Hollow fiber ultrafiltration: The concept of partial backwashing

In dead-end hollow fiber filtration with humic acid solutions, we observed that the humic material accumulates in the final part of the fiber. We proved the hypothesis that backwashing of the last part of the fiber is sufficient to operate the filtration process in a sustainable manner. This strategy works very well for feed solutions that result in either local accumulation of the material at the end of the fiber, or that form a well-defined concentration polarization layer that does not lead to deposition. When deposition throughout the whole module occurs, the proposed cleaning strategy is unsuccessful. This overall deposition occurs when a force balance over the particle, results in net transport to the membrane surface over the entire length of the module. We simulated such conditions by the addition of calcium to the feed solutions. The latter results in large aggregates carried towards the whole membrane area by convective water flow, and an increased interaction between the aggregates and the membrane.