# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 1 Introduction
This relationship is used to monitor the emission of odorous compounds, among others, during aerobic stabilization of digestate and composing of waste [18,19,10,11,12,13,14,15,16,17,18,19,20,21]. OAV is a dimensionless quantity used as a proxy parameter for odour concentration and is applied to identify the odour potential of individual substances contained in an analyzed gas sample [23]. [PERSON] et al. [24] claim that substances with the highest OAV contribute to odour nuisance the most, which is logically explainable. A problem arising from the use of the OAV parameter is the large discrepancy in the literature data in relation to OT of the individual odorants, as well as the failure to consider the interactions between the different substances in the test sample. It can produce different effects on the quality of odour perception [24,25]. However, the OAV helps in determining the major odour contributors in the biological stabilization of waste and in the food industry [10,26,27]. Sensor-based methods for odorant measurement, which have been frequently used in portable gas detectors in recent years, are characterized primarily by uncomplicated operation, relatively low investment, and implementation costs, as well as handiness [28,29,30]. In addition to the advantages indicated, gas detectors are characterized by low sensitivity, which can be affected by factors such as humidity and the presence of other substances [2,12]. The sensors used in the detectors differ from each other, among other things, in construction and principle of operation. There are, among others, electrochemical (amperometric) and photoionization (PID) sensors. In electrochemical sensors (EC), analyte particles diffuse through a membrane and the inner electrolyte towards the surface of the working electrode suitably polarised concerning the reference electrode. The signal obtained from the electrochemical redox reaction is proportional to the concentration of the analyte near the sensor [31,32,33]. The principle of PID sensor operation is based on the phenomenon of photoinization. A high-energy light source is needed to perform the photoionization process. The energy of light, usually with a wavelength shorter than 143 nm, determines the detectable chemical compounds. If the ionization potential of a sample molecule is lower than the ionization energy of the photons, the molecule becomes ionized and generates a current signal proportional to the concentration of ionized compounds [34]. Another example of the use of the detection method is the electronic nose, which is characterized by more complicated operation compared to detectors. The electronic nose consists of a sensor array simulating the receptors of the human olfactory system, a data processing unit, and a pattern recognition system that recognizes the odour patterns of the test substance, a function performed by the brain in the human olfactory system [35]. The characteristic compounds of the decomposition processes of organic matter are primarily volatile organic compounds that include methanol, methylamine, dimethylamine, methyl ethyl ketone, styrene, toluene, methanethiol, ethanethiol, dimethyl sulphide, and acetone, as well as inorganic compounds such as ammonia or hydrogen sulphide [36; 37]. Volatile organic compound (VOC) is a term used for a wide group of organic compounds which have a vapour pressure of at least 0.01 kPa at 20 \({}^{\circ}\)C [38]. VOCs are also characterized by low solubility in water. VOCs are involved in photochemical reactions in the atmosphere that produce photochemical oxidants. This work aimed to analyze the suitability of a portable gas detector as a device that can be a tool for monitoring odorant emission and technological processes. This is particularly important from the viewpoint of taking care of life and health quality, and the well-being of residents. The study also analyzed the correlation between the concentrations of individual compounds emitted in different plants for the mechanical--biological treatment of municipal waste.