Rain water is a natural attribute of the earth's weather system. Air currents in the atmosphere bring evaporated water from the ocean and the earth's surface up into the sky. The evaporated liquid condenses in the cold air, forming moisture-filled rain clouds. Rain water's most well-known and important effect is providing you with water to drink. So we want to collect it. Rainwater harvesting is a technique to store rainwater for reuse before it reaches the aquifer.
But some factors like mercury from coal burning buildings, bird faeces etc may destroy the rain water. So we cannot use it. Also some rooftop material may demolish rain water and that may be very harmful to human health. So that water can only be used for flushing toilets, washing clothes, watering the garden, car washing etc. After purifying this water we can use it as drinking water. Due to the lack of water, people lived in apartments in Chennai uses these purifying water. The methods or technique to purifying rainwater includes chemical purification, solar pasteurization, boiling etc.