Low cross-flow velocity microfiltration of skim milk for removal of bacterial spores

The use of microfiltration membranes in the dairy industry to remove bacterial spores has been applied for some time using the so-called “bactocatch” process. However, these microfiltration units have high energy demands since high linear velocities are required during operation, to avoid problems related to fouling and concentration polarization. In this work, optimization of a backflushing technique combined with reverse asymmetric membranes (‘backshock’ technique) was used to avoid the fouling and concentration polarization problems, allowing the use of low linear velocities and resulting in low-energy costs. In the novel ‘backshock’ technique, the permeate is pressurized during a very short time interval (less than 1 s) and with a frequency around 0.2-1 s⁻¹. The benefit of using a reverse asymmetric membrane is related to the formation of a very open fouling layer just inside the porous support layer and the build-up of a concentration profile of the proteins inside the porous structures. The microfiltration of skim milk using ceramic and polymeric membranes was studied with different membrane structures. The backshock technique, combined with reverse asymmetric membranes of pore size of 0.87 μm, allows the effects of concentration polarization and fouling to be controlled, achieving very high (500 L h⁻¹ m⁻²) and stable fluxes with 100% casein transmission and a high retention of spores (reduction by a factor 10⁴-10⁵) even at low linear velocities (0.5-1 ms⁻¹).