## 3 Results
### Coagulation-Flocculation in Continuous Mode
#### 3.1.2 Organic Matter Removal  
One of the main limitations of recirculating aquaculture systems (RASs) is the buildup of organic matter (OM), primarily originating from uneaten feed and fish feces [38]. This accumulation of organic matter can promote the growth of opportunistic bacteria, which may impair the performance of biofilters. Additionally, excess OM can degrade water quality, compromise fish welfare, and increase the demand for water treatment processes--ultimately raising operational costs [38]. Therefore, removing organic matter is crucial for maintaining fish health, optimizing system performance, and preserving the aquatic environment. Organic matter levels are typically monitored through measurements of dissolved organic carbon (DOC) and UV absorbance at 254 nm (UV\({}_{254}\)), where DOC reflects overall organic load, and UV\({}_{254}\) indicates the presence of natural organic matter (NOM) [34; 39]. Table 2 shows the effectiveness of different coagulants in removing organic matter from aquaculture wastewater in continuous treatment mode while considering both the coagulant concentration and pH level.  
Among the coagulants tested, Tanfloc achieved the highest DOC removal efficiency at pH 7 and a dosage of 25 mg\(\cdot\)L\({}^{-1}\). Among the chemical coagulants, aluminum sulfate showed a strong performance, particularly at pH 8 and 20 mg\(\cdot\)L\({}^{-1}\), while the ferric chloride exhibited lower DOC removal at pH 8 and 25 mg\(\cdot\)L\({}^{-1}\). The chestnut shell-based coagulant demonstrated good DOC removal (47.7%) in continuous flow mode. However, UV\({}_{254}\) removal was lower compared to that obtained in batch mode (about 60% [14]). Overall, it can be confirmed that all the coagulants were efficient in organic matter removal when applied in continuous mode. These results are particularly promising, as natural coagulants are often associated with an increase in organic matter, as reported in the literature [40; 41; 42; 43; 44]. This increase is typically linked to their organic origin (e.g., chestnut shells) and the presence of compounds such as formaldehyde [43]. However, the potential for formaldehyde leaching into the treated water was previously investigated by [PERSON], et al. [25], and it was confirmed that formaldehyde does not leach.  
The evaluation of BOD\({}_{5}\) was also conducted to determine whether using natural coagulants increases the biodegradable organic load in the treated water. In the case of the initial BOD\({}_{5}\) level, the untreated water initially exhibited a value close to 0 mg\(\cdot\)L\({}^{-1}\), as it corresponds to the final effluent, i.e., the water that has passed through all treatment stages and will be returned to the fish tank. However, this parameter increased with the addition of natural coagulants. When 10 mg\(\cdot\)L\({}^{-1}\) of CS was added at pH 8, the BOD\({}_{5}\) increased to 5.4 mg\(\cdot\)L\({}^{-1}\). Similarly, the addition of 25 mg\(\cdot\)L\({}^{-1}\) of Tanfloc at pH 7 resulted in a BOD\({}_{5}\) of 3.8 mg\(\cdot\)L\({}^{-1}\). Even though this parameter increased with the addition of natural coagulants, the reported values are not considered critical for the aquaculture system, as the BOD\({}_{5}\) concentrations remain within the limits set by the Portuguese legislation (Annex XI) [45], which establishes an acceptable range of 3 to 6 mg\(\cdot\)L\({}^{-1}\) for fish farming waters.