## 3 Results and Discussion
### Effect of Flocculant Dosage on Harvesting Efficiency  
The effectiveness of _M. oleifera_ seed powder (MP) to harvest a mixed culture of _Desmodesmus_ sp. and _C. vulgaris_ grown on digestate was investigated with varying dosages from 0.01 g L\({}^{-1}\) to 5 g L\({}^{-1}\). Aluminium sulfate (AS) was chosen as a positive control as a widely used chemical coagulant, but likely unsuitable for feed application due to toxicity effects [31]; the use of iron (III) chloride (IC) for microalgae harvesting has been increasing and this flocculant can be compatible with feed applications within a limited concentration in the final product [32]. The two chemical coagulants were tested in the same range of dosages as _M. oleifera_; a negative control without the addition of flocculants was also included. The obtained flocculation efficiencies after 60 min of sedimentation are shown in Figure 1.  
Interestingly, the negative control, without the addition of flocculants, had an average harvesting efficiency of 53%, indicating that the used culture is a suitable candidate for harvesting by sedimentation and that the addition of a low dosage of flocculants could be a strategy for improving the sedimentation rate and, consequently, the economics of the process. Therefore, the addition of 10 mg L\({}^{-1}\) of flocculants, resulting in a biomass/flocculant ratio of 50, was assessed to test whether this small addition would suffice to improve the sedimentation rates of the _Desmodesmus-Chlorella_ culture. As shown in Figure 1, the addition of 10 mg L\({}^{-1}\) of flocculants did not affect the harvesting efficiency of the culture and yielded similar results to the negative control (\(p>0.05\)). When using higher dosages of flocculants, between 100 mg L\({}^{-1}\) and 5000 mg L\({}^{-1}\), MP resulted in lower maximum harvesting efficiency, of 76%, compared to 95-96% obtained with the chemical flocculants. There was no significant difference in harvesting efficiency between the tested dosages (\(p>0.05\)). Even though _M. oleifera_ yielded the lowest harvesting efficiencies, a longer settling time might have resulted in better yields. While  
Figure 1: Effect of nine dosages of _Moringa oleifera_ seed powder (MP), aluminium sulfate (AS), and iron (III) chloride (IC) on the harvesting efficiency of a mixed culture of _Desmodesmus_ sp. and _C. vulgaris_ (500 mg L\({}^{-1}\)) after a 60-min sedimentation time. Control without the addition of flocculants was also carried out in the same conditions as the assays with the addition of flocculants. The rotation speed and time used were 150 rpm for 5 min, followed by 70 rpm for 20 min in an orbital shaker. Columns represent the average values, and error bars represent the standard deviations of triplicates. Different letters indicate significant differences between the flocculants for a same dosage after a one-way ANOVA followed by a Tukey test (\(p<0.05\)). both chemical coagulants resulted in immediate floc sedimentation, the samples harvested with _M. Oleifera_ had increasing harvesting efficiencies over time (Figure S4). Moreover, the flocculation conditions were not optimized for MP in this experiment, which could result in underperformance; this was further investigated in the following sections. When comparing the chemical flocculants, the highest harvesting efficiency obtained with IC was 96% with the addition of 100 mg L\({}^{-1}\), while AS reached the same harvesting efficiency with a higher dose of 200 mg L\({}^{-1}\). To better compare the two tested chemical coagulants, the molar concentration in terms of their cations (Fe\({}^{3+}\) and Al\({}^{3+}\)) was calculated for the 100 mg L\({}^{-1}\) dosage, yielding 3.7 \(\times\) 10\({}^{-4}\) mol L\({}^{-1}\) and 3.0 \(\times\) 10\({}^{-4}\) mol L\({}^{-1}\), respectively. This small change in molar loading may account for the lower harvesting efficiency found for Al\({}^{3+}\) at 100 mg. L\({}^{-1}\).