High-frequency flow reversal for continuous microfiltration of milk with microsieves

A new filtration method using rotating micro-engineered membranes (microsieves) is described. The method uses constant rotation of the microsieve in combination with high-frequency flow reversal to achieve stable and high fluxes. The high-frequency flow reversal is enabled by a vacuum device placed at the retentate side creating a local negative pressure to purge the microsieve from accumulating particles. The method was validated for milk filtration using microsieves with 0.9μm circular pores with a trans-membrane pressure (TMP) of 0.05-0.30bar and flow-reversal frequencies between 5 and 25Hz. Stable filtration (for at least 4h) was achieved with permeate fluxes for skimmed milk from 8 to 50m³h^-1m^-2 at room temperature, and for whole milk from 7 to 13 m³h^-1m^-2 at 50°C. The effect of the TMP, rotation frequency and flow-reversal strength on the flux was investigated. The flux increased proportional with the TMP up to a critical TMP of about 250mbar. Also the flux increased with rotation frequency up to an optimal frequency of 20Hz. The flow-reversal unit requires a threshold backward flow to create enough negative pressure to overcome the TMP during the removal cycle of the retentate particles. Above this threshold the flux increases strongly with increasing backward flow. The novel high-frequency flow-reversal method enables continuous milk filtration of large volumes during many hours without the need of a chemical cleaning cycle.