I have read that the tap water in Lanzarote is not drinkable because it has an unpleasant taste, and may have been stored in an underground tank for some time before it comes out of the tap.
Several different companies produce portable water filters which remove bacteria, viruses, protozoa, lead and other pollutants. These claim to allow safe drinking of contaminated tap water, or even water taken straight from a river. Examples include:
However these filters cannot remove salt, and tend to clog with salt crystals if the water is brackish.
In Lanzarote the tap water is obtained by desalination of sea water. Is there too much residual salt to use these types of filters?