# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Nutrient Profile  
The nutrient profile of the feedstocks utilized in the composting process plays an important and vital role in how the compost process works. Particularly for CHRSs, nutrients such as C, P, N, and K are vital for the activity of microorganisms, the breakdown of organic material, and the consequent generation of heat. This section provides an examination of the nutrient composition found across different types of compost feedstocks: plant-based materials, animal-derived materials, and non-agricultural feedstocks. The distributions of data collected for the C/N ratio and N, P, and K concentrations are shown in Figure 2 and Table 3.  
The results of the data analysis show a significant variation in the C/N ratio and of N, P, and K concentrations among the feedstocks utilized in the composting process, which are important factors impacting the overall effectiveness of heat recovery systems in the composting operations. These nutrient profiles mentioned above and C/N ratios affect microbial activity, decomposition rate, and overall heat generation during the composting process. The mean C/N ratio for plant-based materials, animal-based materials, and non-agricultural feedstocks was 32.43, 19.36, and 47.82, respectively. The minimum C/N ratio resulted in 5.9 when using poultry manure with wood shavings [12], while the highest ratio of C/N ratio was 144.67, and it was recorded in a blend of paper [13]. A significant difference in C/N ratios was observed between animal-derived materials and non-agricultural feedstocks, but no significant differences were found between animal-derived and plant-based materials or between plant-based materials and non-agricultural feedstocks. Lower C/N ratios generally enhance microbial activity, leading to higher heat generation during composting, as observed by [PERSON] et al. [14]. Their study demonstrated that relative heat generation increases with the C/N ratio, reaching an optimal point at approximately 36.07. Beyond this point, heat production begins to decline, suggesting that, while microbial efficiency improves with lower C/N ratios, there is a threshold at which both microbial activity and heat generation are maximized. This finding underscores the importance of maintaining an optimal C/N ratio to balance decomposition efficiency and heat output during the composting process. On the other hand, excessive C/N ratios can lead to a decrease in microbial efficiency, slowing down decomposition and reducing heat output. This was shown in research by [PERSON] and [PERSON] [15], who found that microbial activity decreased noticeably when the C/N ratio exceeded 29. This suggests that composting systems require careful balancing of the C/N ratio to optimize microbial activity and heat production. The mean N content for animal-derived materials was 22.62 g/kg TS, significantly higher than both plant-based materials (10.99 g/kg TS) and non-agricultural feedstocks (10.53 g/kg TS). These differences highlight the greater N availability in animal-derived materials, which can enhance microbial activity and heat generation in composting processes. The level of N also has an impact on the efficiency of CHRSs: green waste sourced from public and private gardens, which was ground to a particle size of 20 mm along with biowaste (fruits and vegetables) and paper/cardboard, exhibited a minimum N content of 0.1 g/kg TS [14], whereas pig manure combined with wheat straw showed the highest level at 32.2 g/kg TS [16]. Statistically, significant differences were found between animal-derived materials and non-agricultural feedstocks. An increased amount of N can boost the activity of microorganisms in composting processes by speeding up the breakdown of materials and increasing the production of heat, which is beneficial for heat recovery from composting systems.