Dried herbs, teas, and coffee have various grades and degrees of grind but no matter how precise the grading or grinding they all have some bits and pieces that are smaller than the desired size. It is these small parts that are left in your cup as lees, dregs, and fines. That bit of powder that clouds your brew and settles to the bottom of your cup. Those broken pieces that escape the filter.
A filter that allows water through is going to allow small particles through. If the filter is too fine, you don’t get a good steep or infusion. Too coarse and you wind up with too much grit in your brew.
The filter we use for the Cold Morning Cold Brew Coffee and Iced Tea Maker allows anything smaller than 150 microns through. A 150 micron hole is very small. A human hair is 80-100 microns in thickness. 150 microns is a good balance between allowing the flow of water and filtration.
So what do we do about the stuff that came through the filter that we don’t want to drink? I let them settle to the bottom of the jar and then pour off what I want to drink.
After brewing a new batch of coffee, I will often remove the filter, pour off everything except the fines into a separate container, wash my jar and then transfer everything back except the filter.