3.2.2. Spatial Distribution of O2 and CO2 Concentration

Despite the high aeration in piles A1 and A2 during the first two weeks, low O2 concentrations were recorded, together with low temperatures (Supplementary Material Figures S43, S44, S52 and S53). The high rate of airflow would have created optimal conditions for the activity of psychro- and mesophilic microorganisms at that time, leading to depletion of oxygen by respiration. However, the inhomogeneity of the OFMSW and differences in bulk density could also affect airflow through the pores creating local "hot spots". O2 levels were particularly low in most cases at the center of the pile early in the process (e.g., Supplementary Material Figures S46, S47, S54 and S62), but hot spots were discovered in pile A2 at 2.5 m length on day 25 (Table 5). In pile A1, reduced O2 concentrations (~15% lower) occurred on the left side of the pile (Supplementary Material Figures S46–S48), corresponding to the higher temperatures in the same areas (Supplementary Material Figures S4–S6). The above examples were all identified as a hot spot (Table 5). Low O2 concentrations result from the high intensity of biodegradation early in the process (Jiang et al. (2015) especially if aeration is insufficient [38]. Mohajer et al. (2010) note that the O2 consumption strongly increases in the first 4 days of composting to as much as 40 mmol·h−1·kg−<sup>1</sup> d.m., and then decreases with the duration of the process [48].

O2 concentrations increased during the process, related to decreasing O2 consumption, while supply of forced air remained stable (Supplementary Material Table S3, [22]). From day 30, O2 concentrations > 17% were observed in most piles, indicating excellent oxygenation of the waste and sufficient air forced into the pile to promote aerobic digestion.

Data obtained in this experiment shows a generally better aeration system (few locations with less than 15% O2) compared to composting in active aerated and static piles reported by Szanto et al. (2007) [49]. Although the total aeration intensity per waste mass m3·Mg−<sup>1</sup> (Supplementary Material Table S1, [22]) was below the recommended level, >10 m3·Mg−1·h−<sup>1</sup> [4], this did not appear to affect the O2 concentration observed in the piles.

Extremely low O2 concentrations < 5% were observed only at the beginning of the process in pile B1 (Supplementary Material Figure S60). Low concentrations (<10% O2) also occurred in the center of this pile up to day 40 (Figures 6 and 7; Supplementary Material Figures S60–S64) coinciding with high temperatures (Figure 5; Supplementary Material Figures S19–S23). Compared to other piles, B1 contained the greatest number of such hot spots (Table 5). This may indicate that, with an increased amount of waste and high TOC content, the air flow was insufficient during the most intense phase, up to the 4th week of the decomposition process [50].

In pile B2, lower concentrations of O2 were observed at its sides (Supplementary Material Figures S69–S72) with hot spots mainly in the center of the pile (Table 5), which may also confirm insufficient air supply where the total waste mass exceeds 600 Mg. The influence of reactor design during biological waste treatment process has been noted by Mason and Milke, (2005) [20]. Another explanation could be the structure of the waste, consisting mainly of waste fractions < 20 mm (Supplementary Material Figure S3), which could reduce the free air spaces and obstruct the air from the aeration channels. Whether aeration is passive or active, airspace within the substrate plays an important role in the composting process [51]. Air porosity influences not only air permeability, but also determines O2 transport and the removal of water and heat from the pile.

Very high concentrations of O2 > 18% were observed in C1 and C2, where sidewalls were constructed. Low concentrations of O2 < c8% (Supplementary Material Figures S84b and S85b) and high concentrations of CO2 (Figures S128b and S129b) occurred at only a few points (e.g., the cross-section of 17.5 m) indicating hot spots (Table 5). CO2 concentrations are consistent with results obtained by Clemens and Cuhls, (2003) from various types of piles composting municipal solid waste [52]. The spatial distribution of CO2 showed an inverse relationship with O2 (e.g., in pile A1, higher CO2 concentrations occurred on the left side of the pile together with high temperature and lower O2), which is typical for aerobic waste treatment [53,54]. The highest content of CO2 > 10% was observed in B1, mainly up to week 2, but levels were mostly very low (2 to 3%) and occurred in the center of the pile as single hot spots. Similarly, the highest CO2 concentrations were observed in the first phase of composting a mixture of manure and sawdust, and then its gradual reduction as the compost matures [55], and at a small scale during home composting [56].

The influence of the sidewalls in piles C1 and C2 was also noticed as an increase in the occurrence of hypoxic zones near to the border of the walls. This could have resulted from poorly located aeration channels, which were originally designed for non-compacted piles, or from the small amount of air supplied to the pile. Despite such issues of reactor design, an active aeration system is essential, since CO2 may increase to over 25% with inadequate aeration [49].


**Table 5.** Localization of hot spots during biostabilization process.

**Table 6.** Localization of cold spots during biostabilization process (excluding data from first three weeks of biostabilization in piles A1 and A2, due to low temperature in all piles).


It has been shown that the number of hot and cold spots depended on the size of the pile and aeration rate. Comparison between piles A1–2 and B1–2 shows that when the mass of stabilized waste is significantly lower and the aeration rate is two-fold higher the number of hot spots decreases, while cold spots increase. In the case of piles A1–2, the number of hot spots was 6 and 3, respectively, while in piles B1–2, 16 and 8. The opposite situation was in the case of cold spots 5–11 (A1–2) and 2–5 (B1–2) (Table 5). It shows that the application of hot and cold spot monitoring may be a useful tool for the optimization of the AB process. The elimination of hot and cold spots should be the aim, to achieve proper conditions for an efficient process, however, it requires further investigation. Additionally, our results indicated that construction with sidewalls (piles C1–2) decreased the number of hot spots to just 1 and 3, respectively (Table 4). This decrease may be due to improved airflow through the waste by eliminating air escape near the base of the pile. However, in piles B1–2, with similar waste mass and airflow rate, the channelized airflow increased the number of cold spots to 8 and 12, respectively (Table 5), indicating that differing reactor constructions also requires an optimal airflow rate to avoid inadequate aeration.

**Figure 6.** Spatial distribution of O2 changes on day 22 in pile B1, at distances from 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 low O2 concentration in center of pile.

**Figure 7.** Spatial distribution of CO2 changes on day 22 in pile B1, at distances from 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 high CO2 concentration in center of pile.
