Merakeid 5 - Filtering Liquids (Klal 18) Hilchos Shabbos - S0479

We have finished Klal 18, and need to refer back to a few points made by the Chayei Adam in Klal 16 regarding borer (s432-433) which apply over here.

If a person uses a filter to filter impurities, according to one opinion in the Gemara, it is a toldah of merakeid. We learned that there are three levels of unusable liquids which may be filtered:
If the liquid is inherently drinkable as-is, even if there are slight impurities, it is not considered merakeid (tzalul). 
If it is slightly impure (achur ketzas), such as where there is sediment coming from the tap which is not harmful, it is assur miderabanan. 
If it is opaque (achur legamrei), it will be assur mideroaysa to filter it out.

Examples of achur legamrei include:
A person makes tea on Shabbos (which is muttar according to Rav Moshe through use of a kli shlishi) or makes tea essence before Shabbos, and one of the teabags bursts. The person will not drink the tea as-is, and would use a filter to remove those leaves. Filtering in this case would be an issur deoraysa. 
If a person lives in a place in which there are halachic questions regarding bugs in the water (ie, places where bugs are visible or semi-visible in the water), using a filter would be merakeid deoraysa, since people are makpid on it
Placing pasta through a colander, or using a slotted spoon to pick up salad and allow for the liquid to drip out.

An example of achur ketzas is if there are just a few pieces of loose tea inside the water. It would be an issur derabanan.

We will clarify what to do with a teabag left in tea in the upcoming shiur, be’ezras Hashem.

Summary 
Using a filter to remove impurities from a liquid may be considered merakeid. If the liquid is drinkable as-is, there is no issur. If it is slightly impure, it is assur miderabanan, and if it is opaque, it will be assur mideoraysa.