Filtering BIAB wort

I also use BIAB with a bag bought online. The flour from the crushed grain flows pretty freely out of the mash and is present after the boil and chill as a huge cloud settling towards the bottom of my fermenter. Whirlfloc definitely helps clump the stuff together with the cold-crash material, but the goop clogs every filter I have tried.

I've also tried letting the wort settle for 24-36 hours while refrigerating to fermentation temp, then racking to a clean fermenter. That was a huge pain because I did not have an extra gallon to discard, and filtering the remainder at the bottom was impossible. There was also the fear of contaminating my batch by messing around with it in various containers. It seems one could do this effectively with additional equipment like vacuum pumps and Buchner funnels, or maybe a closed system that you could let drip for hours into a giant filter bag...

Long story short, the flour does compact nicely into the trub, and a good cold crash for 2-days at the end of fermentation pretty much takes care of the entire problem for me. On my next batch I am going to try filtering the flour BEFORE the mash by screening it with a #60 screen. Maybe I lose 5-10% of the grain bill that way.

I also have had a similar issue. I switched bags from the handmade swiss voile to one I bought at a homebrew shop which had wider mesh. What happened was pretty disturbing. The beer was suddenly full of fine material which was boiling up and creating a very nasty mess on the side of the pot. I cleaned this crap out several times as it built up over the course of the boil. But there was still material in the wort. I just put it into the fermenter and it settled out on its own and I got a very nice clear beer after fermenting.

Needless to say I went back to my old bag because I liked that it filtered that stuff out. But what I assume is going on is that the mesh on the swiss voile is much finer and is doing this filtering for me. This material is definitely coming from crushing the barley and other grains. So there are a few possibilities to fix this:

1. Crush the grain less or after wetting it to reduce the amount of dust created by crushing.
2. Get a finer mesh bag, I suggest using swiss voile it seems to be pretty much perfect for this.
3. But most importantly don't worry about it, most likely it is going to settle out and be cemented into the trub anyway. Just be careful not to disturb it when you syphon off the yeast cake.