Improvement in electrically induced biomass harvesting of Chlorella sp. MJ 11/11 for bulk biomass production

Oleaginous microalgae could be a promising feedstock for biodiesel production, but their small size, negative surface charge, and diluted suspended biomass create a hindrance in harvesting. In addition, the low value of biofuel makes existing harvesting technologies uneconomical. The aim of the present study was to lower the energy input and investigate the suitability of electroflocculation-sedimentation for harvesting the oleaginous microalga Chlorella sp. MJ 11/11. The Taguchi design of experiments was used to understand the degree of influence of different process parameters (current density, initial pH, biomass concentration, electrode distance, surface/volume ratio, and time) and response surface methodology to determine the suitable conditions for maximization of flocculation efficiency for harvesting using sacrificial ferritic stainless steel (type 430) electrode. Harvested biomass was analyzed by FTIR and SEM to identify the chemical reactivity of functional groups and morphological changes at their surface, respectively. According to the Taguchi analysis, the most influential process parameter was found to be current density followed by initial pH and flocculation time. A multi-parameter interaction study for the maximization of biomass recovery was performed using the response surface methodology. Under the most suitable conditions (current density 11 A m⁻², voltage 12 V, initial pH 6, electrode distance 4 cm, medium conductivity 2.52 mS cm⁻¹, and time 30 min), the maximum flocculation efficiency of 98%, sludge volume index (SVI) value of 91, and lowest energy consumption of 1.85 kWh kg⁻¹ or 1.81 kWh m⁻³ were observed. Thus, electro-harvesting with low energy consumption and high biomass removal can be applied as a promising method for efficient microalgal biomass harvesting.