3.2.1. Spatial and Temporal Distribution of Temperature

The pattern of temperature change during the waste composting process was expected to follow the "classic" pattern described by Cooperband, (2002) [36] and Kowal et al. (2017) [37], with an initial rapid increase in temperature (1st phase of composting) followed by a gradual drop in temperature during maturation/cooling (2nd phase of composting). In contrast, temperatures in piles A1 and A2 (Supplementary Material Figures S1–S3, S10 and S11) remained close to ambient (Figure 3) throughout the entire prism profile (~20 ◦C) during the first two weeks of the process. This lag phase resulted from the high aeration rate (>12,000 m3·Mg<sup>−</sup>1) and the small mass weight in the reactors (300–400 Mg: ~40% less than normal for this treatment plant). From day 20, in both piles, internal temperatures increased to ~60–70 ◦C (Supplementary Material Figures S4, S5 and S12), three hot spots where found (high temperature coupled with low O2: Table 5), and further small increases were observed from weeks 5 to 7. Similar temperature patterns were observed in a reactor without forced aeration by Jiang et al. (2015) [38], during composting of green waste [39], kitchen waste [40], and vermicomposting of duck manure [41]. However, Mulbry and Ahn (2014) showed that, in much larger scale piles, passive aeration allows them to heat up as quickly as during forced aeration [27].

In the other piles, an increase to 60 ◦C was observed at the beginning of the 2nd week. This high temperature was maintained in the piles until the end of the process ~day 50 for piles A1, A2, and B1 (Supplementary Material Figures S8, S16 and S23), ~day 35–40 for B2 and C1 (Supplementary Material Figures S29 and S35), and ~day 30 for C2 (Supplementary Material Figure S39). Temperatures in piles B1, B2, C1, and C2 followed the expected 2-phase pattern.

An interesting trend was observed in pile A1—the highest temperatures, even >70 ◦C, were found at the end furthest from the fan (Supplementary material section 47.5 m Figures S4d and S5c,d) from where the heat spread towards the front of the pile. This could be due to a poorer air supply to the far end of the pile, resulting in self-heating and poor heat removal. In pile A2 this did not occur (Supplementary Material Figures S13–S16), despite a similar air load and a mass of waste greater by about 15% greater. In addition, lower aeration values were noted by about 1.5 m3·Mg−1·h−<sup>1</sup> in the first 3 weeks of the research (Supplementary Material Table S1 Stegenta-D ˛abrowska et al. (2020)) [22]. The opposite effect was noticed in pile B2 (Supplementary Material Figure S30a) which showed greater cooling close to the fan on the final day. This may be the result of faster decomposition of waste located near the fan, which at the end of the process significantly reduced endothermic processes due to the lower activity of microorganisms.

At the same time a few cold spots were observed (Table 6), located mainly at the bottom of pile, due to the forced aeration. The cooling effect of aeration by the three channels below each pile is clear in the visualization (Supplementary Material Figures S13a, S47b and S134b). Typically, this occurs in both the first week of biostabilization when internal heat takes longer to accumulate, and in the final week when the need for O2 is lower.

The observation that, in piles A1 and A2 high temperatures > 50 ◦C remained during the final days of the process despite the continuous high air stream (Supplementary Material Figures S9 and S17) indicates high bio-activity of the stabilized waste and consequent high O2 demand. However, it could result from the unexpected initial two week lag phase in heating the pile.

The highest observed temperatures occurred in pile B1 (from day 16: >70 ◦C inside the pile, >60 ◦C at the edge), which contained the highest TOC and organic substances (VS) of all analyzed samples, as well as having twice the mass of piles A1 and A2, but half the rate of aeration.

In piles C1 and C2 the sidewalls apparently contributed to an overall reduction of temperature (Supplementary material Figures S35–S38, S41 and 42). The cooling of the waste at the border with the walls is visible, especially at the beginning and at the end of the process—a few cold spots were identified near to the sidewalls and at the bottom (Table 6). Despite favorable conditions (large mass of waste; less aeration), no elevation of temperature was noticed.

Experiments carried out under similar conditions by Ermolaev et al. (2012) showed that, despite continuous operation, fans may be unable to maintain an appropriate temperature [26]. Our observations of cold spots (low temperatures) at the base of the pile (e.g., Figure 5; Supplementary Material Figure S30 and Table 6) indicates the impact of pumping large amounts of air, resulting in removal of warm air from the center and overall cooling of the pile. Similar effects of lowering temperature by increased aeration were observed by Shen et al. (2011) and Sołowiej et al. (2010) [42,43]. In bigger piles, B1–2 and C1–2, the optimal temperature for the biostabilization process, around 60 ◦C, was reached much faster, an effect also noted by [44]. Although heat contributes to elimination of pathogenic organisms (Stentiford, (1996) it does not ensure maximum mass removal [45]. According to many authors, greater mass reduction can be achieved at temperatures between 40 and 60 ◦C [45–47]. On the other hand, Richard, (1993) claims that maintaining the temperature in the range of 56–70 ◦C for too long (over a week) reduces biodiversity and increases the intensity of odor compounds [47].

**Figure 5.** Spatial distribution of temperature changes on day 1 in pile B1, at distances from the aeration fan (**a**) 2.5 m, (**b**) 17.5 m, (**c**) 31.5 m, (**d**) 47.5 m, longitudinal sections (**e**) left (**f**) right. Illustration of the problem with irregular temperature in pile.
