2.5. Estimation of Intrinsic Membrane Resistance and Irreversible Resistance

The intrinsic membrane resistance R_m [m⁻¹] was determined before filtration and after chemical cleaning. It was calculated from the pure water flux at a CFV of 0.8 m s⁻¹ (1.6 m s⁻¹ for the 9C HF membrane) and TMPs of 0.1–1.1 bar at 25 °C using Equation (1):

where R_m is the intrinsic membrane resistance [m⁻¹], η is the permeate dynamic viscosity [Pa s], TMP is the transmembrane pressure during filtration [Pa], and J_w is the pure water permeate flux [m³ m⁻² s⁻¹]. The viscosity of water is shown in Table 2. To calculate the irreversible fouling resistance R_{n, irrev} (Equation (2)), the fouled membrane was flushed with deionized water at 50 °C for 20 min using the same process parameters described above and with the permeate valve closed, then with fresh deionized water at 50 °C for 20 min under the same process parameters with the permeate valve open. The index n indicates the fouling resistance of the fermentation broth, medium, or yeast cells, as appropriate.

Viscosity of each solution used in filtration experiments at 25 °C.

The pure water flux of the rinsed membrane was then measured again as described above. The irreversible resistance corresponds to the resistance that can only be removed by chemical cleaning and not water rinsing.