## 3 Results and Discussion
### Circularity Indicator of Nutrient  
Figure 5 presents the CIN results, which exhibit a wide range of values. This indicator conveys how efficient the fertilizer manufacturing process is in maintaining the target nutrient in the final product. Uncertainty levels for this indicator ranged from 5.56% to 11.60%, indicating a good/high data accuracy, following the GHG protocol's data accuracy rating (Table 5).  
The results cover the nature of the manufacturing process:  
* Algae biomass only undergoes drying before being applied to soil; therefore, all the nutrients contained in the plants are maintained in the dried product.
* Spent mushroom substrate has a noticeably reduced CIN due to its manufacturing process, which includes two composing stages and the growing of mushrooms. Part of the nutrients contained in the inflowing waste streams are absorbed by the mushrooms during the cultivation stage, and others are eliminated during composting.
* Something similar can be observed in the case of feather meal, whose manufacturing process includes a hydrolysis stage, in which the constituent molecules are broken down.
* In the case of compost, the bio-based product undergoes an anaerobic digestion followed by a composting stage. Both together reduce the nutrients contained in the raw waste streams.
* Finally, the wood vinegar manufacturing process is the least effective one of all the assessed bio-based products. This is because most of the nutrients contained in the wood biomass ultimately remain in the biochar, which is the solid phase of the waste treatment process. The nutrients in wood vinegar are a minimal part that is emitted in combustion gases and then condensed to obtain the product. Therefore, the function of wood vinegar is more accurately regarded as that of a pesticide and biostimulant rather than a fertilizer stricto sensu.  
The CIN results help identify the most efficient nutrient recovery processes among the five assessed. Although some processes are inevitable (such as growing mushrooms and composting the spent mushroom substrate) and its selection is raw-material dependent (for instance, algae not needing additional treatment beyond drying), having it measured could help industry and policy agents prioritize technologies and waste treatments.  
Figure 5: Results for circularity indicator of nutrient for the five bio-based products and their average.  
It is worth noting that, unlike \"conventional\" products, the ones studied here originate from the valorization of waste streams. If these waste streams did not exist, there would be no reason to produce such fertilizers. Some bio-based fertilizers also depend on other primary processes; for example, increasing the production of spent mushroom substrate would make little sense without a corresponding demand for mushrooms. Therefore, the assessment should not be based on the logic of \"selecting the go-to product\", but rather on evaluating the treatment processes available for a specific waste stream and identifying the most circular option.  
In any case, recovering valuable compounds from waste streams is, by definition, a CBE practice, one that also aligns with the EU's waste hierarchy [65]. Thus, the very production of bio-based fertilizers is preferable to the disposal of residues under the prevailing business-as-usual logic.  
The CIN is fundamentally an efficiency indicator that enables businesses to make better-informed decisions. However, it demands robust quality control and stability practices, which may not be widely established within the bioeconomy sector. For example, an accurate characterization of the feedstock is essential for calculating the CIN. Due to the variability of biowaste streams and limited technical capacity to analyze them, relevant datapoints may be unavailable for a significant number of manufacturers. Another function of this indicator is the fostering of measurement and quality control activities among firms engaged in the CBE.