# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Nutrient Profile
Materials with a lower C/N ratio than 32.2 demonstrated the most efficient composting, according to a study conducted by [PERSON] and [PERSON] [17], which focuses on composting efficiency based on the C/N ratio of different waste materials. This indicates that feedstocks rich in nitrogen, such as those sourced from animals that have a C/N ratio near 32.2, are more effective for heat recovery as they can sustain thermophilic \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 P content & PBM & 5 & 1.22 b & 1.00 & 0.40 & 2.90 & 0.80 \\ (g/kgTS) & ADM & 3 & 13.33 a & 13.50 & 13.00 & 13.50 & 0.02 \\  & NA & 2 & 0.45 b & 0.45 & 0.40 & 0.50 & 0.16 \\ \hline K content & PBM & 5 & 12.60 a & 10.70 & 3.30 & 29.40 & 0.85 \\ (g/kgTS) & ADM & 3 & 11.67 a & 12.00 & 10.50 & 12.50 & 0.09 \\  & NA & 2 & 9.40 a & 9.40 & 1.80 & 17.00 & 1.14 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._conditions longer and generate more heat. Therefore, it is essential to uphold the optimal level of nitrogen to achieve the maximum potential in heat recovery. However, an excessive amount of N can result in higher ammonia volatilization into the atmosphere, as when there is more N present, particularly in the form of ammonium (NH4\({}^{+}\)), there is a greater potential for ammonia (NH3) volatilization, resulting in a decrease in the efficiency of microbial processes and potentially triggering odor issues near CHRS plants [18]. The mean P content for animal-derived materials was 13.33 g/kg TS, significantly higher than both plant-based materials (1.22 g/kg TS) and non-agricultural feedstocks (0.45 g/kg TS). Similarly, for K content, animal-derived materials had a mean content of 11.67 g/kg TS, comparable to plant-based materials (12.60 g/kg TS), while non-agricultural feedstocks showed a slightly lower mean value (9.40 g/kg TS). These results indicate that animal-derived materials are notably richer in P, while K levels are more evenly distributed across the different feedstocks. P content also influences the composing process, particularly its role as a co-nutrient for microbial growth. Statistically significant differences were observed between animal-derived materials and both non-agricultural feedstocks and plant-based materials. Phosphorous supports essential microbial processes (e.g., cell division and energy use and production), thus enhancing overall microbial activity and potentially contributing to increased heat production during composing. Moreover, K content varied less among feedstocks, with no significant differences between categories. However, K is not a direct driver of compost heat generation, but it supports overall microbial health and the metabolic processes essential for effective composing. When K-rich mining waste is introduced into the mix of composing, it can promote the growth of thermophilic bacteria. This helps speed up the breakdown of feedstocks and reduces the time needed for composing by enhancing both oxygen intake and microbial processes during high-temperature stages [19]. Overall, from a nutrient-profile point of view, animal-based materials could be considered as the most suitable choice for CHRSs because of their low and consistent C/N ratios and high N content. These features foster rapid microbial activity and sustain thermophilic conditions, which are essential for maximizing heat generation during composing. On the other hand, plant-based materials with moderate C/N ratios and varying nutrient content can also work well but might lead to less predictable and lower heat output. In order to enhance the efficiency of heat recovery systems using plant-based materials, it would be beneficial to mix them with N-rich materials to balance the C/N ratio, which boosts microbial activity and heat production, or closely manage the composing process to ensure stable microbial activity.