**2. Materials and Methods**

## *2.1. Characteristics of Organic Fraction of Municipal Solid Waste*

The material used in the research was the Organic Fraction (undersize fraction < 80 mm) of Municipal Solid Waste (OFMSW) originated from Warsaw, Poland. The waste was characterized by: moisture content in accordance with Polish standard PN-EN 14346:2011, volatile solids (VS) in accordance with PN-EN 15169:2011, the Total Organic Carbon (TOC) in accordance with Polish standard PN-EN 15936:2013, pH in accordance with Polish standard PN-EN-15011-3:2001, and morphological composition in accordance with Polish

standard PN-93/Z-15006. The tests were performed each time before (6 samples) and after the biostabilization process was completed (6 samples). This gave 12 samples in total.

Total mass of OFMSW loaded into the bioreactor was also recorded before and after the biostabilization process. Each OFMSW sample was collected according to the following procedure: from 4 places along the length of the reactor, in each place, ~10 samples were taken. All collected samples, from one reactor, at the same collection moment, were mixed and using the quartering method were assigned to a representative sample. Properties of the waste before and after treatment are summarized in Figure 1. Detailed analytical results are described in Section 3.1 and in Supplementary Tables S3 and S4 in our previous paper [22].

**Figure 1.** Process configuration and initial and final waste properties. Reactors A1, A2, B1, and B2 were covered by semipermeable membrane. Reactors C1 and C2 were constructed with concrete side walls and covered by semipermeable membrane.
