# The Use of Chemical Sensors to Monitor Odour Emissions at Municipal Waste Biogas Plants
## 2 Materials and Methods  
### Study Methodology  
The research involved the application of a multi-gas detector, MultiRae Pro. The device is equipped with four sensors for gas concentration detection, i.e., ammonia (EC sensor, range 0-100 ppm, resolution 1 ppm, response time (RS) 60 s), hydrogen sulphide (EC sensor, resolution 0-100 ppm, range 0.1 ppm, RS 35 s), methanethiol (EC sensor, range 0-10 ppm, resolution 0.1 ppm, RS \(<\) 35 s), and volatile organic compounds (PID sensor, range 0-2000 ppm, resolution 0.01 ppm, RS 15 s). The chemical compounds are typical for the anaerobic decomposition of organic waste [39; 40]. The operation ranges of the devices and the threshold levels for odour identification for selected chemical compounds are presented in Table 1. For VOCs, toluene was taken as the base compound, which is consistent with [41]. In chromatographic studies aimed at quantitative evaluation of VOCs, the results of which have not yet been published, it has been confirmed that toluene was the compound found at all indicated measuring points in the largest quantity. All sensors were regularly calibrated by a specialist service company. In the case of the VOC sensor, calibration was carried out using a reference gas in the form of 0.01 ppm isobutylene. For the other sensors, the following reference gases were used: 50 ppm NH\({}_{3}\), 10 ppm H\({}_{2}\)S, and 5 ppm CH\({}_{3}\)SH. The detector cooperates with Pro Rae Studio software (Rae System), permitting the reading of results recorded every minute. The measurements by means of the device were conducted at three municipal waste biogas plants located in Poland: in Biala Podlaska (5 July 2019), in Promnik (11 July 2019), and in Wolka Rokicka (18 July 2019). These are the installations that constitute an important element of the municipal solid waste (MSW) management system in the service areas. Biodegradable fractions from MSW--collected selectively or mechanically separated from the mixed waste stream--are processed under anaerobic conditions. Additionally, these installations act as technological lines for sorting out secondary raw materials and fuel from waste. The research was conducted at selected measurement points (1-8) identified as odour nuisance points and located in such places as waste storage, mechanical treatment, fermentation preparation, oxygen stabilization, technological wastewater pumping station, biofiliter with the open surface (in Biaa Podlaska and Wolka Rokicka), and digestate dewatering (in Promnik). Table 2 shows measurement points at analyzed waste treatment plants. The measurement points are marked in Figures 1-3. For points 2, 3, 5, 6, and 8 (biofilter) a static shield with a measuring stub was used for the determination of the chemical compounds--to minimize outside conditions. In addition, the odour concentration (\(\mathrm{c_{od}}\)) was determined at each point using the Nasal Ranger(r) field olfactometer according to the methodology given in [21]. This is not the standardized method. The odour concentration parameter was calculated as the geometric mean of the two measurements analogically to the European Standard EN 13725:2003 (Air Quality--Determination of Odour Concentration by Dynamic Olfactometry) [42]. The mentioned norm is dedicated to dynamic olfactometry in which the \(\mathrm{c_{od}}\) parameter is determined in an olfactometric laboratory after taking a gas sample in a bag. Unlike standard olfactometry, field olfactometry allows for measurements in situ, taking more gas samples and there is no risk of chemical reactions occurring in the gas mixture. At each measuring point, microclimate parameters, inside and outside air temperature (\(\mathrm{T_{i}}\) and \(\mathrm{T_{o}}\)) and relative humidity of inside and outside air (\(\mathrm{RH_{i}}\) and \(\mathrm{RH_{o}}\)) were simultaneously controlled using the Rotronic HydroPalm psychrometer with HygroClip2 HC2-S3 sensor. Measurements were conducted at a height of 1.5 m.  
The results of the studies were analyzed in correlation with STATISTICA 13.1 (StatSoft).