# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 3 Results and Discussion
### Energy-Related Properties
However, it should be noted that the animal-based material data are based on a limited sample size of only three data points. Non-agricultural feedstocks show a VS content of 730.90 g/kg TS based on a single available data point [29], but more data are needed in order to show trend, variability profile, and conduct proper data analysis. While this single point suggests that non-agricultural feedstocks have good energy recovery potential, additional data are necessary to confirm the performance across sources and types of non-agricultural feedstocks. Statistical analyses reveal that there are no significant differences regarding the VS content in plant-based, animal-based, and even non-agricultural feedstock, thus suggesting that energy potential may be comparable in these types of feedstocks. In this regard, the diversity of VS in plant-based materials shows that some materials could perform better than others, potentially for the use in very efficient CHRSs. Moisture level is one of the most critical parameters that affect decomposition rates and heat production during composting. High-moisture feedstocks could even need efficient aeration to achieve proper moisture levels since a lack of oxygen results in the possible obstruction of microbial growth and, in turn, diminishing the heat output. According to a study on composing swine manure, high MC (80%) resulted in anaerobic conditions within the composting pile, which negatively affected the composting process [32]. In contrast, lower moisture levels can limit microbial activity due to insufficient water availability, slowing down decomposition and potentially preventing the composting biomass from reaching optimal temperatures for effective pathogen reduction and nutrient stabilization. Animal-derived materials have a mean MC of 63.16% with a range of 55.00-72.80% [21; 33]. This consistent MC highlights their tendency to retain water; hence, there is the need to conduct aeration management for optimal composting performance. Research indicates that optimal MC for composting materials can vary significantly depending on the specific material composition. For instance, beef manure mixed with bedding materials like sawdust has an optimal MC of around 70%, while mixtures involving rice hulls are most effective at approximately 57% on a wet basis [34]. Plant-based materials, with an average MC of 50.92%, display greater variability, ranging from 5.90% (olive tree pruning) to 74.57% (grass, tomato, pepper, and eggplant) [21; 35]. Their high CV (0.39) suggests inconsistent moisture levels, requiring close monitoring and adjustments. Non-agricultural feedstocks have a lower average MC of 42.0%, ranging from 9.5% to 67.5% [29; 36]. This reduced moisture content increases aeration; however, it must be counterbalanced with enough amount of microbial activity for effective biodegradation and heat generation. The statistical analysis carried out in the present work indicates that there is a statistically significant difference in the average MC of animal waste and non-agricultural feedstocks (p-adj < 0.05), signifying the belief that the increased MC in animal-derived materials might impact their performance in heat recovery systems. Carbon (C) content is essential for compost heat recovery as it provides energy for microbial processes that generate heat. The mean C content of plant-based material was 311.32 g/kg TS, ranging from 185.8 g/kg TS in grasses [37] to 469.4 g/kg TS in plant mixes with tomato, pepper, and eggplant [35]. This variability (CV = 0.35) suggests potential inconsistency in heat output if plant materials are used as the primary feedstock. Although it is practical and also environmentally friendly to use plant-based materials, the variability in the C content means that heat production may be inconsistent unless integrated with sources characterized by a higher C content. Therefore, adding a stable carbon source from plant-based materials could improve the stability of heat recovery.