# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Chemical and Physical Properties  
The physical properties (pH and bulk density) and some chemical constituents (lignin, cellulose, and hemicellulose) of the composing feedstocks play an important role in microbial activity, decomposition rates, and the efficiency of heat generation during composing, which will be discussed in detail in the upcoming section of this paper. This section of the data analysis, as shown in Figure 3 and Table 4, looks closely into these properties of the materials across the considered categories of feedstocks and discusses their implications for optimizing CHRSs. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Variable** & **Feedstock** & **Sample Size** & **Mean** & **Median** & **Minimum** & **Maximum** & **Coefficient of Variation** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Lignin \\ (g/kg TS) \\ \end{tabular} } & PBM & 16 & 132.05 a & 141.50 & 36.70 & 290.00 & 0.64 \\  & ADM & 3 & 52.23 a & 30.00 & 26.68 & 100.00 & 0.79 \\  & NA & 2 & 160.63 a & 160.63 & 131.25 & 190.00 & 0.26 \\ \hline \multirow{3}{*}{\begin{tabular}{c} Cellulose \\ (g/kg TS) \\ \end{tabular} } & PBM & 16 & 208.96 a & 173.00 & 44.10 & 370.00 & 0.50 \\  & ADM & 2 & 315.00 a & 315.00 & 240.00 & 390.00 & 0.34 \\  & NA & - & - & - & - & - & - \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} Hemicellulose \\ (g/kg TS) \\ \end{tabular} } & PBM & 16 & 182.90 a & 200.00 & 19.60 & 457.00 & 0.84 \\  & ADM & 3 & 281.76 a & 215.00 & 200.00 & 415.28 & 0.41 \\  & NA & - & - & - & - & - & - \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Bulk \\ Density \\ (kg/m\({}^{3}\)) \\ \end{tabular} } & PBM & 15 & 556.99 a & 477.00 & 204.80 & 1000.00 & 0.46 \\  & ADM & 3 & 499.33 a & 475.00 & 420.00 & 603.00 & 0.19 \\ \cline{1-1}  & NA & 7 & 528.57 a & 520.00 & 500.00 & 600.00 & 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Lignin, cellulose, and hemicellulose content, bulk density, and pH in different feedstocks (mean values not sharing the same letter are significantly different). Figure 3: Statistical comparison of different feedstocks: (**a**) lignin, (**b**) cellulose, (**c**) hemicellulose, (**d**) bulk density, and (**e**) pH. Lignin plays a crucial role in the decomposition of feedstock because its complex molecular structure makes it highly resistant to microbial breakdown. This resistance slows down the overall decomposition process, leading to reduced microbial activity. Since microbes generate heat during decomposition through their metabolic processes, the slow breakdown of lignin results in decreased heat production. This can significantly affect processes that rely on heat generation during decomposition, such as composting. Therefore, increased lignin levels can slow down microbial activity, resulting in a reduction in the overall efficiency of composting in generating heat. Non-agricultural feedstocks showed the highest mean lignin content (160.63 g/kg TS) with a maximum value of 190 g/kg TS in municipal solid waste organic fractions larger than 2 mm [20]. Plant-based materials showed a mean lignin content of 132.05 g/kg TS with a maximum recorded in palm tree pruning (290 g/kg TS) [21]. Animal-derived materials had the lowest mean lignin content, averaging at 52.23 g/kg TS with a minimum of 26.68 g/kg TS for a mixture of wheat straw and swine manure [22]. However, these findings cannot be generalized due to the small sample size (n = 3) of studies on animal-based materials focusing on this parameter. Studies have shown that the inhibitory impact of lignin on compost effectiveness is widely known, and it is observed that feedstocks with high lignin content tend to be more resistant resulting in slower microbial activity, a reduction in the efficiency of composting, and heat generation [23]. In contrast, the significantly lower lignin content in animal-derived materials allows for faster microbial activity and heat production. Cellulose is a key carbon source in compost, driving microbial activity.