## 3 Results and Discussion
### Compost Characteristics
#### 3.1.4 Heavy Metals (Zn, Cu, Pb, Cr, Ni, Cd, Hg)  
Heavy metal content is another important parameter of compost quality, as it can influence soil fertility, plant health and growth, and ground water pollution. The heavy metal amount in the produced composs was different with respect to the individual elements and ranged from <0.3 mg kg\({}^{-1}\) (C1, C2) for Hg to 174.3 mg kg\({}^{-1}\) (C1)-188.2 mg kg\({}^{-1}\) (C2) for Zn (Table 3). A trend of higher content of Cu, Zn, Cr and Ni in C2 compared to C1 was observed: 1.26, 1.08, 1.12 and 1.11 times (\(p\) < 0.01-0.001), respectively. The recorded values of Cd and Hg were below the limit of quantification of the method used, which implies the presence of negligibly low concentrations, without significant differences between the two composs. The only exception was the Pb concentration, which was 1.30 times higher in C1 than in C2 (\(p\) < 0.001). Because of the same composting method (aerobic composting) applied for both composts, the observed differences in heavy metal concentrations among them suggested different amounts of heavy metals in the initial raw materials (municipal GW and discarded green peppers). It is likely that municipal GW contained lower concentrations of Cu, Zn, Cr and Ni, and higher amounts of Pb than discarded green peppers, while the Cd and Hg levels of all substrates were very low. As the content of heavy metals of the raw materials used to produce both composts has not been analyzed, this hypothesis was not proven. Coefficients of variation for the individual elements of both composts indicated that all were very slightly variable (Cv = 0.54-3.40%). Depending on their amount, the elements in the monitored composts were arranged in the following descending order: Zn > Cu > Pb/Cr > Ni > Cd > Hg.  
The comparative analysis between the obtained results and data of previous studies revealed both similarities and discrepancies. Similar Cu, Pb, Cr, Ni and Cd content and lower Zn content were reported in GW compost from Germany, France and Belgium [84]along with similar Ni, Cr, Pb and Cd content and lower Zn and Cu content in GW used for composting [18]. Other studies demonstrated diverse data on heavy metals levels in GW compost [21; 22; 31; 33], which is completely understandable given the multiple different factors affecting their content in the produced compost in each studied case.  
Referring to the heavy metals ranking by amount, a similar trend was reported for GW compost (Zn \(>\) Cu \(>\) Pb \(>\) Cr \(>\) Ni \(>\) Cd [85] and Pb \(>\) Cr \(>\) Cd \(>\) Hg [22]), while the results in this study partly coincided with the ranking for GW used for composting (Zn \(>\) Ni \(>\) Cu \(>\) Cr \(>\) Pb \(>\) Cd) [18]. Full compliance with the arrangement of the monitored elements was observed for compost from municipal waste [83; 55]. Anyway, this brief analysis illustrates the great variety of the heavy metal contents in GW compost, which is logical given the great diversity in their concentrations in the different types of green wastes used for compost production over the world. It was noteworthy that regardless of the compost type, the amount of Zn was the highest, and that of Cd and Hg was the lowest.