Shear-enhanced microfiltration of microalgae in a vibrating membrane module

The performance of a vibratory shear-enhanced membrane process for dewatering of freshwater microalgae, Chlorella vulgaris, has been studied. Chlorella vulgaris is a potential renewable feedstock for biofuel and bioproduct production. The efficient dewatering of the algal biomass is crucial for scale-up and sustainable design. This dynamic filtration system achieves high shear rates desirable for microfiltration by high-frequency torsional oscillations of the membrane unit. A 0.05 µm (nominal pore size) polyethersulfone microfiltration membrane was evaluated for the separation of suspended algae (0.5–100 g/L). The effect of process parameters such as trans-membrane pressure, surface shear rate, and solute concentration on permeate flux was evaluated and quantified. Algal biomass mixtures were dewatered with high algae rejections for all studies. The effect of trans-membrane pressure on permeate flux showed a classic pattern of a pressure-controlled region at lower pressures transforming to a mass transfer gel layer-controlled region at higher pressures, with quicker transitions at higher algae feed concentrations. The shear rate at the membrane surface was varied by changing the vibrational frequency of the unit. Permeate flux values observed in dynamic filtration mode, compared to those in cross-flow filtration (CFF) mode, were greater by a factor of 4.2–4.9. This process could provide a greener alternative to conventional mechanical and thermal separation systems, as high values of permeate flux and separation efficiency can be maintained with an energy consumption of 1.6 kWh/m³ of water removed.