A PRACTICAL ODOR INDEX FOR EVALUATING DE-ODORANT PERFORMANCE FOR MSWI BOTTOM ASHES

Transcription

1 J. Environ. Eng. Manage., 17(2), (2007) A PRACTICAL ODOR INDEX FOR EVALUATING DE-ODORANT PERFORMANCE FOR MSWI BOTTOM ASHES Chien-Jen Shih, 1, * Cheng-Nan Li, 2 Jia-Kai Chan 2 and Chin-Yung Syu 2 1 Department of Environmental Management Jin-Wen Institute of Technology Taipei 106, Taiwan 2 Research and Development Department Kobin Environmental Enterprise Co. Ltd. Taipei 105, Taiwan Key Words: Odor, bottom ash, olfactometry, panel test ABSTRACT In any bottom ash processing plants, odor remains one of major concerns. It is impractical to identify and quantify the odor-causing compounds through gas chromatography-mass spectrometry, nor measuring odor strength by taking ambient air samples with the so-called olfactometry test. Consequent, a quick and easy approach is developed in the present study. The solid sample was mixed with different amounts of granular activated carbon and then subject to a panel of 9 persons to detect the presence of odor. The developed odor index was then used to evaluate the effectiveness of different de-odorant agents or methods, including chemical oxidizing agents, a commercial product, washing and drying. The lower the number of odor index is, the stronger the odor smelling is encountered, with a maximun odor index being 99 (odorless). The addition of KMnO 4, or H 2 O 2 increases the odor strength due to released odorous compounds upon addition of these oxidizing agents. On the other hand, washing and drying are the most effective methods to reduce odor for treated bottom ash. The application of the developed odor index is discussed. INTRODUCTION The 19 municipal solid waste incinerators (MSWIs) in Taiwan generated over million tons of bottom ash annually [1]. The Kobin bottom ash recycle plant, located in northern Taiwan, has processed about quarter million tons of the bottom ashes since The treatment plant applied screen, crush, magnetic and gravity separation units to recover ferrous metal (8.5%), non-ferrous metal (1%), and sorted out unburned material (0.5%) from bottom ash. The remaining materials (80%) passing through 4.0 mm sieve are mixed with stabilized agent; the final aggregates are used for road construction or as low strength materials for refilling excavated conduit. One of the major problems during operation and final application is the odor problem. The presence of the odor affects operators and surrounding communities during operation and restricts the application of final aggregate products. Compounding the problem is the fact that the complicated characteristics of the bottom ash make it difficult to identify the actual compositions that cause odor and consequently there is no easy way to prevent the odor generation. In addition to identifying and quantifying the odor-causing compounds through gas chromatography-mass spectrometry (GC/MS), odor strength of surrounding ambient air can be measured using an olfactometer [2-5]. In the olfactometry test, the panelists detect the presence of odor in a series of diluted gaseous samples. Both methods, however, are impractical for detecting odor in the bottom ash recycling plant. In plant operation, a quick and easy approach is required to detect the presence of odors to facilitate plant operation. For example, changing the type of stabilizing/oxidizing agent or varying its concentration to reduce odor problems requires an easy approach to optimize the plant operation. A new approach for detecting odor strength was developed for detecting the presence of odor in the bottom ash without taking any air samples. To the best of our knowledge, there is no method currently available for directly detecting the odor from solid sample. The approach of this developed odor index is similar *Corresponding author cjshih@ms2.hinet.net

2 98 J. Environ. Eng. Manage., 17(2), (2007) to the olfactometer test in that a panel, composed of 9 persons, is charged to sniff a series of treated solid samples. Instead of using odorless air for the dilution for gaseous samples, different amounts of the granular activated carbon (GAC) were added to the solid bottom ash since GAC is noted for its adsorption of many odorous compounds. This study presents the detailed procedures of our odor index determination as well as the results of several tests for reducing odor including using a variety of oxidizing agents, drying and water washing. The odor index is currently applied to Kobin plant operation, e.g., those bottom ashes with offensive odor index were placed in a storage area with sufficient ventilation to prevent complaints from surrounding communities. The qualitative odor index can be further applied to other solid waste samples such as compost and biosolids including managing operation and comparison of different operations, among others. 1. Materials MATERIALS AND METHODS The raw bottom ash samples were collected from a particular storage area of Kobin plant. Bottom ashes transported daily from each MSWI to the Kobin plant are unloaded at several storage areas and stay there for about two to six wk before processing. For better control, about five hundred tons of bottom ash from each individual MSWI were treated in a batch. Aggregate was recovered after crush, screen, and magnetic separation (Fig. 1). Samples from treated aggregates were also taken for testing the developed odor index. The washed aggregate was obtained by adding a given amount of aggregate onto 100 mesh (0.15 mm), immersing in a water bath with a given amount of water and shaking for about 5 min. The material remaining on the screen mesh is identified as washed aggregate, and that passing through screen as washed silt. The water content of untreated bottom ash at storage area ranged from 20 to 25%, while the aggregate about 16 to 18%. The dried aggregate was obtained by placing samples at 60 C oven for 2 h. For comparison as background information, two external samples, natural sand and natural mud, were also tested. GAC (Viscarb, type UT-4030) was used as the dilution and adsorption agent to reduce odor of bottom ash samples. The particle size of GAC is 8-30 mesh ( mm), specify gravity , specific surface area about 1000 m 2 g - 1 (BET N 2 ). The purity of hydrogen peroxide (Nacalai Tesgue) is 30% and KMnO 4 (JT Baker) is of reagent grade. Both oxidation agents were applied as a deodorant agent to test their effectiveness on reducing odor. Bio-aid was obtained from one of Taiwan chemical company that normally applied to treat complex organics of wastewater. The major compound of powder (5-6 μm) Zeolite was 60% SiO 2, 15% CaO and 10% Al 2 O Methods 2.1 Test procedure A constant amount of sample (100 g) from different MSWIs was always used along with five or six different amounts of GAC adding into each individual 150 ml plastic bottle. The dosage of GAC ranged between 0.1 to 100 g (or the ratio of GAC dosage to sample from 0.1 to 100%), depending on the characteristic of test material. The bottles were randomly marked, placed in a standard extraction apparatus (Toxic Characteristic Leaching Procedure, TCLP) and mixed for 10 min at 32 rpm. A panel of nine persons was then charged to perform sensory test within 1 h to mark yes (Y) or no (N) for each bottle; Y refers to the detection of the odor, and N represents no detection. Bottom ash screen Fe Shredder Unburned (heavy, bulk) Vibration screen Pneumatic Fine Eddy current Non-Fe metal Manual separation Phosphate stabilization Hammer mill Metal (Fe) Unburned residual Metal (Cu,Al,Pb) Aggregate Fig. 1. Process flow diagram of Kobin bottom ash treatment plant.

3 Shih et al.: A Practical Odor Index for Bottom Ashes 99 Table 1. Hypothetical case for panel evaluation Sample (g) GAC dosage (g) Sample /GAC ratio Log Panel ratio A B C N N N N N N N N N Y N N Y Y N N Y N Threshold Invalid There should be at least 5 effective records for calculation. If not, the sensory test was repeated. 2.2 Odor index The data were first screened and unreasonable results were discarded. For example, if a particular member detected no odor for a given amount of GAC addition, then later detected the odor for the bottle with a much less amount of GAC added, the results are then deemed illogical and subsequently discarded. This is exemplified from the results of Panel A in a hypothetical case presented in Table 1. The sample to GAC dosage ratio is represented by a logarithmic scale. For 100 g sample with 10 g GAC, the ratio is 10 and hence the log ratio is 1 (log 10). The threshold of each panel member was calculated by the average of the answer in Y and N as follows: (log M yi + log M ni ) X i = (1) 2 X i, threshold of panel member i in logarithm; M yi, dosage ratio of GAC as panel i s first Y answer; and M ni, dosage ratio of GAC as panel i s last N answer. An example is shown for Panel B in Table 1 with the threshold number of 2.15 [(2+2.3)/2]. In case of no odor detection through all GAC dilution and adsorption, X i is simply log M ni (Panel C, Table 1). Realistically, one would like to add the amount of GAC such that this case is avoided. The results for the maximum and minimum values of panel members are excluded and the remaining valued then averaged: X1 + X 2 + L + X n 2 X = (2) n 2 X, average threshold of panel in logarithm; and n, number of panel members. Finally, the odor index of each sample is calculated by multiplying the above average (X) with an arbitrary factor of 33. The reason for choosing 33 is that the least amount of GAC used in the present study is 0.1 g (or sample:gac ratio is 1:1000 with log ratio = 3), thus the maximum odor index would be 99 (from a scale 0 to 99 with 99 being odorless). The lower the number of odor index is, the stronger the odor smelling is encountered by the panel. 2.3 Performance by deodorant agent Three major methods, i.e., chemical dosage, drying, and washing were applied to reduce odor of bottom ash or aggregate. For chemical method, several reagents were applied to compare the performance of their capability in reducing the odor. This was done by adding 0.5 or 3 wt% of each reagent to untreated bottom ash, thoroughly mixing in a TCLP extraction apparatus for 10 min. Thereafter, the samples were subject to the above panel test for determining the odor index. From the experience of site operation, odor could be reduced as storage period increases and water content decreases. Thus, odor index for drying and washing of samples were determined to see the extent of odor reduction. For sample drying, bottom ashes were placed inside 60 C oven for 2 h, cool and then subject to panel test. Finally, washing was done by placing 1 kg of aggregate on the surface of 100 mesh (0.15 mm) sieve, shaking in water bath with various amount of water (1-6 L) for 5 min. The material retained on the sieve was tested as washed sample. Both washed sample and silt collected were then subject to odor test. RESULTS AND DISCUSSION Typical panel results for samples from different bottom ashes and aggregates without addition of any deodorant agents are shown in Table 2. The nature sand and mud samples were also performed for comparison. After screening the panel results by excluding some illogical data (striking diagonal lines in Table 2), the odor index was then determined (first column, I od, Table 2). The odor index of natural sand is 94 (close to the odorless sample) and natural mud 74. The bottom ashes from three MSWIs ranged from 17 to 43, whereas the treated aggregates form 17 to 26. Clearly, the processed aggregates still exhibit significant odor. In fact, the odor for treated aggregates is higher than that of raw bottom ash. Some of the 4 different deodorant agents used (Bio-aid, zeolite, H 2 O 2 and KMnO 4 ) for treating aggregates also in fact increase the odor intensity (Table 3). For example, with the addition of 0.5% KMnO 4, the odor index decreased from 28 of the raw aggregate to 13. The reason is apparently due to the formation of odorous compounds after KMnO 4 addition. As KMnO 4 concentration increased (from 0.5 to 3%), the odor was more intensive (odor index further reduced to 8). This simple test clearly indicates the use of oxidizing agent or weak adsorbing agent (e.g., zeolite) is not feasible to reduce the odor present in final aggregate products.

4 100 J. Environ. Eng. Manage., 17(2), (2007) Table 2. Odor index of different samples without addition of deodorant agent Logarithm Panel Material sample/ Average A B C D E F G H I GAC ratio Natural sand 1.00 N N N N N N N N N 1.30 N N N N N N N N N 1.60 N N N N N N N N N 2.00 N N N N N N N N N 2.30 N N N N N N N N N 3.00 Y N N Y N Y Y N N I od = 94 Threshold Natural mud 0.90 N N N N N N N N N 1.00 N N N N N N N N N 1.30 Y N N N N Y N N N 1.70 Y N N N N Y N N Y 2.00 Y N N Y N N Y N N 3.00 Y Y Y Y N Y Y Y Y I od = 74 Threshold BA from MSWI-A 0.30 N N N N Y N N N N 0.60 Y N N N Y N N N N 0.70 Y N N Y Y N N N N 1.00 N N Y N N N N N N 1.60 Y Y Y N Y Y Y Y Y I od = 43 Threshold BA From MSWI-B 0.00 N N N N N N N N N 0.30 N N Y Y Y N N N N 0.60 N Y Y Y N N N Y N 0.70 Y N Y Y Y Y Y Y N 1.00 Y Y Y Y Y Y Y Y Y I od = 17 Threshold BA From MSWI - C 0.00 Y N N N N Y Y N N 0.30 N N N N N Y Y N Y 0.60 Y N Y Y N N Y Y Y 0.70 Y Y Y Y Y N Y Y Y 1.00 Y Y Y Y Y Y Y Y N I od = 17 Threshold Aggregate - A 0.60 Y N N N N N N N N 0.70 Y N N N N N N Y Y 1.00 N Y Y Y Y Y Y N N 1.60 Y Y Y Y Y Y Y Y N 2.00 Y Y Y Y Y Y Y Y N I od = 26 Threshold Aggregate - B 0.60 Y N Y Y N N N N N 0.70 Y N Y Y Y N Y Y Y 1.00 Y Y Y Y N Y Y Y N 1.60 Y Y Y Y N Y N Y Y 2.00 Y Y Y Y Y Y Y Y Y I od = 17 Threshold

5 Shih et al.: A Practical Odor Index for Bottom Ashes 101 Table 3. Performance of different deodorant methods Method Odor index Method Odor index Raw aggregate 28 Washed silt* 54 Bio-aid (0.5%) 22 Washed aggregates* Bio-aid (3%) 8 Dried BA-A 67 Zeolite (3%) 30 Dried BA-B 71 H 2 O 2 (0.5%) 28 Dried BA-C 53 H 2 O 2 (3%) 21 KMnO 4 (0.5%) 13 KMnO 4 (3%) 8 *Water:aggregate = 3 L:1 kg. Table 4. Odor reduction of washed aggregate as function of water quantity Water:solid ratio* Odor index 0 41 ** * Aggregate solid sample = 1 kg. ** Odor index of raw aggregate. Initially, aggregate washing test was performed with 6 different water quantities (from 1 to 6 L). Data in Table 4 indicate effectiveness of water washing in reducing sample odor even with 1 L of water. Further addition of water higher than 3 L does not significantly reduce odor (or from 72 to 85 with 6 L water). Consequently, subsequent water washing test was performed with 3 L water with 1 kg of solid sample. Data in Table 3 further indicate a significant reduction in odor index from water washing as well as dried samples. For example, the odor index was increased from 28 of the original aggregates to 80 after washing; the index for dried sample ranged from 53 to 71. CONCLUSIONS In any bottom ash processing plants, odor remains one of major concerns. It is impractical to identify and quantify the odor-causing compounds through GC/MS, nor measuring odor strength by taking ambient air samples with the so-called olfactometry test. In plant operation, a quick and easy approach is required to identify the presence of odors to facilitate plant operation. For example, changing the type of stabilizing/oxidizing agent or varying its concentration to reduce odor problems requires an easy approach to optimize the plant operation. Odor index developed in the present study is an easy and practical method to evaluate the performance of de-odorant agents or methods. The principle is analogous to the odor detection in aqueous and gases samples in that a panel, composed of 9 persons, is charged to sniff a series of the treated solid samples. Different amounts of the GAC were added to the solid bottom ash since GAC is noted for its adsorption of many odorous compounds. The lower the number of odor index is, the stronger the odor smelling is encountered, with a maximum odor index being 99 (odorless). The odor index of cleaned natural sand and mud are 94 and 74 respectively. The addition of KMnO 4 or H 2 O 2 in fact increases the odor strength due to released odorous compounds upon addition of these oxidizing agents. On the other hand, washing and drying are the most effective method to reduce odor for treated bottom ash. Based on the result of this study, the bottom ashes recycle plant has set up a washing process and constructed the corresponding wastewater treatment facility. Reduced odor in plant operation should maintain good relationship with the neighborhood community. More importantly, the reuse of treated aggregates in practical application would not present any offensive odor problems. REFERENCES 1. Taiwan Environmental Protection Administration (TEPA), Yearbook of Environmental Protection Statistics. TEPA, Taipei, Taiwan (2004). 2. American Society for Testing and Materials (ASTM), Standard Practice for Suprathreshold Odor Intensity Measurement. E ASTM, Philadelphia, PA (1999). 3. ASTM, Standard Practice for Determination of Odor and Taste Threshold by a Forced-Choice Ascending Concentration Series Method of Limits. E ASTM, Philadelphia, PA (2004). 4. Committee for European Normalization (CEN), Air Quality-Determination of Odour Concentration by Dynamic Olfactometry. EN CEN, Brussels, Belgium (2003). 5. Ministry of the Environment (MOE), Odor Index Regulation and Triangular Odor Bag Method. MOE, Tokyo, Japan (2003). Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. Manuscript Received: August 25, 2006 Revision Received: December 26, 2006 and Accepted: January 3, 2007