Easy guide on purifying stored rain-water for drinking

Rainwater is known to be one of the purest sources of water to the occupancies of planet earth. With no discrepancy, the claim has been proven to be true in some but not all cases. If we date back to the era of our great fathers, this water was just harvested and readily consumed with no health hazards .But in the modern world we live in, it isn’t advisable to go by that way of life, due to man’s activities in one way or the other which has been causing tremendous damage in the atmosphere and his immediate surroundings. Rainwater itself is pure because in its natural state, it’s distilled, but gets impure with the release of carbonic oxides, Nitrogen oxides and other likely oxides exposed to the atmosphere by industries which end up polluting the atmosphere. Rainwater purification can’t be discussed without laying emphasis on how to harvest rain water itself.

Rainwater harvesting involves the collection, storage and usage of rain that falls onto a catchment area of a building structure. Storage is done by the use of cisterns/water tanks, barrels and serves as a complimentary source to main water or a partial solution to the non-existence of immediate water in dry lands, coastal regions etc. Rainwater harvesting is collected usually from rooftops or other hard surfaces which allows the runoff of the volume of rain into channels or down-pipes which then directs the flow into cisterns. Rain can be harvested in two major ways, which are:

  • Surface Runoff harvest
  • Rooftop harvest

Rooftop harvest is predominantly used to harvest rain water. It is a technique by which rainwater is being captured from the roof catchment and channeled down into the cisterns with the use of pipes for the purpose of storage.



There are various ways to purify the rainwater for domestic usage. But when we talk about consuming it, optimum care needs to be attained. For a lame man, the very first method known is Boiling, which in all sense is one of the best methods of killing pathogens, bacteria, viruses and algae which may be present in the water. When the rain water goes through boiling, it aids in killing the biological organisms (like bacteria, viruses, algae etc.) present in the rainwater. With that been done, one can’t come into a conclusion that the water is well purified and freed of any harmful substance. One should be very versatile with the fact that boiling for purification only removes the biological contamination but it doesn’t;

  • Remove chemicals or Toxins present in the Rain water

Release of toxins in the air by smoke and soot can be dissolved in rainwater from the air before it gets to the earth surface. Materials used for roofing, gutters, piping and cisterns used for storing the rainwater harvested can introduce harmful substances like lead, asbestos and its likes into the harvested rainwater.

  • Remove particles

Particles present in rainwater can be due to the dirt, dust and germs washed through connected rainwater from the roof. This happens mostly when it rains after several days of dry weather.

However, before going through the process of boiling, which we believe should be the last method to be carried out on rain water. It’s more effective for rainwater to pass through some or all of the following methods of purification:

  1. Sedimentation
  2. Filtration
  3. Chlorination
  4. Iodization
  5. Ultraviolet Irradiation
  6. Reverse Osmosis


In any harvesting system, be it surface runoff or rooftop harvest there’s always a certain amount of sediment which are unavoidable during the process of harvesting. The method of sedimentation simply involves allowing particles (coarse or fine particles) present in the cistern or rain barrel to settle down. When this is attained, the impurities in terms of particles are all settled at the bottom of the cistern. One should know that settling of the particles isn’t 100% guaranteed; some particles still lay at the top portion of the water. This is when filtration comes into play.



Filtration is undoubtedly the most common method of purifying water for consumption after boiling, because of its effectiveness. It is advisable that a cistern or rainwater barrel used for harvesting should be constructed in such a way that the outlet pipe doesn’t take water from the bottom, but rather from a point an inch or two from the bottom of the cistern/rain barrel. Water filtering system comes in various forms and rated by the smallest sizes of particles they are able to filter.

Filtration involves the removal of particles, bacteria, large parasites and some heavy metals present in the rain water by the use of filters. It is advisable for water to go through three stages of filters to ensure the optimum removal of impurities. For rainwater purification, water should be passed at first through a larger filter (50 micron and above) in other to eliminate the coarse or fine particles that may likely pass out through the outlet pipe. The next filter should be a 20 or 10 micron filter to further eliminate particles which by chance wasn’t retained in the first filter. The last filter should be one with a very small pore space (5 to 10micron). The screen of the filters should be one that is easily accessible and should be cleaned frequently if not often.



This is the process by which chlorine is used to disinfect rainwater. The usage of chlorine has been of great reputation and familiarity, mostly used in our swimming pools and industrial water making companies for water purification. when using chlorine to purify rainwater for consumption, one needs to be overly careful in order not you use a great deal of chlorine and allow for it to disinfect the water for at least 30-40 minutes. Chlorine doesn’t kill all pathogens, cysts ,algae and viruses present in the rainwater due to the resilient nature of some microbial organisms to chlorine.

Household bleach is an alternative to chlorine, but it’s of a great concern due to its chemical contents and factors. Too much concentration of chlorine in water purified for consumption may end up given the water an unlikely and unpleasant taste with a likely side effect.



Just like the former, iodizing rainwater in order to purify it for consumption is in tally with the process of chlorination. The use of Iodine in place of chlorine is the difference in this method. Iodine which more popularly known as a powerful disinfectant that helps prevent thyroid problem but also used in water purification system. It can eliminate the presence of Guardia cysts in the rain water which is very detrimental to one’s health when consumed but it’s ineffective against cryptosporidium.

Iodine works effectively just as chlorine. When added to the rainwater it should be allowed to sit for at least 50 minutes instead of the standard 30-40 minutes for optimum disinfection. Iodine is a very powerful substance and can be lethal to one’s health in high dosage. It also leaves an unpleasant taste and smell in the water. In other words, it is not advisable to use iodine as a regular method of purification of rainwater, unless when other methods are unavailable. People with iodine allergy, thyroid problems, pregnant women and children should avoid the usage of iodine for purification.


Ultraviolet radiation;

UV light purification system is greatly effective to eradicate the threat from microbial organisms present in the rainwater due to the wider range of microbes that are defenseless against the concentration of UV radiation. It destroys the DNA of the microbes, thus rendering them harmless to health. The Process of filtration must be carried out on the rainwater in order for the UV lights to work perfectly, effectively and give an accurate result. In cases where filtration wasn’t carried out, the organisms ends up being shaded and thus prevented from concentration of the ultraviolet light which may then lead to growth of algae and more bacteria.

UV water purification system has a lot of similarities to solar (sun) purification, but it produces more ultraviolet light concentration than that of the sun and therefore more powerful and effective than it. It also more effective than chlorination by killing chlorine resilient microbes such as Norovirus and Hepatitis, which is done instantly as opposed to the standard minutes of chlorination. UV radiation has proven to be able to eradicate 99.9% of microbes contaminants like bacteria, waterborne viruses, molds and parasites present in water.

The efficiency of this method in killing microbes depends on the intensity and frequency of the UV light. The higher the concentration and intensity, the more effective is the elimination of microbes and vice versa. Unlike Iodine, UV radiation does not affect the rainwater in any way possible; no unlikely taste not odor will be detected. Though it is effective to doom the life of microbes, UV light can’t remove heavy metals and particles present in water.

The UV light is costly and requires a very high maintenance and needs the presence of electricity. It’s recommended to change it bulb after 9000 hours or approximately annually.


Reverse Osmosis;

Osmosis is a natural occurring phenomenon by which less concentrated solution moves toward a more concentrated solution. Reverse Osmosis as it name implies is the opposite; that is a more concentrated solution moves toward a less concentrated solution. Taking a contaminated rainwater with microbes and some other contaminants(I.e. a more concentrated solution) placed in a RO device, the  solution will be forced through a membrane to a less concentrated solution by removing it’s contaminants in the process.

Reverse osmosis is a process that forces water through a semi permeable membrane in order to eradicate contaminants present in it. It is extremely effective at eradicating microbes such as bacteria and virus, parasite cysts such as Guardia and Cryptosporidium, heavy metals such as lead and mercury, and hard water minerals such fluoride, magnesium, calcium and arsenic. However reverse osmosis is not capable of removing some certain pesticides or solvents that are tiny enough to pass through it membrane.

RO system has been in usage for a very long time as a standard water purification system for the bottled water industries. RO system for household is newly invented; it can be linked to the faucet at homes. The system is expensive and requires frequent maintenance by changing the cartridge.

NOTE: As it is said earlier if at all the above Methods of rainwater purification are to be carried out, it’s advisable to be done before Boiling. After Boiling, the next method should be Distillation.


Other method of purification

  • Electro-dialysis Reversal (EDR)
  • Nano-filtration
  • Micro-filtration
  • Solar pasteurization
  • Solar still
  • Flocculation
  • LED irradiation
  • Bio-remediation
  • Ozonisation