odorous gas emissions during the turning process of sewadge sludge
ODOROUS GAS EMISSIONS DURING THE TURNING PROCESS OF SEWADGE SLUDGE AND GREEN WASTE CO-COMPOSTING WINDROWS Z. DUAN*,°, W. LU* * School of Environment, Tsinghua University, 10084 Beijing, P. R. China ° Current address: Department of Environmental Engineering, Technical University of Denmark, DK-2800 Kgs, Lyngby, Denmark
SUMMARY: Odorous gas emissions during the turning process of sewage sludge and green waste co-composting windrows in a sludge disposal plant in China were investigated. It was found that in summer, more odorous gas was produced during the composting process. Highest odor concentration from the surface of windrows with a material mixing ratio of 3:1 appeared at 0.5h after the turning operation, while for the windrows with higher material mixing ratios, odor concentration peaks were observed at 4h after turning. Sulfur compounds were the dominant odorous compounds for all windrows in both summer and winter. Oxygenated compounds also contributed to the higher odor pollution in summer due to their high concentrations. Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as the odor indicators during the turning process of sewage sludge and green waste co-composting windrows, which need special attention in further studies.
1. INTRODUCTION Along with the rapid growth in the number of sewage wastewater treatment plants in China, the amount of sewage sludge generated in China is increasing dramatically in recent years. In 2013, 6.25 million tons dry solids were produced (Yang et al., 2015); however, more than 80% of the produced sludge are not safely treated and disposed (Feng et al., 2015), which leads to serious secondary environmental pollution. The proper management of sewage sludge has become a big challenge in China. Current existing ways for sewage sludge treatment and disposal in China include incineration, anaerobic digestion, aerobic composting, landfilling with domestic waste, recycle as construction materials, land application, dumping, etc. (Chen et al., 2012); although improper dumping takes more than 80% in most cities (Yang et al., 2015). Aerobic windrow composting followed with land application, due to its low cost and possibility of recycling nutrients that contained in the sludge, is recommended as one of the most feasible disposal technology in China, and has been adopted by some cities (Meng et al., 2011; Feng et al., 2015). Dewatered sludge, with a moisture content of approximately 80%, is first mixed with bulking agents such as garden waste and mature compost to adjust the moisture content, C/N ratio and density and then compacted into large windrows. The windrows are either aerated through installed aeration
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
systems or turned over mechanically with a windrow turner. During the composting process, the temperature could raise to 50-70 °C; pathogens are destroyed and organic components are degraded or transformed into humic compounds (Jouraiphy et al., 2005). The sludge finally turns into stable compost and could be used as fertilizer to improve soil quality. However, the gaseous emissions in the composting process especially in open systems has long been a problem primarily due to the unpleasant odors, which significantly limits the application of composting technology in China. This paper aims to 1) investigate the characterization of odorous gas emission during the mechanical turning process of sewage sludge and green waste co-composting windrows; 2) identify the different emission patterns related with season and material mixing ratios; 3) figure out significant odorous compounds, and provide advice for further odor control activities.
2. MATERIALS AND METHODS 2.1 Site description The sampling was conducted in a biotechnology company which dealt with sewage sludge in Suzhou, China. Sewage sludge produced from wastewater treatment plants was shipped to the composting facility, and then mixed with a certain weight of green waste (mainly clipped branches collected from local gardens and crushed into woodchips with a diameter of 2-3cm) into large windrows with length, width and height of approximately 50, 1.8 and 1m. The properties of the raw sludge and green waste were shown in Table 1. Aerobic co-composting took place in a semi-open shed with upper cover to protect the windrows from rain, while the surroundings were directly open to ambient air. The whole composting process was operated for about 30 days in summer and 45 days in winter, and the windrows were turned over by a windrow turner every one or two days. During the turning process, large amounts of odorous gases were emitted without any odor control system, which often led to complains from adjacent communities. Table 1. Properties of raw sludge and green waste Item Water content (%) VS (%TS) C:N ratio
Sludge 84.63±0.01 62.24±0.19 6.63
Green waste 25.80±1.19 98.59±0.27 22.79
2.2 Sampling and analysis A total of 24 air samples were taken from 2 composting batches on December 2012 (winter) and August 2013 (summer). The 4 windrows, with raw sludge and green waste mixing ratios (in wet weight) of 3:1, 5:1 in winter and 3:1, 4:1 in summer, were all in the thermophilic stage (except the 5:1 windrow) and were turned over in the morning on the selected sampling days. One of two samples were taken at 0.5h before turning and 0.5h, 4h, 8h after turning respectively from each windrow by using a specially designed sampler. Surface gas samples were collected at 5 cm above the surface of the windrows, and internal gas samples were collected 15 cm below the surface of the windrows (Table 2). The composition and concentrations of odorous
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
compounds in each sample were analyzed by a gas chromatography-mass spectrometer (GCMS) system (Agilent 7890A-5975C, Agilent Technologies, Inc., USA) coupled with a three-stage preconcentration system (Entech 7100, USA) using the USEPA TO-15 method. Detailed description of the sampler and analytical method had been reported elsewhere (Duan et al., 2014). Table 2. Sample Numbers Winter Sampling time 0.5h before turning 0.5h after turning 4h after turning 8h after turning
Windrow A (3:1) Surface WA1a WA2a WA3a WA4a
Internal WA1b WA2b WA3b WA4b
Windrow B (5:1) Surface WB1a WB2a WB3a WB4a
Internal WB1b WB2b WB3b WB4b
Summer Windrow Windrow A (3:1) B (4:1) Surface Surface SA1 SB1 SA2 SB2 SA3 SB3 SA4 SB4
3. RESULTS AND DISCUSSION 3.1 Odorous gases composition and concentrations emitted from different cocomposting windrows A total of 89 trace compounds were detected in the samples, including 6 sulfur compounds, 10 oxygenated compounds, 18 aromatics, 3 terpenes, 20 alkanes, 10 alkenes, and 22 halogenated compounds. As could be seen from Table 2, the total odorous gas concentrations in summer was much higher than in winter, which ranged from 582,5 to 4236,6 µg/m3. It was probably because the high ambient temperature as well as higher internal temperatures in the windrows, which accelerated the production and emission of odorous gases. Table 3. Odorous gas concentrations at the surface of co-composting windrows during the turning process
Compound
Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes Alkanes Halogenated compounds Total concentration
Average concentration (µg/m3) Winter Summer Windrow A Windrow A Windrow B Windrow B (5:1) (3:1) (3:1) (4:1) 121.02 ± 91.41 ± 38.29 409.08 ± 233.15 107.12 ± 37.73 66.81 99.8 ± 6.89 102.67 ± 14.77 1103.33 ± 1058.26 514.12 ± 243.09 31.11 ± 8.29 242.42 ± 180.42 77.65 ± 19.51 64.42 ± 11.84 26.49 ± 10.37 40.62 ± 35.12 474.55 ± 321.49 194.55 ± 94.63 8.52 ± 1.83 7.46 ± 1.66 56.4 ± 76.87 9.72 ± 1.55 37.69 ± 6.05 45.28 ± 13.75 75.62 ± 65.79 11.38 ± 4.30 39.89 ± 3.01 45.98 ± 13.75 54.92 ± 36.64 36.22 ± 8.69 364.6 ± 90.63 575.84 ± 247.37 2251.55 ± 1343.84 937.55 ± 303.73
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Relatively high concentrations of sulfur compounds and oxygenated compounds were detected in all samples, while the concentrations of alkenes, alkanes and halogenated compounds were low and showed little variance in different seasons and windrows. Large amounts of terpenes were released from the two windrows in summer, indicating the degradation of green waste (Font et al., 2011). At the surface of the 5:1 windrow in winter, significant amount of aromatics was detected. Sulfur and oxygenated compounds were common products of organic waste decomposition processes both under aerobic and anaerobic conditions (Smet et al., 1999), while aromatic compounds were probably originated from the sludge. The average temperature inside the 5:1 windrow was only around 20 °C, indicating that this windrow was still in the mesophilic stage; and most organic components contained in the raw sludge remained undecomposed, leading to the accumulation of high odorous gas concentrations.
Concentration (µg/m3)
Summer
Winter
4000
Halogenated compounds Alkanes Alkenes Terpenes Aromatics Oxygenated compounds Sulfur compounds
3000
2000
Windrow A (3:1)
Windrow B (4:1)
Windrow B (5:1)
Windrow A (3:1)
1000
4
3
SB
2
SB
1
SB
4
SB
3
SA
SA
2 SA
1 SA
4a
3a
WB
2a
WB
1a
WB
4a
WB
3a
WA
2a
WA
WA
WA
1a
0
Figure 1. Odorous gas emission during the turning process of different co-composting windrows.
Highest odorous gas concentrations at the surface of windrows with a material mixing ratio of 3:1 were found at 0.5h after the turning operation, as is shown in Figure 1, while for the windrows with higher material mixing ratios, the concentrations peaked at 4h after turning. The difference could be attributed to the higher density of 4:1 and 5:1 windrows: the oxygen content inside the windrows increased dramatically after the turning operation, which accelerated the microorganisms’ activities and large amount of odorous gases were produced (Gutiérrez et al., 2017). However, their convection inside the windrows and discharge from the surface into ambient air were significantly limited due to the smaller porosity compared with the 3:1 windrows. Hence, the appearance of odorous gas concentration peaks was also delayed (Shen
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
et al., 2012). 3.2 Surface and internal odorous gas concentrations Surface and internal odorous gas concentrations of windrows in winter were compared to investigate the pattern of odor production and accumulation related with material mixing ratio. It was found that the internal odorous gas concentrations were much higher than at the surface of the windrows before turning (Table 4 and Table 5), while at 0,5h after turning, the surface concentrations exceeded the internal concentrations. As time went by, difference could be seen in the windrows with different material mixing ratios. In windrow A with a material mixing ratio of 3:1, the concentrations at the surface and internal of the windrows were nearly the same, indicating a good ventilation status inside the windrow; while for windrow B, a transformation in the emission pattern could be observed. From 0,5h to 4h after turning, the odorous gas emission was adverse dispersion from surface to internal until they reached balance, and high concentrations were detected due to the release of accumulated odorous gases inside the windrow. Then odorous gases started to accumulate again, and the internal concentration exceeded the surface concentration at 8h after turning. The release of odorous gas could be influenced by their inherited concentrations, production, degradation inside the windrows, adsorption, dissolution and retention in free air space (He et al., 2010). During the turning process, the porosity and oxygen content inside the windrows were changed in different levels, which could promote odorous gas production and their retention time inside windrows. Hence, the emission of odorous gas from the surface of the windrows and their accumulation inside should be taken into consideration simultaneously when assessing possible emission amount of odorous gases during the turning process. Table 4. Surface and internal odorous gas concentrations of Windrow A (3:1) in winter Compound Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes Alkanes Halogenated compounds Total concentration
WA1a 83,92 95,92 29,62 20,8 5,92 30,88 42,98 310,05
WA2a 216 108,92 42,9 41,7 9,125 34,4 41,5 494,55
Concentration (µg/m3) WA3a WA4a WA1b WA2b 117,78 66,4 301,08 104,35 101,32 93,32 6,92 90,22 23,55 28,35 35,55 40,15 24,35 19,1 54,02 24,7 10,2 8,85 17 5,15 41,82 43,65 37,22 33,75 38,98 36,12 92,7 35,2 358 295,8 544,5 333,52
WA3b 50,52 144,15 36,42 29 3,125 43,92 90,78 397,92
WA4b 61,35 97,28 30,65 19,7 6,4 33,7 41,42 290,5
Table 5. Surface and internal odorous gas concentrations of Windrow B (5:1) in winter Compound Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes
WB1a 51,08 108,28 27,58 19,62 5,88
WB2a 85,85 96,42 386,2 26,1 6,3
Concentration (µg/m3) WB3a WB4a WB1b WB2b 143,4 85,3 62,78 79,78 120,08 85,9 109,52 83,92 396,9 159,02 686,65 363,3 93,15 23,62 25,2 9,85 9,38 8,3 14,4 5,42
WB3b 137,48 125,4 392,05 23,95 7
WB4b 71,4 115,45 173,58 34,1 12
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Alkanes Halogenated compounds Total concentration
41,75 54,3 308,48
43,62 31,65 676,15
45,62 60,7 869,22
50,12 37,25 449,52
65,55 29,3 993,4
46,5 30,75 619,52
56,42 45,15 787,45
53,08 94,38 553,98
3.3 Significant odorous compounds 3.3.1 High concentration odorous compounds The most abundant compounds differed in winter and summer, though no apparent difference was observed between windrows of different mixing ratios. Dimethyl disulfide, methylene chloride, pentane, ethanol, diethyl sulfide, α-pinene and dimethyl sulfide were found to be the most concentrated compounds in winter; while in summer, acetone, dimethyl sulfide, α-pinene, β-pinene, acetaldehyde, ethanol and dimethyl disulfide were the most abundant compounds. Zhu et al. ( 2016) designated ammonia, hydrogen sulfide, carbon disulfide, dimethyl disulfide, methyl mercaptan, dimethylbenzene, phenylpropane, and isopentane as concentration indicators in a static forced-aeration sewage sludge composting plant. Compared with their results, high concentrations of terpenes and oxygenated compounds appeared in this study, probably due to the use of green waste as bulking agents. 3.3.2 Odor indicators Theoretical odor concentration (Cod) of each sample was calculated based on ratio of the chemical concentration of each compound to its odor threshold (i.e., dilution multiple) (Capelli et al., 2008), as shown in Figure 3. It was obvious that sulfur compounds were the dominant odor contributors in all samples; besides, oxygenated compounds also appeared to be significant odorous compounds in summer due to their high concentrations.
Others Oxygenated compounds Sulfur compounds
1000
Cod (dimensionless)
800
Summer
Winter 600
400
200
Figure 2. Theoretical odor concentrations of the surface samples.
SB 4
SB 3
SB 2
SB 1
SA 4
SA 3
SA 2
4a SA 1
3a
WB
2a
WB
1a
WB
4a
WB
3a
WA
2a
WA
WA
WA
1a
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Odor indicators were also selected based on the dilution multiples of each compound contained in all the samples. Odorous compounds were ranked top down according to the values of their dilution multiples for each sample, and major odor contributors with dilution multiple > 1 were recognized. Those compounds from each sample were then put together to calculate the frequencies of their appearance in all the samples, and compounds with a frequency > 80% were selected as odor indicators. Results showed that major odor contributors in all the samples turned out to be quite similar despite season and material mixing ratio, and methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as odor indicators for the odor emission during the turning process of the co-composting windrows. Compared with the results reported by Van Durme et al. (1992) and Zhu et al. ( 2016), organic sulfur compounds were the commonly recognized significant compounds in all kinds of composting facilities, which should primary attention in odor control activities.
4. CONCLUSIONS This paper studied the odorous gas emission from sewage sludge and green waste cocomposting windrows during the turning processes. It was found that higher odorous gas emissions were detected in summer, probably because of the higher temperatures in summer. High concentrations of sulfur compounds and oxygenated compounds appeared in all samples, though the oxygenated compounds predominated the emissions in summer. The concentrations of terpenes were much higher in summer than in winter, and high concentrations of aromatics were detected from the 5:1 windrow in winter. Emission peaks were observed at 0,5h after turning for the 3:1 windrows, while for the rest two windrows with higher sewage sludge contents, the peaks appeared at 4h after turning. Sulfur compounds were the major odorous compounds for all windrows in both summer and winter. Oxygenated compounds also contributed to the higher odor pollution in summer due to their high concentrations. Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as the odor indicators.
AKNOWLEDGEMENTS The project was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2011ZX07301-003). The authors also wish to acknowledge with thanks the funding from China Scholarship Council and Prof. Peter Kjeldsen in participating the Symposium.
REFERENCES Capelli, L., Sironi, S., Del Rosso, R., Céntola, P., Il Grande, M., 2008. A comparative and critical evaluation of odour assessment methods on a landfill site. Atmos. Environ. 42, 7050–7058. doi:10.1016/j.atmosenv.2008.06.009 Chen, H., Yan, S.-H., Ye, Z.-L., Meng, H.-J., Zhu, Y.-G., 2012. Utilization of urban sewage sludge: Chinese perspectives. Environ. Sci. Pollut. Res. 19, 1454–1463. doi:10.1007/s11356-
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012-0760-0 Duan, Z., Lu, W., Li, D., Wang, H., 2014. Temporal variation of trace compound emission on the working surface of a landfill in Beijing, China. Atmos. Environ. 88, 230–238. Feng, L., Luo, J., Chen, Y., 2015. Dilemma of Sewage Sludge Treatment and Disposal in China. Environ. Sci. Technol. 49, 4781–4782. doi:10.1021/acs.est.5b01455 Font, X., Artola, A., Sánchez, A., 2011. Detection, composition and treatment of volatile organic compounds from waste treatment plants. Sensors 11, 4043–4059. doi:10.3390/s110404043 Gutiérrez, M.C., Siles, J.A., Diz, J., Chica, A.F., Martín, M.A., 2017. Modelling of composting process of different organic waste at pilot scale: Biodegradability and odor emissions. Waste Manag. 59, 48–58. doi:10.1016/j.wasman.2016.09.045 He, P., Tang, J., Zhang, D., Zeng, Y., Shao, L., 2010. Release of volatile organic compounds during bio-drying of municipal solid waste. J. Environ. Sci. 22, 752–759. doi:10.1016/S10010742(09)60173-X Jouraiphy, A., Amir, S., El Gharous, M., Revel, J.C., Hafidi, M., 2005. Chemical and spectroscopic analysis of organic matter transformation during composting of sewage sludge and green plant waste. Int. Biodeterior. Biodegrad. 56, 101–108. doi:10.1016/j.ibiod.2005.06.002 Meng, X.Z., Meng, X.J., Cao, X.S., 2011. Recycling to Soils: a Sustainable Way of Sludge Disposal and its Practice in China, in: Environmental Biotechnology and Materials Engineering, Advanced Materials Research. Trans Tech Publications, pp. 1417–1422. doi:10.4028/www.scientific.net/AMR.183-185.1417 Shen, Y., Chen, T.-B., Gao, D., Zheng, G., Liu, H., Yang, Q., 2012. Online monitoring of volatile organic compound production and emission during sewage sludge composting. Bioresour. Technol. 123, 463–470. doi:10.1016/j.biortech.2012.05.006 Smet, E., Van Langenhove, H., De Bo, I., 1999. The emission of volatile compounds during the aerobic and the combined anaerobic/aerobic composting of biowaste. Atmos. Environ. 33, 1295–1303. doi:10.1016/S1352-2310(98)00260-X Van Durme, G.P., McNamara, B.F., McGinley, C.M., 1992. Bench-Scale Removal of Odor and Volatile Organic Compounds at a Composting Facility. Water Environ. Res. 64, 19–27. Yang, G., Zhang, G., Wang, H., 2015. Current state of sludge production, management, treatment and disposal in China. Water Res. 78, 60–73. doi:10.1016/j.watres.2015.04.002 Zhu, Y., Zheng, G., Gao, D., Chen, T., Wu, F., Niu, M., Zou, K., 2016. Odor composition analysis and odor indicator selection during sewage sludge composting. J. Air Waste Manage. Assoc. 66, 930–940. doi:10.1080/10962247.2016.1188865
SUMMARY: Odorous gas emissions during the turning process of sewage sludge and green waste co-composting windrows in a sludge disposal plant in China were investigated. It was found that in summer, more odorous gas was produced during the composting process. Highest odor concentration from the surface of windrows with a material mixing ratio of 3:1 appeared at 0.5h after the turning operation, while for the windrows with higher material mixing ratios, odor concentration peaks were observed at 4h after turning. Sulfur compounds were the dominant odorous compounds for all windrows in both summer and winter. Oxygenated compounds also contributed to the higher odor pollution in summer due to their high concentrations. Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as the odor indicators during the turning process of sewage sludge and green waste co-composting windrows, which need special attention in further studies.
1. INTRODUCTION Along with the rapid growth in the number of sewage wastewater treatment plants in China, the amount of sewage sludge generated in China is increasing dramatically in recent years. In 2013, 6.25 million tons dry solids were produced (Yang et al., 2015); however, more than 80% of the produced sludge are not safely treated and disposed (Feng et al., 2015), which leads to serious secondary environmental pollution. The proper management of sewage sludge has become a big challenge in China. Current existing ways for sewage sludge treatment and disposal in China include incineration, anaerobic digestion, aerobic composting, landfilling with domestic waste, recycle as construction materials, land application, dumping, etc. (Chen et al., 2012); although improper dumping takes more than 80% in most cities (Yang et al., 2015). Aerobic windrow composting followed with land application, due to its low cost and possibility of recycling nutrients that contained in the sludge, is recommended as one of the most feasible disposal technology in China, and has been adopted by some cities (Meng et al., 2011; Feng et al., 2015). Dewatered sludge, with a moisture content of approximately 80%, is first mixed with bulking agents such as garden waste and mature compost to adjust the moisture content, C/N ratio and density and then compacted into large windrows. The windrows are either aerated through installed aeration
Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
systems or turned over mechanically with a windrow turner. During the composting process, the temperature could raise to 50-70 °C; pathogens are destroyed and organic components are degraded or transformed into humic compounds (Jouraiphy et al., 2005). The sludge finally turns into stable compost and could be used as fertilizer to improve soil quality. However, the gaseous emissions in the composting process especially in open systems has long been a problem primarily due to the unpleasant odors, which significantly limits the application of composting technology in China. This paper aims to 1) investigate the characterization of odorous gas emission during the mechanical turning process of sewage sludge and green waste co-composting windrows; 2) identify the different emission patterns related with season and material mixing ratios; 3) figure out significant odorous compounds, and provide advice for further odor control activities.
2. MATERIALS AND METHODS 2.1 Site description The sampling was conducted in a biotechnology company which dealt with sewage sludge in Suzhou, China. Sewage sludge produced from wastewater treatment plants was shipped to the composting facility, and then mixed with a certain weight of green waste (mainly clipped branches collected from local gardens and crushed into woodchips with a diameter of 2-3cm) into large windrows with length, width and height of approximately 50, 1.8 and 1m. The properties of the raw sludge and green waste were shown in Table 1. Aerobic co-composting took place in a semi-open shed with upper cover to protect the windrows from rain, while the surroundings were directly open to ambient air. The whole composting process was operated for about 30 days in summer and 45 days in winter, and the windrows were turned over by a windrow turner every one or two days. During the turning process, large amounts of odorous gases were emitted without any odor control system, which often led to complains from adjacent communities. Table 1. Properties of raw sludge and green waste Item Water content (%) VS (%TS) C:N ratio
Sludge 84.63±0.01 62.24±0.19 6.63
Green waste 25.80±1.19 98.59±0.27 22.79
2.2 Sampling and analysis A total of 24 air samples were taken from 2 composting batches on December 2012 (winter) and August 2013 (summer). The 4 windrows, with raw sludge and green waste mixing ratios (in wet weight) of 3:1, 5:1 in winter and 3:1, 4:1 in summer, were all in the thermophilic stage (except the 5:1 windrow) and were turned over in the morning on the selected sampling days. One of two samples were taken at 0.5h before turning and 0.5h, 4h, 8h after turning respectively from each windrow by using a specially designed sampler. Surface gas samples were collected at 5 cm above the surface of the windrows, and internal gas samples were collected 15 cm below the surface of the windrows (Table 2). The composition and concentrations of odorous
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
compounds in each sample were analyzed by a gas chromatography-mass spectrometer (GCMS) system (Agilent 7890A-5975C, Agilent Technologies, Inc., USA) coupled with a three-stage preconcentration system (Entech 7100, USA) using the USEPA TO-15 method. Detailed description of the sampler and analytical method had been reported elsewhere (Duan et al., 2014). Table 2. Sample Numbers Winter Sampling time 0.5h before turning 0.5h after turning 4h after turning 8h after turning
Windrow A (3:1) Surface WA1a WA2a WA3a WA4a
Internal WA1b WA2b WA3b WA4b
Windrow B (5:1) Surface WB1a WB2a WB3a WB4a
Internal WB1b WB2b WB3b WB4b
Summer Windrow Windrow A (3:1) B (4:1) Surface Surface SA1 SB1 SA2 SB2 SA3 SB3 SA4 SB4
3. RESULTS AND DISCUSSION 3.1 Odorous gases composition and concentrations emitted from different cocomposting windrows A total of 89 trace compounds were detected in the samples, including 6 sulfur compounds, 10 oxygenated compounds, 18 aromatics, 3 terpenes, 20 alkanes, 10 alkenes, and 22 halogenated compounds. As could be seen from Table 2, the total odorous gas concentrations in summer was much higher than in winter, which ranged from 582,5 to 4236,6 µg/m3. It was probably because the high ambient temperature as well as higher internal temperatures in the windrows, which accelerated the production and emission of odorous gases. Table 3. Odorous gas concentrations at the surface of co-composting windrows during the turning process
Compound
Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes Alkanes Halogenated compounds Total concentration
Average concentration (µg/m3) Winter Summer Windrow A Windrow A Windrow B Windrow B (5:1) (3:1) (3:1) (4:1) 121.02 ± 91.41 ± 38.29 409.08 ± 233.15 107.12 ± 37.73 66.81 99.8 ± 6.89 102.67 ± 14.77 1103.33 ± 1058.26 514.12 ± 243.09 31.11 ± 8.29 242.42 ± 180.42 77.65 ± 19.51 64.42 ± 11.84 26.49 ± 10.37 40.62 ± 35.12 474.55 ± 321.49 194.55 ± 94.63 8.52 ± 1.83 7.46 ± 1.66 56.4 ± 76.87 9.72 ± 1.55 37.69 ± 6.05 45.28 ± 13.75 75.62 ± 65.79 11.38 ± 4.30 39.89 ± 3.01 45.98 ± 13.75 54.92 ± 36.64 36.22 ± 8.69 364.6 ± 90.63 575.84 ± 247.37 2251.55 ± 1343.84 937.55 ± 303.73
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Relatively high concentrations of sulfur compounds and oxygenated compounds were detected in all samples, while the concentrations of alkenes, alkanes and halogenated compounds were low and showed little variance in different seasons and windrows. Large amounts of terpenes were released from the two windrows in summer, indicating the degradation of green waste (Font et al., 2011). At the surface of the 5:1 windrow in winter, significant amount of aromatics was detected. Sulfur and oxygenated compounds were common products of organic waste decomposition processes both under aerobic and anaerobic conditions (Smet et al., 1999), while aromatic compounds were probably originated from the sludge. The average temperature inside the 5:1 windrow was only around 20 °C, indicating that this windrow was still in the mesophilic stage; and most organic components contained in the raw sludge remained undecomposed, leading to the accumulation of high odorous gas concentrations.
Concentration (µg/m3)
Summer
Winter
4000
Halogenated compounds Alkanes Alkenes Terpenes Aromatics Oxygenated compounds Sulfur compounds
3000
2000
Windrow A (3:1)
Windrow B (4:1)
Windrow B (5:1)
Windrow A (3:1)
1000
4
3
SB
2
SB
1
SB
4
SB
3
SA
SA
2 SA
1 SA
4a
3a
WB
2a
WB
1a
WB
4a
WB
3a
WA
2a
WA
WA
WA
1a
0
Figure 1. Odorous gas emission during the turning process of different co-composting windrows.
Highest odorous gas concentrations at the surface of windrows with a material mixing ratio of 3:1 were found at 0.5h after the turning operation, as is shown in Figure 1, while for the windrows with higher material mixing ratios, the concentrations peaked at 4h after turning. The difference could be attributed to the higher density of 4:1 and 5:1 windrows: the oxygen content inside the windrows increased dramatically after the turning operation, which accelerated the microorganisms’ activities and large amount of odorous gases were produced (Gutiérrez et al., 2017). However, their convection inside the windrows and discharge from the surface into ambient air were significantly limited due to the smaller porosity compared with the 3:1 windrows. Hence, the appearance of odorous gas concentration peaks was also delayed (Shen
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
et al., 2012). 3.2 Surface and internal odorous gas concentrations Surface and internal odorous gas concentrations of windrows in winter were compared to investigate the pattern of odor production and accumulation related with material mixing ratio. It was found that the internal odorous gas concentrations were much higher than at the surface of the windrows before turning (Table 4 and Table 5), while at 0,5h after turning, the surface concentrations exceeded the internal concentrations. As time went by, difference could be seen in the windrows with different material mixing ratios. In windrow A with a material mixing ratio of 3:1, the concentrations at the surface and internal of the windrows were nearly the same, indicating a good ventilation status inside the windrow; while for windrow B, a transformation in the emission pattern could be observed. From 0,5h to 4h after turning, the odorous gas emission was adverse dispersion from surface to internal until they reached balance, and high concentrations were detected due to the release of accumulated odorous gases inside the windrow. Then odorous gases started to accumulate again, and the internal concentration exceeded the surface concentration at 8h after turning. The release of odorous gas could be influenced by their inherited concentrations, production, degradation inside the windrows, adsorption, dissolution and retention in free air space (He et al., 2010). During the turning process, the porosity and oxygen content inside the windrows were changed in different levels, which could promote odorous gas production and their retention time inside windrows. Hence, the emission of odorous gas from the surface of the windrows and their accumulation inside should be taken into consideration simultaneously when assessing possible emission amount of odorous gases during the turning process. Table 4. Surface and internal odorous gas concentrations of Windrow A (3:1) in winter Compound Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes Alkanes Halogenated compounds Total concentration
WA1a 83,92 95,92 29,62 20,8 5,92 30,88 42,98 310,05
WA2a 216 108,92 42,9 41,7 9,125 34,4 41,5 494,55
Concentration (µg/m3) WA3a WA4a WA1b WA2b 117,78 66,4 301,08 104,35 101,32 93,32 6,92 90,22 23,55 28,35 35,55 40,15 24,35 19,1 54,02 24,7 10,2 8,85 17 5,15 41,82 43,65 37,22 33,75 38,98 36,12 92,7 35,2 358 295,8 544,5 333,52
WA3b 50,52 144,15 36,42 29 3,125 43,92 90,78 397,92
WA4b 61,35 97,28 30,65 19,7 6,4 33,7 41,42 290,5
Table 5. Surface and internal odorous gas concentrations of Windrow B (5:1) in winter Compound Sulfur compounds Oxygenated compounds Aromatics Terpenes Alkenes
WB1a 51,08 108,28 27,58 19,62 5,88
WB2a 85,85 96,42 386,2 26,1 6,3
Concentration (µg/m3) WB3a WB4a WB1b WB2b 143,4 85,3 62,78 79,78 120,08 85,9 109,52 83,92 396,9 159,02 686,65 363,3 93,15 23,62 25,2 9,85 9,38 8,3 14,4 5,42
WB3b 137,48 125,4 392,05 23,95 7
WB4b 71,4 115,45 173,58 34,1 12
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Alkanes Halogenated compounds Total concentration
41,75 54,3 308,48
43,62 31,65 676,15
45,62 60,7 869,22
50,12 37,25 449,52
65,55 29,3 993,4
46,5 30,75 619,52
56,42 45,15 787,45
53,08 94,38 553,98
3.3 Significant odorous compounds 3.3.1 High concentration odorous compounds The most abundant compounds differed in winter and summer, though no apparent difference was observed between windrows of different mixing ratios. Dimethyl disulfide, methylene chloride, pentane, ethanol, diethyl sulfide, α-pinene and dimethyl sulfide were found to be the most concentrated compounds in winter; while in summer, acetone, dimethyl sulfide, α-pinene, β-pinene, acetaldehyde, ethanol and dimethyl disulfide were the most abundant compounds. Zhu et al. ( 2016) designated ammonia, hydrogen sulfide, carbon disulfide, dimethyl disulfide, methyl mercaptan, dimethylbenzene, phenylpropane, and isopentane as concentration indicators in a static forced-aeration sewage sludge composting plant. Compared with their results, high concentrations of terpenes and oxygenated compounds appeared in this study, probably due to the use of green waste as bulking agents. 3.3.2 Odor indicators Theoretical odor concentration (Cod) of each sample was calculated based on ratio of the chemical concentration of each compound to its odor threshold (i.e., dilution multiple) (Capelli et al., 2008), as shown in Figure 3. It was obvious that sulfur compounds were the dominant odor contributors in all samples; besides, oxygenated compounds also appeared to be significant odorous compounds in summer due to their high concentrations.
Others Oxygenated compounds Sulfur compounds
1000
Cod (dimensionless)
800
Summer
Winter 600
400
200
Figure 2. Theoretical odor concentrations of the surface samples.
SB 4
SB 3
SB 2
SB 1
SA 4
SA 3
SA 2
4a SA 1
3a
WB
2a
WB
1a
WB
4a
WB
3a
WA
2a
WA
WA
WA
1a
0
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Odor indicators were also selected based on the dilution multiples of each compound contained in all the samples. Odorous compounds were ranked top down according to the values of their dilution multiples for each sample, and major odor contributors with dilution multiple > 1 were recognized. Those compounds from each sample were then put together to calculate the frequencies of their appearance in all the samples, and compounds with a frequency > 80% were selected as odor indicators. Results showed that major odor contributors in all the samples turned out to be quite similar despite season and material mixing ratio, and methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as odor indicators for the odor emission during the turning process of the co-composting windrows. Compared with the results reported by Van Durme et al. (1992) and Zhu et al. ( 2016), organic sulfur compounds were the commonly recognized significant compounds in all kinds of composting facilities, which should primary attention in odor control activities.
4. CONCLUSIONS This paper studied the odorous gas emission from sewage sludge and green waste cocomposting windrows during the turning processes. It was found that higher odorous gas emissions were detected in summer, probably because of the higher temperatures in summer. High concentrations of sulfur compounds and oxygenated compounds appeared in all samples, though the oxygenated compounds predominated the emissions in summer. The concentrations of terpenes were much higher in summer than in winter, and high concentrations of aromatics were detected from the 5:1 windrow in winter. Emission peaks were observed at 0,5h after turning for the 3:1 windrows, while for the rest two windrows with higher sewage sludge contents, the peaks appeared at 4h after turning. Sulfur compounds were the major odorous compounds for all windrows in both summer and winter. Oxygenated compounds also contributed to the higher odor pollution in summer due to their high concentrations. Methyl mercaptan, dimethyl sulfide, dimethyl disulfide, diethyl sulfide, acetaldehyde and ethyl acetate were selected as the odor indicators.
AKNOWLEDGEMENTS The project was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2011ZX07301-003). The authors also wish to acknowledge with thanks the funding from China Scholarship Council and Prof. Peter Kjeldsen in participating the Symposium.
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