# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Chemical and Physical Properties
The cellulose content in materials derived from animals averaged at 315 g/kg TS compared to 208.96 g/kg TS in plant-based materials, though this difference was not statistically significant. The non-significant difference in cellulose content between animal-derived and plant-based compost materials may be partly due to the disparity in sample sizes, with 16 samples for plant-based materials versus only two for animal-derived materials, which might affect the reliability of these findings. Thus, the statistical reliability and representation of the data are affected, potentially skewing the comparative results. The increased content of cellulose in animal-based materials helps maintain a suitable environment for microorganisms and thermophilic conditions that are important for effective heat production, according to studies indicating that microbial consortia with high cellulase activities can speed up the composting of agricultural waste with high cellulose contents and promote beneficial microbial activity under thermophilic conditions, which further enhances heat generation [24]. Plant-based materials showed wide variability, with cellulose content ranging from a minimum value of 44.10 g/kg TS (green waste) [25] to a maximum value of 370 g/kg TS (wheat straw and pig manure) [26]. This inconsistency in cellulose content directly affects their heat production potential. However, the absence of cellulose data for non-agricultural feedstocks complicates the assessment of its impact on heat generation in aerobic processes. Hemicellulose, though less resistant to degradation than cellulose, plays a crucial role in microbial decomposition during composting. The hemicellulose content was found to be higher in animal-derived materials, with an average of 281.76 g/kg TS compared to plant-based materials (182.90 g/kg TS), though this difference was not statistically significant. Plant-based materials displayed significant variation in hemicellulose content with a standard deviation of 152.76 g/kg TS and a coefficient of variation of 0.84. The wide range \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}{*}{pH} & PBM & 15 & 6.38 \({}^{\text{a}}\) & 6.19 & 5.35 & 7.50 & 0.13 \\  & ADM & 14 & 7.46 \({}^{\text{b}}\) & 7.57 & 5.55 & 8.50 & 0.10 \\  & NA & 1 & 5.75 \({}^{\text{a}}\) & 5.75 & 5.75 & 5.75 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: _Cont._of variability in plant-based feedstocks suggests that they may not consistently produce heat. The 95% confidence interval for hemicellulose content in plant-based materials ranged from 108.05 g/kg TS to 242.51 g/kg TS, further indicating the potential for inconsistent performance. The absence of hemicellulose data for non-agricultural feedstocks limits the ability to fully assess its role in heat generation from aerobic processes. In addition, the small sample size (n = 3) of studies concerning animal-based materials suggests that these findings cannot be generalized for this category of materials and for hemicellulose. Bulk density plays also a vital role in how composting works since it impacts airflow and the availability of oxygen, both of which are vital factors for supporting aerobic microbial activity and the generation of heat during the process. When bulk density is high, it can hinder the flow of oxygen, resulting in conditions that can decrease the effectiveness of composting and heat production, while lower bulk density leads to increased free air space and air permeability, resulting in improvement of aeration, which is beneficial for microbial activity and organic matter breakdown [27]. In the data analysis carried out, materials derived from plants showed a mean bulk density of 556.99 kg/m\({}^{3}\), with a maximum value of 1000 kg/m\({}^{3}\). Animal-based materials had a mean bulk density of 499.33 kg/m\({}^{3}\), ranging from a minimum of 420 kg/m\({}^{3}\) up to a maximum of 603 kg/m\({}^{3}\).