Proceedings of the International Conference on Sustainable Solid Waste Management, 5 - 7 September 2007, Chennai, India. pp.250-257
Design and Development of Biofilters for Controlling Odor Emissions from Composting of Municipal Solid Waste S.N.M. Menikpura, S.W.G.R.T.K. Ariyawansha, B.F.A. Basnayake and Y.G.W.G.M.C. Tharinda Department of Agricultural Engineering, Faculty of Agriculture, University of Peradeniya, Sri Lanka Email:
[email protected] ABSTRACT Biological processes that occur in MSW produce odorous gas emissions affecting health of the people and environment. Bio filters are bioreactors most commonly used for odour management. This study focused on evaluation of mature compost based bio filter that can be used in biological treatment facilities in Sri Lanka. The first experiment was with one layer of filter media, whereas the second with two layers having a 10cm gap in between them. One wet and the other dry were maintained at 40% (w/w) and 20% (w/w) moisture contents, respectively. The odorous gases were obtained from an Inclined Step Grate (ISG) composting system. Ammonia was the reference gas. When one layer filter was used 90% of incoming ammonia gas filtered. After modifications due to high moisture content of 40%, high filtration efficiency of 95% from wet bed and remaining 5% from dry bed were achieved. The wet bed prevented cracking and air channelling effect. The temperatures between 22 – 270C and pH 6 to 9 of both filter media and stability of volatile compounds were favourable for microbial growth and survival. Doubled or layered filters with smaller depths and gaps between them would be the best design to achieve higher odour filtration efficiencies. Keywords: MSW, Composting, ISG system, Bio filter, Odour gas, Micro organism 1.0 INTRODUCTION There is an ever increasing interest in the use of composting technology to provide an ecologically sound and cost effective solution to the management of Municipal Solid Waste (MSW). According to the composition of MSW generated in Sri Lanka it has greater potential to convert MSW to compost since 90% of those are biodegradable components. Aside from the many issues regarding optimization of the composting process itself, there have been historically major problems in the sitting of composting plant due to the lack of effective and economical odor control measures. Although several systems developed for controlling odour emissions to the atmosphere during decomposition of MSW, Sri Lanka, like most developing countries need economically viable and environmental friendly methods. Therefore compost based bio filtration is the more appropriate method as the economically viable and low maintenance method for controlling odor (Sheridan et al., 250
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2003; Torkian et al., 2003; Schlegelmilch et al., 2005; Wu et al., 2006). Biofiltration is an odor removal technology in which an odorous air stream is passed through a moist, porous filter medium prior to emission in to the atmosphere. Odorous compounds are removed from the air stream by absorption and diffusion into a moist microbial film on the surface of the filter media (Chen et al., 2005) which is known as the biofilm. Odorous compounds either accumulate in the film, or are digested by the resident microorganisms. The process of digestion, called bio-oxidation, occurs when micro organisms digest the gases, particulate matter, and volatile organic compounds in the presence of oxygen. Thus result of the bio filtration process is a decrease in odor emissions. 2.0 MATERIALS AND METHODS The prototype of open bio filter unit (one layer) with compost as a media was tested at the Meewathura University farm, Faculty of Agriculture, University of Peradeniya. This experimental set up was used for further modification and improved the bio filtration process with two filter media beds. 2.1 Configurations and Elements The filter unit was a polystyrene foam box that sized 42.8cm in length, 36.8cm in width and 50cm in height. The bottom of the box was connected to a plenum chamber on a side. The plenum chamber was made with 300 angles on sides with flat surfaces on the top and the bottom. The top of the box was kept open to escape the exhaust air. A schematic diagram of the bio filter setup is illustrated in Figure1. In the first 18 days of functioning, a blower having a 3.75cm outlet and rotated at 22000rpm was used to exhaust odorous gases from the small-scale Inclined Step Grate (ISG) composting unit to the bio filter bed. After the breakdown of the blower, it was replaced with a 5cm outlet blower which rotated at 3000/3600rpm. According to the configuration, the suction rate of this blower was adequate to suck maximum gas emission rate from the small scale ISG unit. Therefore, this blower was used to exhaust odorous gases from the small scale ISG unit through the bio filter bed. The blower outlet was connected to plenum chamber of the bio filter through a 5cm diameter PVC pipe. A valve system was used to adjust the air flow to the bio filter, since the blower had been manufactured for a fixed suction rate. Two valves were placed just after the exhaust duct of the blower, thus excess gas was diverted to the outside during first 18 days since the flow rate was high when used with the 22000 rpm blower. After changing the blower to low capacity one, exhaust valve was kept closed since the flow rate was just adequate. The outlet pipe of the ISG unit was connected to the blower inlet through a flexible PVC duct. 2.2 Filter Media The filter bed media was compost with high content of inorganic derived from organic materials. This mature compost media had been produced from Inclined Step Grate composting unit at Meewathura farm. It was pre-processed by sieving with a 0.5cm sieve and to remove large inert particles. First trial was done with single layer filter bed. After the modification the filter beds were kept in two layers which were wet and dry to a depth of 7.5cm. A 10cm gap was kept in between these layers. The Figure 1 shows the main component of the filter bed. Two wooden frames with wire mesh were used to hold the media. In order to obtain samples from wet layer, a piece of PVC tube with an end cap was fixed to the wall of filter box just above the wet layer. Adequate water was supplied to maintain the moisture content of the wet layer by a small drip unit made up of 1cm diameter tubes connected to 251
Design and Development of Biofilters for Controlling Odor Emissions from Composting of MSW
a cylindrical water container above the filter bed. A spraying bottle was used to maintain the moisture for dry layer. The water was added daily to attain moisture levels around 20% (w/w). In addition to wet and dry filter beds, wet and dry controls were maintained to compare efficiencies of filter beds. The bulk density of wet compost media was 1411kg/cm3 and dry compost was 864kg/ cm3. Water Container Dry Compost Layer (75mm thick)
Air flow meter Control valve
Sampling Port
Control valve Air Ducting
50cm
Plenum Chamber
Blower
Mesh
Wet Compost Layer (75mm thick)
Biofilter bed
MSW
Odour source-ISG Unit Figure 1 Schematic Diagram of the Experimental Setup
2.3 Maintaining of ISG Unit Fresh MSW that was collected from Kandy market and Yatinuwara Pradeshiya Sabha were used to feed the ISG unit. Also these fresh MSW was added frequently into ISG unit to produce high concentration of odor gases, since it consisted 90% of biodegradable fraction. Adequate amount of water was added to ISG to maintain the waste at high moisture conditions that created optimum conditions for microbial activities while producing odorous gases under semi-anaerobic conditions. 2.4 Performance Measurement Several parameters were monitored to evaluate performance of the bio filter. The filter bed characteristics were evaluated at three days intervals. Samples were taken for evaluating in the filter media as well as in the controls. Bed characteristics were analyzed using pH, moisture content, temperature, Volatile Solid (VS), Organic Carbon content and Total Nitrogen Content. In order to evaluate the bio filter performance, gas samples were collected at the inlet, intermediate gap and the outlet (Sheridan et al., 2003) and analysis was done using ammonia as the reference gas. During the first trial gas chromatography was used for measuring ammonia gas and during the second trial boric acid was used to dissolve ammonia and titrated with concentrated sulphuric acid to determine the concentrations of ammonia.
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3.0 RESULTS AND DISCUSSION 3.1 Particle Size Distribution of Filter Media The particle size distributions of mature compost were determined by dry sieving method and particles sizes ranged between 0.063mm-4.5mm. Particle distributions of the media play a major role in the efficient cleaning of airstreams or in the economical operation of the bio filters. It also plays a major role as media to maintain critical properties of bio filter such as porosity, moisture holding capacity and nutrient content which govern the compost bio filter performance (Nicolai and Schmidt. 2005; Sagastume and Noyola . 2006). Microbial activities generally occur on the surface of the organic particles. Therefore smaller particle sizes through it increases the surface area, would encourage microbial activity and increase the rate of decomposition. When the particles are too small, air circulation is inhibited due to compaction and caused higher pressure drop (Chen et al., 2005) .Therefore more than 60%of particles were above 0.24mm used as the filter media. This range was appropriate for microbial activity as well as to reduce the compaction and pressure drop. 3.2 Moisture Content
Moisture persentage (wet basis)
The moisture content in the wet bed was maintained between 35 – 42% on wet basis see Figure 2. The higher level of moisture content helped to prevent cracking of the media and thus reduced the air channelling effect. On the other hand excessive water content will lead to media compaction and gas clogging that eventually will result in flow channels due to granulometry change (Sagastume and Noyola . 2006). Therefore above wet bed moisture range that was maintained, would have favoured microbial growth and survival, (Nicolai and Schmidt. 2005) which also would have contributed to improving the efficiency of the filter media. In comparison to wet beds, dry beds moisture contents were maintained around 10-20% on wet basis. However under this situation media porosity was high and that helped to reduce pressure drops through the media. In contrast inadequate moisture can allow the media to dry out, deactivating the microbes and creating cracks and channelling of air that results in a reduction of filter efficiency (Nicolai and Schmidt. 2005) Wet control Dry control
50
Wet media Dry media
40 30 20 10 0 0
10
20 Time (30 days) 40
50
60
Figure 2 Moisture Content Variations in Filter Beds and Control Beds
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3.3 Temperature During the monitoring period temperature of filter media and controls were maintained at 23-30oC range. In this temperature range microbes are most active, thus it is the favourable range for microbial growth and development (Nicolai and Schmidt, 2005). These temperatures ranges were less than the environmental temperature ranges. Between wet and dry media temperatures, there were slight fluctuations due to moisture content differences of the two media. Even though nitrifies grow faster at high temperature (25-30°C), once they established, they would have remained at somewhat lower temperature values. 3.4 pH Value Variation with Time The pH value is one particular operating parameter that can markedly impact the microbial population, and corresponding bio filter performance (Wu et al., 2006).Thought out the monitoring period the pH of both wet and dry layers ranged from 6 – 9, which was favourable for microbial growth and development. The wet filter bed, had maintained higher pH values in comparison to others, which ranged 8-8.6 since microorganism activities in this filter could have been higher, specially methanogenic bacteria due to high moisture contents and other favourable conditions in wet media. As explained by Chen et al., (2005), another possibility and a more likely explanation would have been that in the presence of dissolved ammonia, pH level had increased in this wet media. 3.5 Variations of Volatile Compounds
Volatile compound %
The volatile compounds had reduced with time in the wet media as well as in the wet control see Figure 3. This was the usable substrate for microbial growth and development in the bio filter. As Nicolai and Schmidt (2005) has mentioned, the bio filter treatment efficiency depends on the microbial breakdown of volatile organic compounds. Also in the wet media bio filter, volatile compounds were lower indicating that it had developed a live and an active bio filter. Naturally, the volatile compounds in the dry filter media and control had shown fairly high amount than the wet beds throughout the monitoring period. It seems that there were less microbial populations in these dry media resulting from low moisture contents as well as other factors that were not at optimum moisture range for their development. Wet control Dry contorl
34 32 30 28 26 24 22 20 18 0
10
20 30 Time (days)
Wet media Dry media
40
50
Figure 3 Volatile Compound Variation in Beds
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3.6 Total Nitrogen Variation in Filter Beds The total nitrogen concentration of the filter media and controls had fluctuated along with the microbial activities, see Figure 4. As a result of higher microbial activities in the wet filter media had the lowest nitrogen concentrations throughout the monitoring period since nitrogen is the necessary nutrient for microbial survival and one of the most readily depleted nutrients (Wu et al., 2006). In addition, other favourable conditions would have influenced high activation of micro organism in the wet bed. The oxidized nitrogen from the wet beds would have been fixed in the upper dry layer thus it had showed high nitrogen concentrations.
Total nitrogen(mg/g)
Dry control Wet control
Dry media Wet media
160 140 120 100 80 60 40 20 0 0
20
Time(days)
40
60
Figure 4 Cumulative Nitrogen Concentration Variation with Time
3.7 Organic Carbon Content Variation Even though organic carbon content fluctuated throughout the study period, it had reached very low levels in the wet beds towards the end see Figure 5. It had distinctly transformed to more volatile compounds in the wet media, since it had developed live and active bio-film under favourable and conducive conditions for microbial growth and development in the wet media. However under adverse and inappropriate conditions for microbial growth in dry media and dry control, organic contents remained high. 3.8 Gas Analysis - NH3 as the Reference Gas Ammonia gas was considered as the major odor emission gas from ISG unit and other composting systems. As the main finding of this study, the result was very encouraging since ammonia gas destruction in the bio filter was thorough (Chen et al., 2005). It is important to note that 90% of ammonia gas reduction was achieved in the first trial with one filter bed layer. As shown in Figure 6, after the modification with two filter beds it has showed significant reduction of ammonia from the wet layer, amounting to 95% removal. Ammonia reduction through both layers was 100%, since ammonia was not detected in the outlet gas samples. Thus, it could be deduced that other odorous gases would have been controlled by the bio filter.
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Wet control Dry control
Wet media Dry media
Organic carbon %
14 12 10 8 6 4 2 0 0
10
20
30 40 Time (days)
50
60
Inlet Intermediate
15.02.07
10.02.07
07.02.07
05.02.07
Outlet
02.02.07
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
30.01.07
Ammonia concentration (mg/l)
Figure 5 Organic Carbon Percentage Variation during the Functioning Period
Days Figure 6 the Results of the Gas Analysis for Different Days of Sampling
4.0 CONCLUSIONS The study indicated that controlling the moisture content in wet and dry filters was the major parameter for establishing live media. The moisture content approximately 40% (w/w) was optimum to achieve higher filtration efficiency. However, moisture contents in the dry bed were not sufficient to establish live and active media. However, the differences in moisture contents and creating a gap between layers provided ideal conditions for oxidizing ammonia through absorption/adsorption and de-sorption mechanisms. Also the reduction of total N and organic carbon in wet layer as compared to the wet control indicated the activation of micro organisms and the build up of an active filter. The wet filter was the most active media compared to the dryer layers.
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The result after the modification with two filter beds was more encouraging, since 95% of ammonia odour gas controlled through the wet layer which indicated that a live and active media was formed. The rest 5% was reduced from the dry layer. Thus, a deduction can be made from this result that other odorous gases also were reduced from the filter unit. With the incorporation of wet and dry layers together improved the efficiency of filter by 100%. Therefore the compost media bio filter with double layers with a small gap is very effective and efficient to control the odorous gas emissions in composting. ACKNOWLEDGMENT The Authors are most grateful to the Swedish International Development Agency (SIDA) and Asian Instituted of Technology (AIT) for technical and financial support given for the project; Sustainable Solid Waste Management in Asia under the Regional Research Programme on Environmental Technology (ARRPET). REFERENCES Chen, Y.X., Yin, J. and Wang, K.X. Long-term operation of biofilters for biological removal of ammonia. Chemosphere, 58(8), pp.1023-1030(2005). Nicolai, R. and Schmidt, D. Biofilters, Livestock development in South Dakota; Environmental and Health. South Dakota State University, College of Agriculture & Biological Sciences, Cooperative Extension Service / USDA cooperating. pp.2-8 (2005). Sagastume, J.M.M. and Noyola. A. Hydrogen sulfide removal by compost biofiltration: Effect of mixing the filter media on operational factors. Bioresource Technology, 97(13), pp.1546-1553. (2006). Schlegelmilch, M., Streese, J., Biedermann, W., Herold, T. and Stegmann, R. Odour control at biowaste composting facilities. Waste Management, 25 (9), pp.917-927 (2005). Sheridan, B.A., Curran, T.P. and Dodd. V.A. Biofiltration of n -butyric acid for the control of odour. Bioresource Technology 89, pp.199 –205 (2003). Torkian, A., Dehghanzadeh, R. and Hakimjavadi. M. Biodegradation of aromatic hydrocarbons in a compost biofilter. J. chemical. Thechnology and Biotechnology, 78, pp.795-801 (2003) Wu, D., Quan, X., Zhang, Y. and Zhao, Y. Long-term operation of a compost-based biofilter for biological removal of n-butyl acetate, p-xylene and ammonia gas from an air stream. Biochemical Engineering Journal , 32(2), pp.84-92 (2006).
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