In vitro evaluation of critical ultrafiltration fluxes and transmembrane pressure in a high flux dialyzer

In patients treated with haemodiafiltration, high convection volumes are considered beneficial. However it leads to pressure instability and membrane fouling. We aimed to identify critical ultrafiltration fluxes based on different approaches including the maximal global ultrafiltration coefficient (_GK_D-UF max), and to test the influence of ultrafiltration on system stability and membrane fouling. Experiments of cross-flow filtration of a protein-containing fluid (cow milk) were performed. The ultrafiltration rate (Q_UF) was sequentially modified using a peristaltic pump and transmembrane pressure (TMP) was recorded. _GK_D-UF and TMP stability over time were assessed. Q_UF critical values were estimated from the _GK_D-UF, critical flux, irreversible fouling and sustainable flux approaches. Membrane fouling was observed by microscopy. Proteins from the feed, ultrafiltrate and retained on membrane were assessed by protein assays and SDS-PAGE. The _GK_D-UF max approach identified Q_UF critical values close to the irreversible fouling and sustainable flux. When Q_UF exceeded critical values, major increase in TMP over time was observed and more clogged dialyzer fibres were detected. Utilizing Q_UF below the _GK_D-UF max critical value lead to stable TMP over time and fewer clogged fibres therefore _GK_D-UF max is helpful to identify the critical ultrafiltration rate and can be used to optimize ultrafiltration flow that prevent membrane fouling.