**3. Results and Discussion**

#### *3.1. Waste Properties*

As shown by other authors, the composition of the waste used for composting and its mixing has a great impact on the chemical and microbiological changes during the process [23,24]. In all OFMSW reactor piles, moisture contents (36–44%; Figure 1), were below the optimal range of 50–60%, according to Liang et al. (2003), but removal of moisture was more than two-times higher in piles A1 and A2, with a lower initial mass and a high rate of airflow > 12,000 m3·Mg−<sup>1</sup> (Figure 1) [25]. In reactors B1, B2, C1, and C2, the reduced airflow (Figure 1), and the relatively comparable ambient temperature (Figure 3) greatly reduced the loss of moisture in the stabilized material (23–31% of initial value compared to 61–71% in piles A1/A2). Greater removal of moisture in a smaller composting pile was also obtained by Ermolaev et al. (2012) [26]. However, this was much higher than the small decrease (~10%) obtained by Mulbry and Ahn, (2014), using much smaller piles (volume ~1.9 m3) [27]. The effectiveness of the process of moisture removal apparently depends on the amount of air blown in relation to the total waste mass of the pile.

**Figure 3.** Ambient temperature around piles during the biostabilization process of Organic Fraction of Municipal Solid Waste (OFMSW).

Reduction in total mass of the OFMSW was greater in piles A1, A2, and B1 (34–36%), where the process time was longer (9 weeks), compared to the other piles (6 weeks). The final content of organic material (VS: 32–38% d.m.; TOC: 18–22% d.m. (Figure 1) was about half that recorded by [28] during MBT composting of municipal solid waste, and similar values were observed in Komilis et al. (2012) in a larger-scale installation (capacity 250,000 Mg·year−1) [29]. The content of organic substances (both VS and TOC) is typical for MBT composting plants [30]. As with moisture removal, organic matter was most effectively reduced in piles A1/A2 with relatively small size and high airflow. Similarly, Hu et al. (2003) concluded that, for the decomposition of TOC, the moisture and fraction size have a greater influence than the process temperature [31].

The pH value of initial samples of OFMSW was very similar in all analyzed piles (5.4–5.6) (Supplementary Material Table S3 [22]), whereas at the end of the process the pH increased towards neutrality. These values and their change during the biostabilization process are typical for MSW [32].

The fine fraction < 20 mm comprises a large proportion (66–77%) of the OFMSW (Figure 4), with variable amounts, typical for municipal waste, of kitchen waste, paper and plastic and small proportions of textiles (0.3–2.2%), glass (3.8–5.3%), metals (0.8–2.9%), other organic (0.9–2.4%), other minerals (1.7–3.8%), and other materials (0.3–2.6%). After the process, a decrease in the content of kitchen waste and paper was observed in A1, A2, and B1, whereas in B2, C1, and C2, the content remained unchanged or increased, which suggests that the degradation process is more effective with a longer operation time.

**Figure 4.** Changes in the morphological composition of OFMSW—samples were collected during the first and last day of the AB process and represent the entire reactor.

The < 20 mm fraction in the biostabilized material was at least 65% (Figure 4), higher than values of 50%, typical of Polish conditions [33]. In other studies, the content of organic waste was 60–67%, paper 15–17%, glass 7–8%, plastic 6–7%, and metal 3–5% [34]. The efficiency of waste mass removal, ranging from about 20 to 30% (Figure 1), was close to the average values obtained in other similar installations, operating biostabilization technology for municipal solid waste in Poland [35].
