Analysis of odor pollutants in the exhaust gas from thermophilic aerobic fermentation of dead pigs

Abstract: Thermophilic aerobic fermentation (composting) has been emerged as an important approach to dispose of the organic solid wastes. The dead pigs can also be recycled and utilized properly using this biotechnology for resource treatment. However, a large amount of odor gas can be always generated concomitantly in this process, particularly posing a great threat to both air environment and human health as a kind of air contamination. There were some differences in the component and concentration from the odor pollutants produced by thermophilic aerobic fermentation, due to various participating organic wastes. The manure and waste have still been the two most focused raw materials about odor pollutants so far. But there is a rare reference with dead pigs. The purpose of this work was to explore the types and the emitting regularity of odor gas produced from thermophilic aerobic fermentation for dead pigs, in order to control the concentration of odor pollutants for less air contamination during nonhazardous dead pig processing. A three-day experiment was carried out, where 1.2 kg chopped body tissue sample of the dead pig was taken as the raw material, while 0.218 kg crushed maize stovers and 12 g strains were taken as auxiliary materials. The raw material, auxiliary materials, and strains were then mixed well and put together into a 10 L fermenter equipped with a stirring device, further to start fermenting with such an experimental condition that the temperature of interlayer water bath during fermentation was kept at 60℃ and the ventilation rate was retained to 8L/(L·min). After that, 14 times of gas sampling were conducted from the tail gas tank, where three samples were collected each time, and 500 ml of fermentation gas was collected for each sample acting. As such, the primary odorants and the release regularity of odor pollutants were determined in the process of thermophilic aerobic fermentation during a 72-hour fermentation period. Specifically, the sampling interval was set as 3 hours during the first 6 hours, but it changed to 6 hours from the 6th to the 72ed hour. The organic odor components and the concentration of sampling gas were analyzed to determine the concentration of ammonia. Besides, the correlation analysis and principal component analysis were performed on those odor pollutants with the odor activity greater than 1 during fermentation. The results showed that 36 odor pollutants in total were detected during the thermophilic aerobic fermentation of dead pigs, 19 of which were accurately detected qualitatively and quantitatively-including 3 sulfur compounds (methanethiol, dimethyl sulfide, and dimethyl disulfide), 1 alkane compound (n-hexane), 12 aromatic hydrocarbon (benzene, toluene, ethyl benzene, p-xylene, o-xylene, 2-ethyltoluene, 3-ethyltoluene, 4-ethyltoluene, 1,2 ,4-trimethylbenzene, 1,3,5-trimethylbenzene, cumene, and styrene), 1 phenolic compound (p-cresol), 1 amine compound (trimethylamine), and 1 inorganic compound (ammonia). According to the standard and odor activity of odor pollutants, 3-ethyltoluene, 4-ethyltoluene, dimethyl disulfide, dimethyl sulfide, ammonia, p-cresol, methanethiol, and trimethylamine were the main odor pollutants in the exhaust gas of thermophilic aerobic fermentation of dead pigs. The highest concentrations of these eight main odor pollutants reached up to 0.241, 0.350, 0.247, 0.280, 69.063, 0.041, 0.314 and 0.033 mg/m3, respectively, the ratio of which to olfactory thresholds were 2.746, 8.635, 29.326, 36.982, 66.669, 173.315, 374.77 and 432.471, respectively. The OAV of the main odor pollutants also changed with the changing period of fermentation. The correlation analysis demonstrated that the components of the primary odorants were different in the different fermentation periods: trimethylamine, methanethiol, dimethyl sulfide, dimethyl disulfide, ammonia, p-cresol, 3-ethyl toluene, and 4-ethyl toluene were the primary odorants in the fermentation periods of 0-12 h, trimethylamine, methanethiol, dimethyl sulfide, dimethyl disulfide, ammonia, and p-cresol were the primary odorants in the fermentation periods of 12-36 h, while the trimethylamine, methanethiol, dimethyl sulfide, dimethyl disulfide, and ammonia were the primary odorants in the fermentation periods of 36-72 h. Meanwhile, the odor intensity changed a lot with the different stages of fermentation: it increased slowly in the stage of the first 3 hours, but there was a significant drop when it reached the point of the 6th hour, then went up again from the 6th to 18th hour, up to the peak in the 18th hour, and turned stable to the end. This work can provide a strong reference to reduce and control the odor pollutants during the thermophilic aerobic fermentation of dead pigs.