1) Filtering tap water? 2) Straining foods with a sieve? 3) Sifting clumps from powdered baby cereal? 4) Using a salt shaker? 5) Filtering pressed fruit juice? The melacha is the separation and removal of unwanted matter from a mixture by sifting it through a sieve or similar device, eg using a sieve to remove solidified clumps of sugar in a packet of sugar. Note, nothing wrong with using a sieve to spread thin coating of sugar on a cake as nothing is being sifted. This melacha is the act of separating or isolating from a mixture, not merely scattering or sprinkling particles. Not only is sifting included, but so is straining and filtering. So, straining sediment from wine for example is a problem. It applies to non food items too. It's irrelevant whether what you desire or don't desire is removed, either way its a problem. It's only a problem when it "significantly improves". But let's say most people would not be bothered by what's there, you can't define this as a "significant improvement ". So let's look at impurities in water. If it's so bad people wouldn't drink it under almost any circumstances then filtering would be a "significant improvement ". Other examples would include: tea containing many tea leaves, old wine with heavy sediment, fresh juice with lots or pips and peal. But even a level lower than this whereby there is a noticeable amount of impurities that most people would only drink if filtered is also a problem. So this would mean you shouldn't filter tap water that's safe but cloudy enough that most would filter it, tea containing a small number of leaves. It's only liquids containing mixtures that are drinkable to most people that are OK to filter, eg our tap water, so you can filter it on shabbos. Please note if someone is finicky you couldn't filter for them on shabbos. Question 1) Tap water ok to be filtered if in place where most people would be able to drink it without filtering. Question 2) Straining foods with a sieve not ok. Question 3) Sifting clumps from powdered baby food not ok. Question 4) Using a salt shaker is ok as it's only an issue when removing undesired particles. Would be an issue if undesirable matter was being held back in the sieve like cap. Question 5) Filtering pressed fruit juice is ok as long as it would be drinkable to most people in its untrained state. If you personally are finicky you couldn't strain it. In any event this only applies to minor things in juice like pulp and seeds that would bother people.