'Semi- permeable' is to allow certain molecules to pass through but acting like a barrier to others. The semi-permeable membrane permeated impurities like bacteria. So water that pass through is free from toxin and of a high quality.

No. There are microflitration and ultraviolet disinfection. Microflitration is used to get rid of large molecules and ultraviolet disinfection is to assure that all organisms are dormant.

One can heat up the water and let it boil.