This is a very rare occurance. The fine sediment is in fact mineral salts which originate within the activated carbon itself. When spirit runs over activated carbon which contains some mineral salts, some mineral salts can be absorbed into the spirit. Later, once the temperature has dropped, these mineral salts start to become insoluble in the spirit and after a few days a fine sediment appears in the spirit. This fine sediment (sometimes looks like a milky haze, other times it drops to the bottom of the bottle) is the mineral salts originally from the activated carbon. These mineral salts are absolutely 100% safe (in fact essential for life!) but you don’t want them in your spirit.
Under certain circumstances, some of this residual mineral content gets dissolved into distillate spirit as the spirit flows over the activated carbon. Think of it this way, as the spirit passes through the activated carbon, the carbon absorbs the vast majority of ‘volatiles’ from the spirit and holds them within the internal pore structure – however, under certain circumstances, mineral salts contained within the carbon may pass into the spirit. Whether mineral salts do indeed get dissolved into the spirit depends upon 2 main variables:
1. The amount and types of mineral salts within the particular batch of carbon.
2. The pH and chelate chemical (eg organic acids like citrate are a chelating agent) content of the spirit.
Obviously, we have no control over 1. In a perfect world we would persuade the supplier to first wash with an inorganic acid like they currently do and then wash with organic acids to remove the remaining salt content.
These mineral salts remain soluble in the spirit for some minutes / hours because the spirit temperature is warm and so has higher solubility. After the spirit has cooled, these mineral salts will begin to become insoluble. If you had a spectrophotometer to measure even the slightest haze, you would begin to ‘see’ the spirit ‘go hazy’ after just a few hours. To the naked eye, you will not start to see these solids until after 2 or 3 days (may be less or more depending upon the level of mineral salts present).
Re-filtering through activated carbon will not help, but ‘re-filtering’ through an ordinary wine filter or even a coffee filter, say 1 week after the filtering through carbon would remove the insoluble mineral salts and hence solve the problem. But spirit should be stored cold (not frozen) during this week to ensure anything that is going to become insoluble, does become insoluble). If the spirit is left for long enough (3 or 4 weeks?), it should be easy to pour off the bright spirit from a white sediment at the bottom of the bottle.
This problem is likely to be influenced by certain environmental conditions like temperature and water quality.