# Microalgal Growth in Aquaculture Effluent: Coupling Biomass Valorisation with Nutrients Removal
## 3 Results and Discussion
### Wastewater Treatment  
The capacity of _C. vulgaris_ to remove nitrogen in the form of TN and NO\({}_{3}\)-N and phosphorus in the form of PO\({}_{4}\)-P from the aquaculture wastewater was analysed. Figure 3 presents the removal of TN, NO\({}_{3}\)-N and PO\({}_{4}\)-P by microalgae in each assay and the model fit of the modified Gompertz model to the experimental data. The respective legislation limits (Directive 91/271/EEC concerning discharges from urban wastewater treatment plants to sensitive areas subject to eutrophication) are also represented in the figure. Table 3 presents the initial nutrients concentration, the removal efficiencies and the kinetic parameters determined by adjusting the modified Gompertz model to the experimental data. By analysing Figure 3, it is possible to see that in the AW assay: (i) the TN concentration reached values below the EU legislation after 2 d; (ii) the NO\({}_{3}\)-N concentration was already below 10 mg L\({}^{-1}\) after 1 d; and (iii) the PO\({}_{4}\)-P concentration was below the legal limit after 2 d. Therefore, the results show that _C. vulgaris_ removed nitrogen and phosphorus efficiently from aquaculture wastewater, reaching values below the above-mentioned legislation limits in a short period of time. As mentioned before, in the C+ assay, the microalgae were cultivated in a modified OECD medium, containing initial amounts of nutrients different from those present in the AW assay, where aquaculture wastewater was used as the culture medium. Indeed, Table 3 shows that the initial concentration of each nutrient is higher in C+ than in the AW assay. Aquaculture wastewaters usually present high nitrogen and phosphorus concentrations [34]. However, in the present study, the wastewater comes from a small-scale aquaculture plant, which may explain the low concentrations of the N and P obtained. Even though the initial concentration of TN was statistically higher (\(p<0.05\)) in the C+ assay, the removal efficiencies in AW and C+ assays were not statistically different (\(p>0.05\)), being 93.5 \(\pm\) 2.1% and 95.6 \(\pm\) 1.4%, respectively. Comparing the TN uptake rate obtained from the modified Gompertz model, in AW assays, a value of 0.83 \(\pm\) 0.26 d\({}^{-1}\) was achieved, whereas in the C+ assay the nutrient removal rate was 0.44 \(\pm\) 0.07 d\({}^{-1}\). It is also important to mention that the lag time was longer in the C+ assay. Regarding the initial NO\({}_{3}\)-N concentration, the value obtained was statistically lower (\(p<0.05\)) in the AW assay (10.7 \(\pm\) 0.8 mg L\({}^{-1}\)) than in C+ (38.0 \(\pm\) 0.3 mg L\({}^{-1}\)). Both assays achieved high values of NO\({}_{3}\)-N removal efficiencies, 98.0 \(\pm\) 0.1% for AW and 99.4 \(\pm\) 0.1% for C+. The NO\({}_{3}\)-N uptake rate given by the modified Gompertz model was higher in the AW assay (0.92 \(\pm\) 0.21 d\({}^{-1}\)) than in C+ (0.48 \(\pm\) 0.18 d\({}^{-1}\)). The initial PO\({}_{4}\)-P concentration was statistically higher (\(p<0.05\)) in the C+ assay (10.4 \(\pm\) 0.1 mg L\({}^{-1}\)) than in AW (2.2 \(\pm\) 0.1 mg L\({}^{-1}\)). The PO\({}_{4}\)-P removal efficiency was >90% in both assays, but the one obtained in C+ (98.4 \(\pm\) 0.1%) was statistically higher (\(p<0.05\)) than the removal efficiency in AW (92.7 \(\pm\) 0.1%). Additionally, the highest PO\({}_{4}\)-P uptake rate (1.48 \(\pm\) 0.16 d\({}^{-1}\)) and lowest lag time (0.45 \(\pm\) 0.11 d) were achieved in the AW assay. The uptake rate was higher in the AW assay for all the nutrients analysed, which can be related to the lower initial concentrations of these nutrients in the aquaculture wastewater. As the initial biomass concentration was the same, the microalgae were able to assimilate the nutrients faster in AW, due to the nutrient limitation, resulting in a higher uptake rate.