Effects of Temperature in Aerobic Reactor on Stabilization of Biodegradable Waste in MBT System

Transcription

1 Effects of Temperature in Aerobic Reactor on Stabilization of Biodegradable Waste in MBT System Sang-Hagk Kwon, Jong-Sub Ban, Su-Jin Kim, Chae-Gun Phae Department of Environmental Engineering, Seoul National University of Technology Contact Corresponding author: Chae-Gun Phae, Department of Environmental Engineering, Seoul National University of Technology, Cheongung Hall(Room 313) 172 Gongreung 2-dong, Nowongu, Seoul, , Korea Tel: ; Fax: ; phae@snut.ac.kr Executive Summary As the amount of wastes is consistently increasing due to mass production and consumption based on population growth and rapid industrialization, we still need to exert more efforts to efficiently solve these problems. Recently, new disposal methods are being promoted in consideration of the reality and among others, the Mechanical Biological Treatment (MBT) system is being highlighted. Thus, in order to devise more effective MBT methods, this study mixed and filled biodegradable wastes in the aerobic zone of the MBT system. For each reactor, temperature was controlled externally. Impact of warming on decomposition of organic matters was reviewed for analysis of aerobic stabilization of biodegradable wastes. This study reviewed impacts of warming biological reactors in the MBT system based on experiments and analyses with five aerobic stabilization equipments. Warming was conducted through the heating sensor installed in each of the five reactors and temperature varied with nonwarming, 30, 40, 50, and 60 for each reactor. The amount of air flow in each reactor was 2 L/min and there was no change given in terms of the amount of air flowing according to increase of temperature of filling. As for operation of the stirrer, it took place for one minute every three hours with the speed of 10rpm for homogeneous reaction of filling during the reaction period. Wastes filled within each reactor were composed of food 60%, biodegradable wastes which showed high efficiency in aerobic stabilization reaction in the previous experiments.

2 A gas monitor (LMSxi, Gas Data Co., UK) was used for analysis of gas generated and chemical composition of each matter was analyzed through an elemental analyzer (Elementary Analyzer, Leco). To measure changes to weight of filling according to experiments, the whole body of reactor was put on the multi-purpose electronic scale DB-1 (150A) and the reduced amount was measured. of each reactor was analyzed through the bomb calorimeter (AC-350 Calorimeter, Leco). ph, EC and CODcr were analyzed based on liquation of high-phase materials through liquation experiment methods and according to process analysis methods and for analysis of water soluble TOC, Total Organic Carbon Analyzer (1010 TOC Analyzer, O.I.ANALYTICAL) was used. As for changes to weight of the filling, the reactor warmed up to 50 showed 17.7% of decline rate, which was the lowest. LB-1(unwarmed) and the reactor warmed to 40 showed 27.5% and 26.2% of decline rate, respectively, which were the best results. Also in reactors warmed to 50 and 60, which were processed in the dried state, decline rates in weight were 14.5% and 3.0%, the smallest decline rates in weight of the actual filling. ph of the filling in each reactor at the early stage ranged between 6.56 and However after reaction, the LB-1 had 8.15 while the warmed to 30 and 40 had 8.18 and 8.14, respectively, meaning that they have changed to weak alkali. However, the warmed to 50 and 60 had 5.99 and 6.09, respectively, which confirmed that it took place based on simple drying rather than microbial reactions. Based on the results, the conclusion has been drawn out that aerobic stabilization equipments had more advantageous weight reduction and stabilization with fermentation heat produced from decomposition of organic matters through microbes rather than externally made artificial heating. Ⅰ. Introduction In the contemporary society, the amount of various wastes is increasing due to mass production and consumption based on population growth and rapid industrialization. Among others, disposal of domestic wastes is largely divided into reclamation, incineration, and recycling. However, in the contemporary society with this increasingly growing amount of wastes, more fundamental and effective method to treat consistently produced domestic wastes is urgently required because of difficulty to secure land for reclamation and to expand facilities due to the not-in-my-backyard phenomenon in the incineration area as well as air pollution, etc(yang 2007, Taseli 2007). Also management of composition of final wastes is required so that secondary environment pollution from wastes treatment as well as the amount of final wastes land-filled or incinerated can be reduced. Recently, new treatment methods are being promoted in consideration of the reality and the Mechanical Biological Treatment (MBT) facilities for domestic wastes are being highlighted. This study dealt with aerobic stabilization, which belongs to biological treatment in the MBT system, analyzed, and reviewed impacts of warming equipments on aerobic stabilization by using aerobic equipments.

3 . Experiment equipments and methods 1. Experiment equipments In order to analyze impacts of biodegradable wastes, those among separated remnants after disposal and selection through the physical selection equipment in the MBT system, on degree of aerobic stabilization, a stainless-steel cylindrical aerobic reactor, 35 cm in diameter, 85 cm in height, and 60 L in volume, was used as shown in Fig. 1. At the top of the equipment, a stirrer motor was installed to mix the filling and a gas outlet was also installed to collect gas produced from aerobic stabilization reaction of filling. A temperature sensor was installed at about cm from the bottom of the side of the reactor to measure temperature. This was also installed on the exterior of the reactor to analyze impacts of warming of filling and to enable temperature control. Inside the reactor, a screw-type agitator was installed. Each joint of the wings of the stirrer was designed to separate and cut from each other so that stirrer of filling inside the reactor would be easier. An air injection entrance was made at the bottom of the reactor for sufficient aerobic conditions inside the reactor to enable smooth air injection from the bottom to the top. As for general equipments, a control box that can control stirrer speed of filling and hot wires that can control temperature were installed to check and measure aerobic stabilization according to each stirrer speed and changes according to temperature. Also an air compressor that can control the amount of air flow was installed for observation of reactions according to changes of the amount of air flow. At the same time, measurement of temperature according to reaction was made possible through combining each reactor s temperature sensor, control box and thermograph. The generated gas passed through the gas capture pipe and moved to equipment that removed moisture. The moisture eliminator was designed to remove the moisture first through cooling water and then through silica gel. The captured gas whose moisture has been removed was intended to be measured through a gas analyzer (LMSxi, Gas Data Co., UK) that can measure the concentration of COgas through the solenoid box that can emit one gas of each reactor. Fig. 1. Schematic diagram of aerobic stabilization reactor

4 2. Conditions for operation and analysis methods This study reviewed impacts of warming biological reactors in the MBT system based on experiments and analyses with five aerobic stabilization equipments. Warming was conducted through the heating sensor installed in each of the five reactors and temperature varied with nonwarming, 30, 40, 50, and 60 for each reactor. The amount of air flow in each reactor was 2 L/min and there was no change given in terms of the amount of air flowing according to increase of temperature of filling. As for operation of the stirrer, it took place for one minute every three hours with the speed of 10rpm for homogeneous reaction of filling during the reaction period. Wastes filled within each reactor were composed of food 60%, biodegradable wastes which showed high efficiency in aerobic stabilization reaction in the previous experiment(kwon et al 2009) as can be seen from Table. 1, paper 25%, wood 2% and compost 5%. In order to maintain a certain amount of moisture 8% of moisture was additionally provided. Table 1. Experimental condition of aerobic stabilization reactor [Unit: %] Run Food waste Paper waste Wood waste Compost Water Total Heating temperature [] LB LB LB LB LB Table 2. Physicochemical characteristics of biodegradable wastes used in this research Item Korean proximate analysis (%) Water Volatile Ash content solids C/N ratio High heating value (kcal/kg) Water soluble (20:1) CODcr TOC EC ph (mg/l) (mg/l) (ms) Paper waste , Wood waste , Food waste ,183 2,680 1, Compost ,556 5,408 2, A gas monitor (LMSxi, Gas Data Co., UK) was used for analysis of gas generated and chemical composition of each matter was analyzed through an elemental analyzer (Elementary Analyzer, Leco). To measure changes to weight of filling according to experiments, the whole body of reactor was put on the multi-purpose electronic scale DB-1 (150A) and the reduced amount was measured. For judgment of degree of decomposition and stabilization of organic matters according to operation, the caloric value toward samples of each reactor was analyzed through the bomb

5 calorimeter (AC-350 Calorimeter, Leco). ph, EC and CODcr were analyzed based on liquation of high-phase materials through liquation experiment methods and according to process analysis methods and for analysis of water soluble TOC, Total Organic Carbon Analyzer (1010 TOC Analyzer, O.I.ANALYTICAL) was used.. Results and discussion 1. Changes of fermentation temperature and generation of COaccording to warming of reactors To evaluate impact of filling according to warming of biological reactors on aerobic stabilization reaction, ratio of filling content in each reactor was kept same and as for warming temperature of each reactor, LB-1 was not additionally warmed and warming took place with a heating sensor installed in reactors for LB-2(warmed to 30), LB-3(warmed to 40), LB-4(warmed to 50), and LB-5(warmed to 60), and the 400-hour-long aerobic reaction took place. Measurement and analysis of changes of temperature of filling due to aerobic stabilization reaction took place through the temperature measurement sensor installed in reactors. Time-based changes of temperature by filling whose reactor was warmed differently from each other are as presented in Fig. 2 and the results from analysis of COgenerated through aerobic stabilization reaction of filling are as shown in Fig. 3. Fig. 2. Variation of temperature during operation time Fig. 3. Variation of CO2 gas(%) composition during operation time As for changes of temperature among aerobic stabilization reactions, temperature gradually rose up to 60 ~ 70 due to oxidation heat of microbes. However, in this study, LB-4 and LB-5 were warmed up to 50 and 60 from the point of time of start of reaction and temperature drastically rose at the early stage of reaction as can be seen from Fig. 2. Aerobic reaction of microbes was not active in filling and changes were small until the completion of reaction except for temperature

6 maintenance through the heating sensor after 60 hours. Besides, LB-1(unwarmed), LB-2 and LB-3 showed temperature graphs that might appear with progress of general aerobic stabilization reaction and reaction of reactors warmed to 30 and 40 took longer for temperature fall than reactors unwarmed. Also even when reaction was finished, temperature was maintained at 30 and 40 due to warming through the heating sensor. Also in terms of changes of COgas, results of drastic change of gas generation during aerobic stabilization reaction were presented in Fig. 3 based on stirrer of filling at the interval of 1min/3hr. Based on results from calculation of the amount of COgenerated from accumulation for 400 hours in each reactor by using the amount of air flow in the reactor and the amount of COgenerated [amount of air flowed in (ml/min) 60(min/hr) COconcentration (%) 100 ], 1,338 L and 1,685 L of COgas was generated in LB-4 (warmed to 50) and LB-5 (warmed to 60) reactors. This tells greatly lower COgas generation compared to LB-1 ~ LB-3 that generated 2,562 ~ 3,289 L COgas over total reaction time due to inhibition from microbial facilitation after grand-scale reaction at the earlier stage. It is concluded that while appropriate warming is necessary for active reaction of aerobic microbes within the filling, reaction at higher temperature to reduce reaction time will inhibit aerobic stabilization reaction. 2. Changes of moisture content of filling matters Water content decreased from the early reaction with LB-5 whose reactor was warmed to 60 and produced 13.5% of reduction rate. Although LB-4 showed a tendency of increasing water content at the early stage of reaction, 2.8% of reduction rate appeared after completion of aerobic stabilization reaction. This explains that water content declines according to the rising filling temperature regardless of temperature rise through microbial oxidation due to warming of reactors using a heating sensor. As for other reactors, warming was not made for high temperature and water content of filling did not decline. Fig. 4. Variation of Water content among filling material in reactors

7 3. Changes of weight, volume and density of filling matters Results from changes of weight of filling based on aerobic stabilization reaction and changes of weight based on drying of filling are as shown in Fig. 5. As a result from measurement of changes of weight of filling in reviewing impact of warming of reactor on aerobic stabilization reaction, LB- 1 (unwarmed) had 27.5% of weight reduction and LB-3 (warmed to 40 ) had 26.2%, which were relatively higher reduction rates compared to other reactors. As a result of measuring weight of the solid material made through drying moisture of filling, LB-4 (warmed to 50 ) showed 14.5% of reduction rate, which fell below the weight reduction rate due to general aerobic reaction and LB-5 (warmed to 60 ) had 3.0% of reduction rate, which tells that decomposition of organic matters by reaction was insufficient. Granularity of the filling decreases as organic matters are decomposed by aerobic microbes and changes of volume and density based on this are as shown in Fig. 6. Filling of the same composition was filled in each reactor at the early stage, which resulted in the same value of 36.7 L, the early volume. On completion of aerobic stabilization reaction, there was about 50% of volume decline. Fig. 5. Variation of weight among filling material in reactors (A) wet weight, (B) dry weight Fig. 6. Variation of density and volume of filling material during opera operation time

8 LB-4 and LB-5 whose reactors were warmed to 50 and 60, respectively, showed volume decrease of 47.5% and 48.0%, relatively lower than other reactors. Also as a result of measuring density of filling, it was 0.6 kg/l at the early stage and on completion of aerobic stabilization reaction, LB-2 (warmed to 30 ) had 1.0 kg/l, 63.3% of increase rate and the highest density increase rate. On the contrary, LB-5 whose reactor was warmed to 60 showed 53.3% of density increase rate, the lowest in this study. As can be seen from these results, volume of filling matters is closely related to water content while higher water content relates to higher density and smaller volume. When decomposition of organic matters is accompanied, these can be even more lowered. LB-4 and LB-5 whose reactors were warmed to 50 and 60 produced volume decrease rates that were lower than other reactors. Their density increase rate was relatively low, which tells that decomposition of organic matters according to drying of filling due to warming has been small. 4. CODcr and water soluble TOC CODcr and water soluble TOC concentration were measured for supernatant made from dilution with distilled water in 20:1 and liquated after collection of samples before and after decomposition. Changes of CODcr and water soluble TOC concentration are as presented in Fig. 7. Judgment of degree of decomposition of biodegradable matters in wastes is possible through judgment of the amount of organic matters liquated. At the early stage of decomposition process, fugacity takes place based on decomposition from lapse of time after liquation of much biodegradable matters. Fig. 7. Variation of water soluble CODcr and water soluble TOC of filling material during operation time As a result from filling of wastes with the identical composition in each reactor, the early density of CODcr was between 5,464 ~ 6,048 mg /L and the early stage concentration of water soluble TOC was between 2,535 ~ 2,731 mg /L, which is high. The concentration consistently decreased during the 400 hours aerobic stabilization reaction and after completion of aerobic stabilization reaction, CODcr lowered to 2,673 ~ 2,766 mg /L for LB-1 ~ LB-3 and water effluent TOC decreased to 908 ~ 977 mg /L, which produced 50 ~ 60% of reduction rates in CODcr and water soluble TOC as aerobic

9 stabilization reaction progressed. On the other hand, CODcr of LB-4 and LB-5 with hightemperature warming to filling from the early stage decreased to 5,036 mg /L and 5,196 mg /L and water soluble TOC declined to 2,407 mg /L, 2,536 mg /L, making the reduction rate around 10%. This explains that high temperature at the early stage over the 400-hour-long reaction inhibited activities of aerobic microbes, which decreased decomposition of organic matters and influenced aerobic stabilization reaction. Ⅳ. Conclusion This study analyzed and reviewed impact of warming of lab-scale biological reactors in the MBT system on aerobic stabilization reaction and the results from this study are as follows. 1) As for changes of temperature during aerobic stabilization reaction, LB-4 (warmed to 50 ) and LB-5 showed drastic temperature rise at the early stage of reaction based on warming to 50 and 60 from the point of time of the start of reaction. Microbial aerobic reaction in the filling was not active and the change was small until the completion of reaction except for temperature maintenance through the heating sensor until progress up to 60 hours. Besides, LB-1 (unwarmed), LB-2 (warmed to 30), and LB-3 (warmed to 40) showed temperature changes appearing in general aerobic stabilization reaction. Temperature of reactors warmed to 30 and 40 had still later points of time from which temperature began to fall compared to reactors unwarmed. 2) Based on results from measuring accumulated COgas during the 400-hour-long aerobic stabilization reaction, reactors of LB-4 (warmed to 50) and LB-5 (warmed to 60) generated COgas of 1,338 L and 1,685 L. This presents greatly lower gas generation compared to LB-1 ~ LB-3 which generated 2,562 ~ 3,289 L of COgas during the total reaction time. 3) In changes of weight of filling, LB-1(unwarmed) and LB-3 (warmed to 40) had weight reduction rates of 27.5% and 26.2%, respectively, which were comparatively higher than other reactors. As a result from measuring the weight of the solid materials of filling, LB-4 (warmed to 50) and LB-5 (warmed to 60) had reduction rates of 14.5% and 3.0%, which were very low. On the other hand, LB-1 ~ LB-3 had dried weight reduction rates of 30% and higher. 4) Volume decrease rates in LB-4 and LB-5 whose reactors were warmed to 50 and 60 were 47.5% and 48.0%, respectively, which were lower compared to other reactors. Density increase rate was 53.3% in LB-5 whose reactor was warmed to 60, which was relatively lower than other reactors. This explains that decomposition of organic matters according to drying of filling due to high-temperature warming was small. 5) After completion of aerobic stabilization reaction, LB-1(unwarmed) ~ LB-3 (warmed to 40) had density decrease rates of about 50% in CODcr and 60% of density decrease rate for water

10 soluble TOC. On the contrary, LB-4 (warmed to 50) and LB-5 (warmed to 60) that produced poor aerobic stabilization reaction due to high temperature had about 10% of density decrease rate in CODcr and water soluble TOC, which is low. According to the results above, reactors of LB-1 (unwarmed), LB-2 (warmed to 30), and LB-3 (warmed to 40) had active aerobic stabilization reaction while LB-4 (warmed to 50) and LB-5 (warmed to 60) produced very low results in each factor. This explains that while appropriate warming is necessary for active microbial reaction within the filling, reaction at a high temperature for reduction of reaction time will inhibit aerobic stabilization reaction. Acknowledgements This subject is supported by Ministry of Environmental as The ECO-technopia 21 project and Korea Ministry Environment(MOE) ET-Human resource development project. References 1) Yang, S. G.(2007): Status and Future Plan of MBT. Korean Society of Environmental Engineers Vol.29 No.1, pp ) Taseli, B. K.(2007): The impact of the European Landfill Directive on waste management strategy and current legislation in Turkey s Specially Protected Areas, Conservation and Recycling, Vol.52, pp ) Kwon, S. H, Ban, J. S., Phae, C. G., Kim, J. D.(2009): Effect of Food waste Content on Aerobic Stabilization Reaction in MBT System, Korea Society of Waste Management Vol. 26 No.1, pp