Dangers of rainwater harvesting without Purification
Despite the fact that rainwater from rainwater harvesting is usually of a much higher quality than water that is obtained from most of the other sources, it still needs to go thru a rainwater purification process to be safe for consumption.
Cleansing the roof and water tanks and having adequate filters in the rainwater collection system is not adequate.
Sadly a few rainwater harvestors are fooled by the nice clean taste of rain water and the clean look it has, so much so that they think the water may not require further rainwater purification before it can be used for human consumption. This is a mistake that can have deadly repercussions.
To start with, during rain harvesting, when the rain comes into contact with a collection surface like a roof, it will tend to wash contaminants off that surface. Some of these contaminants may be so small as to be difficult to see with the naked eye. And even if they can be seen they will tend to settle at the bottom of the rainwater storage tank where they will be virtually impossible to see when the tank is full, even if one makes an effort to inspect the water tank.
Rainwater contaminants can be mold, bacteria, algae, protozoa and small particles of dust.
But there is an even deadlier group of contaminants that can be present in rainwater from rainwater harvesting. These are micro organisms like bacteria. Not to mention pesticides, lead and arsenic. These are all highly poisonous to the body.
If the rainwater from rainwater harvesting is to be used in the house for drinking and other household chores, then it has to be both filtered and treated so as to kill micro-organisms and remove all the contaminants.
The rainwater must go through various steps of cleansing, filtering and Purification. They include screening, settling, filtering and disinfecting. The screening stage stops debris and large particles from entering the rainwater storage tank. The settling process then further cleans the rainwater by allowing the tiny particles that got through the screening process to settle at the bottom of the rainwater tank.
Filtering is then supposed to remove sediment and contaminants and trap other smaller particles. The final process of disinfecting the rainwater can be done using chlorine, ozone or even ultraviolet light to kill harmful micro-organisms.
Any rainwater from rainwater harvesting that does not go through a careful filtering and disinfecting process, is dangerous for drinking or human consumption of any sort and should be avoided.
There is no point in taking the trouble to install a rainwater collection system and then being casual about the safety of the water you get from your efforts. Researching proper rainwater purification is the smart thing to do.