2.6. Investigation of Total, Fouling, and Reversible Resistance

We determined the total resistance R_{ferm, tot} of each membrane by filtrations with fermentation broth. The feed consisted of 0.75 L medium and biomass to a concentration of 2.5 g L⁻¹ cell dry weight (CDW) mixed with 150 mM citrate-phosphate buffer (pH 5) in a total volume of 1.5 L. When the feed was fully mixed, the pH was adjusted to pH 5 with 37% hydrochloric acid (Carl Roth, Karlsruhe, Germany). Before filtration, the feed was circulated in the system for 10 min at 0.5 bar TMP, a CFV of 0.8 m s⁻¹ (1.6 m s⁻¹ for the 9C_HF membrane) and a temperature of 25 °C with the permeate valve closed. The valve was then opened, and filtration was carried out with manual back-recycling of permeate, until a steady-state flux was established. The total resistance R_{ferm, tot} and the fouling resistance R_ferm were calculated using Equation (3). Single-component resistances were investigated using the 30 nm tubular UF membranes 7C_s and 7C_r. Feed preparation and filtration were then carried out using medium or biomass as described above, and the corresponding resistances R_yeast and R_medium were calculated using Equation (3).

where J_ss is the respective steady-state flux, and the index n indicates the fouling resistance of the fermentation broth, medium, or yeast cells, as appropriate. The respective viscosities are shown in Table 2.

The reversible resistance was calculated by subtracting the irreversible resistance from the fouling resistance using Equation (4).