Research advances in characteristics and reduction technology of VOCs from swine production

Volatile organic compounds (VOCs) as the components of atmospheric pollutants have been potentially harmful to the environment and human health. Swine production, one of the pillar livestock industries, is severely hampered by some air pollution issues, such as the emissions of odor and VOCs. The odor complaints were reported the most, accounting for 12.7% of the whole Chinese livestock industry in 2020, according to the Study "Analysis of National Odor Pollution Complaints in 2018-2020" from the National Ministry of Ecology and Environment. Odor pollution is considered as one of the major environment issues in the large-scale swine production, hindering the sustainable development of the industry. Therefore, it is critical to implement the odor pollution prevention and control measures in the large-scale swine production industry. However, the VOCs, components of the odorous pollutants, vary greatly in the organic molecules in the frequent present of the air, leading to difficulty in mitigation of odor. Generally, the VOCs are the significant precursors of ozone (O₃) and secondary aerosols (SOA). Once the concentration of VOCs exceeds a certain threshold value, a significant negative impact can be imposed on the environment, as well as human and animal health. Therefore, mitigation measures are essential to reduce the VOC emissions, and their negative impact on the environment and public health. In this review, VOCs emitted from the swine production was introduced from the aspects of characteristics and reduction potential. The types, characteristics, and sources of VOCs from pig farms were elucidated, together with the properties of odor-causing VOCs and detection. The factors that could impact the VOCs concentration were also illustrated, such as ventilation in barn, pig breed and manure cleaning. In addition, different testing methods and standards were selected for the variations in the composition of VOCs. The most common VOCs in swine production were found as the volatile fatty acids (VFA), olefins, alkanes, aromatic hydrocarbons, halogenated hydrocarbons, esters, ethers, phenols, indoles, organic sulfur, aldehydes, and ketones. Among them, the VFA, phenol, indole, and organosulfur were the most common odor-causing substances. The sources of VOCs were then analyzed for the efficient odor management. Manure, indoor air, and dust were the three main sources of VOCs in pig barns, which are considered as the primary risk of the environment and human health. The odor annoyance was found to be directly caused in the neighborhood, leading to the formation of ground-level ozone and secondary aerosols when exposed to the air. The detection methods of VOCs generally consisted of the discrete and continuous approaches. The typical VOCs measurement steps of discrete approaches were the sample collection, pre-treatment, and analysis. The commonly used continuous detection instrument was the proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS). Finally, the primary VOCs treatment technologies were evaluated from three aspects of VOCs reduction in pig farms: source reduction, process control, and end treatment. Source reduction of VOCs, i.e., adjustments in the feed rations and the additives could improve the physiology and production performance of pigs. Process management of the VOCs reduction was achieved after adjustments in feeding and ventilation systems. The better manure cleaning practices were applied to limit the off-gassing from swine production. End treatment mainly included the VOCs treatment technologies of manure and ambient air treatment. Traditional VOCs treatment technologies were divided into two categories: recycling and degradation treatments. Photocatalytic oxidation, active material adsorption, low temperature plasma technology and degradation methods with the bio filters can be expected to gradually replace the traditional treatments and become the key technologies for the VOCs treatment. The finding can contribute to the more sustainable development in animal husbandry, in order to mitigate the negative environmental and health impacts caused by the VOC emissions.