuric acid and condensate was determined by the distillation method (Standard Methods, 1995). The total ammoniacal-N (TAN) in the compost mixture was also determined using the same procedure. After the compost had been dried at 105°C for 1 day, it was used in scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis (Hitachi S-4200). 2.3. Determination of oxygen consumption rate To assess the microbial activity in compost, oxygen consumption rate (OCR) was measured by the modi®ed constant volume respirometer (Haug, 1993). All experiments were conducted in an incubator where temperature was maintained at 30°C. Glucose was added at a rate of 0.1 g/g dry compost as an additional substrate. Reactor without glucose addition was also operated as a control. OCR was calculated from the cumulative consumption of oxygen at the initial stage of reaction. 3. Results The well-balanced composting reaction proceeded from the start of reaction in both Runs, as shown in temperature pro®le in Fig. 1. The thermophilic phase (more than 45°C) lasted for about 10 days in both runs. The pH in both runs showed the typical pattern of change observed in conventional composting reactions. The pH dropped down to 5.5 during the ®rst 4 days and rose over 8.5 right after that. The degree of degradation of organic materials in Run A was about 45% after 20-day composting, while it reached only 40% in Run B. This dierence could be regarded as insigni®cant considering that maturation of several months is required after the high-rate fermentation stage in composting. This result, however, indicated that the large addition of Mg and P salts could retard the degradation of organic materials due to an increase in salinity in the compost mixture. The behavior of ammonia showed remarkable differences between the two composting reactions, as shown in Fig. 2. In Run A, ammonia loss was observed from the 7th day, and reached a maximum value on the
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
Fig. 1. Temperature pro®les in both runs.
10th day. The ammonia loss in Run B, however, lagged behind, by about 4 days, that of Run A and the maximum value was much lower than that of Run A. Consequently, the total amount of ammonia lost in Run B was 3.7 g, while it was 16.9 g in Run A. The losses of ammonia in Runs A and B were equivalent to 22.0% and 4.8% of initial nitrogen, respectively. The TAN content in the compost increased gradually and stabilized to a constant value with the progress of the composting reaction. The ®nal content of TAN in Run A reached to 5.6 g/kg TS. This level of TAN in compost is relatively high, considering the usual TAN level is in the range from 2.0 to 6.0 g/kg TS depending on the initial C:N ratio (Switzenbaum et al., 1994; Morisaki et al., 1989). A great amount of TAN was accumulated in Run B, up to 14.3 g/kg TS. This indicated that Mg and P addition had contributed to the conservation of nitrogen during the composting. It can be postulated that the added Mg and P reacted with
131
Fig. 3. OCR after 21 days reaction in both runs.
ammonia that was produced by the mineralization of organic nitrogen during the composting to form the struvite crystals. The TAN content observed in Run B was higher than that reported elsewhere. Since a substantial increase in TAN content could inhibit the microbial activity responsible for the degradation of organic materials, OCR was determined at the end of 20 days of composting. The OCRs in both runs were similar when glucose was used as a substrate, as shown in Fig. 3. The OCR values in control experiments, however, showed slight dierences between Run A and B. These results implied that the elevated TAN content observed in Run B had no signi®cant eects on the microbial activity when substrate provided was sucient. It also meant that most of the ammonia in Run B existed in the form of solid struvite crystals that were not easily dissolved in water at pH over 8.0. The formation of struvite crystals in Run B was veri®ed with SEM±EDS analysis, as shown in Fig. 4. Crystals were observed without magni®cation on the surface of oven-dried compost in Run B. The chemical composition of the crystals was almost the same as the struvite synthesized with pure chemicals, con®rming that the crystals formed in Run B were struvite crystals. Potassium was detected in both samples, because KH2 PO4 was used as a source of phosphate in both cases. Struvite crystals were not observed in compost from Run A. Crystallization of struvite is not restricted to a supersaturated liquid medium. Gonzalez-Munoz et al. (1996) reported that the crystallization of struvite was quickly achieved in the 1.6% solid agar media.
4. Discussion Fig. 2. Change in ammonia loss rate and TAN content in compost during composting reaction.
It is well known that aerobic composting reduces the agronomic quality of the composted product due to
132
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
Fig. 4. Struvite crystals observed with a SEM and results of energy dispersive X-ray analyses of struvite crystals. (a), (c): struvite crystals formed in compost in Run B. (b), (d): struvite standards precipitated with pure chemicals.
signi®cant loss of nitrogen (Tiquia and Tam, 2000). The formation of struvite crystals resulted in a remarkable reduction of ammonia loss and a substantial increase in TAN content of the compost product. The phosphorus that was added to induce the crystallization of struvite is also a major plant nutrient. It has been found that struvite improved plant growth compared with conventional fertilizers for grasses, fruit and high-value crops such as ornamentals and strawberries (Wrigley et al., 1992). The increase in nutrient value of compost via struvite crystallization may help to enlarge the market of compost as an organic fertilizer, because it can oer some additional nutrients that normal compost does not have. Investigation of the eects of the compost reinforced with struvite crystals on the plant growth is needed prior to ®eld application. The addition of magnesium and phosphate salts to compost may increase the total salinity of the compost product. High salinity could reduce the recycling potential of compost. Using such compounds as KH2 PO4 can alleviate this problem, because potassium and phosphorus supplied can also be used as nutrients for plant growth. Schulze-Rettmer (1991) recommended the use of phosphoric acid H3 PO4 and magnesium oxide (MgO) for the formation of struvite to avoid this problem of high salinity in wastewater treatment facili-
ties. Since the addition of extra chemicals such as magnesium and phosphorus to a compost mixture can increase cost, tests are needed to see if Mg and P salts can be replaced with some waste materials containing Mg and/or P in crystallization of struvite. The ammonia that remains in compost is usually transformed into nitrate and escapes as nitrogen gas via nitri®cation and denitri®cation reactions during the maturation stage (Tiquia and Tam, 2000). As a result, inorganic nitrogen content in the matured compost is usually very low. Struvite compost may have a great potential for nitrogen conservation during the maturation stage, because the nitrogen in the form of struvite crystals must be dissolved into water prior to nitri®cation and denitri®cation reactions. The dissolution of struvite crystals may be limited due to the low level of moisture in compost. Further investigation on the behavior of inorganic nitrogen at the maturation stage, however, should be performed to con®rm the ecacy of struvite crystals in conservation of nitrogenous materials. Increased dosing of magnesium and phosphorus salts could suppress the microbial activity responsible for the composting reaction. Therefore optimal doses for magnesium and phosphorus salts should be studied in context with microbial activity and the conservation of nitrogen.
Y.-K. Jeong, J.-S. Kim / Bioresource Technology 79 (2001) 129±133
5. Conclusions It was demonstrated that struvite crystals could be formed in aerobic composting, when sucient Mg and P were added. This crystallization process resulted in a substantial reduction of ammonia loss. Consequently the ammoniacal-N content in compost was as much as 1.4% of dry compost, which is seldom attained in normal composting. It is expected that the struvite crystals in compost could enlarge the use of compost product as an organic fertilizer. The formation of struvite crystals in the aerobic composting process was con®rmed by the SEM±EDS analysis. Acknowledgements This work was supported by the grant from Kumoh National University of Technology in 1999. The authors thank the Taegu Branch of KBSI (Korea Basic Science Institutes) for their assistance in SEM±EDS analysis of struvite crystals. References APHA, AWWA, WEF, 1995. Standard methods for the examination of water and wastewater, 19th ed. Publication Oce APHA, Washington, DC. Bernal, M.P., Lopez-Real, J.M., Scott, K.M., 1993. Application of natural zeolites for the reduction of ammonia emission during the composting of organic wastes in a laboratory composting simulators. Bioresource Technology 43, 35±39. Eklind, Y., Kirchmann, H., 2000. Composting and storage of organic household waste with dierent litter amendments II: nitrogen turnover and losses. Bioresource Technology 74, 125±133.
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