Uses of electro-coagulation-flocculation (ECF) for the pre-concentration of microalgae biomass

The separation of microalgae biomass from diluted cultures found in large-scale production systems is a major challenge for microalgae-related processes. Conventional methods use centrifugation but it requires too much energy (1 kWh/m3), the combination with previous pre-concentration operations such as settling or flotation is needed to reduce this energy consumption. However, both settling and flotation of microalgae biomass require the addition of chemical coagulants/flocculants, thus contaminating the biomass. This work explores the potential use of Electro-Coagulation-Flocculation (ECF) as a method for the pre-concentration of microalgae biomass. Thus, the recovery of freshwater microalgae Scenedesmus almeriensis was studied, both in batch and continuous mode, using this technique to optimize the operating conditions. Data from batch experiments show as a minimum conductivity of 5 mS∙cm−1 is required, the electrical potential of 12 V and current density of 18 mA∙cm−2 allowing for the recovery of 90 % of the biomass, while the biomass recovery increases up to 99 % when increasing the current density to 23 mA∙cm−2. The efficiency of the process increases when increasing the biomass concentration, thus energy consumption reduces by half from 1.0 to 0.5 kWh∙Kg−1 when the biomass concentration increases from 2.8 to 5.0 g∙L−1. To optimize the performance of the system experiments were performed in continuous mode maintaining the voltage and current density at 12 V and 47 mA∙cm−2 respectively, but modifying the flow rate and biomass concentration provided to the electrocoagulation chamber. Data shows as the optimal residence time is 15 s whereas the energy consumption reduces from 3.2 to 0.58 kWh∙Kg−1 when increasing the biomass concentration from 0.75 to 4.20 g∙L−1. Results confirm the reliability of this method for the pre-concentration of microalgae biomass, it being a fast process consuming reasonable energy. Further optimization of the electrocoagulation chamber and electrical conditions will improve the efficiency of this system for further commercial processes while keeping its major advantages of non-contaminating the biomass and allowing to achieve high biomass recovery values in short times.