Hi Hakizimana, millions of ...

Hi Hakizimana, millions of households and rural schools catch rainwater in concrete, galvanized iron or plastic tanks all over the world and use it directly without any treatment.  This is not to say it is 100% safe, but contamination is usually very low.  What is usually recommended is to incorporate a "first flush bypass" system which ensures that the first rainwater from a rainfall event (containing the most contaminants) by-passes the tank and only the clean fresh water then enters the tank.  Using a small amount of chlorine will safeguard the water (about 250 ml household bleach will be adequate for a full 25 m3 tank), but will give it a slight taste (losing it's freshness).  Enjoy the fruit of your labours.