Cross-flow microfiltration of rough non-alcoholic beer and diluted malt extract with tubular ceramic membranes: Investigation of fouling mechanisms

Abstract

The clarification of rough non-alcoholic beer (RNAB) and diluted malt extract (DME) was investigated in a pilot plant consisting of a tubular ceramic membrane with nominal pore diameter of 0.45 μm. The results of the primary experiments show that the concentration of suspended particles in the RNAB (∼0.05 kg/m 3) and DME (∼0.2 kg/m 3) correlates proportionally to turbidity (in NTU) with the factor of 4.45 × 10 −4 (kg m −3 NTU −1). During cross-flow microfiltration (CFMF), flux declined drastically with time due to fouling mechanisms and propensity. According to the characterizing curve of log( d 2 t/ dV 2) versus log( dt/ dV), fouling is initiated by penetration of aggregates through the membrane surface, followed by blocking of the pores for the first 15 min of operation. Cake formation mechanism controls the fouling phenomenon above 20th minute of operation and continues until achieving a quasi-steady-state flux. The specific cake resistance of DME particles was found to change from 6.7 × 10 14 to 36.9 × 10 14 m/kg for the transmembrane pressures (TMP) of 52–207 kPa and the compressibility factor of the cake was evaluated to be 1.027, indicating that the cake layer is highly compressible. This finding is corroborated with previous works in the literature. This explanation for the mechanisms was further verified using scanning electron microscopy (SEM), which showed a distinct cake layer with thickness of about 30–40 μm on the membrane surface and no constriction of pores. The contributions of each, reversible and irreversible fouling were evaluated by comparison of hydraulic resistances after water rinsing, water backwashing and chemical cleaning of the fouled membrane. The reversible fouling including gel and stationary cake layers contributed to more than 95% of the hydraulic resistance and the remainder was attributed to irreversible fouling. The in-pore fouling contributed to 5% of irreversible fouling resistance and 0.2% of total resistance. These observations confirm that microfiltration of RNAB is controlled by formation of cake layer on the surface of the membrane and therefore hydrodynamic techniques may play a significant role in improving the performance of the operation.