# Neutralization of Toxic Malodorous Gases from Cattle Slurry
## 1 Introduction  
Intensive livestock farming generates very large amounts of harmful chemical compounds in the form of odorous gases (odors) and sewage into the environment. Particularly burdensome for the environment are large pig, cattle, or poultry farms that produce significant amounts of natural fertilizers (manner, slurry) [1, 2, 3]. The emission of toxic malodorous gases increases primarily during the use of slurry in agricultural production [4]. Due to the fact that slurry can be a good alternative to mineral fertilizers, its increasing use means that the emitted gases have become a significant air pollutant [5]. Many malodorous gases have been produced in livestock farming including sulfur compounds (e.g., sulfides, mercaptans), nitrogen compounds (e.g., ammonia, amines), and volatile organic compounds (e.g., acids, aldehydes, ketones) [6, 7, 8]. Hydrogen sulfide is mainly formed from the decomposition of sulfur-containing amino acids (e.g., methionine, cysteine) [9]. Gaseous H\({}_{2}\)S has a relatively low odor threshold, thus it is the most odorous in the slurry. This is due to the lower volatility of high-molecular-weight compounds [10]. Olfactory perception is affected by environmental conditionssuch as temperature, air pressure, or humidity. The olfactory thresholds for ammonia and hydrogen sulfide depending on the concentration level are shown in Table 1.  
Emissions of NH\({}_{3}\) have significantly grown in both developed and developing countries [12; 13]. The main sources of anthropogenic NH\({}_{3}\) are agriculture, livestock, and animal production, e.g., manure and slurries [14]. Agriculture contributes to over 80% of all global ammonia emissions [15]. Ammonia and hydrogen sulfide are malodorous gases with strong negative effects on the health of animals, humans, and the environment. Ammonia, as a neurotoxin, can cause respiratory irritation and central nervous system problems in humans and animals [16; 17; 18; 19]. It is estimated that NH\({}_{3}\) concentrations above 300 ppm pose a direct threat to human life [20; 21]. Long-term exposure to these pollutants may be harmful not only to human health but also to the quality of the environment. It was found that ammonia has a direct effect on trees and can cause a eutrophication process [3; 22]. Methods of neutralization of malodorous gases can be divided into three categories:  
* chemical method--thermal oxidation, catalytic oxidation, ozonation,
* physical method--condensation, adsorption, absorption, and UV irradiation,
* biological method--biological filtration membrane method, and biological trickling filter tower method [23; 24]. Among the methods mentioned, the lowest cost and most environmentally friendly is microbial degradation. It is oxidative in nature and produces environmentally safe end products [25; 26; 27]. This process is based on the natural decomposition of organic compounds; therefore, no chemicals are required, and the process itself does not generate other harmful waste [24; 28]. The above-mentioned advantages of using biological methods increase the interest of farmers and become a safe alternative in the environmental aspect [29; 30]. In the biological method of odor neutralization, a very important aspect is the selection of microorganisms that are able to degrade the widest range of odors [1]. _Bacillus_ bacteria are among the microorganisms used in the biological method of odor neutralization. These are gram-positive bacteria that belong to the _Firmicutes_ phylum, which can produce a range of enzymes responsible for the degradation of a variety of chemicals [28; 31]. This bacterium stands out in the great diversity of strains and species and a high adaptation to the environment. It can occur in almost all natural environments including soil, air, water, or plants [32; 33]. Adaptation to different environments is possible because _Bacillus_ has the ability to degrade various carbon sources for energy. The significance of _Bacillus_ bacteria in the field of biotechnology is constantly increasing.