Development of a harvesting technique for large-scale microalgal harvesting for biodiesel production

Abstract

Harvesting imposes a major constraint in microalgal downstream processes and cost-effective production of various high value products. In this study, different harvesting techniques were assessed under a single domain to identify the most suitable one for large-scale harvesting of green microalgae for biodiesel purpose. In the laboratory, Scenedesmus obliquus showed a flocculation efficiency of 83.2% at pH 12 after 1 h. Maximum flocculation efficiencies of 80.2, 95 and 91%, respectively were observed for FeCl₃ at 200 mg L⁻¹, alum at 250 mg L⁻¹ and chitosan at 20 mg L⁻¹ after 1 h. Electro-flotation at 24 V and dissolved air flotation with 1 mg L⁻¹ of alum also revealed flocculation efficiencies of 99 and 91% respectively, after 1 h. For Chlorella vulgaris, similar trends were also observed. Under field trials with 1000 L algal suspension, electro-flotation required a voltage of 60 V to achieve a flocculation efficiency of ∼90% after 24 h. Dissolved air flotation also showed a flocculation efficiency of same magnitude after 7 h, but with 10 mg L⁻¹ of alum. Thus electro-flotation required a profoundly higher voltage with increasing culture volume, whereas for dissolved air flotation a much higher concentration of alum was entailed. Both the processes also depicted a significantly longer time period to achieve the required flocculation efficiency. On the other hand, pH-induced flocculation was found to be the most pertinent one for large-scale set-ups, and emerged to be efficient, cost-effective and eco-friendly as the supernatant can be reused as growth medium by re-supplementing the nutrients and adjusting the pH.