Evaluation of flocculation based harvesting processes for microalgae biomass production based on floc characteristics

The transition of a fossil fuel to a biobased economy in combination with the prospected increase in the world’s population will lead to a growing demand for additional sources of biomass to complement traditional agricultural biomass. Microalgae are such a promising new source of biomass. These aquatic microorganisms receive much interest today as a potential source of biofuels. The most pressing challenge, however, does not lie in the production, but in the energy-efficient separation and downstream processing in a manner suitable for the production of bioproducts. Introducing a flocculation-based bulk harvesting step in the process can lower the energy needs 10-fold. Recently a lot of new studies were published evaluating novel flocculation methods for microalgae harvesting. Our recently published review revealed that most of these studies have focused on a single evaluation criterium, namely flocculation efficiency. In this study, we evaluated 5 different flocculation modes for Chlorella vulgaris and their influence on floc characteristics such as settling velocity, biomass compaction degree and floc size. Based on previous studies, alum sulphate, chitosan, cationic starch, pH induced flocculation and electro- coagulation-flocculation were selected. In addition, the effect of the presence of algal organic matter (AOM) was studied for all flocculation modes. Our result show that the floc characteristics were influenced by the flocculation mode, which depends on the coagulation mechanism. Secondly, the influence of the presence of AOM was evaluated. The presence of AOM led to an increase in needed flocculant dosage for all flocculation modes. This resulted particularly in a decrease of the concentration factor. The floc characteristics upon flocculation using cationic starch were least affected by the presence of AOM, while flocculation using chitosan was most affected. This study demonstrated that the impact on floc characteristics is important to consider next to flocculation efficiency in the assessment of flocculation-based harvesting of microalgae.status: publishe