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Treatment of food wastes using slurry-phase decomposition
Bioresource Technology 73 (2000) 21±27
Treatment of food wastes using slurry-phase decomposition Yeoung-Sang Yun, Jong Ik Park, Min Seok Suh, Jong Moon Park* Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea Received 28 August 1998; received in revised form 21 September 1999; accepted 21 September 1999
Abstract A bioreactor incorporating a slurry-phase reaction was developed for high-rate decomposition of food wastes with an ultimate goal of complete decomposition leaving minimal residue of food wastes when compared to conventional food waste treatment producing composts. In this slurry-phase decomposition, suspended solids in the reactor disappeared with a maximum rate of 7.9 g dry weight dmÿ3 dayÿ1 . The changes in dissolved oxygen concentration and pH were closely related to the decomposition of food wastes. The concentrations of nitrate and phosphate in the liquid phase also gradually decreased and disappeared completely at the end of ®ve days of operation. Approximately 82% of carbonaceous compounds in the initial food wastes were decomposed during this 5-day operation. Long-term operation of slurry-phase bioreactors with daily addition of fresh food wastes was successfully carried out for 46 days. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Food wastes; Aerobic degradation; Slurry-phase reaction; Composting
1. Introduction In Korea, an abundant food culture has resulted in production of large amount of food wastes corresponding to approximately 40% of the total amount of garbage produced every year (Rogoshewski et al., 1983). So far, the food wastes have mainly been dumped in land®ll sites. However, the land®ll of food wastes has created various problems such as putrid smells and leachate polluting ground and surface waters. In addition, the trend of `not in my back yard' (NIMBY) makes it harder to open new land®ll sites. Due to these problems, the Korean government recently made it obligatory for large restaurants and corporate cafeterias to operate facilities for reducing or recycling food wastes (Kim, 1997). Although there has been extensive research on highrate composting of food wastes (Kubota and Nakasaki, 1991; Matthur et al., 1986; Shin et al., 1996; Shin, 1997; Yun et al., 1994), composting is not considered suitable for the Korean food wastes due to the quality of residue containing a high salt concentration (Kim et al., 1995).
*
Corresponding author. Tel.: +82-562-279-2275; fax: +82-562-2792699. E-mail address: [email protected] (J.M. Park).
The high salt content of Korean food mainly originates from the Kimchi, soy sauce and soybean paste, which are fermented and stored in highly saline water. Therefore, the complete decomposition of the volatile matter of food wastes has been considered a feasible solution for the treatment of food wastes (Shin et al., 1996; Seo et al., 1998). In the current study, a slurry-phase bioreactor system was developed for high-rate decomposition of food wastes, which was operated similar to a solid-substrate fermentation. Water was added into the bioreactor in order to make a slurry-phase system and to improve the mixing by agitation with an impeller. This improved mixing enhances the oxygen transfer rate and consequently elevates the decomposition rate. In addition, a slurry phase is believed to supply a more favorable environment for aerobic microorganisms compared to solid phase. As a preliminary study before the full-scale operations, we have characterized the reduction of suspended solid and change of compositions in the solid and liquid phase during the slurry-phase decomposition process. In a long-term experiment presented in this study, fresh food wastes were added daily into the reactor in order to simulate the actual operation of a full scale system for potential use in restaurants or corporate cafeterias where the food wastes would be treated on a daily basis.
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 1 3 1 - 5
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Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
2. Methods 2.1. Food wastes The food wastes used in this study were obtained from a Korean food restaurant on the campus of Pohang University of Science and Technology, Pohang, Korea. The composition of food wastes varied according to the daily menu of the restaurant. Therefore, the food wastes were separated and remixed according to the typical composition of food wastes shown in Table 1 (The Ministry of Environment of Korea, 1992). The remixed food wastes consisted of 48.4% carbon and 6.9% nitrogen with 80.3% water content. The average carbon to nitrogen (C/N) ratio was approximately 7.0. In order to minimize errors in sampling, the food wastes were chopped below 1 cm size. In all experiments, food wastes were used in a wet state, but the weight was described on a dry weight basis. 2.2. Microorganisms The environment in the slurry-phase reactor was dierent from that in the conventional solid-state composting with respect to the water content or activity. Therefore, microorganisms used in our experiment were also isolated in the slurry-phase condition. Various mature composts and mixtures of microorganisms commercially available for solid-state composting were incubated in a 0.5 dm3 ¯ask containing 0.2 dm3 water and 20 g dry weight of food wastes as a substrate on a rotary shaker at 200 rpm and 30°C for 10 days. Thereafter, 50% of the slurry in the ¯ask was removed and 50 g of fresh food wastes and 0.1 dm3 water were added into the ¯ask every week. By this successive incubation, the microorganisms were acclimated to the new environment of slurry-phase conditions and the resulting slurry was used as an inoculum in the following experiments. 2.3. Short-term operation The short-term batch operation was carried out for about two weeks to study basic kinetics of decomposition. Food wastes (46.2 g dw) and 0.01 dm3 of the previously prepared microbial inoculum were introduced into a stirred tank reactor (2 dm3 of total volume) Table 1 Composition of food waste Food types
Composition (%)
Cereals Fruits Vegetables Meats and ®sh
16 2 14 2 51 5 19 2
Fig. 1. Schematic diagram of reactor for slurry-phase decomposition of food wastes. 1: air blower, 2: ¯ow meter, 3: sparger, 4: impeller, 5: reactor vessel (cylindrical tank), 6: motor/speed controller, 7: DO meter, 8: pH meter, 9: DO electrode, 10: pH electrode, 11: sampling port, 12: air outlet.
and the working volume was adjusted to 1 dm3 by adding water. As shown in Fig. 1, air was supplied at a rate of 1.0 dm3 minÿ1 through a sparger and mixing was supplied by a paddle-type impeller at 120 rpm. The reactor was operated at room temperature (25 2°C) and sampling was started at 9 h. 2.4. Long-term operation For the long-term operation, the reactor was scaledup to 12 dm3 but with the same con®guration as that used in the short-term operation. This long-term operation was performed in ®ve steps. In step I, which was a start-up step, 40 g dw of food wastes and 0.005 dm3 of inoculum were introduced into the reactor and the working volume was adjusted to 5 dm3 . Air was supplied at 2 dm3 minÿ1 and mixing was performed with a paddle-type impeller at 200 rpm. All operating conditions in the following steps such as working volume, aeration rate and agitation speed were kept the same with step I. Step II operation was started after eight days of operation of step I, where 9.2 g dw of food wastes was added each day for 15 days. From step II, the amount of daily addition of food wastes was increased in each step. In step III, 14 g dw of food wastes per day was introduced for 11 days followed by 40 g dw of food waste per day for 11 days in step IV and 60 g dw of food wastes per day for 12 days in step V. Each step proceeded to the next step when the concentration of suspended solids in the reactor reached a pseudo-steady state. From step V, which was intended as the ®nal condition of the longterm operation, the operating conditions were changed since the amount of daily addition of food wastes in-
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
creased signi®cantly; 4 dm3 minÿ1 of aeration, 250 rpm of agitation, and 10 dm3 of working volume. 2.5. Determination of oxygen consumption The cumulative amount of oxygen consumption during the slurry-phase decomposition of food wastes was calculated by integrating the oxygen uptake rate (OUR) of the microorganisms: Z t OUR t dt 1 OC t 0
or OC ti
X
OUR ti Dti ;
2
i0
where OC(t) is the cumulative amount of oxygen consumption per reactor working volume (g dmÿ3 ), OUR(t) the oxygen uptake rate at an operation time t per reactor working volume (g dmÿ3 dayÿ1 ), t the operation time (day), and ti is the ith sampling time (day). With a quasi-steady state assumption, OUR becomes the same with oxygen transfer rate (OTR) which can be described with a volumetric oxygen transfer coecient (KL a) and the driving force of mass transfer (dierence between saturated (DO ) and bulk (DO) oxygen concentration): OUR OTR KL a DO ÿ DO:
3 3
ÿ1
The KL a value was determined to be 25.2 ´ 10 day by an unsteady state method as described elsewhere (Shuler and Kargi, 1992) (data not shown). Combining Eqs. (2) and (3), OC(ti ) can be expressed as a function of dissolved oxygen concentration: X KL aDO ÿ DO ti Dti : 4 OC ti i0
The numerical integration in Eq. (4) was carried out by a trapezoidal method (Holland and Liapis, 1983) using a technical computing software (Mathematica 3.0, Wolfram, 1996).
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DO and pH were measured using submerged electrodes (Ingold, USA). Dissolved organic carbon was determined using a TOC analyzer (Shimadzu, Japan). Dissolved nitrate and phosphate were analyzed using ion chromatography (Dionex) as described previously (Yun et al., 1997). IonPac (Dionex, USA) column was used for the separation of anions which were detected by thermal conductivity. The ¯ow rate of eluent (1.8 mM Na2 CO3 and 1.7 mM NaHCO3 ) was 0.002 dm3 minÿ1 . The elemental composition of suspended solids was analyzed using an elemental analyzer (Leco, USA).
3. Results 3.1. Short-term operation The degradation of suspended solids during the short-term operation is shown in Fig. 2. Here, not only the amount but also the size decreased as the operation proceeded and became ®ne particles in the end. The color of ®ltered supernatant became brown which was initially transparent and colorless. The short-term operation was started by introducing 46.2 g dw of food wastes into the reactor with 1 dm3 of working volume. However, the concentration of suspended solids of the ®rst sample withdrawn within 1 h from starting was just 27.0 g dm3 . This large dierence was presumed due to dissolution of the soluble portion of the food wastes (e.g., dissolution of salt, sugar and other soluble ingredients in the food). After approximately ®ve days, most of the suspended solids were degraded at a rate of 7.9 g dw dmÿ3 dayÿ1 . The concentration of suspended solids did not decrease further and the remaining suspended solids were presumed to be non-biodegradable which was equivalent to 14.3% of total food wastes.
2.6. Analyses The working volume of the reactor was adjusted to the initial volume by adding water at every sampling, which was changed due to evaporation and attachment of suspended solids on the reactor wall. These attached solids were resuspended by scraping to achieve a complete suspension, and then removing 0.02 dm3 of the sample. The withdrawn sample was centrifuged at 3000 g for 10 min and the supernatants were ®ltered through a 0.2-lm membrane ®lter (Millipore, USA) and analyzed for dissolved organic carbon, nitrate, and phosphate. The suspended solids were dried in an oven at 105°C for 24 h. After measuring dry weight, the dried suspended solids were homogenized for elemental analysis.
Fig. 2. Degradation of suspended solids in short-term operation. Error bars represent the standard deviations of three samples.
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Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
Fig. 3. Time course of DO and pH changes in short-term operation. Error bars represent the standard deviations of three samples. (d): dissolved oxygen, (n): pH.
Fig. 5. Change of carbon and nitrogen contents in suspended solids. Error bars represent the standard deviations of three samples. (s): carbon content; (n): nitrogen content; (n): carbon to nitrogen ratio.
The time variance of DO and pH during the shortterm operation is shown in Fig. 3. From the beginning, the DO concentration dropped to almost zero but sharply increased from day 5 and reached 7.8 mg dmÿ3 . The pH began to increase after approximately two days and reached 9.0. The increase of pH coincided with the decomposition of suspended solids and the pattern of pH change was similar to conventional solid-state composting (MacGregor et al., 1981; Faure and Deschamps, 1991). The change of major nutrients, nitrate and phosphate in the broth is shown in Fig. 4. These nutrients mainly came from the dissolution of solid food wastes by the microorganisms. The initial concentration of nitrate was 110.6 mg dmÿ3 which disappeared within two days. Meanwhile, 23.7 mg dmÿ3 of initial phosphate was also
completely consumed within two days. It was presumed that the microbial consumption rate of these nutrients after day 2 was higher than the rate of dissolution from suspended solids. Elemental composition of suspended solids is shown in Fig. 5. Variation of carbon and nitrogen content was minimal but the C/N ratio decreased slightly from 7.1 to 5.3 during the short-term operation. As shown in Fig. 6, most of the dissolved organic carbon disappeared via microbial uptake in the early stage of operation but approximately 0.7 g dmÿ3 of dissolved organic carbon remained until the end of the short-term operation (13 days). This residual organic carbon was presumed to be non-biodegradable carbon compounds. The cumulative amount of organic carbon decomposed to gaseous carbon (CO2 ) during the reactor operation was calculated as follows: CDC t FWC 0 ÿ SSC t ÿ DOC t;
Fig. 4. Time course of nutrient concentration change in the broth. Error bars represent the standard deviations of three samples. (d): nitrate, (n): phosphate.
5
where CDC(t) is the cumulative amount of decomposed organic carbon per working volume at an operating time t (g dmÿ3 ), FWC(0) the initial amount of organic carbon per working volume in food wastes introduced into the reactor at the start (g dmÿ3 ), SSC(t) the amount of organic carbon in suspended solids per working volume at an operation time t (g dmÿ3 ), DOC(t) the dissolved organic carbon concentration at an operation time t (g dmÿ3 ), and t is the operation time from the start (day). Since the suspended solids include not only food waste but also microbial biomass, the decomposed organic carbon was thought to be changed to gaseous inorganic carbon (CO2 ). As can be seen in Fig. 6, the increase of gaseous carbon coincided with the decrease of organic carbons in the liquid and suspended solids. In the short-term operation, 17.2 g dmÿ3 of organic carbon was decomposed to gaseous inorganic carbon within
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
Fig. 6. Migration of carbon during slurry-phase decomposition of food wastes. Error bars represent the standard deviations of three samples. (h): gaseous carbon; (m): carbon in dissolved organics (liquid phase); (s): carbon in suspended solid (solid phase).
5 days, which was equivalent to 82.3% of total carbon initially introduced as food wastes. 3.2. Long-term operation It is dicult to accumulate food wastes for long periods of time from restaurants or corporate cafeterias when collected on a daily basis due to sanitary reasons such as production of insects and putrid odors associated with anaerobic decomposition. Therefore, it is
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highly desirable to treat food wastes upon disposal. To simulate this actual situation, food waste was added daily into the reactor in the long-term operation (Fig. 7). In step I, the reactor was operated by introducing 40 g dw of food waste but without daily addition of food wastes in order to allow adaptation and growth of microorganisms. The daily addition of food waste was initiated from step II. Up to step IV, pH was maintained at approximately 8.0 but decreased to 4.3 at the ®nal stage, step V. The DO concentration ¯uctuated between 5.0 and 7.2 mg dmÿ3 in step II but decreased to 2.0 in step III and to near zero in step IV (data not shown). Despite daily addition of food wastes, working volume of the reactor decreased due to evaporation by aeration during steps I, II, and III. The working volume was kept constant by adding water during this period. However, in step IV, the working volume remained more or less constant, which implied that the volume of evaporated water was almost the same with the volume of daily added food wastes. In step V, where 60 g dw of food waste was added daily, the working volume was increased to 10 dm3 in order to reduce viscosity of ¯uid in the reactor. This working volume kept increasing due to the daily addition of food wastes, which was higher than evaporation, even though the aeration and agitation rates were increased. In addition to working volume, the amount of suspended solids also continuously increased, which was accelerated in step V, and continuing operation was practically impossible due to high viscosity and working volume at the late period of step V. Foaming and putrid odors formed at this late period, suggesting an anaerobic decomposition of food wastes due to low DO concentration. This was caused by high viscosity and high amount of suspended solids. As can be seen in Fig. 7, the amount of suspended solids in the reactor increased from step II. This increase of suspended solids was presumed due to accumulation of non-biodegradable matter from the introduced food wastes. 4. Discussion 4.1. Estimation of reactor performance
Fig. 7. Long-term operation of reactor with daily addition of food wastes. 40 g dw of food wastes was introduced at the beginning of operation and the amount of daily addition of food wastes was increased at each step. I: 0.0; II: 9.2; III: 14.0; IV: 40.0; V: 60.0 g dw., respectively.
The evaluation of the decomposition of food waste in the slurry-phase reactor can be made by measuring the suspended solids followed by centrifugation and drying in the oven for 24 h. In addition, the concentration of dissolved organic carbon should be analyzed using a TOC analyzer in order to determine the amount of organic carbon decomposed to gaseous inorganic carbon which represents complete degradation. Therefore, these evaluations required somewhat time consuming and labor intensive procedures. In our experiments, we found that the changes of pH and DO were closely related to the decomposition of food wastes. As can be
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Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
seen in Figs. 2 and 3, pH began to increase as degradation of suspended solids began and DO began to increase when the degradation process was completed. From these observations, the time course of degradation of food waste could be deduced. In addition, as can be seen in Fig. 8, we also observed that the amount of decomposition of organic carbon was closely related to the amount of oxygen consumption which could be calculated using Eq. (4). Therefore, it may be possible to estimate the amount of decomposition of organic carbon by simply measuring the DO during the operation. As shown in Fig. 9, the amount of decomposed (gaseous) carbon was calculated from the amount of oxygen consumption. However, this empirical relationship should be veri®ed with enough data sets obtained at various conditions. As an alternative approach, we are performing more batch operations in order to accumulate enough data sets for the development of an arti®cial
neural network. Using this network program, we may be able to predict the reactor performance at various conditions from pH and DO measurements. In the long-term operation, the behavior of DO and pH re¯ected the performance of the slurry-phase reactor. During the successful operation phase (steps I±IV), pH remained at approximately 8.0, a pH level when most of the organic matter was decomposed in the short-term operation. The DO was over 2.0 mg dmÿ3 up to step III, but declined to near zero after step IV. The short-term operation revealed that the DO began to increase after the degradable organic matter was completely decomposed. Therefore, it was inferred that the reactor was operated with a stable decomposition of food wastes until step III (pH 8, DO > 2 mg dmÿ3 ), and slightly overloaded from step IV (pH 8, DO 0 mg dmÿ3 ), and eventually stopped in step V (pH decreased from 8 to 4, DO 0 mg dmÿ3 ). It was also inferred that the addition rate of food wastes was higher than the decomposition rate from step IV. 4.2. Feasibility of slurry-phase decomposition of food waste
Fig. 8. Relationship between organic carbon decomposition and oxygen consumption. The regression equation is Y 2:53 2:23 10ÿ2 X ÿ 8:46 10ÿ6 X 2 and the correlation coecient R2 is 0.98.
Fig. 9. Calculation of decomposed (gaseous) carbon from the oxygen consumption. (s): experimental values, line: calculated values.
The operating conditions of our reactor were not optimized, but the rate and degree of decomposition of food wastes were substantial compared to conventional solid-state composting. In the short-term operation, 85.7% of the food wastes were decomposed within ®ve days of operation based on the reduction of suspended solids or 82.3% based on the decomposition of organic carbon to gaseous inorganic carbon (CO2 ), where the rate of decomposition was 7.9 g dw dmÿ3 dayÿ1 . In the long-term operation which was continued for 46 days to mimic an actual operation of a food waste decomposer, 827 g dw of food wastes in total was successfully treated. We found that not only the rate of decomposition but also the rate of evaporation of water from the reactor was one of the most important factors in determining the operable time of the reactor. Because the water content of fresh food waste was over 80%, every addition of the food waste resulted in an increase of the working volume which is limited by the reactor size. Operation of the reactor at a higher temperature could be a possible way of enhancing evaporation of water. However, in this case, the microbial consortium could be altered and consequently all operating conditions could be changed. The cost of heating would probably be prohibitive. Viscosity was another important factor in the reactor operation. In step V of the long-term operation where 60 g dw of food wastes were added daily, the viscosity of ¯uid in the reactor was too high to supply adequate mixing and aeration. This high viscosity mainly came from the accumulation of non-biodegradable suspended solids, and eventually forced us to stop the operation. These problems of working volume increase and vis-
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
cosity could be solved by introducing food wastes equivalent to microbial degradation rate and intermittent removal of non-biodegradable suspended solids. It is our ®nal goal to develop a bioreactor which could treat 10 g dw dmÿ3 of food wastes per day without removal of suspended solids for three months. By applying the intermittent removal, we intend to continuously operate this reactor over a period of one year.
5. Conclusion Korean food wastes containing high salt concentration were decomposed in a slurry-phase bioreactor system. The short-term batch operation of the reactor showed that the maximum decomposition rate was 7.9 g dry weight dmÿ3 dayÿ1 and 82% of the initially added suspended solid was decomposed within ®ve days. During the decomposition reaction, the pH and DO could be used as indicators implying the performance of reactor in the both short-term and long-term operations. Moreover, we could correlate the DO with the amount of decomposed carbon, which was used to make an empirical equation and a simulation curve of carbon decomposition. The performance of the slurry-phase reactor system was quite remarkable in respect to the rate and the degree of decomposition. Moreover, we could operate the reactor over 46 days which, we believe, could be extended by optimizing the operating conditions, especially for oxygen supply and water evaporation. The slurry-phase bioreactor system could be useful for the complete decomposition of food wastes which are not suitable for composting. Acknowledgements This work was ®nancially supported in part from Dae-Nam Ind., Ltd. and from the School of Environmental Engineering, POSTECH.
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References Faure, D., Deschamps, A.M., 1991. The eect of bacterial inoculation on the initiation of composting of grape pulps. Bioresource Technol. 37, 235±238. Holland, C.D., Liapis, A.I., 1983. Computer Methods for Solving Dynamic Separation Problems. McGraw-Hill, New York. Kim, K.E., 1997. Overview of Korean solid wastes management policies. Paper presented at Symposium on Composting Technology of Food and Organic Wastes, Seoul, 25 March. Kim, P.J., Chang, K.W., Min, K.H., 1995. Evaluation of the stability of compost madefrom food wastes by the fermenting tank. J. KOWREC 3, 35±42. Kubota, H., Nakasaki, K., 1991. Accelerated thermophilic composting of garbage. Biocycle 32, 66±68. MacGregor, S.T., Miller, F.C., Psarianos, K.M., Finstein, M.S., 1981. Composting process control based on interaction between microbial heat output and temperature. Appl. Environ. Microbiol. 41, 1321±1330. Matthur, R.S., Gaur, A.C., Magu, S.P., Sadasivam, K.V., 1986. Accelerated compost and improved yields. Biocycle 27, 42±42. Rogoshewski, P., Bryson, H., Wagner, K., 1983. Remedial Action Technology for Waste Disposal Sites. Noyes Data Corporation, Park Ridge, NJ. Seo, T.-K., Yang, J.-K., Whang, K., Lee, S.-T., 1998. Screening of dominant microorganisms responsible for composting food wastes in the thermophilic oxic process. J. Korean Soc. Env. Eng. 20, 1061±1072. Shin, H.S., Hwang, E.J., Jeong, Y.K., Kang, S.T., Jeong, Y.J., 1996. Study on operating conditions for high rate garbage composting. J. Korean Solid Wastes Eng. Soc. 13, 202±210. Shin, P.K., 1997. High-rate aerobic degradation of food wastes. Paper presented at Symposium on Composting Technology of Food and Organic Wastes, Seoul, 25 March. Shuler, M.L., Kargi, F., 1992. Bioprocess Engineering: Basic Concepts. Prentice-Hall, Englewood Clis, NJ. The Ministry of Environment of Korea, 1992. Strategies for separate collection and proper treatment of organic wastes. The Ministry of Environment of Korea, Seoul, Korea. Wolfram, S., 1996. The Mathematica Book, Third ed.. Wolfram Research, Champaign, IL. Yun, Y.-S., Lee, S.B., Park, J.M., Lee, C.-I., Yang, J.-W., 1997. Carbon dioxide ®xation by algal cultivation using wastewater nutrients. J. Chem. Tech. Biotechnol. 69, 451±455. Yun, H.Y., Yoo, K.Y., Park, H.W., Kim, K.S., Yu, M.J., 1994. Determination of optimum process variables in the design and operation of aerobic composting for food wastes. J. Korean Solid Wastes Eng. Soc. 11, 377±387.
Treatment of food wastes using slurry-phase decomposition Yeoung-Sang Yun, Jong Ik Park, Min Seok Suh, Jong Moon Park* Department of Chemical Engineering, School of Environmental Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, South Korea Received 28 August 1998; received in revised form 21 September 1999; accepted 21 September 1999
Abstract A bioreactor incorporating a slurry-phase reaction was developed for high-rate decomposition of food wastes with an ultimate goal of complete decomposition leaving minimal residue of food wastes when compared to conventional food waste treatment producing composts. In this slurry-phase decomposition, suspended solids in the reactor disappeared with a maximum rate of 7.9 g dry weight dmÿ3 dayÿ1 . The changes in dissolved oxygen concentration and pH were closely related to the decomposition of food wastes. The concentrations of nitrate and phosphate in the liquid phase also gradually decreased and disappeared completely at the end of ®ve days of operation. Approximately 82% of carbonaceous compounds in the initial food wastes were decomposed during this 5-day operation. Long-term operation of slurry-phase bioreactors with daily addition of fresh food wastes was successfully carried out for 46 days. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Food wastes; Aerobic degradation; Slurry-phase reaction; Composting
1. Introduction In Korea, an abundant food culture has resulted in production of large amount of food wastes corresponding to approximately 40% of the total amount of garbage produced every year (Rogoshewski et al., 1983). So far, the food wastes have mainly been dumped in land®ll sites. However, the land®ll of food wastes has created various problems such as putrid smells and leachate polluting ground and surface waters. In addition, the trend of `not in my back yard' (NIMBY) makes it harder to open new land®ll sites. Due to these problems, the Korean government recently made it obligatory for large restaurants and corporate cafeterias to operate facilities for reducing or recycling food wastes (Kim, 1997). Although there has been extensive research on highrate composting of food wastes (Kubota and Nakasaki, 1991; Matthur et al., 1986; Shin et al., 1996; Shin, 1997; Yun et al., 1994), composting is not considered suitable for the Korean food wastes due to the quality of residue containing a high salt concentration (Kim et al., 1995).
*
Corresponding author. Tel.: +82-562-279-2275; fax: +82-562-2792699. E-mail address: [email protected] (J.M. Park).
The high salt content of Korean food mainly originates from the Kimchi, soy sauce and soybean paste, which are fermented and stored in highly saline water. Therefore, the complete decomposition of the volatile matter of food wastes has been considered a feasible solution for the treatment of food wastes (Shin et al., 1996; Seo et al., 1998). In the current study, a slurry-phase bioreactor system was developed for high-rate decomposition of food wastes, which was operated similar to a solid-substrate fermentation. Water was added into the bioreactor in order to make a slurry-phase system and to improve the mixing by agitation with an impeller. This improved mixing enhances the oxygen transfer rate and consequently elevates the decomposition rate. In addition, a slurry phase is believed to supply a more favorable environment for aerobic microorganisms compared to solid phase. As a preliminary study before the full-scale operations, we have characterized the reduction of suspended solid and change of compositions in the solid and liquid phase during the slurry-phase decomposition process. In a long-term experiment presented in this study, fresh food wastes were added daily into the reactor in order to simulate the actual operation of a full scale system for potential use in restaurants or corporate cafeterias where the food wastes would be treated on a daily basis.
0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 9 9 ) 0 0 1 3 1 - 5
22
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
2. Methods 2.1. Food wastes The food wastes used in this study were obtained from a Korean food restaurant on the campus of Pohang University of Science and Technology, Pohang, Korea. The composition of food wastes varied according to the daily menu of the restaurant. Therefore, the food wastes were separated and remixed according to the typical composition of food wastes shown in Table 1 (The Ministry of Environment of Korea, 1992). The remixed food wastes consisted of 48.4% carbon and 6.9% nitrogen with 80.3% water content. The average carbon to nitrogen (C/N) ratio was approximately 7.0. In order to minimize errors in sampling, the food wastes were chopped below 1 cm size. In all experiments, food wastes were used in a wet state, but the weight was described on a dry weight basis. 2.2. Microorganisms The environment in the slurry-phase reactor was dierent from that in the conventional solid-state composting with respect to the water content or activity. Therefore, microorganisms used in our experiment were also isolated in the slurry-phase condition. Various mature composts and mixtures of microorganisms commercially available for solid-state composting were incubated in a 0.5 dm3 ¯ask containing 0.2 dm3 water and 20 g dry weight of food wastes as a substrate on a rotary shaker at 200 rpm and 30°C for 10 days. Thereafter, 50% of the slurry in the ¯ask was removed and 50 g of fresh food wastes and 0.1 dm3 water were added into the ¯ask every week. By this successive incubation, the microorganisms were acclimated to the new environment of slurry-phase conditions and the resulting slurry was used as an inoculum in the following experiments. 2.3. Short-term operation The short-term batch operation was carried out for about two weeks to study basic kinetics of decomposition. Food wastes (46.2 g dw) and 0.01 dm3 of the previously prepared microbial inoculum were introduced into a stirred tank reactor (2 dm3 of total volume) Table 1 Composition of food waste Food types
Composition (%)
Cereals Fruits Vegetables Meats and ®sh
16 2 14 2 51 5 19 2
Fig. 1. Schematic diagram of reactor for slurry-phase decomposition of food wastes. 1: air blower, 2: ¯ow meter, 3: sparger, 4: impeller, 5: reactor vessel (cylindrical tank), 6: motor/speed controller, 7: DO meter, 8: pH meter, 9: DO electrode, 10: pH electrode, 11: sampling port, 12: air outlet.
and the working volume was adjusted to 1 dm3 by adding water. As shown in Fig. 1, air was supplied at a rate of 1.0 dm3 minÿ1 through a sparger and mixing was supplied by a paddle-type impeller at 120 rpm. The reactor was operated at room temperature (25 2°C) and sampling was started at 9 h. 2.4. Long-term operation For the long-term operation, the reactor was scaledup to 12 dm3 but with the same con®guration as that used in the short-term operation. This long-term operation was performed in ®ve steps. In step I, which was a start-up step, 40 g dw of food wastes and 0.005 dm3 of inoculum were introduced into the reactor and the working volume was adjusted to 5 dm3 . Air was supplied at 2 dm3 minÿ1 and mixing was performed with a paddle-type impeller at 200 rpm. All operating conditions in the following steps such as working volume, aeration rate and agitation speed were kept the same with step I. Step II operation was started after eight days of operation of step I, where 9.2 g dw of food wastes was added each day for 15 days. From step II, the amount of daily addition of food wastes was increased in each step. In step III, 14 g dw of food wastes per day was introduced for 11 days followed by 40 g dw of food waste per day for 11 days in step IV and 60 g dw of food wastes per day for 12 days in step V. Each step proceeded to the next step when the concentration of suspended solids in the reactor reached a pseudo-steady state. From step V, which was intended as the ®nal condition of the longterm operation, the operating conditions were changed since the amount of daily addition of food wastes in-
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
creased signi®cantly; 4 dm3 minÿ1 of aeration, 250 rpm of agitation, and 10 dm3 of working volume. 2.5. Determination of oxygen consumption The cumulative amount of oxygen consumption during the slurry-phase decomposition of food wastes was calculated by integrating the oxygen uptake rate (OUR) of the microorganisms: Z t OUR t dt 1 OC t 0
or OC ti
X
OUR ti Dti ;
2
i0
where OC(t) is the cumulative amount of oxygen consumption per reactor working volume (g dmÿ3 ), OUR(t) the oxygen uptake rate at an operation time t per reactor working volume (g dmÿ3 dayÿ1 ), t the operation time (day), and ti is the ith sampling time (day). With a quasi-steady state assumption, OUR becomes the same with oxygen transfer rate (OTR) which can be described with a volumetric oxygen transfer coecient (KL a) and the driving force of mass transfer (dierence between saturated (DO ) and bulk (DO) oxygen concentration): OUR OTR KL a DO ÿ DO:
3 3
ÿ1
The KL a value was determined to be 25.2 ´ 10 day by an unsteady state method as described elsewhere (Shuler and Kargi, 1992) (data not shown). Combining Eqs. (2) and (3), OC(ti ) can be expressed as a function of dissolved oxygen concentration: X KL aDO ÿ DO ti Dti : 4 OC ti i0
The numerical integration in Eq. (4) was carried out by a trapezoidal method (Holland and Liapis, 1983) using a technical computing software (Mathematica 3.0, Wolfram, 1996).
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DO and pH were measured using submerged electrodes (Ingold, USA). Dissolved organic carbon was determined using a TOC analyzer (Shimadzu, Japan). Dissolved nitrate and phosphate were analyzed using ion chromatography (Dionex) as described previously (Yun et al., 1997). IonPac (Dionex, USA) column was used for the separation of anions which were detected by thermal conductivity. The ¯ow rate of eluent (1.8 mM Na2 CO3 and 1.7 mM NaHCO3 ) was 0.002 dm3 minÿ1 . The elemental composition of suspended solids was analyzed using an elemental analyzer (Leco, USA).
3. Results 3.1. Short-term operation The degradation of suspended solids during the short-term operation is shown in Fig. 2. Here, not only the amount but also the size decreased as the operation proceeded and became ®ne particles in the end. The color of ®ltered supernatant became brown which was initially transparent and colorless. The short-term operation was started by introducing 46.2 g dw of food wastes into the reactor with 1 dm3 of working volume. However, the concentration of suspended solids of the ®rst sample withdrawn within 1 h from starting was just 27.0 g dm3 . This large dierence was presumed due to dissolution of the soluble portion of the food wastes (e.g., dissolution of salt, sugar and other soluble ingredients in the food). After approximately ®ve days, most of the suspended solids were degraded at a rate of 7.9 g dw dmÿ3 dayÿ1 . The concentration of suspended solids did not decrease further and the remaining suspended solids were presumed to be non-biodegradable which was equivalent to 14.3% of total food wastes.
2.6. Analyses The working volume of the reactor was adjusted to the initial volume by adding water at every sampling, which was changed due to evaporation and attachment of suspended solids on the reactor wall. These attached solids were resuspended by scraping to achieve a complete suspension, and then removing 0.02 dm3 of the sample. The withdrawn sample was centrifuged at 3000 g for 10 min and the supernatants were ®ltered through a 0.2-lm membrane ®lter (Millipore, USA) and analyzed for dissolved organic carbon, nitrate, and phosphate. The suspended solids were dried in an oven at 105°C for 24 h. After measuring dry weight, the dried suspended solids were homogenized for elemental analysis.
Fig. 2. Degradation of suspended solids in short-term operation. Error bars represent the standard deviations of three samples.
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Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
Fig. 3. Time course of DO and pH changes in short-term operation. Error bars represent the standard deviations of three samples. (d): dissolved oxygen, (n): pH.
Fig. 5. Change of carbon and nitrogen contents in suspended solids. Error bars represent the standard deviations of three samples. (s): carbon content; (n): nitrogen content; (n): carbon to nitrogen ratio.
The time variance of DO and pH during the shortterm operation is shown in Fig. 3. From the beginning, the DO concentration dropped to almost zero but sharply increased from day 5 and reached 7.8 mg dmÿ3 . The pH began to increase after approximately two days and reached 9.0. The increase of pH coincided with the decomposition of suspended solids and the pattern of pH change was similar to conventional solid-state composting (MacGregor et al., 1981; Faure and Deschamps, 1991). The change of major nutrients, nitrate and phosphate in the broth is shown in Fig. 4. These nutrients mainly came from the dissolution of solid food wastes by the microorganisms. The initial concentration of nitrate was 110.6 mg dmÿ3 which disappeared within two days. Meanwhile, 23.7 mg dmÿ3 of initial phosphate was also
completely consumed within two days. It was presumed that the microbial consumption rate of these nutrients after day 2 was higher than the rate of dissolution from suspended solids. Elemental composition of suspended solids is shown in Fig. 5. Variation of carbon and nitrogen content was minimal but the C/N ratio decreased slightly from 7.1 to 5.3 during the short-term operation. As shown in Fig. 6, most of the dissolved organic carbon disappeared via microbial uptake in the early stage of operation but approximately 0.7 g dmÿ3 of dissolved organic carbon remained until the end of the short-term operation (13 days). This residual organic carbon was presumed to be non-biodegradable carbon compounds. The cumulative amount of organic carbon decomposed to gaseous carbon (CO2 ) during the reactor operation was calculated as follows: CDC t FWC 0 ÿ SSC t ÿ DOC t;
Fig. 4. Time course of nutrient concentration change in the broth. Error bars represent the standard deviations of three samples. (d): nitrate, (n): phosphate.
5
where CDC(t) is the cumulative amount of decomposed organic carbon per working volume at an operating time t (g dmÿ3 ), FWC(0) the initial amount of organic carbon per working volume in food wastes introduced into the reactor at the start (g dmÿ3 ), SSC(t) the amount of organic carbon in suspended solids per working volume at an operation time t (g dmÿ3 ), DOC(t) the dissolved organic carbon concentration at an operation time t (g dmÿ3 ), and t is the operation time from the start (day). Since the suspended solids include not only food waste but also microbial biomass, the decomposed organic carbon was thought to be changed to gaseous inorganic carbon (CO2 ). As can be seen in Fig. 6, the increase of gaseous carbon coincided with the decrease of organic carbons in the liquid and suspended solids. In the short-term operation, 17.2 g dmÿ3 of organic carbon was decomposed to gaseous inorganic carbon within
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
Fig. 6. Migration of carbon during slurry-phase decomposition of food wastes. Error bars represent the standard deviations of three samples. (h): gaseous carbon; (m): carbon in dissolved organics (liquid phase); (s): carbon in suspended solid (solid phase).
5 days, which was equivalent to 82.3% of total carbon initially introduced as food wastes. 3.2. Long-term operation It is dicult to accumulate food wastes for long periods of time from restaurants or corporate cafeterias when collected on a daily basis due to sanitary reasons such as production of insects and putrid odors associated with anaerobic decomposition. Therefore, it is
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highly desirable to treat food wastes upon disposal. To simulate this actual situation, food waste was added daily into the reactor in the long-term operation (Fig. 7). In step I, the reactor was operated by introducing 40 g dw of food waste but without daily addition of food wastes in order to allow adaptation and growth of microorganisms. The daily addition of food waste was initiated from step II. Up to step IV, pH was maintained at approximately 8.0 but decreased to 4.3 at the ®nal stage, step V. The DO concentration ¯uctuated between 5.0 and 7.2 mg dmÿ3 in step II but decreased to 2.0 in step III and to near zero in step IV (data not shown). Despite daily addition of food wastes, working volume of the reactor decreased due to evaporation by aeration during steps I, II, and III. The working volume was kept constant by adding water during this period. However, in step IV, the working volume remained more or less constant, which implied that the volume of evaporated water was almost the same with the volume of daily added food wastes. In step V, where 60 g dw of food waste was added daily, the working volume was increased to 10 dm3 in order to reduce viscosity of ¯uid in the reactor. This working volume kept increasing due to the daily addition of food wastes, which was higher than evaporation, even though the aeration and agitation rates were increased. In addition to working volume, the amount of suspended solids also continuously increased, which was accelerated in step V, and continuing operation was practically impossible due to high viscosity and working volume at the late period of step V. Foaming and putrid odors formed at this late period, suggesting an anaerobic decomposition of food wastes due to low DO concentration. This was caused by high viscosity and high amount of suspended solids. As can be seen in Fig. 7, the amount of suspended solids in the reactor increased from step II. This increase of suspended solids was presumed due to accumulation of non-biodegradable matter from the introduced food wastes. 4. Discussion 4.1. Estimation of reactor performance
Fig. 7. Long-term operation of reactor with daily addition of food wastes. 40 g dw of food wastes was introduced at the beginning of operation and the amount of daily addition of food wastes was increased at each step. I: 0.0; II: 9.2; III: 14.0; IV: 40.0; V: 60.0 g dw., respectively.
The evaluation of the decomposition of food waste in the slurry-phase reactor can be made by measuring the suspended solids followed by centrifugation and drying in the oven for 24 h. In addition, the concentration of dissolved organic carbon should be analyzed using a TOC analyzer in order to determine the amount of organic carbon decomposed to gaseous inorganic carbon which represents complete degradation. Therefore, these evaluations required somewhat time consuming and labor intensive procedures. In our experiments, we found that the changes of pH and DO were closely related to the decomposition of food wastes. As can be
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seen in Figs. 2 and 3, pH began to increase as degradation of suspended solids began and DO began to increase when the degradation process was completed. From these observations, the time course of degradation of food waste could be deduced. In addition, as can be seen in Fig. 8, we also observed that the amount of decomposition of organic carbon was closely related to the amount of oxygen consumption which could be calculated using Eq. (4). Therefore, it may be possible to estimate the amount of decomposition of organic carbon by simply measuring the DO during the operation. As shown in Fig. 9, the amount of decomposed (gaseous) carbon was calculated from the amount of oxygen consumption. However, this empirical relationship should be veri®ed with enough data sets obtained at various conditions. As an alternative approach, we are performing more batch operations in order to accumulate enough data sets for the development of an arti®cial
neural network. Using this network program, we may be able to predict the reactor performance at various conditions from pH and DO measurements. In the long-term operation, the behavior of DO and pH re¯ected the performance of the slurry-phase reactor. During the successful operation phase (steps I±IV), pH remained at approximately 8.0, a pH level when most of the organic matter was decomposed in the short-term operation. The DO was over 2.0 mg dmÿ3 up to step III, but declined to near zero after step IV. The short-term operation revealed that the DO began to increase after the degradable organic matter was completely decomposed. Therefore, it was inferred that the reactor was operated with a stable decomposition of food wastes until step III (pH 8, DO > 2 mg dmÿ3 ), and slightly overloaded from step IV (pH 8, DO 0 mg dmÿ3 ), and eventually stopped in step V (pH decreased from 8 to 4, DO 0 mg dmÿ3 ). It was also inferred that the addition rate of food wastes was higher than the decomposition rate from step IV. 4.2. Feasibility of slurry-phase decomposition of food waste
Fig. 8. Relationship between organic carbon decomposition and oxygen consumption. The regression equation is Y 2:53 2:23 10ÿ2 X ÿ 8:46 10ÿ6 X 2 and the correlation coecient R2 is 0.98.
Fig. 9. Calculation of decomposed (gaseous) carbon from the oxygen consumption. (s): experimental values, line: calculated values.
The operating conditions of our reactor were not optimized, but the rate and degree of decomposition of food wastes were substantial compared to conventional solid-state composting. In the short-term operation, 85.7% of the food wastes were decomposed within ®ve days of operation based on the reduction of suspended solids or 82.3% based on the decomposition of organic carbon to gaseous inorganic carbon (CO2 ), where the rate of decomposition was 7.9 g dw dmÿ3 dayÿ1 . In the long-term operation which was continued for 46 days to mimic an actual operation of a food waste decomposer, 827 g dw of food wastes in total was successfully treated. We found that not only the rate of decomposition but also the rate of evaporation of water from the reactor was one of the most important factors in determining the operable time of the reactor. Because the water content of fresh food waste was over 80%, every addition of the food waste resulted in an increase of the working volume which is limited by the reactor size. Operation of the reactor at a higher temperature could be a possible way of enhancing evaporation of water. However, in this case, the microbial consortium could be altered and consequently all operating conditions could be changed. The cost of heating would probably be prohibitive. Viscosity was another important factor in the reactor operation. In step V of the long-term operation where 60 g dw of food wastes were added daily, the viscosity of ¯uid in the reactor was too high to supply adequate mixing and aeration. This high viscosity mainly came from the accumulation of non-biodegradable suspended solids, and eventually forced us to stop the operation. These problems of working volume increase and vis-
Y.-S. Yun et al. / Bioresource Technology 73 (2000) 21±27
cosity could be solved by introducing food wastes equivalent to microbial degradation rate and intermittent removal of non-biodegradable suspended solids. It is our ®nal goal to develop a bioreactor which could treat 10 g dw dmÿ3 of food wastes per day without removal of suspended solids for three months. By applying the intermittent removal, we intend to continuously operate this reactor over a period of one year.
5. Conclusion Korean food wastes containing high salt concentration were decomposed in a slurry-phase bioreactor system. The short-term batch operation of the reactor showed that the maximum decomposition rate was 7.9 g dry weight dmÿ3 dayÿ1 and 82% of the initially added suspended solid was decomposed within ®ve days. During the decomposition reaction, the pH and DO could be used as indicators implying the performance of reactor in the both short-term and long-term operations. Moreover, we could correlate the DO with the amount of decomposed carbon, which was used to make an empirical equation and a simulation curve of carbon decomposition. The performance of the slurry-phase reactor system was quite remarkable in respect to the rate and the degree of decomposition. Moreover, we could operate the reactor over 46 days which, we believe, could be extended by optimizing the operating conditions, especially for oxygen supply and water evaporation. The slurry-phase bioreactor system could be useful for the complete decomposition of food wastes which are not suitable for composting. Acknowledgements This work was ®nancially supported in part from Dae-Nam Ind., Ltd. and from the School of Environmental Engineering, POSTECH.
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