For water to be drinkable, it needs to be clean of
1. Harmful sediment
2. Pathogens (parasites, bacteria, virus)
3. Harmful substances (heavy metal pollutants, chemicals, dissolved salts) - This is a link to the wiki article for
heavy metal polluted water in the town of Flint, Michigan, USAMake safe 1 by filtering, 2 by boiling, (or chemical means like iodine purifier, bleach, etc) but 3 is a problem if you use well/groundwater. You can get rid of all by distilling the water, but this takes a lot of energy to do.
Rain water is easier to convert to drinking water, but there may be a problem if there is a lot of atmospheric pollution, as rain water absorbs these pollutants as it falls. There is also the problem of local pollution like bird poop from the roof or rotted leaves (spores and fungus) being mixed into the rainwater as it is collected.
In short, I would consider rainwater as
"grey" and use it only as such. I would attempt to obtain drinking water from alternate water sources only in an emergency or survival situation. If you are still keen to go this route,
here is a wiki to a solar still, which you may be able to scale up.
If I were in your situation in Kelantan, I would try to solve it by having multiple storage tanks. But don't say Kelantan... a lot of my friends have multiple storage tanks already in Selangor with all the water cuts happening. Good luck. It absolutely sucks not to have clean water on tap.