Harvesting of microalgae Chlorella vulgaris using electro-coagulation-flocculation in the batch mode

The aim of this study was to evaluate the harvesting of microalgae Chlorella vulgaris by electro-coagulation-flocculation (ECF) using aluminum and iron electrodes, assess the mechanisms responsible for microalgae recovery, quantify the metal contamination in the effluent and biomass, analyze power requirements, and investigate the effect of ECF on lipid and pigment content in the biomass. The influence of six operating parameters (electrode material, sedimentation time, current density, stirring speed, initial pH (pHi) and inter-electrode distance) on the harvesting efficiency was tested. A specific strategy involving flotation and pH-controlled ECF experiments was developed to identify the prevailing mechanism of harvesting: adhesion on flocs was shown to be negligible; flotation contributed to a maximum of 36.6% of microalgae recovery; zeta potential highlighted that the main mechanism responsible for microalgae recovery was charge neutralization at pHi 4 and 6, and sweep flocculation at pHi 8. The most energy saving conditions for the harvesting of Chlorella vulgaris involved aluminum electrodes, and 60 min electrolysis with a current density of 2.9 mA/cm2, pHi 4, stirring speed 250 rpm and an inter-electrode distance of 1 cm. Economic and competitive energy input (1 kWh/kg microalgae) could be achieved by adding 1.5 g/L NaCl. In addition, ECF did not affect significantly the amount of microalgal lipids and pigments.