## 3 Results and Discussion
### Effect of Flocculants on the Composition of the Harvested Biomass  
Bioflocculants are usually proposed for the harvesting of microalgae as an alternative to more traditional chemical flocculants, which are said to contaminate the final biomass with their metallic ions. However, studies usually do not conduct a composition analysis of the harvested biomass to prove (or refute) this statement. Especially with _M. oleifera_, no trace element analyses have been carried out in the harvested biomass to assess its possible further use in food or feed applications. Therefore, the algae biomass harvested with the three tested flocculants (AS, IC, and MP) were characterized for their content of trace elements, and the results are shown in Table 1. These are compared against the maximum tolerable levels (MTL) for swine, cattle, and poultry, which are defined as the amount of a certain mineral that can be fed for a defined period without impairing animal health or performance [48]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Biomass**} & \multirow{2}{*}{**Mn**} & \multirow{2}{*}{**Zn**} & \multirow{2}{*}{**Cu**} & \multirow{2}{*}{**Cd**} & \multirow{2}{*}{**Pb**} & \multirow{2}{*}{**Fe**} & \multirow{2}{*}{**Al**} \\ \cline{2-2} \cline{5-8}  & & & & & **mg kg\({}^{-1}\)** & & \\ \hline Algae + MP & 130.5 & 284.2 & 95.4 & 0.3 & 8 & 10,672.2 & 4614 \\ Algae + AS & 37.5 & 141.4 & 35.9 & BQL & BQL & 4844.6 & 29,687 \\ Algae + IC & 30.9 & 44.3 & 9.5 & BQL & BQL & 47,240.4 & 1727.2 \\ \hline MTL & & & & & & & \\ \hline Swine & 1000 & 1000 & 100 & 10 & 10 & 3000 & 1000 \\ Cattle & 2000 & 500 & 25 & 10 & 100 & 500 & 1000 \\ Poultry & 2000 & 500 & 250 & 10 & 10 & 500 & 1000 \\ \hline \hline \end{tabular}  
* \(\mathrm{BQL}\)—below quantification level.  
\end{table} 
Table 1: Maximum tolerable levels (MTL) of micronutrients in livestock feed [48 and the content of these elements in the harvest microalgae with the different flocculants (MP—M. oleifera seed powder; AS—aluminium sulfate; IC—iron (III) chloride). Figure 4: Effect of six different doses of _M. oleifera_ seed powder water extract (20 g L\({}^{-1}\)) on the harvesting efficiency of a mixed culture of _Chlorella_ and _Desmodes_ (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 30 rpm for 10 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 different dosages of dissolved _M. oleifera_ powder after a one-way ANOVA followed by a Tukey test (\(p<0.05\)). Fe and Al content in all algae samples characterized were above the recommended values. For Fe, the sample harvested with MP had a much higher content than the sample harvested with AS, while only trace amounts of Fe were found in the seed powder; a similar observation can be made for Al, as the algae harvested with MP had more than double the amount of Al found in the samples harvest with IC. Previous research has indicated that MP does not neutralize the charges of the algae cell wall but rather interacts with algae cells by forming bridges that connect clusters of cells, forming flocs. This bridge flocculation results in larger flocs with higher moisture content [21], which may lead to higher retention of trace elements from the medium in the biomass upon drying. The algae used in the present study were grown on digestate, a medium rich in several trace elements; the used digestate, for instance, had an Al content of 2.6 mg kg\({}^{-1}\) and a Fe content of 8.1 mg kg\({}^{-1}\), while usual commercial media used for algae cultivation contain no Al and about 1 mg kg\({}^{-1}\) of Fe.