The stories you are hearing about bio seperating are from backyard bio makers. Commercially made bio will not seperate.
The sludge you are talking about happens to untreated diesel as well. It is called gelling. diesel will freeze but it takes extremely cold weather. How ever in the cold weather we have it thickens up and turns into a jello like substance.
Commercially made diesel that you buy at the pump is treated with winterizers from October-April. Usually during this time people that buy diesel complain about a loss in fuel mileage. The loss in fuel mileage is usually from the additives from the production of winter blend diesel.