A novel method for pH mediated electroflocculation in saltwater systems

Abstract

Microalgae represent an untapped resource for novel biomass production and the subsequent production of feed, food and fuels. Harvesting micro-algal cultures is a technical challenge due to the relatively low concentration of biomass of even dense algal cultures. Flocculating the cells in culture before these steps can simplify harvesting by increasing filterability and settling velocity. Electroflocculation has the advantages of technical simplicity, and potential for integration into a continuous treatment process. In seawater, it is possible to flocculate cells using a non-sacrificial anode such as graphite or titanium, however, this method is complicated by the production of Cl₂ gas and oxidative chloride compounds at the anode. The diffusion of these products into solution causes damage to cells. This study shows that cells can be protected from these oxidative products and flocculated by integrating a regenerated cellulose membrane into the electrolysis system between the culture and anode solution. The viability of this technique is demonstrated by flocculation tests on the productive saltwater microalga Picochlorum celeri TG2, which is difficult to harvest due to slow settling and small cell sizes. By using non-sacrificial electrodes with membrane protection, the settling efficiency of the strain was increased, with an ∼95% clarification and a compaction factor below 0.2 (after treatment and settling for one hour). The added power requirements across a range of current densities and biomass concentrations are also reported. The best volumetric energy consumption tested was at 0.86 A/L requiring an average of 3.1 kWh m³ (±0.1 kWh m³). Settling efficiency and energy requirement on a biomass basis was found to be independent of biomass in the 0.5–1.6 g L⁻¹ range tested, with the lowest biomass energy requirement achieved at 3.0 kWh kg (±0.2 kWh kg).