A module of hollow fiber membranes is re-configured to break up and rearrange a skein into multiple miniskeins each having no more than 250 fibers, all lower miniheaders of the miniskeins being slidably held for vertical movement in vertical slots in an aerator box. Upon shrinkage of the fibers, the lower miniheaders rise in the box. Each miniskein has from 2-12 planar ordered arrays in which the fibers are adhesively secured, preferably without potting the lower miniheaders. The lower ends of the fibers are dead-ended and lower miniheaders are positioned sequentially, transverse to the longitudinal axes of the upper header and the aerator box; its upper miniheader is potted in a choice of configurations. This configuration of miniskeins provides a packing density in each miniheader, whether upper or lower, not attainable in a prior art skein; the use of an aerator box having an area (footprint) no greater than 35% of that of the upper header, provides an effective packing density in the aerator box which provides exceptionally effective use of sparging air and readily removable sludge which drops through the bottom of the aerator box.
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