Influence of organic matter generated by Chlorella vulgaris on five different modes of flocculation

► The importance of algal organic matter (AOM) interference for five different flocculation methods was demonstrated. ► The presence of AOM resulted in a 2–9-fold increase of flocculant dose demand. ► pH-induced flocculation was to be least affected by AOM. Microalgae excrete relatively large amounts of algal organic matter (AOM) that may interfere with flocculation. The influence of AOM on flocculation of Chlorella vulgaris was studied using five different flocculation methods: aluminum sulfate, chitosan, cationic starch, pH-induced flocculation and electro-coagulation–flocculation (ECF). The presence of AOM was found to inhibit flocculation for all flocculation methods resulting in an increase of dosage demand. For pH-induced flocculation, the dosage required to achieve 85% flocculation increased only 2-fold when AOM was present, while for chitosan, this dosage increased 9-fold. For alum, ECF and cationic starch flocculation, the dosage increased 5–6-fold. Interference by AOM is an important parameter to consider in the assessment of flocculation-based harvesting of microalgae.