This equation has been applied to analyze microfiltration data of activated sludge, and it is shown that activated sludge is highly compressible [28].
Hence, a plot of (dt/dV) versus V gives a straight line. The slope (ηαavc/ΔPA2) is common called the average filtration resistance (AFR) of the gel or cake, which has a unit of h/dm6. Fig. 10 shows the results, indicating good linear relations. The values of AFR obtained are listed in Table 1. It is evident that the fouling is more serious for membranes with higher Jw (i.e., YM30). Moreover, for both membranes AFR decreases in the order Zn(II) ⪢ Cu(II) > Ni(II) ∼ Co(II). Satisfactory explanation for such discrepancies cannot be given at this stage, likely due to different conformations of the metal–chitosan aggregates. Practically, all gelatin-like flexible gel or cake layers can be easily removed by flushing with water immediately after UF. In a slightly different analysis method, the filtration resistance for PEI-enhanced UF of metal chelates with the same membranes (0.08–2.11 h/dm6) was much less than that obtained here [14]. Further work on this subject is being undertaken in our laboratory to clarify the differences of fouling phenomena.