Emission characteristics of volatile matter and identification of odor substances during aerobic fermentation of cattle manure

Abstract: In the aerobic fermentation process, a large amount of Volatile Organic Compounds (VOCs) can be produced. They are harmful to human health, while cause environmental pollution, due mainly to the VOCs with the complex component can be important source of odor pollutants. The VOCs vary in a large difference, with various raw materials for aerobic fermentation. So far, there is no report on the emission characteristics of volatile substances that produced by aerobic fermentation of cattle manure. In this study, an aerobic fermentation test was carried out with a period of 30 days, in order to explore the production and emission characteristics of VOCs during aerobic fermentation process of cattle manure. An intermittent forced ventilation way was selected, where the ventilation volume was 10 L/min. The materials were mixed evenly, according to the fresh mass ratio of 18:6, and the mixture was put into the 60 L fermentation device. The C/N ratio of materials was 26:1, and the moisture content was about 62%. In addition, a gas chromatogram-mass spectrometry was used to analyze the components and concentrations of VOCs in different fermentation stages, such as mesophilic, thermophilic, cooling and maturity stage. Boric acid solution absorption and hydrochloric acid titration were used to determine the concentration of ammonia gas. A portable detector was selected to measure the hydrogen sulfide, whereas, three-point comparative odor bag method was used to determine the total odor concentration of different fermentation stages. The results showed that 31 kinds of VOCs were detected during the aerobic fermentation of cattle manure, including carbonyl sulfide, dimethyl sulfide, carbon disulfide, dimethyl disulfide, ethanol, ethyl acetate, acetone, tetrachloroethylene, chloroform, 1,2-dichloroethane, dichloromethane, benzene, toluene, ethylbenzene, m-xylene, p-xylene, o-xylene, m-ethyltoluene, p-ethyl toluene, 1,2,4-trimethylene, heptane, cyclohexane, hexane, 2-methylbutane, 2-Methylpentane, pentane, octane, methylcyclopentane, limonene, 2-methyl-1, 3-butadiene, a-pinene. The above 31 kinds of VOCs were divided into sulfur compounds, alcohol, ester, ketone, halogenated hydrocarbons, benzenes, alkanes, and alkenes, which were 4, 1, 1, 1, 4, 9, 8, and 3, respectively. In the high temperature period of aerobic fermentation, the emission of odor concentration was the maximum of 724, indicating the main period of generation and emission of VOCs. According to the emission standard of odor pollution and Odor Activity Value (OAV), ammonia, hydrogen sulfide, and dimethyl sulfide were included and their OAV values were relatively high. In correlation analysis, the concentrations of ammonia and odor were showed highly significant positive correlation, while, hydrogen sulfide and dimethyl sulfide had no significant correlation with odor concentration, but the odor threshold of hydrogen sulfide and dimethyl sulfide was low, while, the OAV value was large. Sulfur compounds were normally important components of odor gas, suggested that ammonia, hydrogen sulfide, and dimethyl sulfide can be listed as the primary odorants in aerobic fermentation of cattle manure. In GS-MS spectrum and correlation analysis, the detection rates of aromatic compounds, such as ethylbenzene, m-xylene, p-xylene, and o-xylene, were about 100%, and positively correlated with odor concentration (P<0.05). Therefore, these compounds should be focused on monitoring and control. The finding can provide a sound theoretical support for the reduction and control of odorous substances in the aerobic fermentation of cattle manure.