Modeling of fouling reduction by secondary membranes

The external cake of rejected particles plays an important role in filtration processes, often influencing the permeation rates of all species. It may have higher resistance to the permeate flow than the clean membrane itself, but it may also act as a secondary or dynamic membrane which screens the primary membrane from the more strongly fouling species of smaller size. The screening process due to the presence of the secondary membrane is modeled for filtration of a bidisperse suspension of large and small particles. The large particles form an external cake on the surface of the primary membrane, which then acts as a deep-bed filter and captures some of the small particles and prevents them from fouling the primary membrane. The theory predicts an optimum concentration of the large particles which maximizes the permeate volume for a given duration of filtration. To test the model, cross-flow microfiltration experiments were done with yeast suspensions, protein solutions, and yeast–protein mixtures using cellulose acetate membranes. During filtration of yeast–protein mixtures, protein aggregates were captured or screened by the yeast cake, which reduced fouling of the primary membrane by the protein aggregates. Under optimum conditions, the presence of yeast increased the total permeate volume by two fold.