# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 1 Introduction  
Municipal waste management plants are an inseparable element of municipal waste management systems in cities and rural areas, and at the same time, they constitute a potential source of emission of substances causing olfactory nuisance--odorants [1]. It is the emission of odorants that can cause the negative image of municipal waste treatment plants and raises social concerns regarding the well-being of employees operating the plants and residents of nearby areas [2, 3, 4]. For designing and support of minimization plans for odour nuisance, odour emissions are monitored. An important element in implementing appropriate odour and air pollution reduction strategies is the characterization of the compounds that make up the odour mixture [5]. Samples of the test gases can be analyzed by means of analytical methods (e.g., gas chromatography--GC) or sensor methods (e.g., electronic noses or detectors) [6, 7]. The electronic nose allows for monitoring and continuous odour control and reduces the need for multiple sampling. The operation of the electronic nose is based on the use of a sensor matrix that transforms the chemical information provided in the signal. The task of an e-nose is to record and characterize the mixture of volatile substances depending on the concentration of the individual components of the odour. The operation of the devicerelates to the introduction of complex mathematical equations and statistical algorithms responsible for correct interpretation of the results. Detectors, on the other hand, indicate those compounds that result from the sensors. They are installed in the device and are calibrated. They do not require the direction of algorithms, which makes them easier to operate than e-noses. Detectors, depending on the type of sensor, are based on different types of chemical processes [8,9]. In order to implement measures to reduce odour nuisance, the participation and potential of the compounds (or chemical family) in the formation of the odour mixture are also important. For this purpose, an odour activity value (OAV) can be defined, which is the ratio of the chemical concentration of a single compound to its odour threshold (OT). OAVs have been widely employed for odorants monitoring in gas emission from waste biological stabilization [10]. An important element in implementing appropriate odour and air pollution reduction strategies is also the odour nuisance of gas mixture, which can be determined by the use of sensory methods, including dynamic and field olfactory [11]. Chemical methods involve the determination of the quantitative composition of compounds comprising odour. The methods are less costly than olfactory, and their efficiency is not dependent on human response. Odour research should not be based exclusively on the identification of odour gas mixture components. It should be one of the elements of the analysis [4,12]. Malodorous substances in the mixture can mutually strengthen or weaken the intensity and change the hedonic odour quality. Due to this, in the assessment of the odour impact of municipal waste biogas plants, as well as other objects causing odour nuisance, the determination of the odour and odorant concentration is very helpful [13,14,15]. Concentrations of individual chemicals, odorants, can be weighted using their corresponding odour thresholds (OTs). Thanks to this procedure it is possible to identify substances whose concentrations exceed the limits of odour perception. The OT parameter is defined as the chemical concentration of compounds at which 50% of the test population distinguishes the presented gas sample from the odourless gas [16]. The relationship between the concentration of a single odorant and its OT is determined by the odour activity value (OAV) parameter, which is a factor used as an odour surrogate to identify the odorous potential of each substance contained in a gas sample [10,17].