Modeling long-term subcritical filtration of model EPS solutions

Using alginate as a model extracellular polymeric substance, a two-stage TMP profile, with a slow rise followed by a rapid rise, similar to that typically observed with membrane bioreactor effluents was observed during long term filtration. The alginate transmission profile supported the concept of flux redistribution among open pores as a cause of the two stage fouling profiles. A model based on fouling mechanisms was developed to take into account pore plugging of larger pores and flow redistribution among other pores as a function of time. A critical pore size is used in the model for transmission time determination. Using the pore size distribution of polyvinylidenefluoride membranes, the model is able to fit the experimental results with two adjustable parameters. Both parameters can be correlated to the average imposed flux.

By using alginate as the model solution, the results provided a better understanding of the role of EPS in MBR fouling and the two stage of TMP increase observed during long term subcritical flux filtration. The two stage of alginate transmission profile during the long term subcritical flux filtration supported the hypothesis of that redistribution of flow through the remaining pores as a cause of two stage of fouling profiles. Comparison between experimental data and model results indicated that the fouling mechanisms of the long-term subcritical flux filtration of alginate might due to the standard pore blocking in the early subcritical flux stage followed by cake filtration. By fitting the model calculation with the experimental data for different permeate flux, it was found that the proportional of alginate deposited on the membrane pore (k1sp) in the first fouling stage and the speed of cake resistance increase (k2) were in exponential relation with the permeate flux. Thus for a specific permeate flux, the TMP vs. time dependence can be predicted. The increase of the proportion of alginate whose con-vective forces are higher than the energy barrier may be the reason for the exponential relation between k1sp and flux. The exponential relation between k 2 a nd flux supported the assumption of using critical pore size for the transition time deter-mination.