## 2 Materials and Methods
### Assessment of Compost Quality  
The quality of produced composts was assessed in three aspects: (a) fertilizing potential and heavy metal polluting potential; (b) as soil fertilizers according to Bulgarian legislation; and (c) as soil fertilizers according to EU legislation.  
(a) Determination of compost fertilizing potential and heavy metal polluting potential:  
The compost fertilizing potential was determined by the fertilizing index (FI) used for assessment of MSW compost [42] and adapted for other types of compost [54]. FI was determined based on rating scale from one to five, where five corresponds to the best fertilizing potential.  
The FI was computed using the formula [42]:  
\[\text{Fertilizing index (FI)}=\sum\
olimits_{n}{}^{i=1}{(\text{S}_{i}\text{W}_{i} )}/\sum\
olimits_{n}{}^{i=1}{(\text{W}_{i})}\]  
where 'S\({}_{i}\)' is score value of analytical data for TOC, Total N, Total P, Total K and C:N (in five point scale), and 'W\({}_{i}\)' is weighing factor of the '\(i\)'th fertility parameter (in five-point scale); the maximum score of the weighing factor, 5 (TOC), reflects the strongest influence of the factor, and the lowest, 1 (Total K), reflects the weakest, respectively (Table 1).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No**} & \multirow{2}{*}{**Parameters**} & \multicolumn{5}{c}{**Score Value, S\({}_{i}\)**} & \multirow{2}{*}{**Weighing Factor, W\({}_{i}\)**} \\ \cline{3-3} \cline{5-6}  & & **5** & **4** & **3** & **2** & **1** \\ \hline
1 & TOC, \% & \textgreater{}20 & 15.1–20.0 & 12.1–15.0 & 9.10–12.0 & \textless{}9.1 & 5 \\
2 & Total N, \% & \textgreater{}1.25 & 1.01–1.25 & 0.81–1.00 & 0.51–0.80 & \textless{}0.51 & 3 \\
3 & Total P, \% & \textgreater{}0.60 & 0.41–0.60 & 0.21–0.40 & 0.11–0.20 & \textless{}0.11 & 3 \\
4 & Total K, \% & \textgreater{}1.00 & 0.76–1.00 & 0.51–0.75 & 0.26–0.50 & \textless{}0.26 & 1 \\
5 & C:N ratio & \textless{}10.1 & 10.1–15.0 & 15.1–20.0 & 20.1–25.0 & \textgreater{}25 & 3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Criteria for fertilizing index (FI) scoring (Source: [42; 54]). Adapted with permission from [[PERSON], [PERSON], [PERSON]], 2022, Elsevier.  
The compost heavy metal polluting potential was determined by the clean index (CI) [42]. As for FI, the CI determination included a 'weighing factor' of each heavy metal, ranking on a scale from one to five, depending on its biological functions in organisms as well as of its phytotoxicity and mammalian toxicity potential, and a'score value' from one to five, according to the obtained value for the specific element (Table 2).  
The CI values were calculated using the following formula [42]:  
\[\text{Clean index (CI)}=\sum_{i}{}^{j=1}\text{(S}_{j}\text{W}_{j})/\sum_{i}{}^{j=1} \text{(W}_{j})\]  
where 'S\({}_{j}\)' is score value of analytical data for Zn, Cu, Cd, Pb, Ni and Cr (i.e., 1-6) and 'W\({}_{j}\)' is weighing factor of the '\(j\)'th heavy metal (i.e., 1-5) (Table 2).  
(b) The compost quality assessment at a national level was based on 3 physicochemical parameters (moisture content, EC and TOC), 7 heavy metals (Cu, Zn, Pb, Cr, Ni, Cd and Hg) and 2 microbiological parameters (_E. coli_ and _Salmonella_ spp.) according to the Ordinance on separate collection of biowaste and treatment of biodegradable waste [55].  
(c) The compost quality assessment at the EU level was based on the same 7 heavy metals according to Commission Regulation (EC) No 889/2008 [56].