## 4 Discussion
### Behaviour of the Composting Process  
Due to the exothermic nature of the decomposition of organic compounds, temperature change is the main indicator of microbial activity during composting [49]. Microbiological activity, in turn, is influenced by environmental conditions, particularly the bioavailability of organic compounds and the moisture of the composting matrix. Moreover, there is a feedback mechanism: an increase in the temperature to thermophilic conditions leads to hygiisation of the waste [50; 51; 52] and increased evaporation from the reactor/ prism [53; 54]. In the present study, the initial changes in the reactor were typical of composting processes: after the temperature increased relatively rapidly to thermophilic levels (64.3 \({}^{\circ}\)C), it decreased progressively, accompanied by decreases in the respiration activity (AT4) and moisture content. Interestingly, however, on the 30 th day of the composting process, three indicators displayed marked increases: temperature from 23.9 \({}^{\circ}\)C to 36 \({}^{\circ}\)C; AT4 respirometric activity from 14.28 g O\({}_{2}\cdot\)kg\({}^{-1}\) DM to 20.84 g O\({}_{2}\cdot\)kg\({}^{-1}\) DM; and moisture from 52% to 57%. It is assumed that the increase in moisture on the 30 th day of testing caused a temporary increase in microbiological activity, which, in turn, caused an increase in the process temperature. An increase in moisture is known to induce microbial activity in compost under certain environmental conditions [55; 56; 57]. Moreover, a report by [PERSON] et al. [46] indicates that composting of kitchen waste with an initial moisture content of 65% may also be accompanied by an increase in moisture content over the time. In the study presented here, water loss was the main cause of the weight loss in the reactor: out of a total weight loss of 78 kg, 60 kg was due to water loss through evaporation. A temporary increase in moisture on the 30 th day of the process suggests that the mass of metabolic water that was produced (0.5-0.6 g H\({}_{2}\)O/g VS; [58]) could have exceeded the mass of water that evaporated. Mass balance calculations, however, using Equation (2), showed that the amount of water in the reactor was continuously decreasing (Figure 2B). Therefore, it should be assumed that in the tested passive aeration system, the evaporation rate was greater than the rate of metabolic water production. Measurements of four-day respirometric activity (AT4) are commonly used to assess the biological stability of waste [59; 60; 61]. This measurement is an indicator of both microbial activity and the bioavailability of organic compounds. In the present study, the threshold for respirometric activity was set at <10 g O\({}_{2}\cdot\)kg\({}^{-1}\) DM, in accordance with generally accepted standards. In order to reach this threshold, the process lasted 45 days, during which 39% DM of organic matter and 37% DM of total organic carbon were mineralized. These results are very similar to those of other authors. For example, [PERSON] et al. [38] reported that, during the composting of kitchen waste, leaves, and structural material with an initial moisture content of 60%, an initial C/N ratio of 19.6, and an initial carbon content of 43.38%, organic matter was reduced by 32.8%. Similarly, [PERSON] et al. (2016) [62] composted food waste, horse manure, and green waste with an initial moisture content of 54% and an initial C/N ratio of 49.5 and found that the carbon content was reduced by 39%. In a recent study, green waste and food waste were co-composed in proportions similar to those in our study (60% and 40%, respectively) and with a C/N ratio between 14.28 and 20.1: thermophilic conditions were rapidly achieved, and the final product was a high-quality fertilizer [63]. The leachate that is produced during composting comes from three main sources: (1) the water content of the organic waste itself, (2) the water generated by biochemical reactions, and (3) the water added by atmospheric precipitation or adjustment of the moisture content [64].