An ultra-low energy method for rapidly pre-concentrating microalgae

•ECF uses Ni electrodes and 4 s treatment time with pulsed or continuous DC.•Algae rapidly separate from suspension with input energy density of 0.03 kWh/m3.•Max separation after 2 h is 97%; max separation effectiveness is ∼30%/(kWh/m3).•Rapid separation occurs even if untreated algae are mixed with treated saltwater.•Process does not cause significant cell damage.

This study demonstrates that Nannochloropsis sp. can be effectively separated from its growth medium (0.2–0.3 g/L) using electro-coagulation–flocculation in a 100 mL batch reactor with nickel electrodes and a treatment time of only 4 s. Minimum energy density input for effective separation is 0.03 kWh/m3. Both energy input and treatment time are much smaller than reported elsewhere. The process results in rapid separation of microalgae (over 90% in 120 min) with minimal damage to algal cells (>90% still alive after processing). At around 4 V input, algae can be effectively separated even in very low concentrations. Pulsing is equally effective in separating microalgae as continuous direct current of same magnitude and total exposure time. Algae can separate from their growth medium even if the suspension itself is not treated, but is mixed with treated saltwater with same conductivity. The described method has significant advantages including applicability to continuous processing and water reuse.