When flocculation is used in mining, wastewater treatment or fermentation the resultant sludge is disposed of as a waste product. Yet, when flocculation is used to harvest microalgae, the flocculated biomass is the product. The flocculant ends up in the harvested biomass, which may have consequences for the further valorization of the biomass. Contamination of the harvested biomass is an important factor to consider when considering the use of a flocculation method for harvesting micro­algae. For instance, if the flocculant is toxic or contains toxic residues, the harvested biomass or fractions thereof cannot be used for applications such as food or animal feed. This is important when microalgal biomass is used in a biorefinery context where part of the biomass is used to generate energy (e. g., lipids), and other biomass fractions are used as animal feed (e. g., protein fraction) or high-value ingredients for the food or health industry (e. g., carotenoids) (Wijffels et al. 2010). Rwehumbiza et al. (2012) showed that aluminum added during alum-based floc­culation did not contaminate lipids and fatty acid methyl esters, but it might remain in the protein fraction. Contamination of the biomass with flocculants may also influence the recovery efficiency of certain bioproducts. Rios et al. (2013) observed a lower lipid extraction yield in microalgae harvested by flocculation (by pH increase) than in microalgae harvested by membrane filtration. Borges et al. (2011) noted that the choice of the flocculant used for harvesting microalgae influenced the ratio of saturated over unsaturated fatty acids in the biomass.