## 3 Results  
### Physicochemical Properties of Compost  
The course of temperature during the composting of biowaste at the investigated sites is shown in Figure 1. There are some differences in the process itself regarding this indicator. They are related to the slower temperature rise in the Yasno Pole compost (1 A) and the faster one in Harmanli (1 B). Thus, while the initial mesophilic phase is about 20 days in the first case, it is only three days in the second. At the same time, exposure to high temperatures is associated with the degradation of smaller-molecule organic substances. It is favorable for the destruction of available pathogenic microorganisms that would eventually develop [36]. How composting should be carried out is regulated in Bulgaria by the ordinance on the separate collection of biowaste and treatment of biodegradable waste from 2017 [37]. In accordance with this and to ensure decontamination of the composted biowaste, the temperature in the outdoor piles must be maintained for at least 10 days above 55 \({}^{\circ}\)C or at least 3 days above 65 \({}^{\circ}\)C. In the Harmanli trial, temperatures were above 55 \({}^{\circ}\)C for 40 days, with 8 days above 65 \({}^{\circ}\)C. In Yasno Pole, they were above 55 \({}^{\circ}\)C twice for 27 days, and above 65 \({}^{\circ}\)C for 14 days.  
The rest of the physicochemical parameters of compost during thermophilic and mesophilic stages at both sites are shown in Table 1. Differences exist in the parameters between the thermophilic and mesophilic phases at both sites. First, the TOC concentration is about 28% in both thermophilic samples, with a slight advantage of the one from Yasno Pole, while the mesophilic ones are slightly above 22%. As the composting process progressed, a loss of N was found at both sites, with that at Harmanli being higher. This also led to a different rate of decrease in the C/N ratio during the mesophilic phase. Higher values of pH and EC were found at the site in Harmanli and in the mesophilic stage, which is natural because as the composting process progresses, maturing of the compost is observed [38]. A decrease in ammonium nitrogen and an increase in nitrate concentration were observed, which is a regular occurrence in proper composting due to microorganisms' depletion of available ammonium ions. Microbial activity expressed as respiration is also in this direction. A significant reduction was found in composting at Yasno pole, where the reduction in respiration intensity from thermophilic to mesophilic phases was 7.7-fold, while in Harmanli, the reduction was 2-fold. In our opinion, this difference in the reduction of microbiome activity, expressed as respiration intensity, is related to the still incomplete maturation process in the compost from the Harmanli site. Not only the different location  
Figure 1: Temperature changes during the composting process at Harmanli (**A**) and at Yasno pole (**B**) sites. In both figures, the sampling times are indicated—thermophilic and mesophilic.  
of the site (the specific microclimatic conditions), but also the composting technology described in Section 2.1, in particular the way of supplying oxygen and mixing the waste during the composting itself, has an influence here.