## 4 Conclusions  
In this study, the results of pilot research conducted at six biogas plants processing municipal waste in Poland are presented. They show the relations between odour intensity and concentration and the occurring meteorological and ambient conditions and technological factors. The novelty and scientific contribution presented in this paper are related to the analyses of technological aspects and air temperature and humidity impact on odour emissions in municipal waste biogas plants. The literature review shows, that so far, such analyses were conducted mainly in the field of sewage treatment, swim housing, landfilling and animal husbandry. The impact of meteorological and ambient conditions is identified by measuring air temperature and relative humidity and observing their changes. The impact of technological factors was identified by measuring odorant concentration (volatile organic compounds and ammonia) and observing their changes between individual measurement series. The main conclusions and contributions of this work can be summarised as follows:  
1. At most analysed biogas plants, significant relations are observed between odour concentration and air temperature: higher air temperature is accompanied by bigger odour concentrations.  
These relationships are noted even at small differences in both air temperature and odour concentration.
2. The relationships between relative air humidity and odour concentration (bigger odour concentrations at lower air humidity) are observed in the case of air humidity differences above 20%. To capture the full impact of air humidity, research should be conducted under more diverse atmospheric conditions.
3. In planning processes for municipal waste biogas facilities, great care in the choice of technological solutions, technical and organisational measures is called for to reduce the impact of unfavourable atmospheric conditions on odour emissions.
4. At most analysed biogas plants, the influence of technological factors on odour emissions takes place and is clearly noted. It is particularly distinct in the biological part of the installations, but it also takes place in the mechanical part (during waste pre-treatment).
5. The highest VOC concentrations are mainly associated with unit operations related to mechanical waste processing, such as storage, pre-treatment and digestate dewatering. High ammonia emissions are also associated with the elements of the mechanical part of the installations (waste preparation and digestate dewatering), but in addition, they may be accompanied by digestate aerobic stabilisation and the operation of deodorisation facility. Further research should indicate the dominance of some element in the technological line in this respect and show which odorant is responsible for higher odour emission.
6. The experimental results provide an important contribution to the search for more efficient methods for reducing the odour nuisance of biogas plants, which is important for residents of the surrounding areas.
7. Recommendations for further research include subsequent investigating emission of odorants characteristic for waste management, such as volatile organic compounds, ammonia or hydrogen sulphide. Further works should focus on clarifying when meteorological and ambient conditions have a greater impact on odour concentration and intensity, and when technological factors are more important and which of them have a dominant influence.  
M.W.: data curation, investigation, writing--original draft preparation; A.K.: conceptualization, methodology, supervision; K.L.-S.: writing--review and editing, visualisation, validation. All authors have read and agreed to the published version of the manuscript.  
Work was co-financed by the Dean's Grant (No. 504/03693/1110/42.000100): \"Identification and characteristics of the sources at biogas plants processing municipal waste\".  
The authors declare no conflicts of interest.